Television Measurements
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Television Measurements NTSC Systems Copyright © 1998, Tektronix, Inc. All rights reserved. 1 Contents Preface . 3 III. NONLINEAR DISTORTIONS . .41 Differential Ph a s e . .42 Good Measurement Practices . 4 Differential Gain . .46 EQUIPMENT REQUIREMENTS . 4 Luminance Nonlinearity . .50 CALIBRATION . 6 Chrominance Nonlinear Ph a s e . 52 INSTRUMENT CALIBRATION . 6 Chrominance Nonlinear Gain . 53 DEMODULATED RF SIGNALS . 7 Chrominance-to-Luminance TERMINATION . 7 Intermodulation . .54 STANDARDS AND Transient Gain Distortion . .56 PERFORMANCE GOALS . 7 Dynamic Gain Change . .57 Waveform Distortions and Measurement Methods . 8 IV. NOISE MEASUREMENT . .58 I. VIDEO AMPLITUDE AND Signal-to Noise Ratio . .59 TIME MEASUREMENTS . 8 Amplitude Measurements . 9 V. TRANSMITTER MEASUREMENTS . 61 Time Measurements . .12 ICPM . .62 SCH Phase . .15 Depth of Modulation . .64 GLOSSARY OF TELEVISION TERMS . 65 II. LINEAR DISTORTIONS . .18 APPENDICES Chrominance-to-Luminance APPENDIX A - NTSC COLOR BARS . .68 Gain and Delay . .19 APPENDIX B - SINE-SQUARED PULSES 70 Short Time Distortion . .24 APPENDIX C - RS-170A . .71 Line Time Distortion . .26 APPENDIX D - FCC 73.699, FIGURE 6 . .73 Field Time Distortion . .28 Long Time Distortion . .30 Frequency Response . .31 Group Delay . .36 K Factor Ratings . .38 2 Preface To characterize television system New instruments, test signals, A basic knowledge of video is performance, an understanding and measurement procedures are assumed and a glossary of com- of signal distortions and mea- introduced as television test monly used terms is included as surement methods as well as and measurement technology a refresher and to introduce new pr oper instrumentation is needed. evolves. This booklet encom- concepts. This booklet does not This booklet provides informa- passes both traditional measure- provide detailed instructions on tion on television test and mea- ment techniques and newer how to use particular instrum e n t s . surement practices and serves as methods. After a discussion of The basics of waveform monitor a comprehensive reference on good measurement practices, and vectorscope operation are methods of quantifying signal five general categories of assumed. Consult the instrument distortions. This publication television measurements manuals for specific operating deals with NTSC composite ana- are addressed: instructions. log signals. Analog component, I. Video Amplitude and Time digital composite and compo- Measurements nent, and HDTV measurements are outside its scope. II. Linear Distortions III.Nonlinear Distortions IV. Noise V. Transmitter Measurements 3 Good Measurement Practices EQUIPMENT REQUIREMENTS good as the measurement toler- supporting measurements in Television system performance ance. For example, differential other composite and analog com- is evaluated by sending test sig- gain measurement to 1% accura- ponent formats. This eliminates nals with known characteristics cy should be made with a gener- the need for additional signal through the signal path. The sig- ator having 0.5% or less differ- generation equipment where nals are then observed at the ential gain distortion. there is the requirement for mea- output (or at intermediate Television equipment and sys- surements in multiple formats. points) to determine whether or tem performance is generally For synchronization of the not they are being accurately assessed on either an out-of-ser- equipment under test, a black transferred through the system. vice or in-service basis. In broad- burst ref e r ence signal is prov i d e d Two basic types of television test cast television applications, by the TG2000 mainframe. For and measurement equipment are measurements must often be applications requiring the test required to perform these tasks. made during regular broadcast signal source be synchronous Test signal generators provide hours or on an in-service basis. with existing equipment, the the stimulus and specialized This requires a generator capable AGL1 Analog Genlock module oscilloscopes, known as wave- of placing test signals within the provides the interface needed to form monitors and vectorscopes, vertical blanking interval (VBI) lock the TG2000 to an external provide the tools for evaluating of the television program signal. black burst reference signal. the response. Out-of-service measurements, For in-service measurements, the those performed on other than Tektronix VITS200 Generator Test Signal Generators. Tel e v i s i o n an in-service basis, may be made and Inserter is the recommended signal generators provide a wide with any suitable full field test product. The VITS200 provides variety of test and synchroniza- signal generator. a full complement of NTSC test tion signals. Two key criteria in For out-of-service measurements, signals and high degree of flexi- selection of a test signal genera- the Tektronix TG2000 Signal bility in placement of these tor for precision measurements Generation Platform with the signals within the VBI. Signal are signal complement and accu- AVG1 and AGL1 modules is the accuracy is adequate for most racy. The generator should recommended product. The transmission and transmitter provide all of the test signals to AVG1 Analog Video Generator measurement requirements. support the required measure- provides comprehensive signal Both the TG2000 and VITS200 ments and the signal accuracy sets and sufficient accuracy for fully support the measurement must be better than the toler- virtually all measurement capabilities of the 1780R and ances of the measurements to be requirements. The AVG1 is also VM700 Series Video made. If possible, the generator a multiformat unit capable of Measurement Sets. accuracy should be twice as 4 Waveform Monitors and booklet. Many varieties of wave- Vectorscopes. The instruments form monitors and vectorscopes used to evaluate a system's are available today but the response to test signals make up majority of them are not intend- the second major category of ed for precision measurement television test and measurement applications. Most vectorscopes, equipment. Although some mea- for example, do not have preci- surements can be performed sion differential phase and gain with a general purpose oscillo- measurement capabilities. scope, a waveform monitor is The recommended products for generally preferred in television precision measurement applica- facilities. Waveform monitors tions are the Tektronix 1780R provide TV triggering capabili- and the VM700T. Most of the ties and video filters that allow measurement procedures in this evaluation the chrominance and booklet are based on these luminance portions of the signal instruments. independently. Most models also A waveform monitor display of color bars. have a line selector for looking The 1780R provides waveform at signals on individual lines. monitor and vectorscope func- tions as well as many special- A vectorscope, which demodu- ized measurement features and lates the signal and displays R-Y modes that simplify complex versus B-Y, is another important measurements. test and measurement tool. With a vectorscope, the chrominance The VM700T is an automated portion of the signal can be measurement set with results accurately evaluated. available in numeric and graphic form. Waveform and vector dis- When selecting a waveform plays, similar to those of tradi- monitor and vectorscope, care- tional waveform monitors and fully evaluate the feature sets vectorscopes operating in line and specifications to make sure select mode, are also provided. they will meet present and The VM700T Measure mode pro- future needs. This is particularly vides unique displays of mea- true if making accurate measure- surement results, many of which ments of all signal parameters are presented in this book. and distortions described in this A vectorscope display of color bars. 5 CALIBRATION INSTRUMENT CONFIGURATION Most instruments are quite Most of the functions on analog stable over time, however, it is waveform monitor and vec- good practice to verify equip- torscope front panels are fairly ment calibration prior to every st r a i g h t f o rw a r d and have obvious measurement session. Many applications in measurement instruments have internally gen- procedures. A few controls, erated calibration signals that however, might need a bit facilitate this process. In the more explanation. absence of a calibrator, or as an additional check, a test signal DC Restorer. The basic function of directly out of a high quality the DC restorer in a waveform generator makes a good substi- monitor is to clamp one point of tute. Calibration procedures vary the video waveform to a fixed from instrument to instrument DC level. This ensures that the and the manuals contain display will not move vertically The 1780R waveform calibrator. detailed instructions. with changes in signal amplitude or APL (Average Picture Level). Analog CRT-based instruments such as the 1780R have a speci- Some instruments offer a choice fied warm up time, typically of slow and fast DC restorer 20 or 30 minutes. Turn the speeds. The slow setting is used instrument on and allow it to to measure hum or other low fre- operate for at least that long quency distortions. The fast before checking the calibration setting removes hum from the and performing measurements. display so it will not interfere This ensures that the measure- with other measurements. Back ment instrumentation will porch is the most commonly have little or no effect on the used clamp point, but sync tip measurement