WAVEFORM TESTING
Victor Nicholson
CTIC The Urban Institute Washington, DC
have resulted in many complaints by broadcasters, of excessive degradation of their local channels when carried by a cable system. This is especially true of local UHF signals Abstract where interference is often introduced at the Existing subjective and frequency domain head end by the mixing of harmonics of the local testing procedures are shown to have major oscillator from the converter with other UHF. weaknesses in ensuring the delivery of accep Distribution system testing has little value in table television pictures to cable subscribers. resolving these complaints. Recommended, instead, are waveform test techniques using VIT signals as generated by Waveform test procedures have had little the networks. Applications of measurement application in the past to the cable industry as procedures and performance objectives are it was expensive to introduce these test signals discussed for echoes, noise and chrominance I at the head end. In addition, they provided no luminance gain and delay. information as to the quality of incoming tele ·,rision signals. Equipment was necessary to demodulate each channel, insert these diagnostic signals and then remodulate them; all of which added degradation to the desired TV signal.
Today, waveform test procedures are applicable to the cable industry as these diagnostic signals are already injected into the Waveform testing of television signals four major network transmissions; ABC, CBS, introduces a valuable concept to the performance NBC, and PBS. This eliminates the need for testing and maintenance of R. F. distribution a cable operator to own expensive equipment to systems; including cable, satellite and fiber introduce these signals. l\1ost important, in a optics. Waveform techniques, as presently report published in 1975 by the Network Trans used by broadcast engineers to meet perfor mission Committee 1 I there are recommended mance objectives for their video facilities, can test procedures to meet picture impairment also be used by cable engineers to meet perfor performance objectives. Granted these mance objectives as to the picture quality of the objectives for network transmission are more television signals delivered to the subscribers. stringent than required for viewer reception, This is in contrast to present procedures that the signals still provide an excellent reference are primarily designed to provide information of quality and the measurement procedures are on quality and performance of cable~ equipment valuable and easy to use. There need only be and distribution system. reduced, reasonable standards for subscriber reception. Monitoring picture quality to ascertain that the delivered signals meet minimum acceptable This paper compares linear waveform level of impairment to the TV picture also distortion analysis with presently used methods ensures that the distribution system performs of subjective testing and frequency domain satisfactorily: the converse is not true. Know measurements. It also endeavors to show that ing that the distribution system meets 1"0inimum waveform analysis merits consideration by the standards does not ensure the delivery of cable industry to improve subscriber penetration satisfactory pictures. Maintenance procedures, in existing systems and as an aid to gaining based on distribution system characteristics, entry into urban markets. Before going into waveform analysis, this paper discusses the
35 procedures we are using now, including Unfortunately frequency domain testing subjective and frequency domain testing, and provides little information concerning the quality points out their present weaknesses. of the TV pictures delivered for the following reasons: Subjective testing can be defined as evaluation by observation of the television -( 1) Faithful reproduction of the television picture and is a valuable technique in the hands signal, or minimum distortion of the waveform, of qualified viewers in detecting visible degrada requires that delay be considered as well as tion of the picture. In some respects, it is amplitude. The cable industry techniques do not superior to measurements of impairments as it include delay versus frequency measurements. deals directly with the end result rather than causative factors that correlate with picture -(2) As pointed out by Osborne 2/ even "full quality. All distortions are simultaneously knowledge of the amplitude I freq""liency and de lay I visible and are diagnosed by the eye/brain frequency characteristics does not give a direct complex for their acceptability. Other advan indication of the consequent deterioration of tages include high sensitivity to some observable picture quality, save in certain extreme cases, factors, the use of the TV set as test device and and involves a great deal of unnecessary effort. '' the lack of need to disrupt the system. Subjec tive testing has many advantages. -(3) This same viewpoint is emphasized by Thiele 3/ "the deviation in frequency response Subjective testing also has many limitations; (in dB) to produce a given subjective impairment one is the dependence for results on the viewer's in transient response, i.e. a minimum error in competence or motivation. The viewer may be waveform shape, -----will be different at a cable technician without knowledge of the type different frequencies. In a particular case the or magnitude of the distortion observed or may same picture impairment was produced by a be more concerned with concealing defects. A deviation of 0. 4dB at 5Mc Is as by a deviation of major weakness of subjective testing lies in the 0. 5dB at lOOKc/ s." Although these measure field of maintenance; some factors such as hum ments were made using Australian TV standards, modulation can be difficult to see depending on the fact remains that there could be a differential the TV picture content, brightness, contrast of more than 20dB in amplitude/frequency re and ambient light. Other distortions cannot sponse, to produce equivalent impairment. be detected or measured until they are defi nitely visible; therefore they cannot be correct -(4) MacDiarmid 4/ discusses frequency domain ed until they reach this relatively high level. in terms of picture impairment tolerances. He An example is an amplifier in the early part of states "What is perhaps the most important a system which may be introducing an excessible objection to the use of sine-waves arises in amount of distortion, using up most of the system considering the question of tolerances" and then tolerance. Still the picture at that location may "where the amount of distortion to be tolerated not appear degraded. Therefore the cable in is modest, the only economical method is to dustry and the FCC requires that these proce specify and measure the waveform response and dures be supplemented with other techniques that to abandon sine-wave ideas." include frequency domain measurements. These are but a few examples from the Frequencv domain techniques involve intro published literature that encourage the use of ducing equal amplitude sine wave signals at waveform analysis for television signal measure \'arious frequencies into equipment, cable or a ment, and which show that existing methods. do system: and then measuring the amplitude or not ensure acceptable pictures to the viewers. phase response variations that result. These I will now discuss waveform analysis and then techniques are valuable for maintenance of its application to the cable industry. equipment, cable or detecting gross distortions in the cable distribution ~v111tem since amplitude \\'aveform analysis techniques involve variations can affect other noise and overload measurement of the wave shape or the amplitude factors. An important plus is that cable tech versus time response of a test signal of a nicians generally have the test equipment and specified waveform to determine the amount of the know how to make frequency domain measure distortion introduced. The test signals are ments whether using a standard sweep generator selected to provide maximum information as to or a simultaneous sweep technique. The latter the desired parameters, in a manner that is permits analysis of a cable system response easily evaluated, permits rapid testing and is during daytime hours while creating a minimum accurate. These signals conform to the tele amount of distortion to the television picture. vision picture which are of waveforms with
36 similar energy distribution. by the four major networks and AT&T in accor dance with the Network Transmission Committee Waveform analysis signals as used by broad (NTC) Report No. 7. 8/ These are: the cast engineers fall into two categories: the first, Composite Test Signal
These VIT signals are generally available In 1963, AT&T 16/ published a valuable to the cable engineer since they are transmitted tutorial manual (as N;""vised by the Network 37 Transmission Committee) dealing with an This could total about $4000. analysis or television signals. Siocos )2_/ published a paper in 1986 evaluating CBS network -4) There are major savings in labor which will transmissions with slight variations or VIT soon exceed the above equipment cost. These signals. Also about this time Peter Wolr J:!l savings will result from reduced system main proposed the addition or a special 20T pulse for tenance and especially from the minimization or measuring short-time distortions with and with conflicts with subscribers and TV service out a graticule: Everett 19/ derived means for companies. correcting waveform errors: and Rhodes 20/ and Schmid 21/ contributed excellent analytical -5) The performance objectives for network papers dealing with the modulated 12. 5 sine transmission are more stringent than needed for Squared Pulse. subscriber reception. For these less sensitive objectives, the measurements are well within the The IEEE 22/ published a Video Signal capability or a system technician. Trial-Use Standard in 1974 that precisely defined the various terms, discussed measure The advantages or using VIT signals for ment and included, in the appendices, a detailed cable systems apply to many factors. However, description, derivation and measurement or the this paper is being limited to just a rew or these Sin2 Pulse, Sin2 Step, T Step and the Mod that are extremely dUficult or costly to measure 12. 5T Pulse. in any other way: noticably affect subscriber reception and for which the cable engineer has A major step forward in terms or applica some control. In the future additional para tions came in June of 1975 with NTC Report meters will also be useful. 117 23/ which introduced non-regulatory standards or "Performance Objectives" for net -1) Echoes; whether leading, due to direct work transmissions or video links. This report pickup or lagging, for many other reasons. The also defines the transmission parameters, test cable industry often specifies the Paul Mertz 24/ signals and measuring methods to be used. It curve without knowing how to measure compli: also replaced the IEEE use or the sin2 T pulse ance. Orten, Time Domain Reflectometers or with the sin2 2T pulse as the latter introduces other techniques are used that merely measure less irrelevant distortions. Furthermore, the reflective characteristics or a component. these NTC recommended test signals are the ones presently transmitted by the four major For the echo test,the 2T sin2 pulse or the networks, ABC, CBS, NBC and PBS. T bar from the Composite Test Signal can be used. (see figures 2, 3) This 2T pulse is an This is an abridged history or the many excell~nt test signal as its energy spectrum is important papers and texts that have dealt with similar to the waveform of the television signal; waveform testing. Now I return to the present approaching zero at the video cutorr frequency. to discuss the advantages or VIT signals for cable systems. These signals are also used by broadcasters to measure transients, high frequency amplitude There are some important advantages that and phase delay; e. g. waveform distortions from are worth summarizing: 125 ns to 1us. For the cable indus'try it may be wise to ignore short time echoes or less than -1) Most important; tests can be made or para 500 ns as the broadcast signal or the test de meters that greatly affect the picture reception, modulator can also introduce transients. but which have .not been feasible using present Measurements are made or the amplitude and test procudures. This includes measurement or delay of the undesired pulses. Where difficulty echoes, color saturation, color delay and impulse exists in identifying echoes down 30 dB or more, noise. because or noise levels at the end of a system, the use or photographs are helpful since the pulse -2) Tests can be made without disrupting sub is constant and the noise random. scriber service, at any system location and on those channels Cor which the cable operator has The echoes or most concern are from 0. 5 to responsibility for quality. 2 u seconds which can be correlated with its displacement on a TV set. For example, the -3) There is no need to purchase equipment to video part or a TV line is 53 u sec: for a 20" interject these signals: for reception the only wide screen this would be a displacement or test equipment needed is a demodulator with o. 38 inches for 1 us. several input modules plus a waveform monitor.
38 Alternate methods of measuring echoes are Relative Chrominance Time (RCT), where through the examination of the first few micro there is no gain distortion, is equal to ten times seconds of the T bar which has a rise time or the total base displacements in nanoseconds or 125 ns. or the horizontal sync pulse with a rise RCT = + 10 b nanoseconds (figure 4) where b time of 250 ns. between 10 and 901o. is the total base displacement.
It is far easier during antenna surveys to Where both gain and delay distortions exist, observe waveform echoes than to attempt to the equations are more complicated and it is minimize echoes by observing the television easier to use one of the nomographs as developed picture. For example, it can be very difficult by Charles Rhodes of Tektronix~/ (see figure in an urban area to pinpoint an antenna location 5). This nomograph not only gives the delay in with minimum echoes by subjective testing. nanoseconds but also converts the gain to The picture content varies, background light decibels. changes and the eye I brain cannot remember slight variations of quality while taking in Reasonable performance objectives for a changes of color and noise. Using VIT signals, viewer for Chrominance Gain would be + 2dB~' a far better job can be done in much reduced and for Chrominance Delay would be+ 477 nano time in measuring and locating the source of seconda,26/ based on a study by A. Lessman of echoes. Bell Laboratories. The+ 2dB compares with the broadcast network gain performance objective of -2) Chrominance/Luminance Gain and Delay + 0. 5 dB and the + 477 ns. compares with the Inequalities produce noticable effects on the network objective of+ 75 ns. These are quality or a color picture. standards that can be-measured and met by the cable industry, The+ 477 ns. may seem too Chrominance/Luminance gain refers to the permissive, however it involves shaped delay amplitude ratio of frequencies, between the which is less objectionable than the broadcasters low frequencies near the video carrier and the flat delay. '£!_/ high frequencies near the color sub carriers. Improper ratios result in degraded color Some sources of degraded RCL and RCT saturation of the TV picture. in cable systems are the filters for two-way in amplifiers and traps, bandpass filters and signal Chrominance/Luminance delay refers to processors at the head end. the additional time involved in the path or the color energy with respect to the low frequency -3) Random Impulse and Periodic Noise are energy near the video carrier. Additional time also parameters of importance to the quality of a results in delayed chrominance which can show TV picture. and unfortunately are not generally on the TV screen as funny picture effect: for measured in a cable system. Most procedures example, the red color of the lips of a persons measure the random noise as introduced by head face can be displaced from the monochrome end and distribution equipment. This ignores detail of the Ups. In spite of their importance, Impulse noise caused by automobiles, industrial these factors are rarely measured by cable plants, cosmic sources, power lines and video engineers. This is due to the excessive cost of tape or film program sources. Likewise Perio equipment and time, using other techniques. dic noise can be introduced in the broadcast transmission and also at a cable head end from The key to delivering satisfactory color is defective equipment or interfering signals. the ability to measure and correct chrominance inequalities. Color saturation inequalities are A very simple method for measuring the easily correctible by slight amplitude/frequency summation of these noises or for identifying response alignment of a headend channel pro them independently is by the use of the 18 us. cessor or preferably by correcting the source of flat part of the line bar. For low frequency distortion which could be antenna site location, a periodic noise, use can be made of the television narrow band antenna array, incorrectly aligned trap etc. ':' The basis for + 2 dB is past experiments showing noticeably degraded pictures on some For these tests a modulated 12. 5T Chro TV sets resulting from a chrominance level minance Pulse Test Signal is used (see figure 4) reduced by 3 dB. which is also part of the Composite Test Signal. Relative Chrominance Level (RCL) - where there is no delay - is equal to the sum in percent or the peak and base displacements of the pulse or RCL =.! 2a percent. 31 signal itself, observing it at a verticle sync. rate.
This procedure is helpful in locating a cable antenna site or isolating noise sources as measurements are made during the actual signal transmission. This technique provides a means of comparing the total Signal-to-Noise with that introduced by the system.
There are other distortion measurements that can be made just as easily using VIT signals. However, time permits discussing only these few. Some others, such as differential gain and phase, are not presently serious sources of picture impairment although they become more impor tant with the use of cable ancillary services such as satellites or terrestrial microwave. Another VIT signal, the multiburst, could supplement present amplitude I freq·.1ency response techniques in providing information as to total variations. In short this paper has just begun to scratch the surface in describing ways that tests using the VIT signals can contribute to the efficient operation of a cable system with a reduction in cost and time over current measuring procedures.
The measurement techniques discussed in this paper should help ensure that the quality of subscriber service is raised to its full potential, High quality subscriber service is necessary to open up urban markets and reduce broadcaster complaints. In addition high quality subscriber service will improve rela tions with subscribers, and therefore franchise officials. These points are elaborated in a previous paper. 27/ While the desireability of measuring picture impairment has often been acknowledged, the difficulties of measuring these parameters with traditional techniques has been a deterrent. As this paper demonstrates,the test procedures using the VIT signals are not difficult, nor are they new or untried, having been used by broadcasters for years. The VIT signals are generally available for those channels over which the cable operator has some control. Finally the amounts of impairment measured can be compared with the television network perfor mance objectives or be correlated with the Bell Telephone Lab picture quality ratings.
The parameters are important and there is no longer any reason not to measure them. For the future; there will be a need to use waveform analysis not only for measuring other parameters affecting picture quality, but also for isolating sources of problems, automating test proce dures and correlating analog and digital TV reception.
40 FIGURE 3 FIGURE 1 2T Pulse Test Signal The Composite Test Signal IRE IRI! units I COlo 2T 12.5T unitl T Bar
80
60
40 40
20
0 I I I I I I I I I I I I I II 21 HALF-AMPLITUDE DURATION =250ns !0 34 S7 42 46 ~2 '8 Iii! IIICIIOKCOHOS (a) 100 p~ The Combination Test Signal 70 '60 100 . 50 . 40 '30 ~20 :10 I 0 ---=~------~~~_o~~~lO ,20 ~30 0 2T Sm. 2 s·1gnal with severe to Phase Delay
·40 I I I I I I I I I I I I 16 18 24 28 32 36 40 43 46 ~ :14 601 FIGURE 4 (b) I.IICROSECONOS Chrominance Pulse Tes1 Signal FIGURE 2
T
2T 0. 250 us, where fc 4MHz
0.
(a) HAD
2 12. 5T HAD 1570 nanoseconds Freq. MHz
41 FIGURE 4 1b)
Nomograph i 2.5T Modulated Sine-Squared Pulse for NTSC
0
Y2 OR YJ
+40 +SO
The Measurement of Linear Chroma Distortion in NTSC TV Facilities
HANS SCH::'\IID
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. --,- HGHC~:.-~L- •'. OfL•J'fD CHIII'O•, ·'Kr Kclali\'c d1ronm 1irnc.
42 References
(1) "NTC Report No. 7" Video Facility Testing Paired Echoes," Proc. IRE, June 1939, prepared by the Network Transmission pp. 359-385 committee of the Video Transmission Engineer ( 16) "Television Signal Analysis" ing Advisory Committee (Joint Committee of A. T. & T, Long Lines Dept, New York, N.Y. Television Network Broadcasters and the Bell ( 17) C. A. Siocos, Television Transmission System). Published by the Public Broadcasting Testing, "Vertical Interval Test and Reference Service, June 1975 Signals (VITS) in the CBC Television Network" (2) B. W. Osborne, "Picture Quality Assessment 1965, Jour. Soc. Mat. Pict. Telev. Engrs., and Waveform Distortion Correction on Wired pp. 81-84 Television Systems," 1962 (18) P. Wolf, "Modification of the Pulse-and Jour. Brit. Inst. Rad-Engrs. 399-404 Bar Test Signal with Special Reference to (3) A. N. Thiele, "Methods of Waveform Testing, Application in Color Television" Jan, 1966, vol Pulse and Bar - "K" Factor" 75 Jour. Soc. Mot. Pict. Telev. Engrs. Australian Bdcst. Commission Laboratory Report pp. 15-19 No. 26 (19) F. C. Everett, "Recognition and Correction Alderson Bldg. , 508 Pacific Highway of Waveform Errors in TV Transmission" St. Leonards, N. S. W., Australia IEEE Trans. on Bdcst., Vol. BC-18, No. 1, (4) I. F. MacDiarmid, "Waveform Distortion in Mar. 1972. Television Links, Part I - Introduction to Wave (20) C. W. Rhodes, Tektronix Corp. form Distortion" "The 12. 5T Modulated Sine-Squared Pulse for Post Office Elect. Engrs. Jour., 1959, pp. 108- NTSC, " IEEE Trans. on Bdcst. , Vol BC-18, 114 No. 1, Mar. 1972 (5) C. Bailey Neal, "A Brief History of the VIR: (21) H. Schmid, Am. Bdcst. Co. Vertical Interval Reference Signal" "The Measurement of Linear Chroma Distortion Communications/Engineering Digest, Jan 1976, in NTSC TV Facilities" pp. 20-25 IEEE Trans. on Bdcst, Vol. BC-18, No. 3 (6) "EIA Television Systems Bulletin No. 1," Sept. 1972, pp. 77-80 EIA Recommended Practice for Use of a Vertical (22) IEEE Std 511-1974 Interval Reference (VIR) Signal. July 1972, "IEEE Trial-Use Standard on Video Signal Price $1.60 Transmission Measurement of Linear Waveform Electronic Industries Asso., Eng. Dept. Distortion" 2001 Eye St., N. W., Wash. D. C. 20006 IEEE, 345 East 47 St., New York, N.Y. 10017 (7) "EIA Television Systems Bulletin No. 3" (23) NTC Report No. 7 A history of the Vertical Interval Reference (VIR) (24) P. Mertz, "Influence of Echoes on Tele Signal vision Transmission," Jour Soc. Mot. Pict. March 1975, Price $7. 50 Telev. Engrs., 60, pp. 572-596, 1953 (8) NTC Report No. 7 (25) C. W. Rhodes, Same as Reference 20 (9) ArcherS. Taylor, Chairman CTAC Panel 2 (26) A.M. Lessman, "The Effects of Delay Final Report "Subjective Evaluation of Picture Difference of the NTSC Color Television Signa{ Quality" 12-13-14, Table 1 1971, Bell Tel. Labs (10, a) 0. S. Puckle, "The Transient Aspect of (27) Vic Nicholson, "The Case for Picture Wide-Band Amplifiers, " 1935, Quality Standards, " CTIC, Wireless Eng. , p. 251 Communications/Engr. Digest, Vol 1, No. 1 (10, b) 0. S. Puckle, "Time Bases" October 1975 John Wiley & Sons, Inc. (11) A. V. Bedford and G. L. Fredenhall, "Analysis, Synthesis and Evaluation of the Transient Response of Television Apparatus" Inst. of Rad. Eng., 1942, p. 440 (12) N. W. Lewis, "Waveform Responses of Television Links" Proc. Inst. Elect. Eng., 1954, pp. 258-270 (13) I. F. MacDiarmid, "Waveform Distortion in TV Links" Post Office Elect. Eng. J., Parts I, II and III, 1959 (14) B. W. Osborne, See reference 2. (15) H. A. Wheeler, "The Interpretation of Amplitude and Phase Distortion in Terms of
43