了解串行数字视频的抖动测量(英文版)

Understanding

Jitter Measurement for Serial Digital Video Signals

A Tektronix Video Primer

Jitter Measurement for Serial Digital Video Signals

Primer

Contents

1.0Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

2.0Fundamental Concepts and Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

2.1.Encoding method, unit interval, SDI signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

2.2.Decoding process, clock recovery, bit scrambling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

2.3.Time interval error, jitter, jitter waveform, jitter spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

2.4.Decoding errors, normalized jitter amplitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

2.5.Wander, timing jitter, alignment jitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

2.6.Random jitter, deterministic jitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

2.7.Intersymbol interference, equalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

2.8.Pathological signals, SDI checkfield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

2.9.Decoding decision threshold, AC-coupling effects, symmetric signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

2.10.Jitter input tolerance, jitter transfer, intrinsic jitter, output jitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

2.11.Eye diagram, equalized Eye diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

2.12.Equivalent-time Eye, Real-time Eye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

2.1

3.Bit error ratio, Bathtub curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

3.0Specifications on Video Jitter Performance and Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

3.1.Standards documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

3.2.Specifications on jitter frequency bandpass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

3.3.Specifications on signal voltage levels and transition times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19

3.4.Specifications on connecting cables and other system elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19

3.5.Specifications on peak-to-peak jitter amplitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

3.6.Specifications on measurement time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

3.7.Specifications on data patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

3.8.Summary of jitter specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

4.0The Functions Comprising Jitter Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

4.1.Equalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

4.2.Transition detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23

4.3.Phase detection/demodulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

4.3.1.Phase detection/demodulation: Equivalent-time Eye method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

4.3.2.Phase detection/demodulation: Phase Demodulation method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28

4.3.3.Phase detection/demodulation: Real-time Acquisition method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30

4.3.4.Phase detection/demodulation: Summary of methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32

4.4.Measurement filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33

4.4.1.Filter realization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33

4.4.2.Filter accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35

4.5.Peak-to-Peak measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

4.5.1.Peak-to-peak detection methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

4.5.2.Independent jitter samples and normalized measurement time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

4.5.3.Measuring the peak-to-peak amplitude of random jitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37

4.5.4.Measurement times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39

4.5.5.Dynamic range and jitter value quantization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39

4.6.Jitter noise floor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .40

https://www.wendangku.net/doc/5314911932.html,paring jitter measurement methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41

5.0Data Error Rates and Jitter Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44

5.1.Random jitter and BER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44

5.2.Jitter measurement and standards compliance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45

5.3.BER and jitter measurement time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46

5.4.Jitter budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47

6.0Jitter Measurement with Tektronix Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48

6.1.Jitter measurement with the Tektronix WFM700M . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48

6.2.Jitter measurement with other Tektronix video instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48

6.2.1.Wander rejection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49

6.2.2.Measurement of random jitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49

6.2.3.Measurement of deterministic jitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50

6.3.Jitter measurement with Tektronix real-time oscilloscopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50

7.0Recommendations for Measuring Jitter in SDI Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51

7.1.Video system monitoring, maintenance and troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51

7.2.Video equipment qualification and installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51

7.3.Video equipment design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52

8.0Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53

9.0References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54

10.0 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54 Appendix A:Impact of bandwidth limitation in video jitter measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55 Appendix B:Peak-to-Peak and RMS measurement of typical video jitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .57 Appendix C:Limits to clock recovery bandwidth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .58 https://www.wendangku.net/doc/5314911932.html,/video

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https://www.wendangku.net/doc/5314911932.html,/video In this technical guide, we describe the different techniques for measuring jitter in serial digital video signals and how they can lead to different measurement results. We further identify areas where the standards should supply additional specifications and guidance to help ensure more consistent jitter measurements.

1.0 Introduction

Jitter Measurement for Serial Digital Video Signals

Primer

This guide focuses on video jitter measurement techniques typically found in video-specific instruments, e.g., waveform monitors and video measurement sets. General-purpose measurement instruments, e.g., sampling and real-time oscilloscopes, are also used to measure jitter in serial digital video signals. These instruments can offer more extensive jitter analysis capabilities based on sophisticated signal processing.

We will briefly touch on some very basic aspects of video jitter measurement using general-purpose instruments in this guide, specifically related to comparing results with measurements made on video-specific instruments. We will not explore the range of jitter measurement capabilities available on sampling or real-time oscilloscopes, or on other general-purpose instruments.

For the most part, this guide describes jitter measurement methods broadly. It does not give details on specific

implementations in particular instruments. It does describe some aspects of jitter measurement on Tektronix video-specific instruments to illustrate some of the key concepts discussed in the guide.

Timing variation in serial digital signals and the measure-ment of these timing variations are complex technical top-ics. To explain how and why jitter measurements differ, this guide gives a technical overview of jitter measurement techniques and includes technical descriptions of several key concepts. Although we examine jitter measurement in some detail, we do not comprehensively cover all aspects of this topic nor do we explore jitter measurement in extensive technical depth.

Rather, this guide focuses on describing common reasons for differences in measuring jitter in serial digital video signals. In particular, it examines differences associated with the jitter frequencies in the video signal and with the duration of the peak-to-peak amplitude measurements

used to characterize jitter in video systems. It will not cover some topics often mentioned in other discussions of jitter measurement, e.g., techniques for separating random and deterministic jitter components.

The material assumes some understanding of serial digital transmission theory and practice, the design and implemen-tation of signal acquisition systems, the mathematical tech-niques used in characterizing signal transmission, and the properties of random processes.

This guide contains the following major sections:

Fundamental Concepts and Terminology:Reviews the key concepts and terminology we will use to describe jitter measurement.

Specifications on Video Jitter Performance and Measurement:Surveys relevant standards and specifications.

The Functions Comprising Jitter Measurement:

Examines the steps involved in measuring peak-to-peak jitter amplitude, the different ways to implement these steps, and the impacts these differences have on measurement results.

Data Error Rates and Jitter Measurements:Explores the relationship between data error rates in video sys-tems and the requirements for measuring the jitter per-formance of video equipment used in these systems.Jitter Measurement with Tektronix Instruments:Describes implementations of jitter measurement methods in Tektronix instruments and explains differences in measurement results.

Recommendations for Measuring Jitter in SDI

signals:Recommends tactics for effectively using jitter measurement methods and tools.

Jitter Measurement for Serial Digital Video Signals

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In this section we review some fundamental concepts and

terminology needed to describe jitter measurement. This review will briefly touch on several concepts. It does not cover these concepts in any depth.

Those experienced in digital communications will be familiar with many of the concepts reviewed in this section. They may wish to skip this part of the guide, or scan the material to review any less familiar terminology or concepts.2.1 Encoding method, unit interval, SDI signals Distributing digital video over any significant distance requires converting the digital content into a serial digital video signal. Creating these signals involves converting the original digital content into a sequence of individual bits and representing these bits by voltage or light waveforms. A clock signal determines the time interval used to encode a bit in the sequence and an encoding method determines the signal characteristics that represent a ‘0’ or a ‘1’ bit value, e.g., Manchester encoding or NRZ encoding. The time interval corresponding to one bit in these serial data signals is called the unit interval (UI).

The Society of Motion Picture and Television Engineers (SMPTE) has approved standards that define a serial digital interface (SDI) for digital video equipment. SMPTE 259M defines the interface for standard-definition (SD) digital video formats and SMPTE 292M deals with high-definition (HD)video formats. We will refer to serial digital video signals conforming to these standards as SDI signals .

The SMPTE standards define serial digital interfaces for sev-eral different video formats. The information on jitter meas-urement given in this technical guide applies to SDI signals conforming to any of these specifications. In this guide, we will reference two very common types of SDI signals:The 270 Mb/s signals conforming to SMPTE 259M specifications for standard-definition, 4:2:2 component video with either a 4x3 or 16x9 aspect ratio as defined in ITU-R BT.601- 5 (SD-SDI signals)

The 1.485 Gb/s signals conforming to SMPTE 292M specifications for various high-definition video formats (HD-SDI signals)

The SMPTE standards specify that the clock frequency used to create these SDI signals will equal the signal bit rate. As a result, SDI signals encode one bit in one clock cycle, i.e. the unit interval equals the clock period. So, the unit interval of a 270 Mb/s SD-SDI signal equals one period of a 270 MHz clock or 3.7 ns. Similarly, the unit interval of a 1.485 Gb/s HD-SDI signal equals 673 ps or one period of a 1.485 GHz clock.1

The SMPTE standards also specify that SDI signals encode the serialized data-bit values using the NRZI method (Non-return to Zero Inverted). In this method, ‘0’ bit values are encoded as no change in the signal level, while ‘1’ bit values are encoded as a change in the current signal level.If the current signal is high, a ‘1’ bit value causes a transi-tion to the low signal level. If the current signal level is low, a ‘1’ bit value causes a transition to the high signal level (Figure 1).

2.0 Fundamental Concepts and Terminology

Figure 1. Unit interval and encoding method for SDI signals.

1

SMPTE 292M also defines an HD format with a data rate of 1.485 GHz/1.001. This SDI signal has a unit interval of 674 ps.

Jitter Measurement for Serial Digital Video Signals

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https://www.wendangku.net/doc/5314911932.html,/video 2.2 Decoding process, clock recovery, bit scrambling

To extract the digital content from an SDI signal, video equipment samples the SDI signal at the midpoint of the time intervals containing data bits (see Figure 1) and converts these sampled levels to the corresponding bit values. The sampling process uses a clock with the same frequency as the encoding clock, and aligned in time to ensure sampling occurs at the midpoint of the unit interval.Typically, video equipment does not have direct access to the clock used to create the serial data signal. Instead,equipment implements a clock recovery process that uses a phase-lock loop (PLL) to extract the appropriate sampling clock from the received signal. For reliable clock recovery,the edges in the SDI signal, i.e. transitions between the signal levels, must occur at an adequate rate. Long periods of a constant signal level can cause the sampling clock to drift out of synchronization.

Because of the NRZI encoding, long sequences of ‘1’ bit values in the serialized data sequence will have edges at each bit in the sequence. However, serialized digital video content can easily contain extended sequences of ‘0’ bit values. This could create SDI signals with long periods at a constant signal level. To avoid this, the SMPTE standards specify that SDI signal sources will randomize the data before applying the NRZI encoding, using a process known as scrambling .

The scrambling process in an SDI signal source converts the serialized data bits into a pseudo-random bit sequence.SDI receivers implement the inverse of this scrambling process to extract the original data bit sequence from the pseudo-random bit sequence. In most cases, this scram-bling process ensures a fairly large number of bit transitions,although long sequences of ‘0’ bits can infrequently occur.2.3. Time interval error, jitter, jitter waveform,jitter spectrum

Ideally, the time interval between transitions in an SDI signal should equal an integer multiple of the unit interval. In real systems, however, the transitions in an SDI signal can vary from their ideal locations in time. This variation is called time interval error (TIE), commonly referred to as jitter . This timing variation can be induced by a variety of frequency, ampli-tude, and phase-related effects. In this guide, we will view jitter as essentially a phase variation in a signal’s transitions,i.e. a phase modulation of the serial data signal.

As a simple example, suppose the edges in an SDI signal have a sinusoidal variation around their ideal positions

relative to a reference clock. If we viewed this SDI signal on an oscilloscope triggered on the reference clock, the actual edges would appear as a blur around the ideal positions as illustrated in Figure 2. We can fully define this simple sinu-soidal jitter with two parameters, the frequency of the variation and its peak-to-peak amplitude.

In actual SDI signals, jitter will rarely have the simple sine wave characteristics shown in this example. In real systems,a wide variety of factors influence the timing of signal transi-tions. These different sources introduce variations over a range of frequencies and amplitudes. The peak amounts that any particular edge leads or lags its ideal position may differ and there may be long time intervals between edges with large peak-to-peak variation.

The jitter waveform is the amount of variation in a signal’s transitions as a function of time, and the jitter spectrum is the frequency-domain representation of the time-domain jitter waveform. In actual signals, the jitter waveform typically has a complex shape created by the combined effects of various sources, and the jitter spectrum contains a wide range of spectral components at different frequencies and amplitudes.

Figure 2. SDI signal with sinusoidal edge variation (ideal positions

shown in darker lines).

Jitter Measurement for Serial Digital Video Signals

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2.4. Decoding errors, normalized jitter amplitude

In the decoding process, SDI receivers use a reference clock to determine when to sample the input SDI signal.Ideally, the transitions in the input SDI signal occur at

appropriate clock edges and sampling occurs at the mid-point of the unit interval. In the ideal situation shown in Figure 1, the signal transitions align with the clock’s falling edges and sampling occurs at the clock’s rising edges.Real SDI signals, however, have some amount of jitter in their edges. Jitter of sufficiently large amplitude will cause sampling errors. Figure 3 illustrates this situation. It shows an SDI signal that encodes two ‘1’ bit values during two clock periods, n and n+1. In the ideal situation, the sam-pling process would capture a high signal value in clock period n and a low signal value in clock period n+1.In the actual signal, the transitions vary significantly from their ideal locations relative to the reference clock. During clock period n, the actual edge varies by less than one-half the reference clock period. At the sampling time determined by the reference clock, the sampling process captures a high signal value, as it would in the ideal situation.During clock period n+1, however, the actual transition occurs more than one-half the clock period from its ideal position relative to the reference clock. Since the actual edge occurs after the sampling time determined by the ref-erence clock, the sampling process captures a high signal value instead of the low signal value it would have sampled in the ideal situation.

When expressed in seconds, the amount of timing variation needed to generate a decoding error depends on the clock period, i.e. the size of the unit interval. For a 1.485 Gb/s

HD-SDI signal a variation of 340 ps is more than one-half the 673 ps unit interval, while for a 270 Mb/s SD-SDI signal this same variation is less than one-tenth of this signal’s 3.7ns unit interval.

In order to describe these timing variations without referring to specific signal data rates, amplitudes are typically

expressed using unit intervals. In these normalized units, the variation shown in Figure 3 for clock period n+1 has an amplitude value of slightly more than 0.5 UI. An amplitude value of 0.5 UI would equal 1.85 ns in an SD-SDI signal and 337 ps in an HD-SDI signal.

2.5. Wander, timing jitter, alignment jitter

In the preceding examples, we have described variations in the position of signal transitions with respect to an ideal,jitter-free reference clock, i.e. a clock signal in which all

edges occur at their ideal locations in time. Actual reference clocks used in decoding are not jitter free.

As noted in section 2.2, the decoding process typically uses a recovered clock extracted from the received SDI signal. The clock recovery process “locks” the recovered clock to the input signal and the clock will follow timing variations in the input signal that fall within the bandwidth of the recovery process. Hence, the timing variations in the SDI signal introduce variations in the transitions of the recovered clock.

Since the transitions in the recovered clock determine when the decoder samples the SDI signal, using a recovered clock actually reduces the number of decoding errors associated with low frequency variations. The sampling time “tracks” these variations and samples at the correct location inside the unit interval.

The recovered clock does not track variations in signal transitions if the frequency of the variation lies above the bandwidth of the clock extraction process. At these higher frequencies, the position of signal transitions can vary relative to the edges of the recovered clock and these variations can create decoding errors.

As noted in section 2.3, the jitter spectrum in actual SDI signals generally contains a range of spectral components.The recovered clock will generally track spectral compo-nents below the clock recovery bandwidth, but will not track spectral components above this bandwidth. Hence,the impact of jitter on decoding depends on both the jitter’s amplitude and its frequency components. This has led to a frequency-based classification of jitter.

Figure 3. Decoding error caused by a large amplitude variation

in edge position.

Conventionally, the term “jitter” refers to short-term time interval error, i.e. spectral components above some low frequency threshold. For SDI signals, the SMPTE standards set this threshold at 10 Hz and refer to spectral compo-nents above this frequency as timing jitter.

The term wander refers to long-term time interval error. For SDI signals, components in the jitter spectrum below 10 Hz are classified as wander. Since video equipment can gener-ally track these long-term variations, characterizing wander in terms of actual edge positions relative to their ideal posi-tions does not give meaningful information. Instead, wander is measured in terms of frequency offset and frequency drift rate. These parameters characterize the deviation from expected clock rates in normalized units of parts per million (ppm and ppm/sec) or parts per billion (ppb and ppb/s) rather than UI.

Alignment jitter refers to components in the jitter spectrum above a specified frequency threshold related to typical bandwidths of the clock recovery processes. In other words, alignment jitter is a subset of timing jitter that excludes spectral components the clock recovery process can track. The specified frequency threshold differs for

SD-SDI and HD-SDI signals and is defined in the relevant SMPTE standard (see section 3.2). For SD-SDI signals, alignment jitter refers to spectral components above 1 kHz. For HD-SDI signals, spectral components above 100 kHz are classified as alignment jitter.

In general, video equipment does not track alignment jitter, though some equipment may track some low frequency alignment jitter. Thus, high amplitude alignment jitter gener-ally introduces decoding errors. Since video equipment can track wander and low frequency timing jitter, these spectral components often have less impact on signal decoding. While low frequency variations may have less impact on signal decoding, they can have significant impact in other areas. Other processes, e.g., digital-to-analog conversion stages, use this recovered clock, or a sub-multiple of this clock. Since this clock tracks the low frequency jitter in the input SDI signal, its edges vary from their ideal positions. This jitter in the clock signal can introduce errors, e.g.,

non-linearity in D-to-A conversion.

Clock recovery also affects the way jitter and wander accu-mulate in a video system. Reclocking video equipment uses the recovered clock to regenerate the SDI signal. Since

the recovered clock does not track alignment jitter well, reclocking can substantially reduce alignment jitter.However, reclocking may not significantly reduce wander or low-frequency timing jitter since the recovered clock tracks these variations. Hence, low-frequency variations can build through a video system. Amplitudes can eventually grow beyond the tracking capability of clock recovery processes. At this point, decoding errors will appear and the clock recovery hardware might not remain locked to the

input signal.

This guide examines techniques for measuring timing and alignment jitter. We will not examine wander and wander measurement techniques. However, wander does impact jitter measurements since these measurements must exclude contributions from spectral components below

10 Hz. Differences in wander rejection can lead to different measurement results, and we will examine these effects

in later sections.

2.6. Random jitter, deterministic jitter

To fully understand the impact of jitter in video systems,

we need to consider its statistical properties in addition

to its amplitude and spectral content. Commonly used approaches to characterizing and modeling these proper-ties distinguish between two basic jitter types. Random jitter has essentially no discernable pattern. It is best characterized by a probability distribution and statistical properties like mean and variance. Deterministic jitter is more predictable (determinable) and is often characterized by some definable periodic or repeatable pattern with a determinable peak-to-peak extent.

Random jitter

Random processes, e.g., thermal or shot noise, introduce random jitter into an SDI signal. We typically use a Gaussian probability distribution to model this jitter behavior, and we can use the standard deviation of this distribution (equiva-lent to the RMS value) as a measure of the jitter amplitude. However, the peak-to-peak jitter amplitude and the RMS jitter amplitude are not the same. In particular, the peak-to-peak amplitude value depends on the observation time.

In the Gaussian distribution used to model random jitter, small amplitude variations in edge position are most probable, but very large amplitude variations may infre-quently occur. A record of amplitudes made over a short observation time could include a large amplitude value, but probably will not. By contrast, a record of amplitudes made over a long observation time might not contain any large amplitude values, but probably will contain at least one. So, on average, we would expect that a peak-to-peak ampli-

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tude measurement made over a long observation time would have a larger value than a peak-to-peak amplitude measurement made over a short observation time.

The “tails” of a Gaussian distribution can reach arbitrarily large amplitudes. Hence, by observing over a sufficiently large time interval, we could theoretically measure arbitrarily large peak-to-peak jitter amplitude. We describe this prop-erty by saying that random jitter has “unbounded” peak-to-peak amplitude.

Technically, this description applies only to the mathematical model for random jitter. For all practical purposes, however,a Gaussian distribution adequately models random jitter in real systems. Thus, we can say that over any region of interest, random jitter in actual SDI signals has unbounded peak-to-peak amplitude.Deterministic jitter

A wide range of sources can introduce deterministic jitter into an SDI signal. For example:

Noise in a switching power supply can introduce periodic deterministic jitter.

The frequency response of cables or devices can intro-duce data-dependent jitter that is correlated to the bit sequence in the SDI signal (see section 2.7).

Differences in the rise and fall times of transition can introduce duty-cycle dependent jitter (see section 4.2).In addition to these general sources of deterministic jitter,SDI signals can contain deterministic jitter correlated with video properties. For example:

The line and field structure of video data can introduce a periodic deterministic jitter that we will call raster-dependent jitter .

Converting the 10-bit words used in digital video to and from a serial bit sequence can introduce high frequency data-dependent jitter at 1/10 the clock rate, typically called word-correlated jitter .

Deterministic jitter attains some maximum peak-to-peak amplitude within a determinable time interval. Increasing the observation time beyond this time interval will not increase the peak-to-peak jitter amplitude measurement. Unlike ran-dom jitter, repeatable deterministic jitter has a determinable upper bound on its peak-to-peak jitter amplitude.Even if deterministic jitter has infrequent long-term deter-minable behavior, this jitter can be adequately modeled with a predictable pattern that has bounded peak-to-peak

amplitude. Thus, for all practical purposes, deterministic jitter has bounded peak-to-peak amplitude and random jitter has unbounded peak-to-peak jitter amplitude.2.7. Intersymbol interference, equalization In real serial digital signals, transitions from one voltage level to another do not occur instantaneously. They have finite rise and fall times. Further, the frequency-dependent response of devices and communication channels will cause temporal spreading in these transitions. Intersymbol interference (ISI) occurs when the spreading of transitions in earlier bits affect transitions in later bits.

These effects cause transitions to vary from their ideal shapes and locations. In other words, ISI introduces jitter in the signal. Specifically, it produces predictable and repeatable jitter whose magnitude depends on the frequency responses of devices and channels, and on the data patterns in the signal. Hence, ISI produces deterministic, data-dependent jitter.

In particular, cable attenuation greater than 1 dB can introduce significant intersymbol interference. To avoid data errors due to this ISI, receivers typically have cable equalizers that compensate for the 1/√?frequency

response of the cable. Figure 4 shows the typical frequency responses of a cable and equalizer.

Figure 4. Frequency response for 300 m of cable and typical

response of a compensating equalizer.

2.8. Pathological signals, SDI checkfield As noted in section 2.2, clock recovery requires frequent signal transitions, i.e. the signal must have a sufficient transition density . Cable equalization algorithms also need many edges in the signal to determine and maintain the frequency-dependent gain that compensates for the 1/√? frequency response of the cable. Long intervals of constant signal level stress these processes and can lead to decoding errors or synchronization problems. Further, AC-coupling can reduce noise margins in decoding if the input signal remains at the same voltage level for a signifi-cant percentage of time (see section 2.9).

In most cases, scrambling and NZRI encoding ensures that SDI signals have many transitions. Typical SDI signals do not have long intervals of constant voltage that stress clock recovery, equalization, or decoding processes.

However, particular word patterns in digital video content can produce SDI signals with long constant-voltage inter-vals. If the shift register used in the scrambling process has a particular state and the scrambler receives one of several special input bit sequences, the resulting SDI signal after NRZI encoding will have one of the patterns shown in Figure 5. The paper by Takeo Eguchi listed in the References provides additional information.

SDI signals containing these patterns are called pathological SDI signals. Video semiconductor and equipment designers can use these signals to “stress test” clock recovery and equalization processes and to verify the correct operation of clamping or DC-restoration circuits that compensate for AC-coupling effects. As shown, the patterns needed for testing equalization differs from the pattern needed to stress test the clock recovery PLL.

Once initiated, the stress patterns in pathological SDI sig-nals occur only to the end of an active video line. SDI signal formats insert information between lines of active video content, e.g., the SAV (start-of-active-video) and EAV (end-of-active-video) synchronization words. This added information disrupts the special shift register state and bit sequences that create these long constant-voltage inter-vals. Even if the next active line contains the same special bit sequence, the shift register will generally not have the appropriate initial state and the SDI signal will not contain the stress patterns.

Repeating the special bit sequence on multiple video lines will cause the stress pattern to reappear. Eventually, the shift register in the scrambler will enter an appropriate initial state at the right position in the bit sequence. When this occurs, the stress pattern will reappear and will continue to the end of the active video line. The conditions needed to initiate a stress pattern happen infrequently. So, pathologi-cal SDI signals consist of occasional occurrences of video lines containing stress patterns statistically interspersed among many video lines that look like typical SDI signals.Some video test signal generators can create these signals in either full-field formats, or in split-field formats that com-bine the clock recovery and equalizer stress patterns. For testing recovery and equalization in video equipment with SDI inputs, SMPTE developed two recommended practices (RP 178 and RP 198) that define a specific split-field format called an SDI checkfield .

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2.9. Decoding decision threshold, AC-coupling effects, symmetric signals

To determine whether a sampled signal voltage corre-sponds to a “high” or “low” signal level, decoders compare the sampled voltage against a particular voltage level called the decision threshold or decision level . An optimally

chosen decision threshold will equally protect against errors generated by noise on either signal level. If each signal level has the same amount of noise, the optimal decision thresh-old equals the average of the two signal voltage levels.SDI receivers generally use fixed decision thresholds in the decoding process. For optimal performance, signal levels must keep the same relative relationship to this fixed volt-age level. A shift in the signal relative to the decision thresh-old reduces the noise margin for one of the signal levels,which can lead to decoding errors.

SDI receivers typically have AC-coupled inputs that remove DC-offsets in the input SDI signal and maintain a constant average voltage in the AC-coupled signal. In many imple-mentations, this average signal level equals zero volts,although biasing circuitry in the receiver could set the

average signal level of the AC-coupled signal to a non-zero value. Typically, the fixed decision threshold equals the average voltage of the AC-coupled signal. However, the optimal decision threshold may differ from the average signal voltage if one signal level can have more noise than the other.

While AC-coupling filters out DC offsets in the input SDI signal that could lead to decoding errors, it can also shift the signal levels in the AC-coupled signal relative to a fixed decision threshold. Figure 6 illustrates this situation for an implementation of AC-coupling that maintains the average signal level in the AC-coupled signal at zero volts. In this example, the decoding process also uses zero volts as the fixed decision threshold.

Figure 6a shows a segment of an AC-coupled signal

derived from an input SDI signal that does not contain any long duration at the same voltage level. In this case, the high signal level in the AC-coupled signal equals +0.5·V pp and the low signal level equals -0.5·V pp , where V pp is the peak-to-peak amplitude of the input SDI signal. The fixed decision threshold falls at an optimal position midway between the two levels.

Figure 6b shows a segment of an AC-coupled signal

derived from an input SDI signal that stays at the low signal level for long periods of time, e.g., one of the equalizer stress patterns described in section 2.8. In this example,the signal remains at the low signal level 95% of the time.To maintain an average signal level of zero volts, the low signal level in the AC-coupled signal must equal -0.05·V pp ,while the high signal level must equal +0.95·V pp . The low signal level is very close to the fixed decision threshold for decoding, which eliminates the noise margins for this signal level and will lead to decoding errors.

In effect, the AC-coupling has generated intersymbol inter-ference. The values of earlier bits (long strings of ‘0’ bit values after scrambling) have impacted the decoding of later bits.

The amount of shift depends on the coupling time constant.For example, with a coupling constant of 10 μsec, an equalizer stress pattern will shift almost 78% closer to the fixed decision level over one-half of an HD video line. With a coupling time constant of 75 μsec, the stress pattern will shift by less than 33% over an entire HD video line.To compensate for this AC-coupling effect, SDI receivers typically clamp or DC-restore the AC-coupled signal to maintain the relationship between the signal levels and the fixed decision threshold.

Due to scrambling and NRZI encoding, SDI signals are

symmetric , i.e. they spend nearly the same amount of time at each signal level. More specifically, typical SDI signals are symmetric when signal levels are averaged over many unit intervals. Shorter-term, SDI signals can have several periods of constant signal level, with pathological SDI signals as the extreme case.

Figure 6. Shift in AC-coupled signals relative to fixed decision level.

Even in SDI signals with frequent transitions, AC-coupling can introduce a shift in the signal relative to the fixed deci-sion level. If the rise and fall times of signal transitions differ significantly, the signal will spend more time at one of the signal levels. For example, if the signal has fast rise times and slow fall times, it will spend more time in the high signal state. AC-coupling will then shift the high signal level closer to the fixed decision threshold, reducing noise margin. Typically, SDI signals have symmetric rise and fall times, but asymmetric line drivers and optical signal sources (lasers) can introduce non-symmetric transitions. While significant, these source asymmetries do not have especially large impacts on signal rise and fall times. In particular, cable attenuation will generally have a much larger impact on signal transition times.

Without appropriate compensation or other adjustments, asymmetries in SDI signals can reduce noise margins with respect to the decision threshold used in decoding and can lead to decoding errors. As we explore in section 4.2, these same asymmetric conditions can also impact jitter measurements.

2.10. Jitter input tolerance, jitter transfer, intrinsic jitter, output jitter

SDI signal receivers can differ in their implementations of the processes described in the preceding sections. A par-ticular receiver’s clock recovery process may not track jitter as well as others, or it may not sample the SDI signal near the midpoint of the unit interval. The design and hardware a receiver uses to implement equalization, clock recovery and decoding processes may introduce a significant amount of additional jitter into the signal. Thus, a particular receiver may have multiple data errors when decoding SDI signals that other receivers can decode without error. Such a receiver has a lower jitter input tolerance.

A receiver’s jitter input tolerance depends on the jitter fre-quencies in the SDI signal. As noted in section 2.5, clock recovery can track low frequency jitter, so receivers typically have a higher tolerance for low frequency jitter. The jitter input tolerance drops significantly for jitter frequencies above the clock recovery bandwidth.Some video equipment, e.g., a distribution amplifier, pro-duces an SDI output from an SDI signal applied at an input. Typically, jitter in the input SDI signal does not directly trans-late to jitter in the corresponding output. In particular, clock recovery can filter out high frequency jitter, or may amplify some jitter in the input signal. Jitter transfer is the jitter on an SDI output resulting from jitter in an input SDI signal, and the jitter transfer function is the ratio of output jitter to applied input jitter as a function of frequency.

Like receivers, source and regeneration equipment also has internal jitter. This internal jitter will appear on an SDI output signal even if the associated SDI input has no jitter. Intrinsic jitter is the amount of jitter at an SDI output in the absence of input jitter. Output jitter is the total amount of jitter at an SDI output resulting from intrinsic jitter and the transfer of jitter in any associated SDI input.

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2.11. Eye diagram, equalized Eye diagram Engineers commonly use Eye diagrams to analyze serial data signals and diagnose problems. Measurement instru-ments create Eye diagrams by superimposing short segments of the serial data signal. The finite rise and fall times of these transitions create the characteristic ‘X’

patterns in the Eye diagram (see Figure 7). The eye-shaped area without transitions gives the display its name. We will call the point where the rising and falling edge transitions intersect the crossover points in the Eye diagram. The time interval between the crossover points in the Eye equals the unit interval. In the ideal case, the decoding process samples the signal at the mid-point between the crossover points and the decision threshold corresponds to the widest part of the Eye opening (Figure 7).

To make the Eye diagram, the instrument aligns the seg-ments using a reference clock signal. Typically this reference clock is extracted from the data signal, but may be a sepa-rate reference clock signal. It can be externally supplied,e.g., through the trigger input on an oscilloscope, or extracted within the measurement instrument.

If the transitions in the input signal align with the edges in this reference clock they will lie on top of each other in the Eye diagram. Any transitions that vary from the nominal positions determined by this reference clock will appear in different locations. If the instrument uses a recovered clock to form the Eye diagram, the reference clock will track jitter below the loop bandwidth of this clock recovery process.Thus, the Eye diagram will only show jitter components with frequencies above this bandwidth threshold, called the Eye clock recovery bandwidth .

For signals with a small amount of jitter, the edges in the aligned segments occur in nearly the same location. The small variations in edge position create only a slight “blur”around the nominal edge positions (see Figure 7). Most of the space between the crossover points is free of transitions. In this situation, we say the Eye is “open.” As the amplitude of the jitter increases, more transitions move into the open space between crossover points, i.e.the Eye starts to “close” (see Figure 8).

Using Eye diagrams, engineers can quickly form a qualita-tive impression of the jitter in a signal and the potential for decoding errors. Overall, a signal that forms a large,wide-open Eye is less likely to produce decoding errors than a signal that forms a small or closed Eye. However, in making this qualitative assessment, one of the key factors engineers need to consider is the difference between the Eye clock recovery bandwidth and the bandwidth of the receiver’s clock recovery process.

If the clock recovery bandwidth in the receiver equals the Eye clock recovery bandwidth, the size of the Eye opening correlates reasonably well with the potential for decoding errors. If the input signal forms a large, “wide-open” Eye,the decoding process will most likely sample the signal before the transition to the next bit.

If the clock recovery bandwidth in the receiver is less than the Eye clock recovery bandwidth, the signal may contain jitter frequencies below the Eye clock recovery bandwidth that impact the decoding process but do not appear in the Eye diagram. The decoding process may generate errors even though the Eye diagram has a large Eye opening.

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Figure 7. Eye diagram for signal with very small amplitude jitter.

Figure 8. Nearly closed Eye caused by large amplitude jitter.

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On the other hand, if the clock recovery bandwidth in the receiver is greater than the Eye clock recovery bandwidth,the Eye diagram may show jitter that does not impact the decoding process. The receiver may decode the signal without errors even though the Eye diagram has a small Eye opening or is completely closed.

Other factors also influence the qualitative assessment of signal jitter using Eye diagrams. If receivers introduce a significant amount of internal jitter or do not consistently sample near the middle of the unit interval, they may generate more decoding errors than suggested by the size of the Eye opening.

Thus, in using an Eye diagram to assess the potential for data errors, engineers need to consider the combined effects of the receiver’s clock recovery, equalization and decoding processes. In other words, they need to consider the receiver’s jitter input tolerance (see section 2.10). A receiver with low jitter input tolerance can generate errors in decoding a signal that forms a wide-open Eye diagram,while a receiver with high jitter input tolerance may correctly decode a signal that forms a closed Eye diagram.

As noted in section 2.7, frequency-dependent cable attenu-ation “spreads” transitions in SDI signals. This intersymbol interference can significantly reduce or completely close the Eye opening in an Eye diagram constructed from a signal at the end of a long cable (see Figure 9).

However, a small or closed Eye opening in the Eye diagram of a non-equalized signal at the end of a long cable does not necessarily indicate a high potential for decoding errors.The cable equalization used in receivers will restore the signal’s transitions and “re-open” the Eye. With adequate equalization, the ISI from cable attenuation will not signifi-cantly impact the decoding process. Without adequate equalization, the data-dependent jitter introduced by cable effects can lead to decoding errors.

While equalization can compensate for cable effects, the equalized signal can still contain signal jitter or amplitude noise that reduces or closes the Eye opening. To qualita-tively assess the remaining potential for decoding errors after equalization, engineers can use an equalized Eye diagram constructed from the equalized version of the input signal.

Eye diagrams can also show AC-coupling effects. Signal level shifts due to AC-coupling (section 2.9) causes a corresponding shift in the superimposed segments that form the Eye diagram. This can occur even if the measure-ment instrument forming the Eye-diagram has a DC-coupled input. Other equipment in the system may have AC-coupled inputs, causing shifts in the SDI signal before it reaches the measurement instrument.

Figure 9. Eye diagram for SD-SDI signal showing a nearly closed

Eye due to attenuation in a 100 m cable.

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Figure 10a shows a pathological SDI signal containing

an equalizer stress pattern in an Eye display set to a sweep rate equal to several video lines. At this slow sweep rate,the resulting waveform contains thousands of individual Eyes per division. This display clearly shows that the signal levels shift higher because of AC-coupling effects due to the long intervals at a low signal level (top pattern in Figure 5b).

Figure 10b shows an Eye display for the same signal using a much lower sweep rate that displays a full video field. This display demonstrates the effects from the two different equalizer stress patterns shown in Figure 5b.2.12. Equivalent-time Eye, Real-time Eye The instruments most commonly used to monitor and measure signal jitter construct Eye diagrams by sampling the input signal. They acquire these samples using two different methods.

Many instruments, including waveform monitors and other video-specific measurement instruments, use equivalent-time sampling techniques to create the Eye diagram. These techniques use under-sampling to approximate an over-sampled acquisition. We will refer to an Eye diagram constructed in this manner as an Equivalent-time Eye . Measurement instruments use different equivalent-time sampling techniques with a wide range of capabilities and characteristics. Broadly speaking, because of the under-sampling used in this approach, the “edges” in the Eye

diagram represent the composite effect of many separated edges in the actual signal, possibly widely-separated edges.The sampling rate used to construct the Eye can strongly influence the results of peak-to-peak jitter amplitude measurements (section 4.3.1).

Real-time digital oscilloscopes can construct an Equivalent-time Eye diagram using the equivalent-time technique mentioned above. They can also construct Eye diagrams using real-time sampling techniques that over-sample the input signal. These instruments use software-based clock recovery techniques in creating the Eye.

We will refer to an Eye diagram constructed using this real-time sampling technique as a Real-time Eye . In this technique, the edges in the Eye diagram are actual edges in the input signal. The amount of acquisition storage and the sampling rate will influence the results of peak-to-peak jitter amplitude measurements (section 4.3.3).

Figure 10. AC-coupling effects.

2.1

3. Bit error ratio, Bathtub curve

All SDI signals contain some amount of random jitter. As noted in section 2.6, random jitter has no discernible pattern. Thus, decoding errors due to random jitter in a signal will not occur at determinable times or rates. In place of error rates, the combined impact of deterministic and random jitter on decoding can be usefully characterized by a bit error ratio (BER), the ratio of the number of incorrectly decoded bits to the total number of bits decoded.For example, consider an HD-SDI signal with a small

amount of random jitter and a receiver that always samples at the midpoint of the unit interval. Suppose that the total jitter in this signal, i.e. the combined effects of deterministic and random jitter, causes sampling errors in this receiver at an average rate of 1 per minute. In one minute, the 1.485 Gb/s HD-SDI signal transmits 8.91 x 1010bits. So, the total jitter in the signal corresponds to a BER of at least 1.12 x 10-11in this ideal receiver. For a 270 Mb/s SD-SDI signal, an average of one decoding error per minute corresponds to a BER of at least 6.17 x 10-11. Due to error propagation effects in the SDI receiver associated with bit scrambling and NRZI to NRZ conversion, one sampling error can lead to multiple bit errors, producing a higher BER.

Now imagine moving the sampling location away from the midpoint of the unit interval and towards one of the crossover points in the Eye diagram. Figure 11a illustrates this process with a sketch of an Eye diagram that has accumulated edges long enough for large amplitude ran-dom jitter to nearly close the Eye. As the sampling location moves closer to a crossover point, smaller jitter amplitudes can now cause transitions to occur in the wrong position relative to the sample location.

In random jitter, smaller amplitude variations happen more frequently than larger amplitude variations. Thus, as the sampling location moves towards a crossover point, random jitter can more frequently shift transitions into the wrong positions relative to the sampling location. This leads to an increased number of decoding errors and an increased BER.

The sketch in Figure 11b shows this relationship between the BER due to jitter in a signal and the sampling location in the unit interval. This is called a Bathtub plot or a Bathtub curve because the shape looks like a cross-section of a bathtub.

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https://www.wendangku.net/doc/5314911932.html,/video Figure 11. Bathtub curve – BER vs. location in Eye Diagram.

Bathtub curves are useful in assessing whether a video system can achieve a target BER. For example, suppose an operation wants the BER in a video system to stay below 10-10. Consider two different sources within the

system whose output SDI signals contain different amounts of random and deterministic jitter. At a particular receiver’s input, suppose the total jitter in the two signals has the same RMS amplitude and generates the Bathtub curves shown in Figure 12.

The shapes of the Bathtub curves offer insight into the signal jitter. The steeper curve on the signal from Source A indicates a lower amount of random jitter compared to the signal from source B. As the number of bits observed increases, the peak-to-peak jitter amplitude increases less in the signal from source A than in the signal from Source B. Since the total jitter in the signals have equal RMS jitter amplitudes, the ratio of deterministic jitter to random jitter is greater in the signal from Source A compared to the signal from Source B.

For a BER of 10-10, the sides of the Bathtub curve for the SDI signal from Source A define a 0.5 UI region centered in the unit interval. Presuming that any signal transition in this region causes a decoding error, we can say that the Eye opening for this signal equals 0.5 UI except for 1 transition in 1010bits. By contrast, the Eye opening for the SDI signal from Source B equals 0.33 UI except for 1 transition in 1010bits.

To meet the 10-10BER target, the receiver must sample the SDI signal from Source B inside a 0.33 UI region around the midpoint of the unit interval. The receiver has greater margin in sampling the signal from Source A. It can sample this

signal anywhere inside a 0.5 UI region centered in the unit interval.

As described in section 2.5, receivers track signal jitter at frequencies below the bandwidth of their clock recovery process and can adjust the sampling location to compen-sate for this variation. However, clock recovery cannot track these variations perfectly.

Suppose that timing errors in the clock recovery process could cause the sampling location of the receiver used in this example to fall anywhere within a 0.4 UI region cen-tered in the unit interval. Then, signals from source B will most likely not meet the 10-10BER requirement due to the larger random jitter component in these signals. Signals from Source A can more easily meet this BER requirement. Except for 1 transition in 1010bits, the Eye opening in the SDI signals from Source A is greater than the potential variation in the receiver’s sampling location.This includes some margin to allow for small, occasional increases in internal jitter or variation in sampling location.The primer “Understanding and Characterizing Timing Jitter” listed in the References contains additional informa-tion about Bathtub plots and the impact of random and

deterministic jitter.

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Figure 12. Bathtub curves at the receiver input for SDI signals from two sources.

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https://www.wendangku.net/doc/5314911932.html,/video Consistency in jitter measurements necessarily starts with the standards. The industry develops and adopts these standards to ensure that equipment will perform satis-factorily in video production, distribution, and transmission systems. Video equipment manufacturers must design and deliver products that meet these standards. Video test equipment manufacturers must fully understand the stan-dards, implement test procedures that conform to the requirements documented in these standards, and make these implementations as accurate as possible within the constraints of their specific implementations.

However, implementing test procedures in conformance to the relevant video standards does not ensure consistent measurements. In particular, the current video standards allow for significantly different jitter measurement methods that can yield noticeably different results. Hence, any

discussion of jitter measurement and variability in measure-ment results must begin by looking at the relevant stan-dards and specifications.

3.1. Standards documents

SMPTE publishes standards, recommended practices (RP),and engineering guidelines (EG) for the video industry. The Institute of Electrical and Electronics Engineers (IEEE) also publishes video standards. Table 1 lists the standards and recommended practices that apply to video jitter and briefly describes their jitter-related content.2

RP 184 gives the framework for specifying jitter perform-ance, including jitter input tolerance, jitter transfer, and output jitter. This includes methods for specifying the jitter frequencies included in peak-to-peak amplitude measure-ments. This recommended practice only describes the form of jitter specifications. All parameters are in symbols without specific performance limits.

In particular, RP 184 does not give values for measurement bandpass cutoff frequencies or peak-to-peak jitter limits.These measurement parameters depend on the particular SDI format and are listed in the standard defining the

format. Also, RP 184 defers specification on measurement time to other standards or recommended practices.RP 192 gives examples of jitter measurement techniques that conform to RP 184 and describes these particular techniques in detail. However, RP 192 does not preclude other techniques that conform to RP 184. This recom-mended practice does not specify particular measurement times, but does describe a procedure for determining the minimum measurement time for oscilloscope-based jitter measurement.

SMPTE 259M, section 3.5, deals with jitter in SDI signals carrying standard-definition digital video content. SMPTE 292M, section 8.1.8, deals with jitter in SDI signals carrying high-definition digital video content. For their respective formats, these standards specify the performance limits

Document SMPTE RP 184SMPTE RP 192SMPTE 259M

SMPTE 292M SMPTE EG 33IEEE Std 1521

Title

Specification of Jitter in Bit-Serial Digital Systems

Jitter Measurement Procedures in Bit-Serial Digital Interfaces 10-Bit 4:2:2 Component and 4?sc Composite Digital Signals—Serial Digital Interface

Bit-Serial Digital Interface for High-Definition Television Systems Jitter Characteristics and Measurements

IEEE Trial-Use Standard for

Measurement of Video Jitter and Wander

Content description

Methods and performance templates for specifying jitter input tolerance, jitter transfer, and output jitter. Methods for carrying out the jitter performance measurements identified in RP 184.

Specifications on performance limits for jitter at the SDI outputs of SD-SDI signal sources.Specifications on performance limits for jitter at the SDI outputs of HD-SDI signal sources.Guidance on jitter measurement and minimizing jitter in video systems.

Specifications for output jitter and wander only,including performance templates, and methods for measuring jitter, including jitter measurement frequency response.

Table 1. Standards and other documents that apply to video jitter.

3.0 Specifications on Video Jitter Performance and Measurement

2

The ITU also publishes video standards containing specifications on jitter performance, e.g., ITU-R BT.656, ITU R-BT.799, and ITU-R BT. 1363. In Japan, the ARIB standards contain specifications in this area. To a significant extent, the guidelines and specifications in these documents agree with those in the SMPTE and IEEE documents described in this guide.

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on jitter from “the serial output of a source derived from a parallel domain signal whose timing and other characteris-tics meet good studio practices.”

Hence, these standards define performance limits only on output jitter. In particular, they assign specific values for the parameters identified in RP 184 for measuring output jitter. These include the measurement bandpass corner frequen-cies, peak-to-peak jitter limits, and the test signal to use in making jitter measurements. These standards do not specify a peak-to-peak amplitude measurement time.

EG 33 gives engineers more detailed information on jitter in SDI signals and guidance on jitter measurement techniques. It describes some of the impacts jitter can have on system operations and suggests design approaches that minimize or mitigate these impacts.

IEEE Standard 1521 describes requirements for specifying the measurement of jitter and wander for both analog and digital video. As with RP 184, it gives only the form of the specification. It does not give values for measurement filter corner frequencies or peak-to-peak jitter limits. It also describes three methods for making jitter and wander measurements.

In this technical guide, we consider only the measurement of output jitter. The video standards specify performance limits only on this type of jitter. These are the most com-monly performed measurements, and they have generated the greatest confusion.

3.2. Specifications on jitter frequency bandpass

As described in section 2.5, video jitter is classified based on frequency. To measure the amount of jitter in these dif-ferent classes, measurements must be restricted to specific frequency ranges. RP184, RP192, and IEEE Std. 1521 all contribute specifications on bandpass shapes. The relevant SDI specification gives the bandpass corner frequencies. As an example, Figure 13 shows the bandpass for measur-ing timing jitter in an HD-SDI signal. The values shown

in the figure combine the specifications from all relevant standards and recommended practices.

SMPTE 292M specifies the low-frequency cutoff at ?1= 10 Hz, consistent with the definition of timing jitter. It specifies that the high-frequency cutoff, ?4, shall be > 1/10 the clock rate, which equals 148.5 MHz for HD-SDI signals.RP 184 recommends at least a 20 dB/decade slope on the highpass filtering at ?1. RP 192 recommends at least a 40 dB/decade slope, and IEEE Std. 1521 recommends at least a 60 dB/decade slope. The IEEE standard requires the steeper 60 dB/decade slope to separate jitter from wander (See Figure 1 in IEEE Std. 1521). To conform to the IEEE standard and both recommended practices, jitter measure-ment equipment must use at least a 60 dB/decade high-pass ramp, as shown in Figure 13.3

RP 184 recommends at least a –20 dB/decade slope on the lowpass filtering at ?4. It also recommends an in-band ripple less than ±1 dB, but does not give any guidance on the accuracy of the highpass corner frequency, ?1. The lowpass corner frequency, ?4, can have any value above

1/10 the clock rate.

To determine the amount of alignment jitter we also need to measure over a range of frequencies, but one with different highpass corner frequency and slope (Figure 14).

SMPTE 292M specifies the low-frequency cutoff for meas-uring alignment jitter as ?3= 100 kHz. In agreement with the timing jitter specification, it requires that the high-fre-quency cutoff, ?4, shall be at least 1/10 the clock rate. Specifications of the highpass corner frequency in the bandpass for measuring alignment jitter reflect expectations about the bandwidths of clock recovery processes that track low frequency jitter.

Figure 13. Frequency bandpass for measuring timing jitter in an

HD-SDI signal.

3Based on discussions currently underway, the recommendation in RP 184 and RP 192 for the highpass slope will likely change to at least 60 dB/decade.

SMPTE selected the value for ?3shown in Figure 14 with the expectation that equipment handling HD-SDI signals will have clock recovery bandwidths of at least 100 kHz.Equipment handling SD-SDI signals may have smaller clock recovery bandwidths, especially legacy equipment. Hence,SMPTE 259M specifies that ?3= 1 kHz in the bandpass for measuring alignment jitter in SD-SDI signals.

RP 184 recommends at least a 20 dB/decade slope on the highpass filtering at ?3, while RP 192 recommends at least a 40 dB/decade slope. To conform to both recommended practices, jitter measurement equipment must use at least a 40 dB/decade high-pass slope, as shown in Figure 14.4RP 184 recommends at least a –20 dB/decade slope in the low-pass filtering at ?4. As with timing jitter, RP 184 recom-mends an in-band ripple less than ±1 dB and does not give any guidance on the accuracy of the highpass corner frequency, ?3. The lowpass corner frequency in the band-pass for measuring alignment jitter, ?4, can have any value greater than 1/10 the clock rate.

3.3. Specifications on signal voltage levels and transition times

For SD-SDI outputs, SMPTE 259M specifies a peak-to-peak signal amplitude of 800 mV ± 10% with a DC offset equal to 0.0 V ± 0.5V . The transition between voltage levels can take no less than 0.4 ns and no more than 1.5 ns, and the rise and fall times cannot differ by more than 0.5 ns.For HD-SDI outputs, SMPTE 292M specifies the same signal amplitude conditions. The transition between voltage levels can take no more than 270 ps, and the rise and fall times cannot differ by more than 100 ps.

Hence, the SMPTE standards allow asymmetric rise and fall times in SDI signals and significant DC offsets. As noted in section 2.9, these signal characteristics can impact decod-ing. They can also impact jitter measurement, as we describe in section 4.2.

3.4. Specifications on connecting cables and other system elements

For SD-SDI signals, SMPTE 259M specifies that measure-ments of source output signal characteristics shall be

made across a resistive load connected by a “short coaxial cable.” For HD-SDI signals, SMPTE 292M specifies a “1-m coaxial cable.” Hence, for both SD- and HD-SDI signals,the standards only specify jitter performance near the source output as measured over a short cable length.For SDI signal receivers, the standards place some require-ments on the SDI inputs, including impedance and return loss. They do not, however, define any performance limits on the jitter input tolerance of an SDI receiver. Also, the standards do not define performance limits on jitter transfer in system elements.

The standards do not specify particular cable types, but both do require that coaxial connections have the 1/√? frequency response needed for the correct operation of cable equalizers. For HD-SDI signals, SMPTE 292M goes somewhat further and specifies the cable return loss.Neither standard places performance limits on the data-dependent jitter introduced by ISI on long cables. They do say the receivers should nominally operate with cable loss-es up to 20 dB at one-half the clock frequency. This is not a performance limit, however, as they also say that “receivers that operate with greater or lesser signal attenuation are acceptable.” The standards do not specify performance characteristics on other sources of ISI in video systems,including reflections on connectors in patch panels.

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https://www.wendangku.net/doc/5314911932.html,/video Figure 14. Frequency bandpass for measuring alignment jitter in an

HD-SDI signal.

4

Based on discussions currently underway, the recommendation in RP 184 for the high-pass slope will likely change to at least 40 dB/decade.

Jitter Measurement for Serial Digital Video Signals

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3.5. Specifications on peak-to-peak jitter amplitude

SMTPE 292M specifies that the timing jitter in the HD-SDI output of a source derived from a parallel domain signal will have peak-to-peak amplitude less than 1.0 UI (673 ps). It also specifies that the alignment jitter in the SDI output will have peak-to-peak amplitude less than 0.2 UI, which equals 135 ps (0.2 x 673 ps).

SMTPE 259M specifies that both the timing and the align-ment jitter in the SD-SDI output of a source derived from a parallel domain signal will have peak-to-peak amplitude less than 0.2 UI, which equals 740 ps (0.2 x 3.7 ns).

Note that these two standards only specify the maximum peak-to-peak amplitude of output jitter allowed in the SDI signal at the output of a source that derives this signal from a parallel domain input. They do not specify the maximum peak-to-peak amplitude of output jitter allowed in SDI sig-nals at the output of devices that derive the output signal directly from an SDI input.

3.6. Specifications on measurement time The measured peak-to-peak jitter amplitude depends on the time interval used to make the measurement. Section 2.6 describes this dependency for random jitter. It also applies to peak-to-peak jitter amplitude measurements made on signals containing deterministic jitter. Figure 15gives a simple illustration of this dependency.

In this example, the SDI signal contains periodic, determin-istic alignment jitter that consists of well-separated pulses.One advances transitions from their ideal positions; the other delays transitions. An instrument that makes the peak-to-peak measurement over a 50 ms observation window will only measure a single jitter peak and will indicate that the signal has 0.15 UI of alignment jitter peak-to-peak. This amount of jitter is within the specified performance limit. However, an instrument that makes the peak-to-peak measurement over 150 ms will detect both the advance and delay peaks. This instrument will indicate that the signal has 0.3 UI of alignment jitter peak-to-peak,above the specified performance limits.

While SDI signals can have deterministic jitter behavior of the kind shown in Figure 15, it is not a typical pattern.

However, all SDI signals have some amount of random jitter.As noted in section 2.6, random jitter can be modeled by a Gaussian probability distribution of jitter amplitudes and, for all practical purposes, does not have an upper bound on peak-to-peak jitter amplitude. Extending the time interval for

the peak-to-peak measurement increases the probability that some larger amplitude variations will occur during the measurement period, which increases the measured peak-to-peak jitter amplitude. We examine these effects in more detail in section 4.5.3.

As noted in section 3.1, the standards offer very limited guidance on peak-to-peak measurement time. Thus, differ-ent manufacturers of video jitter measurement equipment can, and do, measure the peak-to-peak jitter amplitude over different time intervals. Variations in measurement time typically lead to discrepancies in the measured values. To enable greater consistency in measuring peak-to-peak jitter amplitude, the standards will need to specify measurement times.

Figure 15. Peak-to-peak measurement value depends on

measurement time.

农村广播系统在农村的重要作用

农村广播系统在农村的重要作用 随着时代的进步，以及政府的体制改革，农村广播系统逐步走向衰败。90年代中期，农村广播彻底瘫痪。90年代后期，有限电视网络开始兴起，各地广播电视部门开始组建组建的有限电视网络，并迅速延伸到农村地区，而当地的有关部门的工作重心也向有线电视转移，从而导致全国只有极个别地区保留了农村广播。然而，中国地大物博，人口众多，农村村庄的分布点广，从而演变成，农村治安问题成了当今和谐社会经济发展、建设新农村急需解决的一大问题，农村广播系统成了解决以上问题的最佳选择。 在建设社会主义社会实现四个现代化的历程中，农村无线广播为社会主义建设起了很大的作用。在当今经济高速发展的环境中，有线喇叭已经跟不上时代的发展，满足不了中国特色的社会主义新农村的建设需要,无线数字语音广播报警系统综合起来有以下功能： 一、县、乡镇级领导可通过分控实现1--50 千米内有效无线广播、控制下级各村无线广播的开机、关机、语音呼叫、下达通知等;也可实现对辖区内村庄的无线广播单独控制和使用。同时具有寻址功能，县、乡镇领导或派出所即可对全乡镇村庄或个别村庄下达通知、抢险救灾、治安联防调度等. 二、村委会发送主机与各接收终端之间 5 千米内无需线路架设，省物省力免检修，不易被破坏。村委会领导或村巡防人员即可通过控制村无线广播的开机、关机、语音呼叫、下达通知、抢险救灾、人员调度、治安联防等. 三、机器设备只要接通电源无人看管，既可自动接收县、乡镇广播通知和村委会广播通知，又可单独讲话扩音，方便耐用。 四、及时将通知、科普信息、磁带歌曲广播到村民中去。 五、有太阳能配套设备，在没有电力线路的情况下也能使用，是抢险救灾时信息沟通，调度管理的得力工具。 六、可自动接收地方广播电台节目，定时播放。 七、农村无线广播无线通讯方式、太阳能供电，没有电线线路存在，不会受自然灾害影响中断广播和通讯，确保应急救援时的正常广播。

公共广播系统方案

公共广播系统方案 Revised as of 23 November 2020

前言： ? 一套完整的公共广播系统设计方案，大家可以参考一下。 ? 正文： 一、公共广播设计思想 先进性和可扩展性： 现代信息技术的发展，新产品、新技术层出不穷。因此本系统在投资费用许可的情况下应充分利用现代最新技术，以使系统在尽可能长的时间内与社会发展相适应。但由于现代科学技术的飞速发展，故必须充分考虑今后的发展需要，设计方案必须具备前瞻性和可扩展性。这种可扩展性不仅充分保护了甲方的投资，而且具有较高的综合性能价格比。本设计对此均作了充分考虑，预埋了必要的管线，预留了各种接口，极便于系统的扩展和升级。 科学性和规范性： 公共广播系统与一般广播系统不同，是一个先进复杂的综合性系统工程，必需从系统设计开始，包括施工、安装、调试直到最后验收的全过程，都严格按照国家有关的标准和规范，做好系统的标准化设计和科学的管理工作。最后提交正规的测试验收报告及全套施工图纸和技术资料供甲方存档。特别作为政府拨款项目，必须确保整个工程经得起各方面的和较长时间的严格考验。 安全性和可靠性： 公共广播系统的建设，直接影响着用户的使用效果、外部形象及投资回报，因此系统设计必须安全、可靠，本方案已充分考虑采用成熟的技术和产品，在设备选型和系统的设计中尽量减少故障的发生。并从线路敷设、设备安装、系统

调试以及对甲方人员的技术培训等方面，都必满足可靠性的要求。特别重要的一点是本方案选用的所有主要关键设备，均取得该设备的生产厂家或代理商的授权证书，并承诺在工程设备的提供、技术支援及售后服务等方面给予全力支持。（容后付上授权证书）这一点是国际国内工程招标项目重点考核的关键条件之一。 二、公共广播设计原则及依据 设计原则 从投资合理、外观美观、设计规范的思想出发，日常广播和紧急广播二个系统的设计，在功能上互相独立，在设备及器材上有机结合。根据规范要求，紧急广播的控制具有最高优先权，并采用智能的联动和自动火灾报警广播方案。设有音量调节器的扬声器，平时在接收日常广播时可以调节音量或关闭，紧急广播时扬声器不受音量调节器控制，都将处于紧急广播状态。 设计依据 本设计主要依据以下标准进行： 1. 《智能建筑设计标准》GB /T 50314-2015 2. 《火灾报警与消防联动控制》（JGJ/ 3. 《火灾自动报警系统设计规范》（GB50116-2008） 4. 《火灾自动报警系统施工及验收规范》（GBJ50166-2007） 5. GB50116-2008《火灾自动报警系统设计规范（摘录）》 6. 《民用建筑电气设计规范》JGJ/T16-2008 7. 《调音台基本特性测量方法》 GB9003 8. 《工业企业通信接地设计规范》 GBJ－79-85

05广播电视传输系统技术能手竞赛试题(完整答案版)

广播电视传输系统技术能手竞赛试题 一、填空题（每空0.5分，共30分） 1、广播电视节目的传输按其传输媒质可分为：_有线_和____无线___。 2、目前常用的多路复用技术有_____频分复用___和_____时分复用________。 3、模拟通信可采用_调频__、___调幅__、___调相__三种调制方式，采用数字调制时，相应地称为_______FSK______________、_______ASK__________、________PSK______________三种键控方式。 4、网络管理的五大功能模型是：____故障___管理；____性能_____管理；_配置_____管理；____计费____管理；_____安全_____管理。 5、数据通信开放系统互连模型的七层协议中的第二层和第七层分别是：_____数据链路层______、____应用层_________。 6、1毫瓦在75 欧姆上所对应的电平为_274__毫伏。 7、电视信号包括图像和伴音信号，图像信号频谱为__0Hz-6MHz_________，伴音信号频谱为 ___________20Hz-20KHz___。我国的电视标准采用__625__行和___25____帧的隔行扫描制。行扫描频率为_____15625Hz________,场扫描频率为___50Hz_________。 8、在卫星接收和发射系统中按馈源与反射面的相对的位置可分为：___前______天线、______后_____天线、___偏________天线。 9、国际电联按频率把全世界卫星通信与广播频段划分为__三__个区，中国属于第___三__区，Ku波段的下行频段为__ 11.7 -12.2GHz _______和____ 12.2 -12.7GHz _____。 10、国内卫星通信中采用的电波极化方式为_____线极化_________。 11、当电波传播空间参数发生变化，使接收电平改变的现象称为__衰落__现象。 12、对于模拟微波传输系统来说，一般在单向传输时，出现下述情况之一，并连续10S以上既为中断不可用：视频加权信杂比低于图象质量评价标准______分以下；声音节目信号低于主观评价等级________分以下；接收信号功率低于调频_____________电平以下。 13、在光纤通信中，按调制信号的不同，分为两大类，即__调频_调制和__调幅___调制。

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