Preface
To characterize television system performance, an understanding of signal distortions and measurement
methods as well as p roper instrumentation is needed.
This booklet provides information on television test and measurement practices and serves as a comprehensive reference on methods of quantifying signal distortions.
New instruments, test signals, and measurement procedures continue to be introduced as television test and measurement technology evolves. This booklet encompasses both traditional measurement techniques and
newer methods. After a discussion of good measurement practices, five general categories of television measurements are addressed:
I. Amplitude and Timing Measurements
II. Linear Distortions
III.Nonlinear Distortions
IV. Noise Measurements
V. Transmitter Measurements
A basic knowledge of video is assumed and a glossary of commonly used terms is included as a refresher and to introduce newer concepts. The basics of waveform monitor and vectorscope operation are also
assumed. Consult the instrument manuals for specific operating instructions.
This publication deals with PAL composite analogue signals. Analogue component, digital composite and component, and HDTV measurements are outside its scope.
Good Measurement Practices
EQUIPMENT REQUIREMENTS
Television system performance is evaluated by sending test signals with known characteristics through the signal path. The signals are then observed at the output (or at intermediate points) to determine whether or
not they are being accurately transferred through the system.
Two basic types of television test and measurement equipment are required to perform these tasks.
Test signal generators provide the stimulus and specialized oscilloscopes known as waveform monitors and vectorscopes are used to evaluate the response.
Test Signal Generators
Television signal generators provide a wide variety of test and synchronization signals. Two key criteria in
selection of a test signal generator for precision measurements are signal complement and accuracy.
The generator should provide all of the test signals to support the required measurements and the
signal accuracy must be better than the tolerances of the meas u rements to be made. If possible,
the generator accuracy should be twice as good as the measure m e n t tolerance. For example, differential
gain measurement to 1% accuracy should be made with a generator having 0.5% or less differential gain distortion.
Television equipment and system performance is generally assessed on either an out-ofservice or in-service basis. In b roadcast television applications, measurements must often be made during regular broadcast
hours or on an in-service basis.
This requires a generator capable of placing test signals within the vertical blanking interval (VBI) of the television program signal.
Out-of-service measurements, those performed on other than an in-service basis, may be made with any suitable full field test signal generator.
For out-of-service measurements, the Tektronix TG2000 Signal Generation Platform with the AVG1 and AGL1 modules is the recommended product. The AVG1 Analogue Video Generator provides comprehensive signal sets and sufficient accuracy for virtually all measurement requirements. The AVG1 is also a multiformat unit capable of supporting measurements in other composite and analogue component formats. This eliminates the need for additional signal generation equipment where there is the requirement
for measurements in multiple formats. For synchronization of the equipment under test, a black burst reference signal is provided by the TG2000 mainframe. For applications requiring the test signal source be synchronous with existing equipment, the AGL1 Analogue Genlock module provides the interface needed to lock the TG2000 to an external black burst reference signal.
For in-service measurements, the Tektronix VITS201 Generator and Inserter is the recommended
product. The VITS201 provides a full complement of PAL test signals and high degree of flexibility
in placement of these signals within the VBI. Signal accuracy is adequate for most transmission and transmitter measurement requirements.
Both the TG2000 and VITS201 fully support the measurement capabilities of the 1781R and VM700T Series Video Measurement Sets.
Wa v e f o rm Monitors and Ve c t o r s c o p e s .
The instruments used to evaluate a system's response to test signals make up the second major category of television test and measurement equipment.
Although some measurements can be performed with a general purpose oscilloscope, a waveform
monitor is generally preferred in television facilities.
Wa v e f o rm monitors automatically trigger on the television synchronizing pulses and provide a voltage
versus time display of the video signal. These instruments are equipped with specialized video clamps and filters that facilitate separate evaluation of the chrominance and luminance portions of the signal. Most
models also have a line selector for looking at signals in the vertical interval.
A vectorscope is designed for accurate evaluation of the chrominance portion of the signal. This instrument demodulates the PAL signal and displays the V (R-Y) colour difference component on the vertical axis
and the U (B-Y) colour diff e re n c e component on the horizontal axis.
When selecting waveform monitors and vectorscopes, carefully evaluate the feature sets and specifications to make sure they will meet the measurement needs. This is particularly true if making accurate measurements
of all the signal parameters and distortions described in this booklet. Many varieties of waveform
monitors and vectorscopes are on the market today but the majority of them are not intended for precision measurement applications. Most vectorscopes, for example, do not have precision differential phase and gain
measurement capabilities.
The recommended products for precision measurements are the Tektronix 1781R and VM700T and most of the measurement procedures in this booklet are based on these instruments.
The 1781R provides waveform monitor and vectorscope functions as well as many specialized measurement features and modes that simplify complex measurements.
The VM700T is an automated measurement set with results available in numeric and graphic form. Waveform and vector displays, similar to those of traditional waveform monitors and vectorscopes operating in line select mode, are also provided.
The VM700T Measure mode provides unique displays of measurement results, many of which are presented in this book.
CALIBRATION
Most instruments are quite stable over time, however, it is good practice to verify equipment calibration prior to every measurement session. Many instruments have internally generated calibration signals that facilitate this process. In the absence of a calibrator, or as an additional check, a test signal directly out of a high quality enerator makes a good substitute.
Calibration procedures vary from instrument to instrument and the manuals contain detailed instructions.
Analogue CRT-based instru m e n t s such as the 1781R have a specified warm up time, typically 20
or 30 minutes. Turn the instrument on and allow it to operate for at least that long before
checking the calibration and performing measurements.
This ensures that the measurement instrumentation will have little or no effect on the measurement results.
Computer-based instruments such as the VM700T also specify a warm up time but the operator does not need to verify or adjust the calibration settings. The VM700T will automatically calibrate itself when it is turned on and will continue to do so periodically during operation. For best results, wait 20 or 30
minutes after initial turn-on b e f o re making any measure m e n t s .
INSTRUMENT CONFIGURATION
Most of the functions on waveform monitor and vectorscope front panels are fairly straightfor -
ward and have obvious applications in measurement pro c e d u re s .
A few controls, however, might need a bit more explanation.
DC Restorer.
The basic function of the DC restorer in a waveform monitor is to clamp one point in the video waveform to a fixed DC level. This ensures that the display will not move vertically
with changes in signal amplitude or Average Picture Level (APL).
Some instruments offer a choice of slow and fast DC restorer speeds. The slow setting is used to measure hum or other low frequency distortions. The fast setting removes hum from the display so it will not interfere
with other measurements. Back porch is the most commonly used clamp point, but sync tip clamping has some applications at the transmitter.
Automatic Frequency Control (AFC) versus Direct Triggering.
The AFC/DIRECT selection in the 1781R CONFIGURE menu provides a choice between two methods of triggering the waveform monitor's horizontal sweep. The ramp that produces the horizontal sweep is always
synchronous with the H (line) or V (field) pulses of the reference video and can be started either by the pulses themselves (DIRECT) or by their average (AFC). In the DIRECT mode, the video sync pulses directly trigger the waveform monitor horizontal sweep. The DIRECT setting should be used to remove the
effects of time base jitter from the display in order to evaluate other parameters. Since a new
trigger point is established for each sweep, line-to-line jitter is not visible in this mode.
In the AFC (Automatic Fre q u e n c y Control) mode, a phase-locked loop generates pulses that represent
the average timing of the sync pulses. These averaged pulses are used to trigger the sweep. The AFC mode is useful for making measurements in the presence of noise as the effects of noise-induced horizontal jitter
are removed from the display.
The AFC mode is also useful for evaluating the amount of time base jitter in a signal. The leading edge of sync will appear wide (blurred) if much time base jitter is present. This method is very useful for comparing signals or for indicating the presence of jitter but be cautious about actually trying to measure it.
The bandwidth of the AFC phase-locked loop also affects the display.
Vectorscope Gain: 75%/100% Bars.
Several different kinds of colour bars are used in PAL systems and many generators produce at least two types. In order to accommodate the various types of colour bars, some vectorscopes have a 75%/100%
selection on the front panel which changes the calibration of the vectorscope chrominance gain. The 75% setting corresponds to 100.0.75.0 colour bars, often referred to as EBU Bars.
The 100% setting corresponds to 100.0.100.0 colour bars. The 75%/100% distinction refers to chrominance amplitude, not to saturation or white bar level.
Colour bar parameters and nomenclature are discussed in detail in Appendix A.
It is important to know which colour bar signal is in use and to select the corresponding setting on the vectorscope. Otherwise chrominance gain can easily be misadjusted.
DEMODULATED RF SIGNALS
All of the baseband measurements discussed in this booklet can also be made on demodulated RF signals. It is important, however, to eliminate the demodulator itself as a possible s o u rce of distortion. Measurement
quality instruments such as the Tektronix TV1350 and 1450 Television Demodulators will eliminate the likelihood that the demodulator is introducing distortion.
TERMINATION
Improper termination is a very common source of operator error and frustration. Double term i n a t e d or unterminated signal paths will seriously affect signal amplitude. It is essential that each video signal in a facility be terminated in one location using a 75 Ohm terminator. If a signal is looped through several pieces
of equipment, it is generally best to terminate at the final piece of equipment in the signal path.
The quality of the terminator is also important, particularly when trying to measure very small distortions. Be sure to select a terminator with the tightest practical tolerance as incorrect termination impedance can cause amplitude errors as well as frequency response problems and pulse distortions.
Terminators in the 1/2% to 1/4% tolerance range are widely available and are generally adequate for routine testing.
DEFINITION OF THE PAL TELEVISION STANDARD
The most widely used definition of the PAL standard is probably Report 624 of the CCIR (International Radio Consultative Committee), which specifies amplitude, timing and colour encoding parameters for all of
the major television standards.
This report was last reviewed in 1990 making version 624-4 the most current at this time.
There are a number of variations of PAL (M, N, B, G, H, I, D, etc.).
With the exception of PAL-M, which is a 525-line system, the differences between the standards are fairly minor at baseband and usually involve only a bandwidth change. The default standard for this publication is
PAL-B/G, which has a 5-MHz bandwidth and is used in much of Europe.
Governments of the various countries which use the PAL standard, as well as broadcasting organizations (such as the EBU, BBC, IBA, etc.), also publish standards documents. You may find discrepancies between the various standards. These can be difficult to resolve since there is no absolutely ""correct'' answer.
In general, documents from the local broadcasting authority should take precedence when there are conflicts.
PERFORMANCE GOALS
Acceptable levels of distortion are usually determined subjectively, however, a number of broadcasting organizations publish documents that specify recommended limits. In some cases government regulations may require that certain published criteria be met. While these documents can be useful as performance guidelines, each facility must ultimately determine its own performance goals.
Only experience can reveal what is practical with the equipment and personnel at a given facility.
While there is usually agreement about the nature of each distortion, definitions for expressing the magnitude of the distortion may vary considerably from standard to standard. A number of questions should be kept in
mind. Is the measurement absolute or relative? If it is relative, what is the reference?
Under what conditions is the re f e rence established? Is the peakto- peak variation or the largest peak deviation to be quoted as the amount of distortion?
A misunderstanding about any one of these issues can seriously affect measurement results so it is important to become familiar with the definitions in whatever standards are used. Make sure those involved in measuring system performance agree on how to express the amount of distortion. It is good practice to
record this information along with measurement results.
Lihat juga
Table of Contents
To characterize television system performance, an understanding of signal distortions and measurement
methods as well as p roper instrumentation is needed.
This booklet provides information on television test and measurement practices and serves as a comprehensive reference on methods of quantifying signal distortions.
New instruments, test signals, and measurement procedures continue to be introduced as television test and measurement technology evolves. This booklet encompasses both traditional measurement techniques and
newer methods. After a discussion of good measurement practices, five general categories of television measurements are addressed:
I. Amplitude and Timing Measurements
II. Linear Distortions
III.Nonlinear Distortions
IV. Noise Measurements
V. Transmitter Measurements
A basic knowledge of video is assumed and a glossary of commonly used terms is included as a refresher and to introduce newer concepts. The basics of waveform monitor and vectorscope operation are also
assumed. Consult the instrument manuals for specific operating instructions.
This publication deals with PAL composite analogue signals. Analogue component, digital composite and component, and HDTV measurements are outside its scope.
Good Measurement Practices
EQUIPMENT REQUIREMENTS
Television system performance is evaluated by sending test signals with known characteristics through the signal path. The signals are then observed at the output (or at intermediate points) to determine whether or
not they are being accurately transferred through the system.
Two basic types of television test and measurement equipment are required to perform these tasks.
Test signal generators provide the stimulus and specialized oscilloscopes known as waveform monitors and vectorscopes are used to evaluate the response.
Test Signal Generators
Television signal generators provide a wide variety of test and synchronization signals. Two key criteria in
selection of a test signal generator for precision measurements are signal complement and accuracy.
The generator should provide all of the test signals to support the required measurements and the
signal accuracy must be better than the tolerances of the meas u rements to be made. If possible,
the generator accuracy should be twice as good as the measure m e n t tolerance. For example, differential
gain measurement to 1% accuracy should be made with a generator having 0.5% or less differential gain distortion.
Television equipment and system performance is generally assessed on either an out-ofservice or in-service basis. In b roadcast television applications, measurements must often be made during regular broadcast
hours or on an in-service basis.
This requires a generator capable of placing test signals within the vertical blanking interval (VBI) of the television program signal.
Out-of-service measurements, those performed on other than an in-service basis, may be made with any suitable full field test signal generator.
For out-of-service measurements, the Tektronix TG2000 Signal Generation Platform with the AVG1 and AGL1 modules is the recommended product. The AVG1 Analogue Video Generator provides comprehensive signal sets and sufficient accuracy for virtually all measurement requirements. The AVG1 is also a multiformat unit capable of supporting measurements in other composite and analogue component formats. This eliminates the need for additional signal generation equipment where there is the requirement
for measurements in multiple formats. For synchronization of the equipment under test, a black burst reference signal is provided by the TG2000 mainframe. For applications requiring the test signal source be synchronous with existing equipment, the AGL1 Analogue Genlock module provides the interface needed to lock the TG2000 to an external black burst reference signal.
For in-service measurements, the Tektronix VITS201 Generator and Inserter is the recommended
product. The VITS201 provides a full complement of PAL test signals and high degree of flexibility
in placement of these signals within the VBI. Signal accuracy is adequate for most transmission and transmitter measurement requirements.
Both the TG2000 and VITS201 fully support the measurement capabilities of the 1781R and VM700T Series Video Measurement Sets.
Wa v e f o rm Monitors and Ve c t o r s c o p e s .
The instruments used to evaluate a system's response to test signals make up the second major category of television test and measurement equipment.
Although some measurements can be performed with a general purpose oscilloscope, a waveform
monitor is generally preferred in television facilities.
Wa v e f o rm monitors automatically trigger on the television synchronizing pulses and provide a voltage
versus time display of the video signal. These instruments are equipped with specialized video clamps and filters that facilitate separate evaluation of the chrominance and luminance portions of the signal. Most
models also have a line selector for looking at signals in the vertical interval.
A vectorscope is designed for accurate evaluation of the chrominance portion of the signal. This instrument demodulates the PAL signal and displays the V (R-Y) colour difference component on the vertical axis
and the U (B-Y) colour diff e re n c e component on the horizontal axis.
When selecting waveform monitors and vectorscopes, carefully evaluate the feature sets and specifications to make sure they will meet the measurement needs. This is particularly true if making accurate measurements
of all the signal parameters and distortions described in this booklet. Many varieties of waveform
monitors and vectorscopes are on the market today but the majority of them are not intended for precision measurement applications. Most vectorscopes, for example, do not have precision differential phase and gain
measurement capabilities.
The recommended products for precision measurements are the Tektronix 1781R and VM700T and most of the measurement procedures in this booklet are based on these instruments.
The 1781R provides waveform monitor and vectorscope functions as well as many specialized measurement features and modes that simplify complex measurements.
The VM700T is an automated measurement set with results available in numeric and graphic form. Waveform and vector displays, similar to those of traditional waveform monitors and vectorscopes operating in line select mode, are also provided.
The VM700T Measure mode provides unique displays of measurement results, many of which are presented in this book.
Figure 1. A waveform monitor display of colour bars.
Figure 2. A vectorscope display of colour bars.
CALIBRATION
Most instruments are quite stable over time, however, it is good practice to verify equipment calibration prior to every measurement session. Many instruments have internally generated calibration signals that facilitate this process. In the absence of a calibrator, or as an additional check, a test signal directly out of a high quality enerator makes a good substitute.
Calibration procedures vary from instrument to instrument and the manuals contain detailed instructions.
Analogue CRT-based instru m e n t s such as the 1781R have a specified warm up time, typically 20
or 30 minutes. Turn the instrument on and allow it to operate for at least that long before
checking the calibration and performing measurements.
This ensures that the measurement instrumentation will have little or no effect on the measurement results.
Computer-based instruments such as the VM700T also specify a warm up time but the operator does not need to verify or adjust the calibration settings. The VM700T will automatically calibrate itself when it is turned on and will continue to do so periodically during operation. For best results, wait 20 or 30
minutes after initial turn-on b e f o re making any measure m e n t s .
INSTRUMENT CONFIGURATION
Most of the functions on waveform monitor and vectorscope front panels are fairly straightfor -
ward and have obvious applications in measurement pro c e d u re s .
A few controls, however, might need a bit more explanation.
DC Restorer.
The basic function of the DC restorer in a waveform monitor is to clamp one point in the video waveform to a fixed DC level. This ensures that the display will not move vertically
with changes in signal amplitude or Average Picture Level (APL).
Some instruments offer a choice of slow and fast DC restorer speeds. The slow setting is used to measure hum or other low frequency distortions. The fast setting removes hum from the display so it will not interfere
with other measurements. Back porch is the most commonly used clamp point, but sync tip clamping has some applications at the transmitter.
Figure 3. The 1781R waveform calibrator.
Figure 4. The 1781R vectorscope calibration oscillator.
The AFC/DIRECT selection in the 1781R CONFIGURE menu provides a choice between two methods of triggering the waveform monitor's horizontal sweep. The ramp that produces the horizontal sweep is always
synchronous with the H (line) or V (field) pulses of the reference video and can be started either by the pulses themselves (DIRECT) or by their average (AFC). In the DIRECT mode, the video sync pulses directly trigger the waveform monitor horizontal sweep. The DIRECT setting should be used to remove the
effects of time base jitter from the display in order to evaluate other parameters. Since a new
trigger point is established for each sweep, line-to-line jitter is not visible in this mode.
In the AFC (Automatic Fre q u e n c y Control) mode, a phase-locked loop generates pulses that represent
the average timing of the sync pulses. These averaged pulses are used to trigger the sweep. The AFC mode is useful for making measurements in the presence of noise as the effects of noise-induced horizontal jitter
are removed from the display.
The AFC mode is also useful for evaluating the amount of time base jitter in a signal. The leading edge of sync will appear wide (blurred) if much time base jitter is present. This method is very useful for comparing signals or for indicating the presence of jitter but be cautious about actually trying to measure it.
The bandwidth of the AFC phase-locked loop also affects the display.
Vectorscope Gain: 75%/100% Bars.
Several different kinds of colour bars are used in PAL systems and many generators produce at least two types. In order to accommodate the various types of colour bars, some vectorscopes have a 75%/100%
selection on the front panel which changes the calibration of the vectorscope chrominance gain. The 75% setting corresponds to 100.0.75.0 colour bars, often referred to as EBU Bars.
The 100% setting corresponds to 100.0.100.0 colour bars. The 75%/100% distinction refers to chrominance amplitude, not to saturation or white bar level.
Colour bar parameters and nomenclature are discussed in detail in Appendix A.
It is important to know which colour bar signal is in use and to select the corresponding setting on the vectorscope. Otherwise chrominance gain can easily be misadjusted.
DEMODULATED RF SIGNALS
All of the baseband measurements discussed in this booklet can also be made on demodulated RF signals. It is important, however, to eliminate the demodulator itself as a possible s o u rce of distortion. Measurement
quality instruments such as the Tektronix TV1350 and 1450 Television Demodulators will eliminate the likelihood that the demodulator is introducing distortion.
TERMINATION
Improper termination is a very common source of operator error and frustration. Double term i n a t e d or unterminated signal paths will seriously affect signal amplitude. It is essential that each video signal in a facility be terminated in one location using a 75 Ohm terminator. If a signal is looped through several pieces
of equipment, it is generally best to terminate at the final piece of equipment in the signal path.
The quality of the terminator is also important, particularly when trying to measure very small distortions. Be sure to select a terminator with the tightest practical tolerance as incorrect termination impedance can cause amplitude errors as well as frequency response problems and pulse distortions.
Terminators in the 1/2% to 1/4% tolerance range are widely available and are generally adequate for routine testing.
DEFINITION OF THE PAL TELEVISION STANDARD
The most widely used definition of the PAL standard is probably Report 624 of the CCIR (International Radio Consultative Committee), which specifies amplitude, timing and colour encoding parameters for all of
the major television standards.
This report was last reviewed in 1990 making version 624-4 the most current at this time.
There are a number of variations of PAL (M, N, B, G, H, I, D, etc.).
With the exception of PAL-M, which is a 525-line system, the differences between the standards are fairly minor at baseband and usually involve only a bandwidth change. The default standard for this publication is
PAL-B/G, which has a 5-MHz bandwidth and is used in much of Europe.
Governments of the various countries which use the PAL standard, as well as broadcasting organizations (such as the EBU, BBC, IBA, etc.), also publish standards documents. You may find discrepancies between the various standards. These can be difficult to resolve since there is no absolutely ""correct'' answer.
In general, documents from the local broadcasting authority should take precedence when there are conflicts.
PERFORMANCE GOALS
Acceptable levels of distortion are usually determined subjectively, however, a number of broadcasting organizations publish documents that specify recommended limits. In some cases government regulations may require that certain published criteria be met. While these documents can be useful as performance guidelines, each facility must ultimately determine its own performance goals.
Only experience can reveal what is practical with the equipment and personnel at a given facility.
While there is usually agreement about the nature of each distortion, definitions for expressing the magnitude of the distortion may vary considerably from standard to standard. A number of questions should be kept in
mind. Is the measurement absolute or relative? If it is relative, what is the reference?
Under what conditions is the re f e rence established? Is the peakto- peak variation or the largest peak deviation to be quoted as the amount of distortion?
A misunderstanding about any one of these issues can seriously affect measurement results so it is important to become familiar with the definitions in whatever standards are used. Make sure those involved in measuring system performance agree on how to express the amount of distortion. It is good practice to
record this information along with measurement results.
Lihat juga
Table of Contents
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3
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4
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4
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6
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6
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8
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8
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8
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8
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9
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9
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10
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12
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15
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18
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19
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24
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26
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28
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30
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31
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36
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38
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41
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42
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46
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50
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5 2
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5 3
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54
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55
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56
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57
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58
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6 0
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61
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63
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64
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APPENDICES
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67
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