NTSC Video Measurements

Copyright © 1997, Tektronix, Inc. All rights reserved. About this Booklet

Over the years, Tektronix has In this booklet we’ve arranged a Page numbers and figure num- made available many articles, sequence of topics which leads bers used here indicate both the application notes, booklets, from relatively basic concepts section of the booklet and the video tapes, and other materials toward the more advanced. The sequence within the section. For to help you, our customers, bet- ideas involved, however, do example, page 2-3 is the third ter understand the interaction of interrelate and support each page of the second section; your daily activities and Tek’s other — you may find it helpful Fig. 1-5 is the fifth figure in the products. This booklet is a com- to take the time to look through first section. pilation of several shorter pieces the whole booklet even if As always, Tektronix is very which are (or were) offered by your initial interest is for a interested in your feedback and Tektronix’ Television Division. particular topic. comment and may be contacted at any of the offices listed in the back of this booklet.

Credits:

Portions of this document have been reprinted with permission as chapters of this book:

• Government and Military Video, P.S.N. Publications, Inc., New York, NY, 1992

• Video Systems, Intertec Publishing Corp., Overland Park, KS, 1992 Table of Contents

1. General Concepts Why Test Video Signals...... 1-1 Test Methodology...... 1-2 Connecting and Terminating Instruments...... 1-3 2. Basic Video Testing — Waveform Monitor Techniques Connecting the Instruments...... 2-1 Understanding the Waveform Display...... 2-2 Understanding the 1710J Controls...... 2-3 Making Measurements...... 2-4 3. Basic Video Testing — Vectorscope Techniques Understanding the Display...... 3-1 Using the Vectorscope Controls...... 3-2 Checking the Display...... 3-3 Checking Chrominance Phase...... 3-3 Checking Chrominance Gain...... 3-4 Checking White Balance...... 3-4 4. Setting up a Genlocked Studio Genlock Basics...... 4-1 Adjusting TBCs...... 4-2 Adjusting for Synchronization...... 4-2 5. Intermediate NTSC Video Testing Setting Up the Test Equipment...... 5-2 Insertion Gain...... 5-2 Frequency Response Testing...... 5-4 Chrominance to Luminance Gain...... 5-6 Nonlinear Distortions...... 5-7 Luminance Nonlinearity...... 5-7 Differential Gain...... 5-8 Differential Phase...... 5-10 6. Beyond Bars — Other Tests and Test Signals ...... 6-1 7. The Color Bar Signal — Why and How ...... 7-1 8. Glossary ...... 8-1 General Concepts

Why Test Video Signals? A television picture is conveyed by an electrical signal (Figure 1-1) . Working in video, the quality of This video signal is carried from your final product depends on one place to another by cables many factors. Scripting, casting, (coax) or by radio-frequency directing, and many other ingre- (RF) waves. Along the way, it dients add together to make must pass through various (or break) a production. But even pieces of equipment such as when all the creative efforts are video tape machines, switchers, perfectly balanced, poor techni- character generators, special cal quality — and the resulting effects generators, and transmit- poor picture quality — can ters. Any of this equipment can obscure all the hard work and change or distort the signal in skill invested. undesirable ways. This holds true in any facility Since picture quality is largely from the largest production determined by signal quality, it’s house or network studio, to Figure 1-1. A television picture is created by an electrical video signal. In important to detect and correct this case, the picture of color bars is created by a color bars test signal. corporate studios, and to the any signal distortions. The signal smallest freelance production has to be right before the picture shop. The diffe r ence is, the large r can be right. facilities have a technical staff devoted to keeping equipment Many video facilities rely heavily performance and picture quality on picture monitors for quality in peak form. Smaller commer- checks at various stages of pro- cial, business, and industrial duction and distribution. A facilities have smaller staffs. pi c t u r e monitor, after all, displays Sometimes a staff of one! the final product, the picture. It’s a quick and easy means of Whether you’re a video veteran ensuring that there are no or the newest intern, it behooves obvious picture impairments. you to know something more about video quality than what But picture monitors do not tell you see on a picture monitor. the whole story. In fact, relying The goal of this booklet is to solely on picture monitors for help you acquire additional video quality checks can be an Figure 1-2. Waveform display of a color bars signal. technical knowledge, and be open invitation to disaster. more comfortable and capable First of all, not every picture To avoid such surprises, you need with video testing. impairment is obvious on a to look at more than pictures. You must, however, exercise picture monitor. Minor problems You need to look at the video caution and common sense. are easily overlooked. Some signals that convey the picture Be sure to heed all warnings cannot be seen at all. For exam- information through the various printed on equipment covers. ple, a marginal video signal devices and interconnecting And never remove any equip- can still produce a seemingly cables of a video system. ment casings or panels unless “good” picture on a forgiving Specialized instruments have you are qualified for internal monitor. This can produce a been developed to process and servicing of that equipment. The false sense of security as signal display these signals for analysis. information in this booklet is degradations accumulate throu g h intended only for use in externa l various production stages. The monitoring and adjustment of end result can be a nasty surprise user accessible controls on that can lead to costly remakes video equipment. and missed deadlines.

1-1 A waveform monitor is an ensure the overall gain of a of the video system under test. instrument used to measure record/playback system is cor- For all practical purposes, these luminance or picture brightness rect, i.e., a recording made with test signals are “perfect” signals. as well as a high frequency color a standard amplitude video signal Other instruments such as wave- signal called chrominance. An at the machine’s input will pro- form monitors, vectorscopes, instrument called a vectorscope duce standard amplitude video combinations of waveform and is required for quality control of at its output during playback. vector monitors, or specialized video chrominance, especially In the first case (specific signal) video measurement sets are used in more complex systems. we’ll need test equipment that to evaluate the test signal at the When properly used, these tools enables observation of the signal output of the path under test. As allow you to spot signal prob l e m s and knowledge of what the an example, Figure 1-2 shows a before they become picture appropriate limits or characteris- waveform monitor display of a problems. You can make certain tics are. The second case (equip- color bars signal. This display is that the signals are optimal, ment or system testing) is more also called a waveform — it is rather than marginal. Plus, with general and is assumed in the actually a graph of the changing regular video system testing, you following discussion. voltage of the signal (plotted can catch minor degradations The usual method of evaluating vertically) and time (plotted and performance trends that video equipment is with a well- horizontally). Calibrated scales indicate a need for system main- defined, highly stable test signal on the waveform monitor’s tenance. This allows you to avoid having known characteristics, screen allow the various ampli- costly failures, both in equipment such as a color bars signal. tude (voltage) and time parame- and in program production or ters of the waveform to be mea- distribution. Video testing is based on this sured. Other test signals and simple principle of applying a their related instrumentation Test Methodology known test signal to the video and displays are discussed in system or equipment input and Th e r e are two somewhat diffe r i n g the following sections. observing the signal at the out- ideas used in testing video. Our put. Any distortion or impair- Test signal generators and signal ap p r oach is to test characteristics ment caused by the system is evaluation instruments are of a specific signal to ensure that observed and measured on the available in a wide variety of it meets certain artistic or tech- output signal. If there are distor- models. These can range from nical requirements. Or, we may tions, the equipment is adjusted simple production-oriented test the characteristics of indi- to eliminate or minimize them. instruments to highly sophisti- vidual pieces of equipment or The point is, if the system can cated engineering instruments. several pieces in the video signal pass the test signal from input to Wav e f o r m monitors, vectorscopes, path to determine that signal output with little or no distor- video test sets, and other spe- distortions introduced by this tion, it can cleanly pass picture cialized equipment to display equipment are acceptable — or, signals as well. and/or evaluate the signal are at least, minimized. also available in a wide variety The signals necessary for such An example of the first case of configurations. The following testing are obtained from a test might be monitoring the output sections will acquaint you with signal generator. This instrument of a video source (camera, char- some of these tools and with produces a set of precise video acter generator, etc.) to ensure it methods of using them to signals with carefully defined is not producing signals that enhance your effectiveness in and controlled characteristics. exceed black or peak white signal maintaining video quality. Each signal is ideal for verifying limits. As an example of the one or more specific attributes second case, we may wish to

1-2 Connecting and Terminating Also, not all cables are well Instruments shielded and signals may cross- talk from outside the cable into The Tektronix instruments dis- the video path inside. In other cussed in this booklet, and most words, the interconnecting others, have rear panel BNC cables themselves may be connectors. For many video introducing signal distortions. tests, you only need to use one connector on each instrument. While small, flexible cables are This is the connector marked convenient to handle, they are TEST SIGNAL on the signal not without technical cost. generator and the connector Consideration should be given to marked CH A INPUT on the using larger, double-shielded wa v e f o r m monitor or vectorscope. cables in long or critical runs. The techniques in this booklet Notice there are actually two can be used to evaluate the dis- “loop-through” CH A connectors tortions introduced by various on both the waveform monitor lengths and/or quality of cable. and vectorscope (Figure 1-1). If Fi g u r e 1-3. Loop-through inputs on instruments allow the same video source you feed the signal in one side While on the subject of cables to be viewed simultaneously in waveform, vector and picture forms. of these inputs and out the other, and interconnections, always the signal will pass through the remember to properly terminate There are three ways a signal instrument unaffected. This type each signal path. If a signal path path can be properly terminated: of loop-through input lets you is left “open” at the end — such • Some instruments have a connect the same signal to more as a high impedance loop-throu g h single-connector input with a than one instrument. with nothing connected on built-in terminator. You can one side — several problems For example, after running the connect such an instrument can result. test signal from the signal gener- “as is” at the end of a signal ator through the system under Most obvious will be a change in path, but you can’t loop the test and from there to a waveform amplitude of the signal on that signal through it. monitor, you can then loop the path. With an open termination • Some instruments have a two- signal through the waveform the amplitude will be higher connector input with a built-in monitor into a vectorscope. Using than expected (usually about terminator and a switch for the same method, you can also twice amplitude, but actually selecting loop-through or loop through the vectorscope to depending on the signal source terminating connection. You a picture monitor. This connec- impedance). With a double ter- can connect such an instr u- tion method allows you to look mination, such as will result if ment anywhere in the signal at the same test signal on all an internally terminated loop- path, but you must set the three instrument displays through and an external termina- switch correctly for the (Figure 1-3). The order in which tor are used on the same path, intended use (either Hi-Z for the instruments are connected the amplitude will be decreased. loop-through or 75 ohm for do e s n ’ t matter — if the connecting A double termination will often end-of-line termination). cables are short. result in a two-thirds amplitude • Some instruments, such as signal, again depending on Coaxial cable does have signal the 1720 and 1730, have two- source impedances. loss — the signal’s amplitude connector inputs with no decreases as it progresses down Amplitude is not the only effect built-in terminators. You can the cable. For runs of a few feet, of misterminations. Don’t be connect such an instrument this decrease in amplitude is tempted to make up for improp e r anywhere in the signal path. typically 1% or less and is usually termination by adjusting the But, if you connect it at the ig n o r ed. For longer cable lengths, signal amplitude. Mistermi n a t i o n s end of the signal path, you must or when precision measurements also introduce problems with attach a 75 ohm terminator to are being made, the loss must be frequency response (amplitude the unused connector. taken into account or corrected becomes a function of signal with a compensating distribution frequency) and with differing amplifier. Not only is coax lossy, response depending on location the loss is a function of signal along the signal path. Use the frequency — higher video correct terminating resistor on frequencies are attenuated more the end of each coaxial path. than the low frequencies.

1-3 Basic Video Testing — Waveform Monitor Techniques The focus of this section is on Connecting the Instruments tests in production facilities using Once you’ve identified the a waveform monitor. It will be instruments to be used, the next helpful if you have already read step is to connect them to the the first section of this booklet, video system to be tested. Often Figure 2-1. The Tektronix TSG 100 NTSC Television Generator produces a General Concepts. The following variety of special signals for testing video equipment and systems. (NTSC this has already been done for assumes you are already familiar stands for National Television Systems Committee, the committee that you, especially when the instru- developed the television system we use in the United States.) with that material. ments ar e mounted in equipment To provide examples of tests in a racks. Still, it pays to know how production environment, the the connections should be made Tektronix TSG 100 NTSC so you can make sure they have Television Generator (Figure 2-1) been done correctly. and Tektronix 1710J NTSC The Tektronix instruments dis- Waveform Monitor (Figure 2-2) cussed here have rear-panel BNC are used since they are models connectors. These connectors are often found in the production used for connecting cables to the environment. However, all system under test. For basic waveform monitors and test video testing, you only need to signal generators, no matter how use one connector on each sophisticated, include the same instrument. Refer to Connecting basic functions described here. Figure 2-2. The Tektronix 1710J NTSC Waveform Monitor displays video and Terminating Instruments on A vectorscope (Figure 2-3) is signals for amplitude and timing measurements. page 1-3 for further information. another useful instrument for video signal evaluation; its use Ensure everything is connected is covered in the next section of as shown in Figure 2-4 (you may this booklet. not be using a vectorscope or picture monitor — just be sure to Vectorscopes and waveform terminate the end of the signal monitors complement one path). With all equipment turned another and provide a full repre- on, a test signal also needs to be sentation of all information selected. For the examples in about the video signal. Because this section, a color bars signal of this, both instruments are is the only test signal that often mounted side-by-side in you’ll need. instrument racks. In some cases, both functions may even appear The TSG 100 has a series of Figure 2-3. The Tektronix 1720 Vectorscope complements a waveform in the same instrument. The rest signal selection push buttons on monitor by providing additional information about the video signal’s color of this section shows you how to its front panel (Figure 2-1). Each (chrominance) content. use test signal generators and push button is labeled to indicate waveform monitors for basic the type of signal selected by video testing. For further details that button. Select the color on these instruments and their bars signal. additional uses, you may want to refer to: Basic Waveform Monitoring Vid e o t a p e (2 4 W- 7 0 2 9 ) . This tape is available through your local Tektronix Sales Office or by calling 1-800-TEK-WIDE, extension TV.

Figure 2-4. Loop-through inputs on instruments allow the same video signal to be viewed simultaneously in waveform, vector and picture forms. 2-1 If you are using a picture moni- into major and minor divisions. tor, you should see the familiar These divisions are used for color bars display on that moni- measuring time intervals on the tor. You should also see a color video signal. The value of each bars signal on the waveform division — the time per division monitor display. This waveform — is determined by the wave- is shown in Figure 2-5. This color form monitor’s horizontal bars signal is called SMPTE color control settings. bars because it conforms to the To interpret the display in signal specifications set by the Figure 2-5, you should be aware Society of Motion Picture and that an NTSC television picture Television Engineers (SMPTE). consists of 525 horizontal lines. Understanding the Waveform These lines are formed by scan- Display ning from left to right in a raster pa t t e r n. This pattern is illustrated Notice in Figure 2-6 that there are in Figure 2-7. various lines and numbers on Figure 2-5. Waveform monitor display of SMPTE color bars waveform Every line of a video signal con- the front of the waveform moni- shows video signal voltage variations with time. tains a negative-going synchro- tor’s CRT. These markings are nizing pulse. This ‘sync’ pulse referred to as the graticule and starts the next line of video and allow measurement of waveform is contained in the signal’s hori- amplitude and time parameters. zontal blanking interval. This The nominal video signal level blanking interval is a short period for television studios and pro- of time between the scan lines. duction facilities is one volt The 1710J displays what appears (1 V) peak-to-peak. (The term to be two lines of video side by peak-to-peak means from the side in what is generally called a bottom of the signal to the top. It 2H mode (two horizontal lines). is often abbreviated as p-p.) This is actually a display of all Without such an amplitude 525 lines laid on top of each standard, signals from different other, with half of the lines on sources might not be compatible the left and half of the lines on with each other. Keep in mind the right. It also puts a complete that this is for a nominal signal, Fi g u r e 2-6. One line of a SMPTE color bars signal drawn to show key details. horizontal blanking interval near one that contains the brightest the center of the screen. This is possible (peak white) picture shown in Figure 2-5. Some information. Many test signals fit waveform monitors use a 1H this description. Actual picture mode which shows one line of information often does not con- video rather than two lines side tain any peak white levels and by side. This would be like will therefore not measure 1 V looking at just the left half of the peak-to-peak. display in Figure 2-5 expanded For making peak-to-peak and horizontally to fill the screen. other amplitude measurements, A Closer Look at the Color the vertical axis of the waveform Bars Waveform monitor graticule is marked in IRE units. An IRE unit (named for The drawing in Figure 2-6 shows the Institute of Radio Engineers) one complete line of a SMPTE is a relative unit of measure color bars waveform. This wave- equaling 1/140th of the peak-to- form consists primarily of a brightness signal (called lumi- Figure 2-7. A raster pattern is followed in forming a 525-line television peak (p-p) video amplitude. Since picture. The picture lines are scanned from left to right in a sequence from a video signal should be 1 V p-p, nance) and a high-frequency top to bottom. an IRE unit is about 0.00714 V, color signal (called chrom i n a n c e ) . or 7.14 mV, in this case. The luminance and chrominance are added together to form the Notice also that there is a hori- overall waveform. zontal scale on the waveform monitor graticule at 0 IRE. The scale is divided by tick marks

2-2 The luminance signal is a series All of the color bars test signals of voltages or “levels” that deter- illustrated so far were fed direc t l y mine brightness variations fr om the test signal generator into across the picture. Each of the an accurately calibrated wave- colors in the color bars signal form monitor. This means they has a different luminance level, are undistorted, their ampl i t u d e s and the bars are arranged by and timing are correct, and they level from highest to lowest represent the ideal condition. (white, yellow, cyan, green, But suppose the signal had been magenta, red, blue, and black). passed through other equipment The chrominance signal is a sine first, an amplifier or recorder for wave. Because of this signal’s example. Since you know what high frequency, the sine wave the test signal is supposed to cycles appear to run together in look like, you’ll be able to most displays. Individual cycles quickly tell if the signal has can be seen, however, when the been distorted or changed by the Figure 2-8. This 75% color bars signal includes a 100% white reference display is expanded horizontally equipment — and by how much. level. An eighth black bar is also included. as in Figure 2-9. That’s why it’s so important to Notice in Figure 2-6 that the use test signals with known peak-to-peak amplitude of the characteristics when evaluating chrominance signal varies from video systems. one color bar to the next. The Understanding the 1710J first bar, being white, has no Controls chrominance. All of the other bars have the correct amount of Before attempting to make any chrominance amplitude to measurements, you need to produce a full intensity (100% know how to use the 1710J saturated) color. The last bar Waveform Monitor controls. It’s (Black) also has no chrominance. also important to realize that when you adjust these controls, Color bar test signals fall into two you’re only changing the way general categories: 100% bars the waveform looks on the moni- (full amplitude) and 75% bars tor’s display. The waveform (reduced amplitude). You should monitor controls do not in any always use 75% bars for basic way affect the video signal Figure 2-9. When a chrominance signal is expanded horizontally, individual testing. This is because 100% cycles of the sine wave can be seen. itself. The same is true of picture bars contain signal levels that monitors and vectorscopes. If may be too high to pass through testing reveals something wrong a system without distortion. with the video signal, you must At some point, however, you will use the controls on the equipment need a 100% white reference under test to correct the prob l e m . level for checking overall signal Referring back to Figure 2-2, amplitude. Many 75% color bar notice that the 1710J front-panel test signals do provide a 100% controls are grouped into func- white level for this purpose tional blocks. The following (Figure 2-8). discussion describes those The SMPTE color bars signal co n t r ols block by block. Although shown in Figures 2-5 and 2-6 are this discussion is specific to the 75% bars with a 75% white 1710J, much of it also applies in level. This signal also incorpo- general to all waveform monitors. rates a 100% white level on some lines. This 100% white level is easiest to see on a wave- Figure 2-10. The 1710J dual-filter display is convenient for making luminance and chrominance measurements from the same display. The form monitor used in either the luminance display (L-PASS) is shown on the left, and luminance plus low-pass or dual-filter mode chrominance (FLAT) is on the right. (Figure 2-10).

2-3 INPUT block. The INPUT con- You need to select A or B, Making Measurements trols select the input signal to be depending on which set of Now that you understand the displayed, which parts of it are connectors is being used for basic concepts of testing, test displayed, and how it’s synchro- connection to the equipment signals, and the instruments nized. These controls include under test (Figure 2-4). used for measurements, you are the FILTER, REFerence, and VE R TICAL block. The VERTI C A L ready to make measurements. INPUT buttons. controls adjust the size and First, feed the color bars test FILTER Button — position of the displayed wave- signal into the system you want FL A T displays the entire signal, form. (Remember, changing the to test. This signal can come such as shown in Figure 2-5. display doesn’t change the video straight from the test signal signal being looped through generator or it can be a color L PASS removes the high- the monitor.) frequency chrominance signal bars signal previously recorded from the display, leaving GAIN Button — on video tape. just the luminance signal GA I N magnifies the vertical size Second, connect the system for observation. of the waveform display x5. output to the waveform monitor CH R M displays only the POSITION Knob — CH A input. Remember to prop- ch r ominance informa t i o n . erly terminate loop-throughs. POSITION moves the dis- Then, set the waveform monitor Because you need luminance played waveform up and controls as follows: plus chrominance for some down on the screen. POWER ON amplitude measurements, but HORIZONTAL block. The only luminance for others, both HORIZONTAL controls adjust INPUT the FLAT and L PASS displays the waveform’s horizontal size FILTER FLAT are useful. Another option is to and position on the display. REF INT use the 1710J dual-filter display. INPUT A SWEEP Button — This shows both waveforms at VERTICAL once with L PASS on one side and SWEEP selects whether the GAIN x1 FLAT on the other (Figure 2-10). monitor displays two lines POSITION Blanking level To obtain a dual-filter display, (2H) or two fields (2FLD) of aligned with the simply press and hold the video. Press and hold for a horizontal axis 1710J’s FILTER button. This one line display. as shown in display allows you to check MAG Button — Figure 2-5 luminance and chrominance MAG expands the display HORIZONTAL levels without switching back and horizontally. When used with SWEEP 2H forth between FLAT and L PASS. a 2H sweep, MAG provides a MAG 1µs REF Button — calibrated sweep speed of one POSITION Sync pulse INT synchronizes the wave- mi c r osecond per major division approximately form monitor to the displayed on the horizontal graticule centered as waveform. For the tests scale. This setting is used for shown in described in this application most timing measurements. Figure 2-5 note, always use INT (in t e rn a l ). POSITION Knob — DISPLAY Adjust all con- trols for comfort- EX T sy n c h r onizes the waveform PO S I T I O N moves the displayed able viewing monitor to an external source. waveform left and right on CAL displays a built-in cali- the screen. With all controls set as described above, you should see the same bration signal for adjusting the DISPLAY block. The DISPLAY display as shown in Figure 2-5 waveform monitor’s gain. This controls adjust the display for or Figure 2-8. will be discussed later in comfortable viewing. more detail. FOCUS changes the focus of INPUT Button — the trace. A displays any signal connected SCALE varies the graticule to the CH A input BNC illumination. connectors on the back panel. INTENS controls waveform B displays any signal connected trace brightness (intensity). to the CH B input BNC connectors on the back panel. BO T H is a press and held button for a display of both A and B inputs simultaneously.

2-4 Checking luminance levels Next, check the sync pulse level. After verifying that the waveform The sync pulse is the narrow monitor is calibrated, you can pulse in mid-screen in Figure 2-8 begin making video signal level extending down from the blanking measurements with confidence. (0 IRE) level. The flat bottom of the sync pulse should be at - First, check the video system’s 40 IRE. If it’s more than a few overall gain. This is done by IRE off, see if the equipment checking the amplitude of the under test has a sync amplitude color bars signal coming from adjustment. If it does, adjust the the system. If the amplitude is sync amplitude to -40 IRE. The too high (Figure 2-11), the luminance level and sync level picture is said to be “hot.” It’s must both be correct, i.e., the like an overexposed photograph. 0 IRE reference level luminance On the other hand, an amplitude must extend to 100 IRE at peak that is too low is like an under- white and sync must extend to exposed photo and results in a -40 IRE. If the signal cannot be Figure 2-11. An example of a “hot” signal — notice how parts of the signal dark picture. exceed 100 IRE. adjusted to match the graticule All amplitude measurements are at all three points (white at Verifying monitor calibration made relative to the horizontal 100 IRE, blanking at 0 IRE, Be f o r e making any measurem e n t s , axis at 0 IRE. To do this, make and sync at -40 IRE) the equip- it is wise to make sure the wave- sure the blanking level (labeled ment requires servicing by a form monitor is properly cali- in Figure 2-6) is aligned with the qualified technician. brated. This is an easy step to 0 IRE graticule line. Then make A word of caution take with the 1710J’s built-in sure that the 100% white refer- calibration signal. ence level coincides with the If there is more than one piece of 100 IRE graticule line. Also, equipment with gain controls in To view the calibration signal, make sure the black level the path you’re testing, you must press and hold the REF button (sometimes called setup) is at make sure that each unit is located in the INPUT block. 7.5 IRE. Notice there is a special adjusted for standard input and When the calibration signal is dashed graticule line at 7.5 IRE output levels. This is done by selected, the front panel CAL for verifying black level. (Be sure moving the test equipment con- LED will be illuminated. The you do not confuse the black nection to the output of the first color bars signal display will level with the blanking level.) unit, then to the second, etc., disappear and the calibration and adjusting each unit in turn. If the video signal’s 100% white signal — a 1 V p-p square wave Failure to do this may allow level does not correspond to — will appear on the screen. a system to have higher- or 100 IRE, use the gain control on To check calibration, position lower-than-standard levels at the system under test to adjust the calibration square wave some points in the path, which the top of the signal to 100 IRE. vertically so that its flat bottoms will almost always introduce This should also bring the black align with the -40 IRE graticule increased distortions. To put this level to 7.5 IRE. However, there line. The flat tops of the signal another way, the effects of non- may be a separate black level should then line up with the standard levels at some point in control on the equipment under 100 IRE graticule line. If this the system cannot be fully test that can be used for final is not the case, the waveform corrected “downstream.” adjustment of black level. If monitor is out of calibration there is a separate black level and should be checked by a co n t r ol, adjust it first, then adjust qualified technician. the gain control for 100 IRE. Making adjustments in this order should minimize interaction between them.

2-5 Checking chrominance gain Next, adjust the vertical position After verifying correct luminance control to position the pulse’s gain, you need to check chromi- top (blanking level) at +20 IRE nance gain. If the chrominance as shown in Figure 2-12. The amplitudes are wrong with res p e c t bottom of the pulse should now to luminance, the picture’s be at -20 IRE. Now, the 50% color intensity (saturation) level of the pulse corresponds is affected. to 0 IRE. First, make sure the color burst With the pulse displayed in this ranges from -20 to +20 IRE. Then ma n n e r , you can read pulse width check the maximum chrom i n a n c e di r ectly from the horizontal 0 IRE levels of the first two color bars axis. In 2H MAG, each major (yellow and cyan). Both should division on the axis equals 1 µs. be at exactly 100 IRE. If they are Using this scaling to measure not, adjust the chrominance gain pulse width at the 50% level, on the equipment under test to the sync pulse width should be correct the chrominance levels. 4.7 µs. If the width isn’t close to Fi g u r e 2-12. Sync pulse display horizontally magnified (MAG) and positioned 4.7 µs and there’s no sync width vertically so the pulse’s 50% levels are on the 0 IRE horizontal axis. Checking sync width adjustment on the equipment The next step Sync width should be checked under test, you should have the after verifying correct luminance equipment serviced. The instruments and techniques and chrominance gain. Sync presented in this section can Checking sync pulse width width is not as critical as signal solve or avoid many picture completes the series of basic amplitude, but it still should problems often encountered in measurements for objectively be checked to make sure the video facilities. Being able to verifying video signal quality. equipment is operating within apply these skills enhances your Again, the verification relies on defined limits. confidence and professional value. applying a known ideal test To accurately measure sync pulse signal to the input of the equip- You should, however, be aware width, you need to magnify the ment under test. The signal that there are many other, more pulse on the waveform monitor should pass through the equip- specialized, tests and measure- screen. To do this, press the ment without any changes in ments that can be made on video MAG button. If the sync pulse is luminance and chrominance systems. The various test signal at center screen when MAG is amplitudes or sync pulse width. selection buttons on the TSG pressed, the magnified sync If there are changes in any of the 100 (Figure 2-1), other than pulse should still appear near parameters, the equipment either color bars, offer only a hint of center screen (Figure 2-12). If it needs adjustment or servicing. the range of other tests and doesn’t, adjust the waveform measurements possible. monitor’s horizontal position control to bring the pulse to center screen.

2-6 Basic Video Testing — Vectorscope Techniques

In the section on waveform mon- Understanding the Display itor techniques, it was explained There are two parts to a vec- that a waveform monitor displays torscope display — the graticule a graph of the video signal — and the trace. The graticule is a amplitude (voltage) on the verti c a l scale that is used to quantify the axis and time on the horizontal parameters of the signal under axis. A vectorscope also graphs examination. Graticules may be portions of the video signal for screened either onto the face- test and measurement purposes, plate of the CRT itself (internal but the vectorscope display graticule) or onto a piece of glass differs from a waveform display. or plastic that fits in front of the While a waveform monitor CRT (external graticule). They displays amplitude information can also be electronically gener- about all parts of the signal, a ated. The trace represents the vectorscope displays informa t i o n video signal itself and is elec- about only the chrominance tronically generated by the Figure 3-1a. The vectorscope screen shows a display of a SMPTE color bars (coloring) portion of the video demodulated chrominance test signal with all of the dots in their boxes and the color burst correctly signal — it does not respond to signals (Figure 3-1). placed on the horizontal axis. other parts of the video signal. All vectorscope graticules are There are two important parame- designed to work with a color ters of the chrominance signal bars signal. Remember, the color that may suffer distortions leading bars signal consists of brightness to noticeable picture problems. information (luminance) and These are amplitude (gain), and high-frequency color information phase (timing). Amplitude is an (chrominance or chroma). independent measurement and Each bar of the color bars signal can actually be made with a cr eates a dot on the vectorscope’s waveform monitor. Phase is the display. The position of these relationship between two signals dots relative to the boxes, or — in this case, the relationship targets, on the graticule and the between the chrominance signal phase of the burst vector are the and the reference burst on the major indicators of the chromi- video. The processing within a nance (color) signal’s health. vectorscope and the display of (Burst is a reference packet of the processed signals is designed subcarrier sine waves that is sent Figure 3-1b. The vector dots are rotated with respect to their boxes, to readily detect and evaluate on every line of video.) indicating a chroma phase error. both phase and gain distortions of the chrominance.

3-1 The graticule and upper-right corners of The graticule is usually a full the screen. These boxes, along circle, with markings in 2- and with the vertical line that bisects 10-degree increments. The cross the circle, are used for making point in the center is the ref e re n c e stereo audio gain and phase mark for centering the trace. measurements with a Lissajous Within the circle you also see display. While this capability is six target shapes each containing useful in certain applications, smaller, sectioned shapes. The we will not discuss it in this smaller shapes are where each booklet, but rather concentrate dot of the color bars signal on video measurements. should fall if the chroma gain The trace and phase relationships are The vectorscope’s display is correct. (In practice a dot that created by decoding the chromi- will fall entirely within the nance portion of the video signal smaller target is only created by into its two components, B-Y a very low noise signal, such as Figure 3-2. Proper white balance of a video camera is indicated by a fuzzy (blue minus luminance) and R-Y directly from a color bars genera- spot centered on the vectorscope display when the camera is viewing a (red minus luminance). These tor. Expect the dots to be much white object. two signals are then plotted larger and “fuzzier” on signals against each other in an X-Y from tape players or off-air Using the Vectorscope fashion, with B-Y on the hori- receivers. Note also the camera Controls zontal axis and R-Y on the signal in Figure 3-2.) The first rule-of-thumb to vertical axis. While waveform remember when you begin to The horizontal line bisecting the monitors lock to sync pulses in operate the vectorscope is that circle at zero and 180 degrees the video signal, vectorscopes the controls do not in any way (9 o’clock to 3 o’clock positions) lock to the 3.58 MHz color burst affect the video signal itself. The is used as a ref e r ence for correc t l y and use it as the phase reference only way to adjust the signal, positioning the burst display. for the display. rather than just its vectorscope (Burst is indicated by the porti o n Three color bars signals are com- display, is with the controls of of the trace that lies from the monly used with vectorscopes the equipment providing or center part way toward the zero — 75% and 100% full field transferring the video signal. degree position.) The markings color bars and the SMPTE color toward the left of the burst All the basic controls for powering bars. The 100% and 75% labels display indicate correct burst up the instrument and adjusting refer to the amplitude of the amplitude for 75% or 100% the display for comfortable signal, not the saturation of the color bars. Note that burst does viewing (power and display: colors; both are 100% saturated. not change electrical amplitude focus, scale illumination, and In this application note, we will — the gain in the vectorscope’s intensity) are self-explanatory. use the SMPTE color bars signal processing is increased when it The gain controls provide a way exclusively, because it is an is set to display 75% amplitude to calibrate the display’s gain for industry standard. SMPTE bars bars in the targets — and that either 75% or 100% color bars. are 75% amplitude color bars. in c r eased gain causes the display A variable control expands the of the fixed amplitude burst You should always use 75% bars trace to compensate for low signal to be longer. for basic testing. This is because signal levels, or so you can 100% bars contain signal levels analyze the signal in greater At the left edge and near the too high to pass through some detail. For white balancing outer circumference of the types of equipment undistorted, cameras, use of this control also graticule is a grid used to even when the equipment is allows more visibility of the measure differential gain and operating properly. For more detail in the white and black phase. These measurements are information on the various color information near the center of discussed in the later section on bars signals, refer to pages 2-2 the vectorscope display. Intermediate Video Testing. and 2-3 in the Basic Video Some vectorscopes also have Testing — Waveform Monitor square boxes outside the gratic- Techniques section. ule’s circle in the lower-left

3-2 The input controls typically There are also a few things you This rotation of the dot pattern consist of the input channel, should examine on the waveform means that the chrominance reference, and mode selections. monitor. The waveform monitor phase is incorrect relative to the The input for channel A, for should indicate the following: color burst. This phasing error example, is likely to be video 1. Waveform black level at causes hues to be wrong — from a switcher’s output (with a 7.5 IRE. (Black level adjust- people’s faces appear green or VTR, camera, signal generator or ments are made with the setup purple, for example. other equipment providing the control on the equipment Correct chrominance phase is input to the switcher). Reference under test while viewing the critical, even if hue errors aren’t may be internal, where the vec- waveform monitor.) obvious on a picture monitor. To torscope locks to the color burst 2. Waveform luminance (white correct a chrominance phase of the selected input channel, or er ror , perfo r m the following steps: ex t e r nal, where lock is to a second bar) at 100 IRE. (Amplitude signal. In this section we are adjustments are made with the 1.Ensure the vectorscope’s burst only using internal reference. video level, or video gain display is aligned with the Externally referencing a vec- control on the equipment horizontal axis (phase control torscope is imperative for the under test while viewing the on the vectorscope). purpose of matching the chroma waveform monitor.) Refer to 2.Adjust the hue control or phase of various sources to the Waveform Monitor chroma phase control on the eliminate color shifts at edit Techniques section, “A Word equipment in the path to get points. This application is of Caution” (page 2-5) if there the dots as close to their discussed in the Setting Up a is more than one video gain or respective graticule boxes as Genlocked Studio section. level control in the path possible. They may be too you’re testing. Internally referenced displays near the center of the display only show the phase rel a t i o n s h i p Using the test signal generator’s or too far away, but they will between the burst and color bars color bars as a reference signal, lie in the proper direction signal of the selected input signal you should check to make from the center when phase itself — not phase relationships certain the signals appear on is correct. between different signals. the waveform monitor and vec- If you are working with a camera torscope as mentioned above. If the vectorscope has an externa l in your system, the adjustments Again, use the phase control to are no different. You just have to X,Y input for displaying audio, ensure the color burst vector is a mode control allows you to look at the cameras output, or at properly positioned and the the output of the camera control enable this input for display vector color dots are in their with or without the regular unit (CCU) if you are using one, graticule boxes. With this refer- when you make the adjustments. vector display. ence position established, any The phase control is one you differences in other signals you It is often best to go back and will use often. In the internally select will be obvious. forth between the chrominance referenced mode it must be used gain (discussed next) and phase to align the burst vector on the Checking Chrominance Phase adjustments to get the dots in their graticule boxes. The dots horizontal axis (pointing toward Now that you’ve established a will rarely be exactly centered zero degrees, or 9 o’clock). If the reference position on the vec- on the cross points in the smaller color burst is not in this correct torscope, the next step for boxes. It is, however, acceptable position, the signal is not prop- checking the signal from the if they fall somewhere within erly aligned with the graticule studio VTR equipment against the small boxes rather than the and provides no really useful the reference signal is to play larger boxes. information. As you rotate the back a videotape with the 75% phase control, the whole trace color bars recorded. Select the Remember, these tests are only rotates about the center point. equipment under test on the checking the bar signal relative video switcher (VTR, TBC or to the color burst. Not all equip- Checking the Display proc amp), or by appropriately ment requires or makes available When you first check the display connecting cables, and look at this adjustment. Phasing (or of the color bars signal on a the vectorscope display. timing) several sources or paths so their signals may be switched vectorscope, you should see: If the burst phase vector lines or mixed requires external refer- 1.A bright dot in the center of up on the horizontal axis, but ence for the vectorscope and the display. the dot pattern is rotated with somewhat different techniques. respect to the boxes, there is 2.Color burst straight out on the These are discussed in the a chroma phase problem with horizontal axis extending to Setting Up a Genlocked Studio the equipment under test the 75% mark. section beginning on page 4-1. (Figure 3-1b).

3-3 Checking Chrominance Gain vectorscope in the internal refer- (Amplitude) ence mode — checking the Adjusting chroma phase alone white balance of video cameras. may not put the dots in their White balance is the process of boxes because they may fall balancing the camera’s red, gree n short of, or extend too far out- and blue channels. When these side the boxes (Figure 3-3). This channels are properly balanced, indicates a chroma gain error. the camera’s output signals will For checking chroma gain on a reproduce whites in a scene VTR, you will again use the without adding color. In ord e r color bars signal recorded at the for the camera to rep ro d u c e head of the tape — other equip- colors correctly, it must be able ment may require a bars signal to reproduce white accurately. as its input. Again, you should The signals from the green, check luminance levels on the blue, and red sensing elements waveform monitor, and correct in the camera must be properly balanced to ensure there is no Figure 3-3. This display shows the vector dots extending beyond their any errors, before you make any boxes, indicating a chroma gain error. measurements with the vec- ch r ominance signal at the camera torscope. (Remember the vec- output when it is viewing white. blue gain). It may help to remem- torscope does not display any Be f o r e you can set white balance, ber the red balance control will information about luminance.) you must first check the camera move the spot vertically and for proper black balance: the blue control will move it 1.Use the setup control (if pro- horizontally. To manually set vided) to position the black 1.Connect the video camera white balance, the following level at 7.5 IRE. signal to the vectorscope steps are recommended: 2.Use the video level controls input, or switch to the camera on the video switcher. 1.Aim the camera at a white on the equipment under test to reference. (Before attempting adjust the white level on the 2.Cap the cameras lens. to correct a white balance waveform monitor to 100 IRE. If the black balance is correct, problem, make sure you have While the tape is playing the the camera’s signal will produce the correct filter selected on recorded bars, dots will appear a fuzzy spot in the center of the the camera for the type of on the vectorscope. If the dots vectorscope display, the same lighting in use. The wrong are beyond the boxes, the as for proper white balance filter selection will cause chroma amplitude is too high. (Figure 3-2). If the spot is dis- an apparent white balance If the dots fall short of the tended away from the center in problem on the vectorscope.) boxes, chrominance is too low. any direction, proceed as follows: 2.Adjust the red and blue gain 3. Adjust the chroma gain control 3.Adjust the black balance controls on the camera to get of the equipment under test control on the camera until the fuzzy spot in the center of until the dots fall into the spot returns to the center the graticule on the vectorscope. their boxes. of the display. Some cameras with an automatic If adjusting the chroma gain To check white balance, uncap white balance feature may not control doesn’t get the dots into the camera and point it at a pure offer manual adjustment and their boxes, you may need to white target. If white balance is will have to be referred to a re-adjust hue or have the correct, the resulting signal from service technician if the auto- equipment serviced. the camera should again prod u c e matic balancing feature does not achieve the desired results. If you happen to run the tape a fuzzy spot in the center of the past the color bars signal and vectorscope display (Figure 3-2). Conclusion If any color channel in the camera into active video, you’ll notice This section has dealt with three is out of balance, the fuzzy that the vector display looks basic uses of a vectorscope — spot on the vectorscope will be very different. This is because evaluation of chrominance to distended or moved toward the there are typically no large areas burst phase, chrominance gain, corresponding color’s graticule of primary or secondary colors and color balancing of a camera. box. For example, too much red in the picture to create the bar There are other, more extensive, signal moves the spot toward dots. The display looks fuzzy or me a s u r ements that you can make the red box. blurred because the vector with a simple vectorscope, and shows the blend of hues within If the vectorscope display indi- others that require instruments the picture. cates incorrect color balance, it with enhanced capabilities. can be corrected by using the Several of these more advanced Checking White Balance camera’s white balance controls. techniques are addressed in the Along with checking for chroma Cameras that allow manual white following sections: Setting Up phase and gain, there is another balance adjustment usually a Genlocked Studio and studio application that uses a have two controls (red gain and Intermediate Video Testing.

3-4 Setting Up A Genlocked Studio

Color Burst generator. Equipment that is Genlock Basics synchronized by a master gener- 7.5 IRE Black Level (Setup) There are three basic require- 0 IRE Blanking Level ator is often referred to as being ments for setting up a genlocked generator locked, or “genlocked” studio. You need: for short. Horizontal • Video equipment that can be Sync Pulse Genlocked equipment works genlocked together in synchronization with Figure 4-1. The black burst signal used in genlocking is composed of the • A genlock signal source horizontal sync pulse, subcarrier color burst, and a 7.5 IRE black signal. a master sync signal. It’s much like the members of a marching • Adjustment for synchronizing band “locking” their steps to the equipment with the genlock beat of the bass drum. Without source. the steady beat of the drum — The first of these requirements the synchronization pulse — the — video equipment that can be band members wouldn’t have a Figure 4-2. The Tektronix TSG 200 provide multiple black burst signal out- genlocked — is relatively easy to puts for economically genlocking small studios. Additional features on the common reference for staying in meet. Most professional video TSG 200 can be used for basic video testing and other odd jobs around step with each other. the studio, including ID generation, setting up picture monitors and equipment is genlockable. To see blacking tapes. Similarly, video equipment that if your cameras, VCRs or VTRs, isn’t genlocked has difficulty and other equipment meet this working together. Trying to requirement, check the equip- dissolve from the output of one ment for a genlock loop-through VTR to another results in picture input or an external reference tearing or roll until synchroniza- input (Ext. Ref.). Also, the equip- tion is reestablished. With gen- ment must have some provision locked equipment, the dissolve for adjustment of timing, whether is smooth and “in step” with the it is a single control for adjusting master sync. both H-timing and SC-phase, The black burst signal (Figure 4-1) or separate controls for is often used for genlocking adjusting each. equipment. It is a composite The genlock or external ref e re n c e signal with a horizontal sync input connector is necessary for Figure 4-3. A black burst signal is distributed from the test signal generator pulse and a small packet of applying an external synchro- for genlocking studio equipment. If the signal generator has multiple black 3.58 MHz color subcarrier nization signal — typically black burst outputs, as is the case with the TSG 200, a distribution amplifier may (color burst). The term black burst — to the equipment. The not be necessary since they have five black burst outputs. burst arises from the fact that timing controls are necessary for the active picture portion of the adjusting the equipment for signal is at 7.5 IRE (black) and it synchronization with the refer- Fades and dissolves are a basic contains color burst. ence’s horizontal (H) sync pulse part of most professional video Establishing a genlocked studio and subcarrier (SC) color productions. Even the smallest is simple and need not require burst phase. studios use them. Before dissolves expensive equipment. This With a genlock signal source, or other editing effects can be section will show you how to such as a Tektronix TSG 200, a used, though, the studio equip- genlock your equipment with black burst signal can be distrib- ment must be synchronized. the Tektronix TSG 200 NT S C uted to the studio equipment as Synchronization is done by Generator (Figure 4-2). This gen- shown in Figure 4-3. locking cameras, VTRs, editors, erator is an economical source switchers, and other video of multiple black burst signal equipment to a master sync outputs for genlocking.

4-1 There are several important Adjusting TBCs Adjusting for Synchronization things to note about this distrib- TBCs have several controls, in As mentioned before, the hori- ution diagram. First, the signal addition to the timing controls zontal sync pulse and subcarrier generator used for genlocking just mentioned, that are used to burst of the black burst signal should have multiple black burst compensate for gain deficiencies are like the drum beat to which outputs. A unique black burst and color errors on tapes. Setup members of a marching band signal is not needed for each (or black level), video gain, synchronize their steps. piece of equipment to be gen- chrominance gain, and hue are Everyone’s footsteps should locked. Looping-through equip- commonly used adjustments on strike the ground in time with ment in different locations TBCs. Each of these adjustments the drum. Additionally, for the within a facility is possible, but has already been covered in this whole band to be in step, every- presents one major problem — application note and will not be one’s left foot should be striking too long a cable run will prod u c e repeated here. The purpose of the ground at the same time. delays beyond the adjustment this section is to link the nearly Occasionally, however, a band range of the equipment universal inclusion of color bars member will be out of step, being locked. on tape leaders to TBC adjust- and you’ll see them do a little Generally speaking, a single black ments and discuss a few guide- “skip-step” as a timing adjustment burst feed for each cluster of lines to follow while making to get in step with the rest of equipment is sufficient, with that these adjustments. the band. single feed being looped through Video levels and hues recorded Similarly, each piece of video just one group of equipment. on tape sometimes differ from equipment must have some If multiple black burst outputs facility to facility, and VTR to timing adjustments made in are not available, a multiple out- VTR. This is a fact of life. When order for it to be “in step” with put signal distribution amplifier differences exist they must be the rest of the video system. should be used with one output co r rected. By rec o r ding color bars Adjusting system timing is one for each cluster of equipment to on the beginning of every tape, of the most fundamental — be genlocked. operators can make the necessary and critical — procedures in Also notice in Figure 4-3 that the TBC adjustments quickly and the studio. black burst signal is distributed simply with just a waveform System timing adjustments to the Time Base Corrector (TBC) monitor and vectorscope. involve setting the H-timing and for each VTR, rather than the The TBC’s horizontal timing and SC-phase controls on each piece VTR itself. The reason for this is subcarrier phase controls of equipment. The adjustments that the TBC is actually the final shouldn’t require checking or are made so that each piece of program output of the VTR. The adjustment each time you play equipment’s horizontal sync TBC is where genlocking and back a tape from a different pulse and color burst phase line timing adjustments need to occur. source. Since the TBC always up with the sync pulse and burst Similarly, if you use camera replaces the sync and burst of phase of the reference genlock control units (CCUs), connect the recorded signal, playing back signal (black burst). the black burst output to the different tapes should not affect Making sure that each piece of CCUs, not to the cameras. the TBC’s system timing. Many equipment is “in step” with the Camera timing adjustments, experienced operators make TBC genlock signal prevents horizontal unlike TBC/VTR adjustments, adjustments in the following jumps and color shifts of the are made at the camera, not at order to minimize interaction picture when switching between the CCU. between the adjustments: 1) setup, video sources. Matching the 2) video gain, 3) hue, and timing of the sync pulses makes Finally, note that one of the 4) chrominance gain. black burst signal outputs from sure the scanning of each picture the test signal generator is loop- Remember that TBC adjustments source is in step (e.g., the images th r ough connected to the externa l must be made when the VTR will stay in the same position, reference inputs of a waveform connected to it is playing back a without roll, tearing or “jumps” monitor and vectorscope. (Don’t recorded color bars signal, not during a cut or fade). Matching forget the 75 ohm terminator at when the VTR is in the electrical- the burst phase of the various the end of each chain of genlock to-electrical (E-to-E) mode. sources maintains the correct or external reference loop- color of the images during th r oughs.) The waveform monitor editing transitions. and vectorscope are also loop- th r ough connected to the Prog r a m Output of the studio switcher. These connections to a wave- form monitor and vectorscope are necessary for the final requirement — adjusting the equipment for synchronization with the genlock source.

4-2 the switcher’s active input. This burst vector aligned with the applies the generator’s test 9 o’clock position on the vec- signal to the Channel A inputs torscope display? If it isn’t, use of both the waveform monitor the SC-phase control on the and vectorscope. selected piece of equipment to On the waveform monitor, make set color burst to the 9 o’clock the necessary adjustments to zero reference. expand the display on the hori- In the case of a single timing zontal sync pulse. Be sure the adjustment (where SC-phase waveform monitor is in the and H-timing have a fixed rela- external reference mode, i.e., tionship), simply observe the displaying the program output of vectorscope and adjust the the switcher, but referenced to equipment such that the color the black burst. Use the waveform burst vector lines up with the monitor’s horizontal positioning 9 o’clock reference line on the control to place the leading edge vectorscope’s graticule. Some Figure 4-4. Waveform monitor display of the reference sync pulse edge of sync on one of the display’s equipment has a wide range adjusted to correspond to a major timing mark. This timing mark can now major timing marks. This timing adjustment which will continue serve as the zero reference for individually adjusting the H-phase of each piece of video equipment in the system. mark is now the zero-time refer- to move the sync timing beyond ence for inputs from all other the first place where the burst video equipment, and the hori- phase is correct — if so, continue zontal position control should to adjust the control in the not be moved during the rest of ap p r opriate direction. This should the system timing adjustments. move the H sync pulse to the zero- Figure 4-4 shows an example of reference marker as well. If it the reference sync edge display does not, there is either an interna l with the sync edge positioned H-timing adjustment that is set on a major timing mark. incorrectly or the equipment The next step is to zero ref e re n c e needs servi c i n g . In either case, the vectorscope to the reference an experienced technician signal’s color burst subcarrier should attend to the problem. phase. This is done by using the Setting sync and color burst to vectorscope’s phase control to the established zero references position the color burst vector to completes timing alignment for the 9 o’clock position. Here the selected piece of equipment. again, be sure the vectorscope is That piece of equipment is now Figure 4-5. Vectorscope display of the reference color burst vector adjusted in the external ref e r ence mode — in step with the black burst to the 9 o’clock position. All other pieces of equipment should be SC-phase adjusted to correspond to the 9 o’clock reference. locking to the external black burst. reference signal. This is shown in Figure 4-5. Now, using the switcher, select Be f o r e making timing adjustments, Once the reference signal’s color another piece of equipment and you should check the video gain burst is positioned to 9 o’clock, adjust its H-timing and SC-phase from each piece of equipment. do not touch the vectorscope’s controls to zero reference its The SMPTE color bars signal phase control again during the sync and color burst output. from the TSG 200 can be used rest of the timing adjustments. Continue this equipment for this. If necessary, correct any With sync and color burst zero selection and zero-referencing video gains before going on to referencing completed, use the process for all other pieces of system timing adjustments. switcher to select the signal video equipment in the system. System timing adjustments are output from the first piece of Remember that for VTRs, the made with a waveform monitor video equipment to be adjusted. timing adjustments are done at and vectorscope connected to The waveform monitor will the TBC. the switcher output as shown in display the horizontal sync When these adjustments have Figure 4-3. Make sure that both pulse from the selected piece of been completed for all pieces of the waveform monitor and vec- equipment. If the sync edge does equipment in the system, each torscope are set to trigger on the not line up on the previously piece of equipment will be in external reference signal. established zero-reference mark, synchronization and properly The next step is to set up zero adjust the H-timing control on timed with all other pieces of timing references on both the the selected piece of equipment equipment. You now will be waveform monitor and vec- to place the sync edge on the able to switch smoothly between torscope. The first step in this zero-reference marker. video sources and make clean “zeroing” process is to select the Look at the color burst display edits without picture roll or test signal generator’s output as on the vectorscope. Is the color horizontal jumps.

4-3 Intermediate NTSC Video Testing

1720 Vectorscope. 1730 Waveform Monitor.

TSG 170A NTSC Television Generator.

Figure 5-1. Video system testing requires a signal generator, a waveform monitor, and a vectorscope.

Previous sections of this booklet Test equipment first As for test equipment capabilities, have discussed making basic To ensure valid test results, you all the tests described in this measurements and adjustments must use good test equipment. application note can easily be in video systems. This section Es s e n t i a l l y , there are two primary performed with the Tektronix goes on to discuss intermediate requirements for this. First, the in s t r uments shown in Figure 5-1: level tests using three pieces of test equipment’s specified • 1720 Vectorscope, used to te s t - a n d - m e a s u r ement equipment performance must exceed the evaluate the chrominance — a signal generator, waveform expected performance of the (color) information in a monitor, and vectorscope. This system being tested. And second, video signal section describes how to use the test equipment must be • 1730 Waveform Monitor, used that test equipment to make the performing to its specifications. following tests: for video signal level and The latter requirement is really a timing measurements • insertion gain two-stage requirement. The test • TSG 170A NTSC Television equipment must always be setup • frequency response Generator, used to generate the using standard operator calibra- • ch r ominance-to-luminance gain necessary standard test signals tion procedures. For highest • luminance nonlinearity confidence, the equipment This test equipment is excellent • differential gain should also be fully calibrated for all basic and intermediate periodically at a qualified testing requirements. Other simi- • differential phase service center. (Full and precise lar test equipment may be avail- The picture impairments caused calibration of test equipment able to you and can be used in by the distortions these tests are requires special skills and labo- essentially the same manner. In designed to detect are also ratory instruments not normally some instances, the waveform described where appropriate. found in a video facility.) monitor and vectorscope functions

5-1 complete in-service or accep- monitor. Study this display tance testing. (In-service testing carefully. It’s your reference of Generator also requires a VITS inserter, what the “perfect” signal should such as the Tektronix VITS200, look like. VITS100 or 1910, to generate Ideally, you should see exactly Signal and insert vertical interval the same “perfect” display after Path test signals.) the test signal is passed through Being Tested Setting Up the Test Equipment the video path being tested. However, if there are distortions Regardless of the specific test in the signal path, you’ll see equipment models used, the corresponding changes in the basic test concepts and setup displayed signal. Various types Waveform remains the same. The diagram of distortion are described furth e r Monitor in Figure 5-2 shows the setup for in the following discussion of testing any video system, no test procedures. matter how simple or complex. The block labeled Signal Path Insertion Gain Vectorscope Being Tested can be any part of Insertion gain reflects the video the video system. It can be a path’s ability to maintain correct single piece of equipment such signal amplitudes from input to Figure 5-2. The basic test setup for any video as a playback deck or a rec o rd i n g output. The general test proce- system involves applying a standard test signal at the path input and observing or measuring the deck. Or the path can include a dure is to apply a standard resulting signal at the path output. switcher along with other equip- NTSC 1 volt (140 IRE) signal to ment such as cameras, VTRs, the video path or equipment distribution amplifiers, character/ input. The test signal is then will be combined in a single title generators, special effects measured at various points along instrument. This is the case, generators, and so forth. In any the signal path to verify its for example, with the Tektronix case, all testing is done in essen- correct amplitude. 1740A and 1750A Series tially the same manner — inject It is important to verify insertion Waveform/Vector Monitors. With a signal at the start of the path gain before doing any other tests. these instruments, you select the (the input), and observe or mea- If there are insertion gain errors wa v e f o r m monitor or vectorscope sure any distortions to the signal and they are not corrected, sub- mode by pressing the Waveform at the end of the path (the output). sequent tests will be incorrect as or Vector button on the front panel. For additional details, see the well. This is because most tests earlier section on Connecting are based on the presumption Similarly, more advanced, multi- and Terminating Instruments function instruments, such as that insertion gain is correct. starting on page 1-3, and Also equipment that is proc e s s i n g the Tektronix 1780R or VM700T “A Wor d of Caution” on page 2- 5 . Video Measurement Sets, allow a nonstandard level signal will push-button selection of wave- Before actual testing begins, often introduce other distortions. form monitor or vectorscope however, you should check the Insertion gain errors are usually modes. Additionally, the 1780R test equipment against its own expressed as a percent variation and VM700T provide numerous internal calibration signal. This from the nominal value. There- other features that simplify the is typically referred to as a user fo r e, the goal is zero insertion gain. measurements described in this calibration procedure, and it will Positive insertion gain errors section as well as many other be outlined in the instrument indicate an increase (gain) in measurements. Some of these operator’s manual. signal amplitude from input to features include microprocessor- Also, as a matter of reference, output. This can lead to distor- assisted or fully automatic you should view an undistorted tions from signal overload. me a s u r ements, user-p ro g r a m m a b l e test signal on the test instrum e n t ’ s Negative insertion gain indicates measurement sequences, graphic display. To do this, connect a decrease (loss) in signal ampli- display of measurements, the test signal generator’s output tude. This causes dark pictures and brighter displays in line- directly to the vectorscope and also reduces the signal-to- select mode. or waveform monitor noise ratio (SNR). The more sophisticated capabili- input terminals. ties and enhanced precision of With the instruments properly these instruments makes them connected and terminated, you'll ideal for all measurements see the test signal displayed on including applications such as the vectorscope or waveform

5-2 In s e r tion gain errors affect overall When there is an insertion gain picture brightness and may also error, the equipment under test affect apparent color saturation. should be adjusted to remove The human eye is very sensitive the error. To do this, use the to even small brightness varia- appropriate control on the tions, particularly when they equipment under test to align occur rapidly. Because of this, the color bars reference bar with its especially important to make the waveform monitor’s 100 IRE sure that all video switcher graticule line. In doing this, be inputs are matched when sure to keep the blanking level different shots of the same scene on 0 IRE by using the waveform are being combined. It should monitor’s position control if also be kept in mind that small necessary. With the reference bar gain errors in several system on 100 IRE, the sync and color components can quickly cascade burst amplitudes should both into big errors. be 40 IRE peak-to-peak. You Insertion gain testing is done may need to make separate Figure 5-3. A waveform monitor, set for one-line sweep and flat response, by applying either a SMPTE adjustments to make sure these displays a properly adjusted SMPTE color bars signal. This test signal is used for detecting insertion gain and color burst amplitude errors. color bars signal or a full-field, levels are correct. 75% color bars test signal having In making gain adjustments, it’s a 100% white reference level. important to start first with A waveform monitor is then equipment at the beginning of used to observe the color bars the video signal path. Otherwise, signal and make insertion you may simply mask rather gain measurements. than correct errors occurring The TSG 170A supplies the earlier in the signal path. SMPTE color bars signal. Since After verifying that insertion the white reference for SMPTE gain is correct, you need to check color bars is at 75%, the 100 IRE chrominance amplitudes. This Y signal that overlays the first can be done using either a wave- two color bars, as shown in form monitor or a vectorscope. Figure 5-3, is more convenient On the waveform monitor, for insertion gain tests. Figure 5-4 chrominance amplitude is shows a full-field, 75% color checked by first measuring the bars signal with a 100% white color burst amplitude. It should Figure 5-4. Full-field, 75% color bars signal with 100% white reference. reference. (Either of these signals extend from -20 IRE to +20 IRE can also be used to check (40 IRE peak-to-peak). Then chrominance amplitude and check the maximum level of the chrominance-to-burst phase on first two color bars (yellow and a vectorscope.) cyan). Both should be at exactly The waveform monitor should 100 IRE. If they aren’t, adjust the be set for a one- or two-line chrominance gain on the equip- sweep with the filter selection in ment under test to correct the flat. (Other filter positions alter chrominance levels. the waveform monitor response Checking chrominance levels on and may mask a gain error.) the vectorscope is done with the With the sweep and filter selected, display shown in Figure 5-5. use the vertical position control The color burst vector should to align the trace’s blanking level emanate from center screen with 0 IRE on the graticule. As along the horizontal axis shown in Figure 5-3, the 100% (9 o'clock position). If it doesn't, white reference (Y signal) should adjust the vectorscope’s phase Figure 5-5. A vectorscope display of a SMPTE color bars test signal. All of now coincide with the 100 IRE control to put the color burst in the dots should be in their boxes when the color burst is aligned with the graticule line, and the sync tips the 9 o'clock position. Then horizontal axis. should be at 40 IRE. Insertion check to see if all of the other gain error, if any, is determined vector dots fall into their proper by noting the position of the boxes, as shown in Figure 5-5. white bar and subtracting 100%. For example, if the bar is at 93 IRE, insertion gain error is 93 - 100, or -7%.

5-3 If the color vector dots do not distortions. A line bar, such as fall within their boxes, you’ll the one found in the pulse and need to make chrominance gain bar signal (Figure 5-6) is used for or phase adjustments on the checking for line-time distorti o n s . equipment under test. When the Field-time distortion testing is dots are beyond the boxes, done with either a field square toward the display edges, wave or a windowed bar signal. chrominance amplitude is too The pulse and bar signal prov i d e d high. If the dots fall short, by the TSG 170A is a windowed toward the display center, signal, consisting of 130 lines of chrominance is too low. If the the pulse and bar signal in the dots are rotated out of their center of the field. On a picture boxes, chrominance phase is monitor, this creates the window incorrect relative to color burst effect shown in Figure 5-7. If the and needs to be adjusted on the video path can faithfully convey equipment under test. this signal, you can assume that Figure 5-6. The line bar in this pulse and bar signal is used to check for If all of the dots cannot be the system’s low-frequency low-frequency distortions at the line rate. adjusted to fall in their respec- response is satisfactory. tive boxes, other distortion To check field-time distortion, errors exist in the system. These the waveform monitor should be could include frequency res p o n s e set for a two-field sweep in the errors or other gain or linearity flat-response mode. Also, the dc errors. Finding and correcting restorer should be in either these errors requires further slow-restore mode or turned off. testing and adjustments by a qualified service technician. If the video path’s low-frequency response is correct, you should Frequency Response Testing see perfectly flat horizontal lines. Any tilt in these lines, For optimum performance, the such as shown in Figure 5-8, frequency response of the entire represents a field-rate impair- video system should be as flat as ment. Such an impairment possible. Frequency response would cause brightness varia- testing determines how flat the tions between the top and system actually is. If the system bottom of the picture. Provided is not flat, signal amplitudes will Figure 5-7. The windowed pulse and bar signal is used to check for there is no insertion gain error, low-frequency distortions at the field rate. become distorted as a function the field-time distortion can be of frequency. For example, the measured as the percent varia- system may attenuate higher tion from the normal flat level, frequency components more excluding the first and last 0.2 than lower frequency components milliseconds of the bar. or vice versa. Similar observations are made For test purposes, television with the line bar signal for signals are divided roughly at line-rate response problems. 1 MHz into low- and high- Any tilt in the line bar would frequency ranges. Even though produce brightness variations some of the most important between the left and right sides signal information falls below of the picture. With active video, 1 MHz — namely synchronizing line-time distortion produces pulses, general brightness level, horizontal “streaking” — usually and some active video — low- seen as light and/or dark streaks frequency testing is often extending to the right of horizontal neglected. This can be a trouble- transitions in the picture. In some and costly oversight. Figure 5-8. Waveform monitor display of a windowed pulse and bar signal quantifying this measurement, showing about 6% field-time distortion. You should routinely check exclude the first and last system response in the low- microsecond of the bar, since frequency range. Low-frequency distortions near the transition testing is further divided occur at frequencies above the into line time and field time line rate.

5-4 If your equipment exhibits line This can be significantly less than time or field time distortions 4.2 MHz, which means that you beyond the limits specified by should expect to see attenuation the manufacturer, it must be of the high-frequency packets. se r viced by a qualified technician. For multiburst measurements, Th e r e are no external adjustments the waveform monitor should be for either low frequency set to a one- or two-line sweep response or high frequency and flat response. For a perfect response errors. system response, the multiburst Response to higher frequencies, display would show all packets those above 1 MHz, is often as having the same peak-to-peak checked with a multiburst test amplitude. Any significant signal. Response problems above amplitude variations in packets 1 MHz can cause impairments indicate a frequency response in either or both the chrom i n a n c e variation — it is an error only if and monochrome detail it is outside the equipment’s of pictures. specifications. Some video Figure 5-9. To display this multiburst signal, the waveform monitor is set for equipment, such as distribution one-line sweep and flat response. This signal shows a flat response for high The multiburst signal tests frequencies (500 kHz to 4.2 MHz). response by applying packets of amplifiers or switchers, may discrete frequencies ranging pass the multiburst packets from about 500 kHz to 4.2 MHz. with very little frequency The Tektronix TSG 170A NTSC response distortion. Television Generator provides The response at a specific fre- the multiburst signal shown in quency can be expressed as a Figure 5-9. percent of nominal value or in The multiburst signal is com- decibels. Either method of posed of six frequency packets. expression is based on peak-to- The second packet from the right peak amplitude measurements. has a frequency of 3.58 MHz and Again, the absolute frequency is used to check color subcarrier response is often not the issue of response characteristics. Notice greatest concern. Instead, the also that the multiburst signal response relative to a particular starts with a low-frequency specification or to earlier mea- signal (bar, sine wave, or, as is surements is used to indicate the case in Figure 5-9, a square equipment performance. A Figure 5-10. High-frequency rolloff is apparent in this multiburst signal wave). This low-frequency signal difference between past and display. The maximum error occurs where the signal is about 50 out of is used as an amplitude ref e re n c e pr esent response measurements is 60IRE, which is -1.6 dB. in measuring the relative a sign of equipment performance amplitudes of the other packets. changing and may indicate a Be aware there are many diffe re n t need for service. configurations of multiburst High-frequency rolloff, such as signals — this test signal has a shown in Figure 5-10, is prob a b l y long history and meets many the most common type of differing needs. When testing a response distortion. When this VTR, you should use a reduced occurs, luminance fine detail is amplitude (60 IRE vs. 100 IRE) degraded. Many VTRs will show multiburst signal. This is to a much greater rolloff and still avoid intermodulation between be within specifications. the multiburst frequencies and Hi g h - f r equency peaking is another the FM recording system in the type of distortion. This is shown VTR. Such intermodulation can in Figure 5-11. It is usually cause signal distortions even caused by incorrect equalizer when there actually is nothing adjustment or misadjustment of wrong with the VTR. Also, when some other compensating Figure 5-11. This multiburst signal suffers from high-frequency peaking. evaluating multiburst amplitudes, device. This problem causes The maximum error occurs where the signal is 80 IRE instead of the you need to take into consideration noisy pictures — you see sparkles nominal 60 IRE (1.3dB increase). the VTR’s specified bandwidth. and overly emphasized edges.

5-5 Center-frequency dipping or Measurements are made using peaking can also occur. When the special chrominance pulse this affects a broad range of fre- shown in Figure 5-12. This quencies, it can be detected with pulse is the modulated 12.5T the multiburst signal. On the sine-squared pulse which is other hand, peaking or dipping included in many combination may only affect a narrow range test signals. For example, it’s of frequencies. When this is the included in both the pulse and case and the affected frequencies bar and NTC 7 Composite signals occur between multiburst pr ovided by the TSG 170A NTSC packets, the distortion can go Television Generator. The pulse undetected by this test technique. and bar signal also includes a To catch such narrowband line bar and a 2T pulse. response problems, a sweep The chrominance pulse consists signal or other continuous-band of a low-frequency, sine-squared response measurement technique luminance component that’s Figure 5-12. A 12.5T chrominance pulse, shown in the center of this is needed. Fortunately, narrow- been added to a chrominance display, is used to evaluate chrominance-to-luminance gain and delay band peaking or dipping does packet having a sine-squared errors. For zero gain and delay errors, the negative peaks of this modulated sine-squared pulse should line up on the pulse baseline. not occur often. And when it modulation envelope. These does occur, it can be detected as combined pulse components noticeable ringing on sync pulses have characteristics that allow or other sharp signal transitions. gain and phase errors to be seen Such ringing indicates the need as distortions of the pulse base- for more thorough response line. In the case of Figure 5-12, evaluation using frequency there are no errors and the sweep, multipulse, or (sin x)/x baseline is flat. pulse techniques. To learn more Figure 5-13 shows what happens about these techniques, refer to when there’s a relative chroma Tektronix publications Using the level distortion. The upward Multipulse Waveform to Measure bowing of the baseline (the Group Delay and Amplitude negative waveform peaks) Errors (20W-7076), and indicates that chrominance is Frequency-Response Testing reduced relative to luminance. Using a (Sin x)/x Signal and the If chrominance were increased VM700A Video Measurement relative to luminance, the Set (20W-7064). baseline would bow downward. Figure 5-13. A single, symmetric peak in the chrominance pulse baseline indicates a chrominance-to-luminance gain error. This example shows an Chrominance to Luminance Chrominance-to-luminance gain error of about 20%. Gain er ror can be measured directly on the waveform monitor graticule. When a signal passes through a This is done by comparing the video system, there should be no peak-to-peak amplitude of the change in the relative amplitudes chrominance component of th e of chrominance and luminance. 12.5T pulse to the norma l i z e d In other words, the ratio of the white level reference bar. In the chrominance and luminance case of Figure 5-13, the chromi- gains, which is sometimes nance amplitude is 80 IRE, referred to as relative chroma indicating a 20% error. This level, should remain the same. measurement approach is valid If there are relative chroma only if there is no low-frequency level errors, the pictures color amplitude distortion and there saturation will be incorrect. is negligible chrominance-to- Relative chroma level is checked luminance delay. by measuring chrominance-to- A chrominance-to-luminance luminance gain. Measured gain- gain error is easy to correct if the ratio errors can be expressed in equipment under test has an IRE, percent, or dB. When external chroma gain control. If chrominance components are it does, simply adjust the chrom a peaked relative to luminance, gain control for a flat chrom i n a n c e the error is a positive number. pulse baseline. If it does not, the When chrominance is attenuated, equipment must be serviced by a the error is negative. qualified technician.

5-6 Chrominance-to-luminance more definitive measurement delay, on the other hand, is a results can often be obtained by more common error. Its presence using a generator, such as the is indicated when the chromi- Tektronix TSG 170A, that nance pulse baseline has a sinu- provides test signals at several soidal distortion such as shown different APLs. When using such in Figure 5-14. When there is a generator, run the test with at delay error only, the sinusoidal least two APLs, one low and one lobes are symmetric and the high, and report the worst result. pulse amplitude should match The three remaining measure- the white level reference bar ments to be discussed in this amplitude (100 IRE). This is the section fall into the category of case shown in Figure 5-14. determining the degree of signal Asymmetrical lobes along with linearity (or nonlinearity). These peaking or attenuation of the are the tests for luminance pulse amplitude indicate the nonlinearity, differential gain, presence of combined gain and and differential phase. These can Figure 5-14. A sinusoidal distortion of the chrominance pulse baseline indi- delay errors. be conducted with test signals cates that chrominance is either advanced or delayed relative to luminance. Since there are no user adjust- provided by the same signal ments for chrominance-to- generator used in the previous luminance delay on composite tests, the Tektronix TSG 170A NTSC equipment, correcting this NTSC Television Generator. problem requires a trip to a local Luminance nonlinearity service center. The luminance nonlinearity test Measuring chrominance-to- is used to determine if luminance luminance delay is beyond the gain is affected by luminance level. scope of intermediate video This measurement is also ref e r red system testing. However, you can to as differential luminance. learn about these more advanced measurements by referring to Luminance nonlinearity errors Tektronix publications occur when the video system Television Measurements For fails to process luminance NTSC Systems (063-0566-00) or information consistently over Using the Multipulse Waveform the entire amplitude range. to Measure Group Delay and When the progression from one Figure 5-15. The five-step luminance staircase signal is used to measure Amplitude Errors (20W-7076). brightness level to another is luminance nonlinearity. nonlinear, the accuracy with Nonlinear Distortions which the picture display brightness levels in the nonlinear Thus far, the focus has been on range is affected. For example, distortions having equal effects shades of gray that should be for signals of differing amplitudes. distinct may appear the same. These are linear distortions because the amount of signal Luminance nonlinearity can distortion varies linearly with be evaluated using a staircase signal amplitude. In other signal having five or ten steps, words, a linear distortion causes with or without high-frequency the same percent error on a modulation. However, the small signal as on a large signal. modulated staircase may give a different measure of luminance Nonlinear distortions, by contrast, nonlinearity — the high-freq u e n c y are amplitude dependent. They signals may affect the lower- may be affected by changes in frequency processing. The Average Picture Level (APL) as TSG 170A contains both a five- well as instantaneous signal Figure 5-16. The five-step staircase of the NTC 7 Composite test signal can step luminance staircase and a level. In other words, nonlinear also be used to measure luminance nonlinearity. To make the measurement, five-step modulated staircase though, the waveform monitor’s filter must be set to low-pass to remove di s t o r tion causes diffe r ent perce n t (contained in the NTC 7 the chrominance information. errors depending on signal Composite signal). The five-step amplitude. An overdriven luminance staircase and the amplifier, for example, causes NTC 7 Composite signal are nonlinear distortion when it shown in Figures 5-15 and co m p r esses or clips signal ampli- 5-16, respectively. tu d e peaks. Since APL changes should be taken into account,

5-7 To make the luminance nonlin- Some waveform monitors — such earity measurement on the as the Tek t r onix 1780R — include waveform monitor using a a diffe r entiated step filter. This luminance staircase, ensure the filter can be used for lumina n c e waveform monitor’s filter selec- nonlinearity measurem e n t s on an tion is set to flat. When making unmodulated staircase signal. the measurement on a modulated The differentiated step filter staircase, you must first engage converts each step to a spike of a the low-pass filter (LPASS). This height proportional to the step eliminates the high-frequency height (Figure 5-19). Any varia- modulation, as shown in tions in step height become Figure 5-17 (but remember, it immediately apparent as changes does not eliminate it from the in spike heights. By using the equipment under test). Then waveform monitor’s variable calculate the nonlinearity error gain control, the largest spike as the diffe r ence in height between can be set to 100 IRE and the Figure 5-17. With the waveform monitor in low-pass filter mode, the the largest and smallest steps. percent nonlinearity error can be undistorted modulated staircase appears as five luminance levels with The error is then expressed as a read directly from the graticule. equal steps. percentage of the largest step. If the amount of luminance non- In Figure 5-18, for example, the linearity measured exceeds the largest step covers about 23 IRE, manufacturer’s specifications, and the smallest step covers about the equipment requires service 16 IRE. From these two values, or repair. the luminance nonlinearity error is computed to be 30%. This is Differential gain a fairly large error that was Differential gain testing allows chosen simply for the purpose you to determine when chromi- of illustration. nance gain is being affected by More precise measurements can luminance level. Differential be made by using the waveform gain, sometimes referred to as monitor’s X5 magnifier to diff gain or dG, occurs when a expand the steps. With X5 video system doesn’t process the magnification, a 20 IRE step chrominance signal consistently vertically covers 100 IRE on the at all luminance levels. This can graticule scale. By positioning cause unwanted chrominance Figure 5-18. This staircase shows increased gain (taller steps) around the the waveform vertically, you can amplitude increases and middle of the luminance range and decreased gain (shorter steps) at the view each step individually decreases, all in the same signal. highest luminance level. The maximum linearity error on this signal is In other words, chrominance can about 30%. over the full graticule for high- resolution measurements. be too high at one luminance level and too low at another.

Figure 5-19. This differentiated staircase, which is displayed on the 1780R Video Measurement Set, shows a luminance nonlinearity of about 10%.

5-8 Differential gain causes color To determine the amount of saturation to have an unwarra n t e d differential gain, find the differ- dependence on luminance level. ence in peak-to-peak values for This is seen in the television the largest and smallest packets pi c t u r e as saturation being affe c t e d of the distorted signal. The by variations in brightness. difference is then expressed as a When a colored object moves percentage of the larger number. from sunlight to shade, for For example, the differential example, the color intensity will gain in Figure 5-21 is about 20%. increase or decrease abnormally. This is a fairly large distortion, A five-step modulated staircase, used for purposes of illustration. found on the NTC 7 Composite You can also use a vectorscope signal, can be used to measure to check differential gain. On the differential gain with either vectorscope, the modulated a waveform monitor or a staircase will appear as shown vectorscope. in Figure 5-22. Figure 5-20. The waveform monitor’s chrominance filter eliminates A modulated ramp, available To evaluate differential gain, varying luminance levels from the modulated staircase or ramp signal. in the TSG 170A, can also be adjust the vectorscope’s variable Theflat top and bottom of this chrominance signal indicate an absence of used, as well as a 10-step gain so that the dot representing differential gain. modulated staircase. the staircase chrominance To make measurements on a touches the graticule circle waveform monitor, set the (Figure 5-22). (If undistorted, instrument for one- or two-line the staircase chrominance will sweep with the chrominance overlay the burst vector — they filter selected. This filter passes are each 40 IRE p-p at reference only the 3.58 MHz chrominance phase at the generator.) Any signal and eliminates the lumi- horizontal elongation of the dot nance steps. Also, for convenience, is due to differential gain. adjust the gain control so the maximum peak-to-peak amplitude of the signal is 100 IRE. Disregarding the color burst signal, the top and bottom of the chrominance signal should be flat, as shown in Figure 5-20. If differential gain is present, Figure 5-21. This waveform monitor display of a chrominance-filtered packet amplitudes will not be modulated staircase shows top and bottom distortions representing a un i f o r m, as shown in Figure 5-21. differential gain of about 20%.

Figure 5-22. This vectorscope display of a modulated staircase test signal shows little or no differential gain. The vectorscope’s variable gain has been increased for this measurement.

5-9 amounts of distortion. But for gain is adjusted so the end of the more precise measurements, modulated staircase vector especially on smaller amounts of touches the graticule circle. Any distortion, you’ll need a more widening of the dot along the sophisticated measurement tool graticule circumference repre- with a special DIFF GAIN mode. sents differential phase. This can The Tektronix 1780R Video be measured using the special Measurement Set, for example, graticule marks provided on provides a DIFF GAIN mode. most vectorscopes (Figure 5-24). Differential gain, like other Higher resolution measurements, nonlinear distortions, cannot be however, need to be done with a corrected by a simple front panel more sophisticated vectorscope adjustment. Internal adjustments having a DIFF PHASE mode. A or repairs are necessary to correc t DIFF PHASE mode is provided, excessive amounts of diff gain. for example, on the Tektronix Tektronix has produced a video- 1780R Video Measurement Set. Figure 5-23. Elongation of the chrominance vector dot indicates about 15% The VM700T Video Measurem e n t differential gain. tape that provides further details on differential gain and how to Set also provides a DG DP mode measure it. You can obtain this for automatic measurement of videotape by ordering Tektronix differential gain and phase. This Television Division Measurement is particularly convenient since Series #1, “Differential Gain” differential gain and phase often (068-0330-04, NTSC VHS). occur simultaneously, as shown in Figure 5-25. Differential phase Co r recting diffe r ential phase, like Measuring differential phase di ff e r ential gain, req u i r es servi c i n g de t e r mines whether chrom i n a n c e by a qualified technician. phase is affected by luminance level. Differential phase, also Tektronix has produced a video- referred to as diff phase or dP, tape that provides further details occurs when the video system on differential phase and how it fails to process chrominance is measured. To obtain this consistently at all luminance videotape, order Tektronix levels. This is similar to differ- Television Division Measurement ential gain, except luminance Series #1, “Differential Phase” Figure 5-24. Widening of the chrominance vector dot along the graticule (068-0331-04, NTSC VHS). circumference indicates 5 degrees of differential phase. level changes affect chrominance phase rather than gain. Conclusion Differential phase causes the To maintain optimal signal chrominance signal phase to qu a l i t y , regular system testing is a either lead or lag the desired must. The tests described in this phase. Such phase errors cause booklet, while not exhaustive, colors to change hue when do detect most of the distortions picture brightness changes. For commonly responsible for example, when an object moves picture quality problems. More from sunlight to shadow, it importantly, these tests can warn appears to change color. The you of impending problems that incorrect reproduction of color may not yet be visible as actual is most likely to occur in the picture impairments. high-luminance portions of It’s wise to establish standard the picture. test procedures for each system Differential phase can be tested and document the results from using the same five- or ten-step each test. Such test histories Figure 5-25. This display shows simultaneous differential gain and phase of modulated staircase signal used allow you to spot system deteri- about 10% and 9 degrees. for differential gain testing. For oration trends even before the Most vectorscopes have special digital systems, however, the video signal violates acceptable graticule marks, such as shown modulated ramp signal is limits. The reward for this effort in Figure 5-23, to help measure generally preferred. will be video productions of the amount of differential gain. Measurements are made using a consistently high technical These graticule markings allow vectorscope. As with differential quality while avoiding costly you to measure fairly large gain, the vectorscope’s variable reshoots and reediting.

5-10 Beyond Bars — Other Tests and Test Signals

Most folks consider them obnox- Testing 1, 2, 3 Standard values ious. After all, who hasn’t dozed The principles behind video sig - Most studio, production and off watching late-night television nal testing are quite simple. The post facilities use a variety of only to be startled by the blare of output of a test signal generator equipment and systems. These the 400 Hz tone — you know, is applied to the input of a piece require a broad corresponding the one after the tranquility of of video equipment or distribu- selection of signals for setup, “The Star Spangled Banner” — tion system. The test signal, after maintenance and calibration. accompanying the video test passing through the components, Calibrating a picture monitor signals used to check systems is displayed on a waveform requires SMPTE color bars and a after programming finishes? It’s monitor and vectorscope. Any crosshatch (convergence) signal. hard to expect viewers to appre- significant change in the signal Color bars are used to set chrom a , ciate, especially at 4 a.m., that becomes fairly conspicuous, and hue and brightness adjustments. those annoying pictures and the type of change gives many The convergence signal helps sounds are for their benefit. clues about the possible source align the red, green and blue Color bars are the most ubiquitous of the problem. beams. The multiburst signal is of all test signals. When not Color bars are handy for setting used to check the picture following a TV station sign-off, up such equipment as time base monitor’s horizontal resolution. they are most often perceived as correctors, or TBCs. However, Ob s e r ving technical values within a preprogram place holder — measuring the performance, a system on a waveform monitor something like a blank electronic such as the bandwidth of a TBC, or vectorscope requires a genera- film leader. But, as technical requires more specific testing. tor with test signals such as pros know, color bars provide Periodic measurement of overall pulse-and-bar, modulated both serious at-a-glance confir- system performance helps spot st a i r case, multipulse or multiburst. mation of signal path completion minor problems before they However, evaluation with each and confidence of video grow into noticeable picture test signal is time consuming. signal acceptability. problems. The result is consis- Combination signals, such as Merely observing bars on a TV tently higher picture quality. NTC 7 Composite, FCC Composite monitor, however, is a subjective What test signals ensure video and NTC 7 Combination, contain assessment. The monitor will signal quality? Color bars are two or more test signals as display a signal problem, but it most obvious. They monitor or elements on each video line. reveals little about the source, measure several amplitude, Each element of a combination nature or degree of the signal timing and color parameters. signal is basically a narrow ver- impairment. To extract detailed However, no one test signal sion of regular signals, allowing information about video signal defines all of the amplitude and two or more to fit side by side fitness requires careful examina- timing relationships of the NTSC on a single line of video. tion of one or more test signals signal. Signal path impairments on a waveform monitor and system performance are Many generators also produce and/or vectorscope. often best detected with several matrix test signals that combine two or more different signals in Color bars are only one of many types of test signals. You’ll find a single field of video. Different test signals. Test signals are the most common of these matrix signals are available for precisely defined electronic signals in the Summary of Video transmission and system testing signals that serve as performance Test Signals chart. and picture monitor setup appli- measurement benchmarks. The The types of signals suited for an cations. A matrix typically purpose of each signal is to make application depend mainly on consists of 40 or more consecu- one or more types of video dis- two factors: the environment tive video lines of one signal tortion easy to see and quantify. and the video format. followed by the same number of lines of the next signal, throughout the active video lines of a field.

6-1 Like the combination signals, More than test Distribution distortions matrix signals speed the process Other signals and functions Distribution paths can be subject of equipment and system evalua- pr ovided by test signal generators to myriad distortions. A distribu- tion and adjustment. With several can simplify several routine tion path may be as simple as a signal types, you can observe pr oduction tasks. A safe-title/ camera output looped through a multiple problems (or the safe-action-area signal helps monitor to a switcher and termi- absence of multiple problems) operators position and size the nated, or as complicated as a without changing the test signal. critical parts of a scene so they 1,000-machine duplication Continuous technical monitoring don’t appear somewhere off the system. That’s why multipurpose of a source is enhanced with edge of the presentation screen. combination signals, such as the Vertical Interval Test Signal Blacking tapes is a task that FCC and NTC 7 Composite and (VITS) insertion. By including some generators can reduce to the NTC 7 Combination, are one or two test signals on unused the push of two buttons. (You so widely used as VITS in lines in the vertical interval, you need only press Record on the transmission testing. can make objective judgements VTR and select a countdown Multiburst (which is part of the about the extent of the signal sequence on the generator.) NTC 7 Combination signal), degradation instantaneously, at Evaluating the performance of multipulse and (sinX)/X are all any point in the program. Most 2-wire (Y/C) or 3-wire analog used extensively for frequency- TV stations and networks use component systems requires response testing of various VITS to monitor and evaluate special signals in the appropriate distribution systems. Multipulse technical parameters without component form, in addition to and (sinX)/X indicate group interrupting programming. the composite signals. Bowtie is delay as well, but (sinX)/X Once you’ve decided which an essential signal for 3-wire requires a spectrum analyzer or signals and formats you need, component systems designed automated video measurement th e r e are still more considerations specifically for precise amplitude set for display. Big ticket items, for choosing a test signal genera- and timing adjustments. An such as spectrum analyzers or tor. Space is always important; economical alternative to multiple automated measurement systems, usually the smaller the generator’s test signal generators is a multi- often aren’t a liability, because package size, the better, prov i d e d format generator with signals many other standard transmission the unit supplies the signals in NTSC, Y/B-Y/R-Y and and distribution tests require you need. Depending on the Y/C formats. these instruments. To optimize performance, particularly in this complexity of the facility, other Cameras usually don’t require era of higher-resolution formats, video gear in the system may external test signals, but in the good test equipment is a basic need to be genlocked. Some test studio they must be genlocked to signal generators eliminate the requirement, not an option. the system. In smaller facilities need for an extra distribution or Electronic Field Production amplifier (DA) by providing (EFP) applications, a generator multiple black burst outputs. with multiple black burst outputs System performance checks might eliminate the need for a require only a few other signals. separate master sync generator The NTC 7 Composite and or DA. A black burst input to a Combination signals or a system pr oduction switcher is commonly test matrix signal can provide used as the source of black for a all components necessary for quick checks of system linearity, fade-to-black. insertion gain, frequency response, differential phase and gain, chrominance-to-luminance gain and delay, and other short- and long-time distortions.

6-2 Maintaining performance Operational equipment in need Reliable, high-quality, uninter- Maintenance areas have some of routine maintenance or rupted video — the type clients special requirements. Flexibility troubleshooting can’t always go pay for — requires extensive is the key, as the proliferation of to the shop. So, with the proper testing and preventative mainte- interconnect and recording signals and formats, a small, nance to catch and fix declining formats continues. lightweight package can make it system performance before a lot easier to bring the tools to visible picture impairment It’s not uncommon to find com- the problem. occurs. Once quality starts to posite NTSC gear in use with slip, so will the attention of component analog or Y/C gear. The economics of testing your viewers, as well as future Although all of this gear has One thing is certain today — business from your clients. NTSC inputs and outputs, viewers expect high video quality, equipment in each video format all the time. Pr oducing video without adequate requires test signals in the same test and measurement gear is as Gone are the days when a single, format to fully exercise its risky as driving a car at night snowy TV channel was revered circuitry. In addition to the without headlights. Today’s as a miracle of modern technology luminance and color difference digital-based test signal generators and occasional problems from signals primarily used with com- provide many high-precision technical difficulties were toler- ponent analog video equipment, signals in small, economical ated. Today, home viewers fix the time-compressed versions packages. Waveform monitors technical difficulties on their (CTCM or CTDM, the actual and vectorscopes come in a own without leaving their recording formats used) must variety of affordable packages, armchairs by switching channels also be available. and sometimes both functions with remotes. Viewers of corpo- are combined for even greater Serial digital video is finding rate and other nonbroadcast economy. With compact, easy- favor as an interconnect format video have similar expectations to-use gear, the serious video within many video facilities. and reactions, except instead of producer can’t afford not to use With it comes the need for yet tuning out with a remote control , test equipment every day. another test signal format and, of they tune out their brains. course, new testing issues. Serial digital requires the same analog test signals — you simply need to convert them to the new digital format and add a number of very complex digital goodies to the vertical interval.

6-3 Summary of Video Test Signals Test Signal Signal Shape Use and Benefits Color Bars General amplitude and timing measurements. Most widely available test signal. Used in all aspects of system setup and testing from ENG/EFP units to the broadcast transmitter.

Black Burst Commonly used for synchronizing video gear. Also used for noise measurements.

Multiburst Contains packets of six different frequencies. Used for basic frequency-response checks of equipment and distribution paths in ENG/EFP and studio work.

Modulated Staircase Available in 5- and 10-step forms. Tests differential gain/phase and luminance linearity. Used in ENG/EFP, studio and distribution.

Pulse and Bar Used for amplitude, timing, and distortion measurements. Modulated pulse portion tests chrominance- to-luminance gain and delay. Used in ENG/EFP, studio and distribution.

Multipulse Contains pulses modulated at different frequencies for comprehensive measurement of amplitude and group delay errors over the video baseband. Especially important for transmitter testing.

(Sin x)/x Provides frequency-response and group-delay test coverage of all baseband frequencies. Can be used as a VIT signal, making it ideal for in-service transmitter testing.

NTC 7 Combination Combines multiburst and modulated pedestal for frequency-response and distortion tests. Designed for distribution and transmission system testing.

NTC 7 Composite Contains various signal elements allowing amplitude, phase and some distortion measurements. Designed for studio and distribution testing. Rise time too fast for broadcast transmitter use.

FCC Composite Offers the same uses and benefits as the NTC 7 Composite signal. Its slower rise time makes it appropriate for VITS use with broadcast transmitters.

Modulated Ramp Used the same as modulated staircase but provides finer-grained results.

Sweep Provides a continuous sweep of video baseband frequencies, usually with embedded 1 MHz markers. Used for detailed frequency-response testing, but not VITS compatible.

Bowtie Component analog video (CAV) test signal used for high-precision measurement of component channel gain and delay inequalities.

6-4 The Color Bars Signal —

Monochrome Video cameras, scanning in parallel, The information to reconstruct each with a Red, Green or Blue G B R a black and white image is rela- filter over the lens to produce tively easy to understand — color video. The display also 100 each point in the image is used multiple images, filters scanned in the familiar line and optics to combine the Red, pattern and at each point along Green and Blue images into one the line the camera produces a full-color picture. voltage representing the amount The Color Bars Signal — RGB 0 of light at that point. The rapid Setting up a system with three succession of points scanned — cameras, three video transmission and the corresponding succession paths, and three displays is a of signal voltages — creates the Figure 7-1. Video waveforms of the three channels for RGB-color bars chore! Matching the three (100% bars). video waveform. This waveform channels to “track” from black to is “one dimensional,” i.e., only peak brightness is especially Encoded color — NTSC one piece of information is needed im p o r tant. Test signals have been The RGB system, with parallel for each point in the image. developed for this sole purpose. full-bandwidth video in three Color Video — History In this RGB system, each video channels, makes excellent The situation gets quite a bit path is normalized so the signals pictures but has several practical more complex with color video. match at black level (zero light) disadvantages. Perhaps the The problem with color video is and at peak level (maximum most severe is the very wide it’s not one dimensional, but light). A good test signal consists bandwidth needed in broadcast three dimensional — three of a square wave with black and service or over other long pieces of information are needed peak levels in each of the three interconnect paths. channels. If the square waves are for each point in the image. There have been several timed diffe r ently in each channel, There are several ways to convey attempts at “compressing” color all combinations of high and information about color. Most video information so it is more low levels in the channels are take advantage of a characteristic practical to record, interconnect, exercised (Figure 7-1). Since of human vision known as the and broadcast. The winner, by there are three channels, each tristimulus theory — we can fa r , is the NTSC system developed with two possible levels, there satisfactorily reproduce a wide in the early 1950s. This ar e two cubed (eight) possibilities. range of color using only three “encoding” scheme manages to These eight combinations exerci s e colors of light mixed in certain put the essential parts of the the primary colors individually ways. Color photography and three-dimensional color signal (Red, Green, and Blue), the color printing are based on these into the spectrum needed for a secondary colors (combinations same ideas — three filters, three monochrome signal — putting it of two primaries: Yellow, Cyan, dyes, three inks, etc. (The four- all in one path. (Work is still and Magenta), White, and Black. color printing process uses continuing today with complex The colors (color bars) are black ink in addition to make digital filters, advanced modula- arranged in order of decreasing dark or gray areas in the image tion techniques and “smart” total brightness: White, Yellow, easier to control.) compression algorithms, but Cyan, Green, Magenta, Red, Blue, these are beyond the scope of While other combinations are Black. The color bars signal is this tutorial.) possible, there are some practical useful in setting up a RGB-color reasons for choosing Red, Green video system, and it gives a and Blue (RGB) as the three pleasing image that also conveys “p r i m a r y” colors for a color video some indication of the “health” system. In fact, the earliest color of the color video system. television systems used three

7-1 NTSC must still maintain three channel, taking into account the 100 information paths, inherent in luminance coefficients of each the nature of color. The specific primary, i.e., Y = 0.59G + 0.30R set of three signals, however, is + 0.11B. If the input signal for changed from RGB to a more this process is a RGB color bars manageable set. The three NTSC signal, the resulting luminance signals are Luminance, and two video waveform is the familiar Color Difference components. decreasing “staircase” shown in Figure 7-2. 0 The symbol used for luminance is “Y.” It is closely related to Note the steps in this staircase movement in the “y” direction are not equal amplitude — they in a common x vs y graphical depend on the luminance coeffi- Figure 7-2. Color bars luminance signal (100% bars). rep r esentation of color. The color cients. Consider, for example, difference signals also contain Y the Green to Magenta transition and one of the primary colors — (in the center of the bars wave- usually B-Y and R-Y. For example, form). The luminance level for B-Y is the Blue video signal Green is 59%. Magenta consists minus the luminance signal. of Blue and Red only, its lumi- Luminance nance value is 0.11 + 0.30 = 0.41 or 41%. The amplitudes of The NTSC luminance component the other levels may be is very much like the signal for similarly calculated. mo n o c h r ome video — containing information about the amount of Color difference signals — light in each element of the B-Y and R-Y picture. This is one of the key Once the luminance signal is reasons the NTSC system is available, it takes only a little “compatible” with older more processing to derive the monochrome equipment — color difference signals. For monochrome sets simply ignore example, if luminance is sub- the color information and display tracted from the B component of using luminance only. the RGB video the result is B-Y. Luminance is derived from RGB This process, and the parallel color signals. While the tristimu- one that produces R-Y, are fairly lus theory req u i r es three diffe re n t easy to accomplish with electron i c colored lights, it does not imply circuits. (Sometimes, another set that each light contributes the of color difference signals were same amount to our perception used — I and Q. We’ll discuss I and Q a little later.) Figure 7-3. Unmodulated and modulated color difference signals and of brightness of a point in the modulated chrominance for color bars. image. In fact, for the particular With the color information now colors chosen (a certain Green, a in the form of luminance (Y) certain Blue, and a certain Red), and color difference components Green contributes about 59% of (B-Y and R-Y) we still have our perception of brightness in a three signals and the associated white part of the image. The Red bandwidth and interconnect light contributes about 30%, and problems. A bit more processing Blue only about 11% of the is needed to get it all into the brightness of white. These “‘one-wire” NTSC signal format. numbers are called the luminance Encoded color (NTSC) co e f ficients of the primary colors. Another characteristic of human (A certain color of white must be vision is we can’t see fine detail chosen as well, but that and the nearly as well for changes in math involved are beyond our scope here.) coloring as we can for changes in luminance. In other words, the The luminance signal is prod u c e d picture won’t suffer very much if by combining (adding) the video we reduce the bandwidth of the signals from each primary color coloring components, provided

7-2 we can maintain essentially full Some adjustment of amplitudes bandwidth of the luminance sig- is needed when the luminance nal. In fact, this is a good reason and chrominance signals are for developing color difference combined. Figure 7-4 shows components in the first place. the result if the signals for full- Even a full bandwidth luminance amplitude bars are combined signal doesn’t have very much without these adjustments. Note energy in the upper end of its the peak level of the chrom i n a n c e sp e c t r um — the higher freq u e n c y is very much above 100% white signals are quite a bit lower luminance. This signal will be amplitude almost all the time. seriously distorted in passing These two facts (less bandwidth th r ough a video system, especially req u i r ed for the color informa t i o n a system originally designed for handling monochrome signals. and some “room” available in Figure 7-4. Full amplitude bars without gain adjustments. the luminance spectrum) allow In Figure 7-5, the level of B-Y is the NTSC system to place the reduced by a factor of 2.03 and color components in only R-Y level is reduced by a factor the upper portion of the of 1.14. These specific factors luminance spectrum. are chosen to limit the peak of the waveform to 100% white Color subcarriers level with a 75% amplitude bars The technique for getting the signal (Figure 7-6). This 75% R-Y and B-Y signals moved up bars signal has become the in frequency is amplitude modu- most common for testing NTSC lation — a carrier (subcarrier) is systems. (Refer to the discussion modulated by each of the color of various bars signals on difference signals. Remember, pages 2-2 and 6-1.) there are two different signals, Conclusion so two carriers are needed. Both Figure 7-5. Full amplitude bars. signals are at the same frequency There are several aspects of the (about 3.58 MHz) but are main- generation of NTSC video signals tained in phase quadrature that underlie recommended (90 degrees apart) so the B-Y monitoring and test methods. and R-Y information can be kept 1.Luminance and Chrominance separated. This process is known are a “matched set” and must as Quadrature Amplitude remain balanced in amplitude Modulation (QAM). Balanced and timing in order to accu- modulators are also used so the rately reconstruct the RGB ca r riers themselves are suppres s e d signal (and the color picture). at the modulator output. Only 2.The color difference compo- the sideband energy generated nents are only kept separate Figure 7-6. Corrected 75% bars signal (with 100% white). in the modulation process is by differences in subcarrier saved. This modulated color phase. Small phase distortions difference information is usually can create large color distor- called chrominance. tions in the end picture. The NTSC composite signal 3. Ch r ominance information is all When luminance and chromi- at the upper end of the video nance are combined, the result spectrum and maintaining is the NTSC composite color frequency response flatness signal. Figure 7-3 shows the B-Y th r oughout the system is critical. and R-Y signals for color bars The color bars signal, whether in and the resultant envelope at the RGB or NTSC form, offers a output of each modulator, as well known, accurate signal that as the combined chrominance exercises the practical limits of envelope containing both B-Y the color video system. and R-Y information.

7-3 Glossary

audio signals — XLR connectors chrominance-to-luminance differential phase — Variation in provide dual-channel audio sig- gain — The difference between the phase of the chrominance nals. The left channel can be set the gain of the chrominance subcarrier as the luminance to click as a means of easily dis- portion of the video signal and signal on which it rides is varied tinguishing the left channel from the gain of the luminance porti o n from blanking to white level. the right channel in audio tests. as they pass through a system. fr equency response — A system’s average picture level (APL) — color burst — The burst of color gain characteristic versus fre- The average signal level with subcarrier added to the back quency. Frequency response is respect to blanking during the porch of the composite video often stated as a range of signal active picture time. APL is signal. It serves as a frequency fr equencies over which gain varies expressed as a percentage of the and phase reference for the by less than a specified amount. difference between the blanking chrominance signal. graticule — The calibrated scale and reference white levels. composite video — A single for quantifying information on a chrominance — The color in f o r - video signal containing all of wa v e f o r m monitor or vectorscope mation in a television picture. the necessary information to screen. The graticule can be silk- Chrominance affects two proper- reproduce a color picture. screened onto the CRT face plate ties of color: hue and saturation. co n v e r gence — Used for adjusting (internal graticule), silk-screened Also called chroma. co n v e r gence of the green, blue and onto a piece of glass or plastic chrominance-to-burst phase — red beams on picture monitors. that fits in front of the CRT (external graticule), or it can be The difference between the decibel — A logarithmic unit expected phase and the actual electronically generated as part that expresses the ratio between of the display. phase of the chrominance a signal and a reference signal. portion of the video signal For voltages, dB = 20 log horizontal blanking interval — relative to burst phase. (See artwork below.) (Vmeasured/Vnominal). chrominance-to-luminance insertion gain — The gain (or differential gain — Variation in delay — The difference in time loss) in overall signal amplitude the gain of the chrominance that it takes for the chrominance introduced by a piece of equip- signal as the luminance signal portion of the video signal to ment in the signal path. Inserti o n on which it rides is varied from pass through a system relative to gain is expressed as a percent blanking to white level. the time it takes for the luminance (Vout - Vin)/Vin x 100. portion. Also called relative chroma time.

BLANKING 10.9 µ SEC ± 0.2 µ SEC 100 IRE REF WHITE (714 mv) SYNC TO BLANKING END 9.4 µ SEC = ± .1 µ SEC

FRONT SYNC TO BURST END COLOR PORCH 7.8 µ SEC BACK 1.5 µ SEC PORCH ± .1 µ SEC BREEZWAY 1.6 µ SEC .6 µ SEC BURST 2.5 µ SEC REF BLACK SYNC LEVEL 4.7 µ SEC ± .1 µ SEC 7.5 IRE 4 IRE 40 IRE 4 IRE REF BURST BLANKING AMPLITUDE LEVEL 20 IRE Z Z 40 IRE REF SYNC (286 mv) AMPLITUDE

Figure 8-1. Horizontal blanking interval.

8-1 For further information, contact Tektronix: World Wide Web: http://www.tek.com; ASEAN Countries (65) 356-3900; Australia & New Zealand 61 (2) 888-7066; Austria, Eastern Europe, & Middle East 43 (1) 701 77-261; Belgium 32 (2) 725-96-10; Brazil and South America 55 (11) 3741 8360; Canada 1 (800) 661-5625; Denmark 45 (44) 850700; Finland 358 (9) 4783 400; France & North Africa 33 (1) 69 86 81 08; Germany 49 (221) 94 77-400; Hong Kong (852) 2585-6688; India 91 (80) 2275577; Italy 39 (2) 250861; Japan (Sony/Tektronix Corporation) 81 (3) 3448-4611; Mexico, Central America, & Caribbean 52 (5) 666-6333; The Netherlands 31235695555; Norway 47 (22)070700; People’s Republic of China (86) 10-62351230; Republic of Korea 82 (2) 528-5299; Spain & Portugal 34 (1) 372 6000; Sweden 46 (8) 629 6500; Switzerland 41 (41) 7119192; Taiwan 886 (2) 722-9622; United Kingdom & Eire 44 (1628) 403300; USA 1 (800)426-2200

Copyright © 1997, Tektronix, Inc. All rights reserved. Tektronix products are covered by U.S. and foreign patents, issued and pending. Information in this publication supersedes that in all previously published material. Specification and price change privileges reserved. TEKTRONIX and TEK are registered trademarks of Tektronix, Inc. All other trade names referenced are the service marks, trademarks or registered trademarks of their respective companies. 7/97 FL4996/XBS 25W–7247–1