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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Table of Contents

Preface ...... 3 III. NONLINEAR ...... 41 Differential Ph a s e ...... 42 Good Measurement Practices ...... 4 ...... 46 EQUIPMENT REQUIREMENTS ...... 4 Luminance Nonlinearity ...... 50 CALIBRATION ...... 6 Nonlinear Ph a s e ...... 52 INSTRUMENT CONFIGURATION ...... 6 Chrominance Nonlinear Gain ...... 53 DEMODULATED RF SIGNALS ...... 8 Chrominance-to-Luminance TERMINATION ...... 8 Intermodulation ...... 54 DEFINITION OF THE PAL Transient Sync Gain . . . . .55 TELEVISION STANDARD ...... 8 Steady-State (Static) Sync Gain PERFORMANCE GOALS ...... 8 Distortion ...... 56

Waveform Distortions and Measurement Methods . . 9 IV. MEASUREMENTS ...... 57 I. VIDEO AMPLITUDE AND Signal-to-Noise Ratio ...... 58 TIME MEASUREMENTS ...... 9 Amplitude Measurements ...... 10 V. TRANSMITTER MEASUREMENTS . . 60 Time Measurements ...... 12 ICPM ...... 61 SCH Phase ...... 15 Depth of Modulation ...... 63

II. LINEAR DISTORTIONS ...... 18 GLOSSARY OF TELEVISION TERMS ...... 64 Chrominance-to-Luminance Gain and Delay ...... 19 APPENDICES Short Time Distortion ...... 24 APPENDIX A - PAL COLOUR BARS . . . .67 Line Time Distortion ...... 26 APPENDIX B - Field Time Distortion ...... 28 SINE-SQUARED PULSES ...... 69 Long Time Distortion ...... 30 Frequency Response ...... 31 Group Delay ...... 36 K Factor Ratings ...... 38

Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Preface

To characterize television system encompasses both traditional A basic knowledge of video is performance, an understanding measurement techniques and assumed and a glossary of com- of signal distortions and mea- newer methods. After a discus- monly used terms is included surement methods as well as sion of good measurement as a refresher and to introduce pr oper instrumentation is needed. practices, five general categories newer concepts. The basics This booklet provides informa- of television measurements of and tion on television test and mea- are addressed: vectorscope operation are also surement practices and serves I. Amplitude and Timing assumed. Consult the instrument as a comprehensive reference Measurements manuals for specific operating on methods of quantifying instructions. II. Linear Distortions signal distortions. This publication deals with PAL New instruments, test signals, III.Nonlinear Distortions composite analogue signals. and measurement procedures IV. Noise Measurements Analogue component, digital continue to be introduced as composite and component, and V. Transmitter Measurements television test and measurement HDTV measurements are outside technology evolves. This booklet its scope.

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EQUIPMENT REQUIREMENTS tolerance. For example, differen- other composite and analogue Television system performance tial gain measurement to 1% component formats. This elimi- is evaluated by sending test sig- accuracy should be made with a nates the need for additional nals with known characteristics generator having 0.5% or less signal generation equipment through the signal path. The differential gain distortion. where there is the requirement signals are then observed at the Television equipment and for measurements in multiple output (or at intermediate system performance is generally formats. For synchronization of points) to determine whether or assessed on either an out-of- the equipment under test, a not they are being accurately service or in-service basis. In black burst reference signal is transferred through the system. br oadcast television applications, provided by the TG2000 main- Two basic types of television test measurements must often be frame. For applications requiring and measurement equipment are made during regular broadcast the test signal source be syn- required to perform these tasks. hours or on an in-service basis. chronous with existing equip- Test signal generators provide This requires a generator capable ment, the AGL1 Analogue the stimulus and specialized of placing test signals within the Genlock module provides the known as waveform vertical blanking interval (VBI) interface needed to lock the monitors and vectorscopes are of the television program signal. TG2000 to an external black used to evaluate the response. Out-of-service measurements, burst reference signal. those performed on other than For in-service measurements, the Test Signal Generators. Television an in-service basis, may be made Tektronix VITS201 Generator signal generators provide a wide with any suitable full field test and Inserter is the recommended variety of test and synchroniza- . product. The VITS201 provides tion signals. Two key criteria in For out-of-service measurements, a full complement of PAL test selection of a test signal generator the Tektronix TG2000 Signal signals and high degree of flexi- for precision measurements are Generation Platform with the bility in placement of these signal complement and accuracy. AVG1 and AGL1 modules is the signals within the VBI. Signal The generator should provide all recommended product. The accuracy is adequate for most of the test signals to support the AVG1 Analogue Video Generator transmission and transmitter required measurements and the provides comprehensive signal measurement requirements. signal accuracy must be better sets and sufficient accuracy for Both the TG2000 and VITS201 than the tolerances of the mea- virtually all measurement fully support the measurement su r ements to be made. If possible, requirements. The AVG1 is also capabilities of the 1781R the generator accuracy should be a multiformat unit capable of and VM700T Series Video twice as good as the measurem e n t supporting measurements in Measurement Sets.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Wav e f o r m Monitors and Vec t o r s c o p e s . needs. This is particularly true if The instruments used to evalu- making accurate measurements ate a system's response to test of all the signal parameters and signals make up the second distortions described in this major category of television test booklet. Many varieties of wave- and measurement equipment. form monitors and vectorscopes Although some measurements are on the market today but the can be performed with a general majority of them are not intended purpose , a wave- for precision measurement form monitor is generally applications. Most vectorscopes, preferred in television facilities. for example, do not have preci- Wav e f o r m monitors automatically sion and gain trigger on the television synchro- measurement capabilities. nizing pulses and provide a volt- The recommended products for age versus time display of the precision measurements are the video signal. These instruments Tektronix 1781R and VM700T are equipped with specialized and most of the measurement Figure 1. A waveform monitor display of colour bars. video clamps and filters that procedures in this booklet are facilitate separate evaluation of based on these instruments. the chrominance and luminance portions of the signal. Most The 1781R provides waveform models also have a line selector monitor and vectorscope for looking at signals in the functions as well as many vertical interval. specialized measurement features and modes that simplify A vectorscope is designed for complex measurements. accurate evaluation of the chrominance portion of the The VM700T is an automated signal. This instrument demodu- measurement set with results lates the PAL signal and displays available in numeric and graphic the V (R-Y) colour difference form. Waveform and vector component on the vertical axis displays, similar to those of tra- and the U (B-Y) colour diffe re n c e ditional waveform monitors and component on the horizontal axis. vectorscopes operating in line select mode, are also provided. When selecting waveform moni- The VM700T Measure mode tors and vectorscopes, carefully Figure 2. A vectorscope display of colour bars. provides unique displays of evaluate the feature sets and measurement results, many of specifications to make sure they which are presented in this book. will meet the measurement

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com CALIBRATION INSTRUMENT CONFIGURATION Most instruments are quite Most of the functions on wave- stable over time, however, it is form monitor and vectorscope good practice to verify equip- front panels are fairly straightfor- ment calibration prior to every ward and have obvious applica- measurement session. Many tions in measurement proc e d u re s . instruments have internally A few controls, however, might generated calibration signals that need a bit more explanation. facilitate this process. In the absence of a calibrator, or as an DC Restorer. The basic function of additional check, a test signal the DC restorer in a waveform directly out of a high quality monitor is to clamp one point in generator makes a good substi- the video waveform to a fixed tute. Calibration procedures vary DC level. This ensures that the from instrument to instrument display will not move vertically and the manuals contain with changes in signal amplitude Figure 3. The 1781R waveform calibrator. detailed instructions. or Average Picture Level (APL). Analogue CRT-based instrum e n t s Some instruments offer a choice such as the 1781R have a speci- of slow and fast DC restorer fied warm up time, typically 20 speeds. The slow setting is used or 30 minutes. Turn the instru- to measure hum or other low ment on and allow it to operate frequency distortions. The fast for at least that long before setting removes hum from the checking the calibration and display so it will not interfere performing measurements. with other measurements. Back This ensures that the measure- porch is the most commonly ment instrumentation will used clamp point, but sync tip have little or no effect on the clamping has some applications measurement results. at the transmitter. Computer-based instruments such as the VM700T also specify a warm up time but the operator does not need to verify or adjust

Figure 4. The 1781R vectorscope calibration oscillator. the calibration settings. The VM700T will automatically cali- brate itself when it is turned on and will continue to do so periodically during operation. For best results, wait 20 or 30 minutes after initial turn-on be f o r e making any measurem e n t s .

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Automatic Frequency Control (AFC) In the AFC (Automatic Freq u e n c y Vectorscope Gain: 75%/100% Bars. versus Direct Triggering. The Control) mode, a phase-locked Several different kinds of colour AFC/DIRECT selection in the loop generates pulses that repre- bars are used in PAL systems 1781R CONFIGURE menu sent the average timing of the and many generators produce at provides a choice between two sync pulses. These averaged least two types. In order to methods of triggering the wave- pulses are used to trigger the accommodate the various form monitor's horizontal sweep. sweep. The AFC mode is useful types of colour bars, some The ramp that produces the for making measurements in the vectorscopes have a 75%/100% horizontal sweep is always presence of noise as the effects selection on the front panel synchronous with the H (line) or of noise-induced horizontal jitter which changes the calibration of V (field) pulses of the reference are removed from the display. the vectorscope chrominance video and can be started either gain. The 75% setting corre- The AFC mode is also useful for by the pulses themselves sponds to 100.0.75.0 colour bars, evaluating the amount of time (DIRECT) or by their average (AFC). often referred to as EBU Bars. base jitter in a signal. The leading The 100% setting corresponds to In the DIRECT mode, the video edge of sync will appear wide 100.0.100.0 colour bars. The sync pulses directly trigger the (blurred) if much time base jitter 75%/100% distinction refers to waveform monitor horizontal is present. This method is very chrominance amplitude, not to sweep. The DIRECT setting useful for comparing signals or saturation or white bar level. should be used to remove the for indicating the presence of Colour bar parameters and effects of time base jitter from jitter but be cautious about nomenclature are discussed in the display in order to evaluate actually trying to measure it. detail in Appendix A. other parameters. Since a new The bandwidth of the AFC trigger point is established for phase-locked loop also affects It is important to know which each sweep, line-to-line jitter is the display. colour bar signal is in use and to not visible in this mode. select the corresponding setting on the vectorscope. Otherwise chrominance gain can easily be misadjusted.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com DEMODULATED RF SIGNALS DEFINITION OF THE PAL PERFORMANCE GOALS All of the baseband measure- TELEVISION STANDARD Acceptable levels of distortion ments discussed in this booklet The most widely used definition are usually determined subjec- can also be made on demodulated of the PAL standard is probably tively, however, a number of RF signals. It is important, Report 624 of the CCIR broadcasting organizations however, to eliminate the (International Radio Consultative publish documents that specify demodulator itself as a possible Committee), which specifies recommended limits. In some so u r ce of distortion. Measurem e n t amplitude, timing and colour cases government regulations quality instruments such as encoding parameters for all of may require that certain pub- the Tektronix TV1350 and the major television standards. lished criteria be met. While 1450 Television Demodulators This report was last reviewed in these documents can be useful will eliminate the likelihood 1990 making version 624-4 the as performance guidelines, each that the demodulator is most current at this time. facility must ultimately deter- introducing distortion. There are a number of variations mine its own performance goals. of PAL (M, N, B, G, H, I, D, etc.). Only experience can reveal what TERMINATION With the exception of PAL-M, is practical with the equipment Improper termination is a very which is a 525-line system, and personnel at a given facility. common source of operator error the differences between the stan- While there is usually agreement and frustration. Double termi n a t e d dards are fairly minor at base- about the nature of each distor- or unterminated signal paths band and usually involve only a tion, definitions for expressing will seriously affect signal bandwidth change. The default the magnitude of the distortion amplitude. It is essential that standard for this publication is may vary considerably from each video signal in a facility be PAL-B/G, which has a 5-MHz standard to standard. A number terminated in one location using bandwidth and is used in much of questions should be kept in a 75 Ohm terminator. If a signal of Europe. mind. Is the measurement is looped through several pieces Governments of the various absolute or relative? If it is of equipment, it is generally best countries which use the PAL relative, what is the reference? to terminate at the final piece of standard, as well as broadcasting Under what conditions is the equipment in the signal path. organizations (such as the EBU, ref e r ence established? Is the peak- The quality of the terminator is BBC, IBA, etc.), also publish to-peak variation or the largest also important, particularly standards documents. You may peak deviation to be quoted as when trying to measure very find discrepancies between the the amount of distortion? small distortions. Be sure to various standards. These can be A misunderstanding about any select a terminator with the difficult to resolve since there is one of these issues can seriously tightest practical tolerance as no absolutely ""correct'' answer. affect measurement results so it incorrect termination impedance In general, documents from the is important to become familiar can cause amplitude errors as local broadcasting authority with the definitions in whatever well as frequency response should take precedence when standards are used. Make sure problems and pulse distortions. there are conflicts. those involved in measuring Terminators in the 1/2% to 1/4% system performance agree on tolerance range are widely avail- how to express the amount of able and are generally adequate distortion. It is good practice to for routine testing. record this information along with measurement results.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Waveform Distortions And Measurement Methods I. VIDEO AMPLITUDE AND TIME MEASUREMENTS

This section deals with two fun- When setting video amplitudes, checked less freq u e n t l y , however, damental properties of the sig- it is not sufficient to simply it is still important to under- nal, amplitude and time. In these adjust the output level of the stand the measurement methods. two dimensions, problems are final piece of equipment in the A periodic verification that all more frequently caused by oper- signal path. Every piece of timing parameters are within ator error than by malfunction- equipment should be adjusted to limits is recommended. ing equipment. Correction of appropriately transfer the signal This booklet does not address amplitude and pulse width prob- from input to output. Television system timing issues which deal lems often simply involves prop- equipment is generally not with relative time relationships er adjustment of the equipment designed to handle signals that between the many signals in a the signal passes through. deviate much from the nominal television facility. Although Two kinds of amplitude mea- 1-volt peak-to-peak amplitude. system timing is critical to pro- surements are important in tele- Signals which are too large can duction quality, it is outside the vision systems. Absolute levels, be clipped or distorted and sig- scope of this publication. On such as peak-to-peak amplitude, nals which are too small will the following pages, only those need to be properly adjusted. suffer from degraded signal-to- timing measurements that relate The relationships between the noise performance. to a single signal are addressed. parts of the signal are also Video amplitudes are monitored important. The ratio of sync to and adjusted on a daily basis in the rest of the signal, for example, most television facilities. Signal must be accurately maintained. timing parameters are usually

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DEFINITION waveform monitor vertical gain PAL signals are in the calibrated setting (1 volt nominally 1 volt peak-to-peak. full scale), the signal should be 1 Amplitude measurement volt from sync tip to peak white techniques are used to verify (see Figure 7). The graticule in that the signals conform to this the VM700T WAVEFORM mode nominal value and to make the can be used in a similar manner. appropriate gain adjustments Added Calibrator Method. Some when needed. Similar methods waveform monitors have a fea- of evaluating the waveform are ture that allows the internal cali- used for both measurement and Figure 5. 100.0.75.0 colour bars. brator signal to be used as a ref- adjustment of signal levels. erence for amplitude measure- Measurements of the peak-to- ments. This feature is known as peak amplitude of the video WFM + CAL in the 1781R. In the signal are sometimes called 1481 it is accessed by depressing “insertion gain” measurements. both the CAL button and the OPER buttons. PICTURE EFFECTS The WFM + CAL display consists Insertion gain errors cause the of two video traces vertically off- picture to appear too light or too set by the calibrator amplitude. dark. Because of the effects of This display is obtained by ambient light, apparent colour adding the incoming signal to a Figure 6. Pulse and bar test signal. saturation is also affected. calibrated square wave of known amplitude. Signal amplitude is TEST SIGNAL equal to the calibrator amplitude Insertion gain can be measured when the bottom of the upper with any signal that contains a trace and the top of the lower 700 mV white portion. Colour trace coincide. bars and pulse and bar signals The WFM + CAL mode is most are most frequently used (see commonly used to set insertion Figures 5 and 6). Many of the gain which requires a 1-volt standard ITS signals also contain calibrator signal. When using a a 700 mV bar and can be used to 1781R, select a calibrator ampli- measure or adjust video gain. tude of 1000 mV. In the 1481R, MEASUREMENT METHODS the DC RESTORER setting deter- mines which of two calibrator Waveform Monitor Graticule. Signal amplitudes is selected. The cali- amplitude can be measured with brator amplitude is 1 volt when a waveform monitor by compar- SYNC TIP is selected and 700 mV ing the waveform to the vertical when BACK PORCH is selected. Fi g u r e 7. A 1-volt signal properly positioned with respect to the 1781R graticule. scale on the graticule. With the

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com In s e r tion gain is set by externa l l y NOTES adjusting the signal amplitude 1. Sync to Picture Ratio. When the until sync tip of the upper trace signal amplitude is wrong, it is and peak white of the lower important to verify that the prob- trace coincide. Figure 8 shows a lem is really a simple gain error properly adjusted signal. Since rather than a distortion. This can the waveform monitor vertical be accomplished by checking the gain need not be calibrated in ratio of sync to the picture signal this mode, the gain can be (the part of the signal above increased for greater resolution. blanking), which should be 3:7. The 1781R has a variable ampli- If the ratio is correct, proceed tude calibrator so the WFM + with the gain adjustment. If the CAL mode can be used to mea- ratio is incorrect, there is a prob- sure signal amplitudes other lem and further investigation is than 1 volt. Measurements are needed. The signal could be made by adjusting the calibrator suffering from distortion, or amplitude (with the large front equipment that re-inserts sync Figure 8. The WFM + CAL mode in the 1781R indicates that insertion gain panel knob) until the bottom of and burst may be malfunctioning. is properly adjusted. the upper trace and the top of 2. Sync & Burst Measurements. the lower trace coincide. At this Sync and burst should each be point the calibrator amplitude 30% of the composite video equals the signal amplitude and amplitude (300 millivolts for a can be read from the screen. The 1-volt signal). Most of the meth- example in Figure 9 shows the ods discussed in this section can WFM + CAL mode being used to be used to measure sync and measure sync amplitude. burst amplitudes. When using the 1781R voltage cursors, the 1781R Voltage Cursors. Some TRACK mode is a convenient waveform monitors, such as the tool for comparing sync and 1781R, are equipped with on- burst amplitudes. In this mode, sc r een voltage cursors for making the separation between the two accurate amplitude measure- cursors remains fixed and they ments. Peak-to-peak amplitude can be moved together with can be measured by positioning respect to the waveform. one cursor on sync tip and the other on peak white (see Figure 3. Measurement Accuracy. In gen- Fi g u r e 9. The WFM + CAL mode can also be used to measure sync amplitude. 10). The 1781R vertical gain eral, the added calibrator and control affects the cursors and voltage cursor methods are more the waveform in the same accurate than the graticule tech- manner so vertical gain can be nique. However, some cursor increased to allow for more ac c u - implementations have far more rate positioning of the cursors. resolution than accuracy, crea t i n g When setting insertion gain, it an impression of measurements may be convenient to first set more precise than they really the cursor separation for 1000 are. Familiarity with the specifi- mV. The video signal amplitude cations of the waveform monitor should than be adjusted to and an understanding of the match the cursor amplitude. accuracy and resolution avail- able in the various modes will VM700T Cursors. Manual ampli- help make an appropriate choice. tude measurements can be made with the VM700T by selecting 4. Using the Luminance Filter. CURSORS in the WAVEFORM When setting insertion gain with mode. The horizontal baseline in a live signal rather than a test Figure 10. 1781R voltage cursors positioned to measure peak-to-peak the middle of the screen is used signal, it may be useful to enable amplitude. as a reference. To measure peak- the luminance or lowpass filter to-peak amplitude, first position on the waveform monitor. This sync tip on the baseline. Touch filter removes the chrominance the RESET DIFFS selection on information so that peak white the screen to reset the voltage luminance levels can be used for difference to zero. Now move setting gain. the waveform down until the white bar is on the baseline and read the voltage difference from the screen.

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DEFINITION Horizontal and vertical synchro- nization pulse widths are mea- sured in order to verify that they fall within specified limits. Other synchronization parame- ters such as rise and fall times and the position and number of cycles in burst are also specified and should occasionally be mea- sured to verify compliance. CCIR Report 624 is a widely accepted standard for PAL timing values and tolerances. The CCIR horizontal timing information for PAL systems is reproduced in Figure 11.

PICTURE EFFECTS Small errors in pulse widths Figure 11. CCIR horizontal pulse width requirements. will not affect picture quality. However, if the errors become so large that the pulses cannot be properly processed (by equip- ment), picture breakup may occur.

TEST SIGNAL Timing measurements can be made on any composite signal that contains horizontal, vertical and subcarrier (burst) synchro- nization information.

MEASUREMENT METHODS Waveform Monitor Graticule. Time intervals can be measured by comparing the waveform to the marks along the horizontal baseline of a waveform monitor graticule. In order to get adequate resolution, it is usually neces - sary to magnify the waveform display horizontally. Select the setting that provides as much magnification as possible while still keeping the interval of inter- est entirely on-screen. The scale factor, typically microseconds per major division, changes with horizontal magnification. The 1781R displays the microsec- onds per division setting on the screen. For the 1481, time per division is obtained from the switch setting.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Most PAL pulse width measure- To make a pulse width measure- ments are specified between the ment, position the cursors on the 50% points of the rising and 50% points of the transitions falling edges. Such measure- and read the cursor separation ments can usually be made with directly from the screen. An the vertical gain in the calibrated example of a horizontal sync position. To measure horizontal width measurement is shown in sync width, for example, posi- Figure 13. If necessary, use the tion the waveform so that the vertical graticule scale to help sync pulse is centered around locate the 50% points. the graticule baseline (blanking Alternatively, the voltage cursors level at 150 mV above the base- in the RELATIVE mode can be line and sync tip at 150 mV used to locate the 50% points. below the baseline). The time scale is now at the 50% level VM700T Cursors. The cursors in and the pulse width can be read the VM700T WAVEFORM mode directly fr om the graticule (see can be used to make pulse width Figure 12. Horizontal sync width measurement at the 50% amplitude points. Fi g u r e 12). measurements. After establishing the 100% and 0% points of 1781R Time Cursors. Some wave- sync, the cursors can be moved form monitors and oscilloscopes to the 50% point to obtain a are equipped with cursors to time measurement (see Figure facilitate the measurement of 14). Consult the manual for time intervals. The time cursors detailed instructions on how to in the 1781R appear as bright use the cursors. dots on the waveform, an imple- mentation that allows for very accurate positioning on wave- form transitions.

Figure 13. The 1781R time cursors positioned to measure horizontal sync width at the 50% amplitude points.

Figure 14. The VM700T cursors can be used to make horizontal sync width measurements.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com VM700T Automatic Measurement. 6. Checking the Vertical Interval. The H TIMING selection in the The number of pulses in the VM700T MEASURE menu dis- vertical interval, as well as the plays all horizontal blanking widths of the equalizing pulses interval timing measurements and vertical serrations, are also (see Figure 15). The AUTO mode specified. CCIR nominal values also provides measurements of and tolerances are shown in the individual parameters. Figure 17. It is good practice to occasionally NOTES verify that all of these parame- 5. Rise and Fall Time ters are correct. The V Blank Measurements. Many standards selection in the VM700T include specifications for the MEASURE mode provides a rise and fall time of the sync convenient means of checking pulse (also referred to as build- the format of the vertical interval up time). These measurements and the timing of the individual Figure 15. The VM700T H Timing display. are indicators of how fast the pulses (see Figure 16). transitions occur and are typically made between the 10% and 90% points of the signal. The methods used for measuring pulse widths can generally be applied to rise and fall times. However, for 10%-to-90% mea- surements, it is generally most convenient to use the waveform monitor variable gain control to normalize the pulse height to 500 or 1000 mV. The 10% and 90% points can then easily be located with the graticule. In the 1781R RELATIVE mode, the Figure 16 The VM700T Vertical Blanking display. voltage cursors can be used to locate the appropriate levels.

Figure 17. CCIR vertical interval specifications.

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DEFINITION PICTURE EFFECTS of Signal B must occur at the SCH (SubCarrier to Horizontal) SCH phase becomes important same time). When this condition Phase refers to the timing rela- only when television signals is achieved, the two signals are tionship between the 50% point from two or more sources are said to be ""colour framed''. It is of the leading edge of sync and combined or sequentially important to remember that the zero crossings of the refer- switched. In order to ensure that colour framing is inextricably ence subcarrier. Errors are horizontal jumps do not occur tied to other system timing para- ex p r essed in degrees of subcarri e r when a switch is made, the sync meters and is by no means an phase. The official EBU defini- edges of the two signals must be independent variable. Only if tion, taken from EBU Technical accurately timed and the phase two signals have the same SCH Statement D 23-1984 (E), is as of colour burst matched. Since phase relationship and are prop- follows: “The subcarrier-to-line both sync and subcarrier are erly colour framed can the sync sync (Sc-H) phase is defined as continuous signals with a fixed timing and burst phase matching requirements be achieved. the phase of the +E’u component relationship to one another, it is of the colour burst extrapolated possible to simultaneously Since signals must have the to the half-amplitude point of achieve both timing conditions same SCH phase relationship in the leading edge of the synchro- only if the two signals have the order to be cleanly combined, nizing pulse of line 1 of field 1.” same SCH phase relationship. standardization on one value of Since there is no burst on line 1, Because of the complex relation- SCH phase will clearly facilitate measurement of SCH phase on ship between the sync and sub- the transfer of programme mater- line 7 of field 1 has become the carrier frequencies, the exact ial. This is one reason for trying generally accepted convention. SCH phase relationship for a to maintain 0 degrees of SCH Target tolerances of ± 20 degrees given line repeats itself only phase error. Another motivation have been established although, once every eight fields (see Note for keeping SCH phase within in practice, much tighter toler- 7). In order to achieve the sync reasonable limits is that various ances are generally maintained. and burst timing conditions pieces of equipment need to be Modern facilities often try to required for a clean switch able to distinguish between the ensure that SCH phase errors do between two signals, the eight- colour frames in order to process not exceed a few degrees. field sequence of the signals the signal properly. This cannot must be properly lined up (i.e. be done accurately if the SCH Field 1 of Signal A and Field 1 phase is allowed to approach 90 degrees.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com TEST SIGNALS The 1781R must be internally SCH phase measurements can be referenced to measure the SCH made on any signal with both phase of a single signal. Sync sync and colour burst present. and burst of the selected signal are compared to each other in MEASUREMENT METHODS this mode. When external refer- Polar Display. Some instruments, ence is selected, both burst and such as the 1781R, are equipped sync of the selected signal are with a polar SCH display that displayed relative to burst of the consists of the two burst vectors external reference signal. This and a dot representing the phase display allows determination of of sync. The dot is in the centre whether or not two signals are of a "window'' in the large circle colour framed. Assuming that that appears as part of the dis- both the reference signal and the play (see Figure 18). This circle selected signal have no SCH is a result of the 25 Hertz offset phase error, the sync dot will be Figure 18. The 1781R polar SCH phase display showing a 10 degree error. (see Note 7) which changes the between the burst vectors if the SCH phase from line to line. signals are colour framed and The circle itself contains no 180 degrees away when they relevant information. are not.

The SCH phase is 0 degrees VM700T Automatic Measurement. when the dot is at an angle mid- Select SCH PHASE in the way between the two bursts. If VM700T MEASURE menu to there is an SCH phase error, its obtain a polar display of SCH magnitude can be determined by phase (see Figure 19). The vector measuring the angle between the in this display directly repre- sync dot and the midway point sents SCH phase error (there are of the two bursts. The graticule not separate vector representa- can be used for this purpose tions of sync and burst). The when the bursts are properly dual SCH display provides a positioned on their +135 and simultaneous view of the SCH -135 degree points. The preci- vectors for two signals. The full sion phase shifter in the 1781R field SCH display provides a can also be used to quantify field-rate display that plots the Figure 19. The VM700T SCH Phase Measurement display. the error. SCH phase of each line in

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com the field. the equation that there are an this is why SCH phase is defined odd number of subcarrier quar- on a given line in PAL. It is NOTES ter-cycles in a line. This implies im p o r tant to rem e m b e r , however, 7. The PAL Eight-Field Sequence. that SCH phase changes by 90 that the existence of the eight- The eight-field sequence exists degrees every line. field sequence is determined in PAL because of the relation- Since there are also an odd num- only by the sync-to-subcarrier ships between the line, field ber of lines in a frame, the exact relationship and is independent and subcarrier frequencies. phase relationship between sync of the 25 Hertz offset, the Bruch Remember that subcarrier and and burst for a given line repeats blanking sequence, and the H sync can be thought of as two only once every eight fields alternate-line V-axis inversion. continuous signals with a fixed (four frames). relationship to one another. 8. For More Information. For a This relationship is defined Due to the 25 Hz offset, which is comprehensive discussion of mathematically as: added to interleave chrominance SCH phase and colour framing dot patterns in the picture, the Fsc = (1135/4 X Fh) + 25 issues, see Tektronix Application line-to-line change in SCH phase Note (20W-5614-1), “Measuring which yields a subcarrier fre- is actually somewhat more than and Monitoring SCH Phase with quency (Fsc) of 4,433,618.75 Hz 90 degrees. Keep this in mind the 1751A Waveform/Vector for a line frequency (Fh) of when making measurements, as Monitor”. 15,625 Hz. It can be seen from

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com II. LINEAR DISTORTIONS

sponding to a familiar television ments look at amplitude versus Waveform distortions that are time interval, have been identi- frequency characteristics while independent of signal amplitude fied. (The range of time intervals group delay measurements ar e ref e r red to as linear distorti o n s . for each category may vary examine phase versus frequency These distortions occur as a somewhat from definition to characteristics. Unlike the mea- result of a system's inability to definition.) These categories are: surements classified by time uniformly transfer amplitude SHORT TIME (100 nanoseconds interval, frequency response and and phase characteristics at to 1 microsecond) group delay measurements per- all frequencies. mit separation of amplitude dis- LINE TIME (1 microsecond to tortions from delay distortions. When fast signal components 64 microseconds) such as transitions and high-fre- In addition to these measure- FIELD TIME (64 microseconds ments, there is one specific case quency chrominance are affected to 20 milliseconds) differently than slower line-rate that needs to be examined in or field-rate information, linear LONG TIME (greater than 20 detail. The phase and amplitude distortions are probably present. milliseconds) relationships between the chrominance and luminance These distortions are most com- This classification is convenient information in a signal are criti- monly caused by imperfect because it allows easy correla- cal. Chrominance-to-luminance transfer characteristics of the tion of the distortions with what gain and delay are therefore equipment in the signal path. is seen in the picture or in a measured in order to quantify a However, linear distortions can waveform display. A single system's ability to process also be externally introduced. measurement for each category chrominance and luminance in Signals such as power line hum takes into account both ampli- correct proportion and without can couple into the video signal tude and phase distortions relative time delays. and manifest themselves within that time range. as distortions. Sine-squared pulses and rise While the combination of these times are used extensively in the One method of classifying linear four categories covers the entire measurement of linear waveform distortions involves grouping video spectrum, it is also useful distortions. It may be helpful to them according to the duration to have methods of simultane- review the information in of the signal components that ously evaluating response at Appendix B which discusses the are affected by the distortion. all frequencies of interest. use of sine-squared pulses in Four categories, each corre- Frequency response measure-

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Chrominance-to-Luminance Gain and Delay television testing. combination ITS signals include such a pulse. DEFINITION The frequency spectrum of a Chrominance-to-luminance gain composite pulse includes energy inequality (relative chrominance at low frequencies and energy level) is a change in the gain centered on the subcarrier fre- ratio of the chrominance and quency. Selection of an appro- luminance components of a priate pulse width is a trade-off video signal. The change is between occupying the PAL expressed in percent or dB with chrominance bandwidth as fully the number negative for low as possible and obtaining a pulse Figure 20. A combination signal that includes a 20T modulated pulse chrominance and positive for with sufficient sensitivity to (CCIR Line 17). high chrominance. delay errors. The 10T pulse is Chrominance-to-luminance more sensitive to delay errors delay inequality (relative than the 20T pulse, but does not chrominance time) is a change occupy as much of the chromi- in the time relationship between nance bandwidth. CCIR specifi- the chrominance and luminance cations generally recommend the components of a video signal. use of 20T pulses while 10T The change is expressed in units pulses are commonly used in of time, typically nanoseconds. the U.K. The number is positive for A modulated bar is also some- delayed chrominance and nega- times used to measure chromi- tive for advanced chrominance. nance-to-luminance gain inequalities. PICTURE EFFECTS Gain errors most commonly MEASUREMENT METHODS Figure 21. The chrominance and luminance components of a modulated appear as attenuation or peaking Conventional chrominance-to- sine-squared pulse. of the chrominance information. luminance gain and delay mea- This shows up in the picture as surements are based on analysis incorrect colour saturation. of the baseline of a modulated Delay distortion will cause sine-squared pulse. (See colour smearing or bleeding, Appendix B for a definition of particularly at the edges of the time interval T.) This pulse objects in the picture. It may is made up of a sine-squared also cause poor reproduction of luminance pulse and a chromi- sharp luminance transitions. nance packet with a sine-sq u a re d envelope (see Figure 21). TEST SIGNALS Chrominance-to-luminance gain and delay inequalities are mea- sured with a 10T or 20T modu- lated sine-squared pulse. Many

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Modulated sine-squared pulses ways. A single peak in the base- converts the baseline measure- offer several advantages. First of line indicates the presence of ments into gain and delay numbers. all, they allow evaluation of both gain errors only. Symmetrical To make a measurement, first gain and delay differences with positive and negative peaks indi- normalize the pulse height to a single signal. A further advan- cate the presence of delay errors 100% (500 mV or 1000 mV is tage is that modulated sine- only. When both types of errors generally most convenient). The squared pulses eliminate the are present, the positive and ne g - baseline distortion can be mea- need to separately establish a ative peaks will have diffe re n t sured either by comparing the low-frequency amplitude refer- amplitudes and the zero crossing waveform to a graticule or by ence with a white bar. Since a will not be at the centre of the using voltage cursors. Using a low-frequency reference pulse pulse. Figure 22 shows the effe c t s nomograph (see Figure 23), find is present along with the high- of various types of distortion. the locations on the horizontal frequency information, the and vertical axes which corre- Waveform Monitor and Nomograph. amplitude of the pulse itself spond to the two measured can be normalized. One method of quantifying distortion peaks. At the point chrominance-to-luminance The baseline of the modulated where perpendicular lines inequalities involves measuring pulse is flat when chrominance- drawn from these two locations the peaks of the modulated to-luminance gain and delay dis- intersect, the gain and delay pulse baseline distortion and tortion is absent. Various types numbers may be read from applying these numbers to a of gain and delay distortion the nomograph. nomograph. The nomograph affect the baseline in different

Figure 22. Effects of gain and delay inequalities on the modulated sine- squared pulse.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Figure 23. Chrominance-to-luminance gain and delay nomograph for a 20T pulse.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com When making measurements in This measurement is made by this manner, it is important to normalizing the white bar ampli- know whether the signal is a tude to 100% and then measur- 10T or a 20T pulse. The same ing the amplitude difference nomograph can be used for both between the modulated pulse but a correction factor must be top and the white bar. This dif- applied. The nomograph in ference number, times two, is Figure 23 is for a 20T pulse and the amount of chrominance-to- the result must be divided by luminance gain distortion in two when using a 10T pulse. percent. Note that when the pulse top is higher or lower than 1781R Semi-Automatic Procedure. the bar, the bottom of the pulse The CHROMA/ selection is displaced from the baseline by in the 1781R MEASURE menu the same amount. Thus the eliminates the need for a nomo- peak-to-peak difference between graph. The on-screen readout the modulated pulse and the bar guides the user through cursor Figure 24. Results obtained with the CHROMA/LUMA selection in the 1781R is actually twice the difference MEASURE mode. measurements of the various between their peak values, hence parameters required to obtain a the factor of two. number from a nomograph. The lines at the centre of the After all parameters have been baseline on the 1781R and 1481 entered, the instrument calcu- external graticules can be used lates the results (see Figure 24). to estimate chrominance-to- The accuracy and resolution of luminance delay errors. This this method are roughly equiva- method yields valid results only lent to using the graticule and if gain errors are negligible (the a nomograph. baseline distortion should Waveform Monitor Graticule appear symmetrical). To use Approximations. When a system is these graticule marks, first use free of significant nonlinearity the variable gain to normalize and delay distortion is within the modulated pulse height to certain limits, chrominance-to- 700 mV. Then centre the pulse luminance gain inequalities can on the two graticule lines which be measured directly by compar- cross in the centre of the base- ing the height of the modulated line (see Figure 25). The gratic- Figure 25. The 1781R graticule indicates that this signal has approximately ule lines indicate 200 nanosec- 200 nanoseconds of chrominance-to-luminance delay. pulse to the white bar. This method and the nomograph will onds of delay for a 20T pulse yield identical results when and 100 nanoseconds for a 10T there is no delay distortion. It is pulse. With X5 vertical gain generally considered a valid selected (in addition to the vari- approximation for signals with able gain required to normalize delay distortion in the 100 to the pulse), the lines indicate 40 200 nanosecond range and is nanoseconds of delay for the accurate to within a few percent 20T pulse and 20 nanoseconds for signals with several hundred for the 10T pulse. nanoseconds of delay.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com VM700T Automatic Measurement. NOTES Chrominance-to-luminance gain 9. Harmonic Distortion. If harmonic and delay errors can be mea- distortion is present, there may sured by selecting CHROM/LUM be multiple aberrations in the GAIN DELAY in the VM700T baseline rather than one or two MEASURE mode. Numeric clearly distinguishable peaks. In results are given in this mode this case, nomograph measure- and both parameters are simulta- ment techniques are indetermi- neously plotted on the graph nate. The VM700T, however, is (see Figure 26). Delay is plotted capable of removing the effects on the X axis and gain inequality of harmonic distortion and will on the Y axis. These measure- yield valid results. Minor dis- ments are also available in the crepancies between the results of VM700T AUTO mode. the two methods may be attrib- utable to the presence of small Calibrated Delay Fixture. Another amounts of harmonic distortion Figure 26. The Chrom/Lum Gain Delay display in the VM700T method of measuring these dis- as well as to the higher inherent MEASURE mode. tortions involves use of a cali- resolution of the VM700T method. brated delay fixture. The fixture allows incremental adjustment of the delay until there is only one peak in the baseline indicat- ing all delay errors have been nulled out. The delay value can then be read from the fixture and gain measured from the gratic- ule. This method can be highly accurate but requires the use of specialized equipment.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Short Time Distortion

DEFINITION It is very important a T rise time Short time distortions cause bar be used with the short time amplitude changes, ringing, distortion graticule. Many com- overshoot, and undershoot in mon test signals have 2T rather fast rise times and 2T pulses. than T rise times and are not The affected signal components suitable for this measurement. It range in duration from 0.100 should also be noted that T rise microsecond to 1.0 microsecond. time signals will suffer signifi- cant distortion when passed For PAL systems, distortions in through a TV transmitter as they the short time domain are most contain spectral components often characterized by measuring that will be removed by the K2T or Kpulse/bar. These measure- transmitter 5 or 6 MHz lowpass ments are described in the filter. Short time distortion mea- K Factor Ratings section of this surements made on transmitted booklet. Alternatively, the aber- signals will therefore evaluate rations in a T rise time bar can only those components in be described in terms of the approximately the 200 nanosec- "percent SD'' method described ond to 1 microsecond range. in this section. MEASUREMENT METHODS PICTURE EFFECTS Measurements of the under- Figure 27. A T rise time bar has a 10% to 90% rise time of nominally Short time distortions produce 100 nanoseconds. shoot, overshoot, and ringing at fuzzy vertical edges. Ringing can the edge of a T rise bar are not sometimes be interpreted as generally quoted directly as a chrominance information (cross percent of the transition ampli- colour) causing colour artifacts tude, but rather in terms of an near vertical edges. amplitude weighting system that yields results in ""percent SD''. TEST SIGNALS This weighting is necessary Short time distortion can be because the amount of distortion measured with any signal that depends not only on the distor- has a T rise time white bar. A T tion amplitude but also on the rise time bar has a 10%-to-90% time the distortion occurs rise time of nominally 100 with respect to the transition. nanoseconds (see Figure 27). Although results can be calculated See Appendix B for a discussion from the time and amplitude of of the time interval T. the measured ringing lobes, special graticules, conversion tables, or nomographs are used in practice.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Waveform Monitor Graticule. 11. Pulse-to-Bar Ratios. The Graticules for measurement of amplitude ratio between a 2T short time distortion are not pulse and a line bar is some- included in the 1781R. However, times used as an indication of some organizations use custom short time distortion. To make a graticules that indicate, for pulse-to-bar measurement with a example, 2% and 5% SD limits. waveform monitor, first normal- The measurement procedure ize the bar amplitude to 100%. involves normalizing the gain Now measure the pulse ampli- and positioning the rising or tude, in percent, to obtain pulse- falling edge of the bar in the to-bar ratio reading. The 1781R's graticule. The largest graticule voltage cursors can be used in limit touched by the waveform the RELATIVE mode to make indicates the amount of measurements of this type. distortion. Other values can A pulse-to-bar measurement can be interpolated. be obtained from the VM700T Figure 28. The VM700T Short Time Distortion display. by selecting K FACTOR in the VM700 Automatic Measurement. MEASURE mode. Both pulse-to- Select SHORT TIME DISTOR- bar ratio and K results TION in the VM700T MEASURE pulse/bar (see Note 17) are provided in mode to obtain a SD result and a this mode. tracking graticule (CCIR 421). The user can also define custom graticules in this mode. NOTES 10. Nonlinearities. If the device or system under measurement is free of nonlinear distortion, the rising and falling transitions will exhibit symmetrical distortion. In the presence of nonlinearities, however, the transitions may be affected differently. It is prudent to measure, or at least inspect, both the positive and negative transitions.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Line Time Distortion

DEFINITION departure of the bar top from the Line time distortion causes tilt level at the centre of the line bar in line-rate signal components is most often quoted as the such as white bars. The affected amount of distortion. In some signal components range in cases the peak-to-peak level vari- duration from 1.0 microsecond ation is given, particularly when to 64 microseconds. The amount a 10 microsecond bar is used. of distortion is expressed as a The measurement methods in percentage of the line bar ampli- this section are described in tude at the centre of the bar. terms of peak results but can readily be adapted for peak-to- Figure 29. Pulse and bar signal. Distortions in the line time peak measurements. domain can also be quantified In either case, the tilt is by measuring Kbar as discussed in the K FACTOR Ratings expressed as a percentage of the section of this booklet. level at the centre of the bar. The first and last microsecond of the PICTURE EFFECTS bar should be ignored as errors near the transition are in the In large picture detail, this dis- short time domain. tortion produces brightness vari- ations between the left and right Waveform Monitor Graticule. The sides of the screen. Horizontal graticule on a waveform monitor streaking and smearing may also can be used to quantify this dis- be apparent. tortion. Measure the maximum TEST SIGNAL deviation from the centre of the bar and express that number as a Line time distortion is measured percentage of the level at bar with a signal that includes a 10 centre. It is generally most con- microsecond or 25 microsecond venient to use the variable gain white bar. Rise time of the bar is to normalize the centre of the not critical for this measurem e n t . bar to 500 or 1000 mV. Deviations in the top of the bar MEASUREMENT METHODS can then be read directly from Line time distortion is quantified the graticule in percent. by measuring the amount of tilt Remember to ignore the first and in the top of the line bar. For last microsecond. PAL systems, the maximum

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com 1781R Voltage Cursors. Waveform The 1781R time cursors are con- monitor voltage cursors in the venient for locating the appro- RELATIVE mode can be used to priate time interval in the centre measure line time distortion. of the bar. Set the time separa- Define the amplitude difference tion to the bar time (usually 10 between and the or 25 microseconds) minus 2 bar centre as 100%. Leave one micro-seconds. Put the time cursor at the bar centre and cursors in the TRACK mode, and move the other cursor to mea- move the two cursors together sure the peak positive and peak until they are centered on the negative deviations in the top of bar (see Figure 30). the bar. The largest of these numbers (ignore the sign) is the VM700T Automatic Measurement. amount of line time distortion. Select BAR LINE TIME in the VM700T MEASURE menu to obtain a line time distortion result (see Figure 31). Line time Figure 30. The 1781R voltage and time cursors can facilitate line time distortion can also be measured distortion measurements. in the AUTO mode.

Figure 31. The VM700T Bar Line Time display.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Field Time Distortion

DEFINITION MEASUREMENT METHODS Field time distortion causes Field time distortions are quanti- field-rate tilt in video signals. fied by measuring the amount of The affected signal components tilt in the top of the field bar (the range in duration from 64 700 mV part of the field square microseconds to 20 millisec- wave signal). The maximum onds. The amount of distortion departure of the field bar top is generally expressed as a per- from the level at the centre of centage of the amplitude at the the field bar is generally quoted centre of the line bar. as the amount of distortion although peak-to-peak results are K50 Hz measurements, which are discussed in the K FACTOR sec- sometimes given. The measure- tion of this booklet, provide ment methods in this section are another method of describing described in terms of peak field time distortions. results, but can readily be adapt- Figure 32. The field square wave test signal. ed for peak-to-peak measure- PICTURE EFFECTS ments. The centre of the line bar is usually used as the reference Field time linear distortion will amplitude and the first and last cause top-to-bottom brightness 250 microseconds (about 4 lines) inaccuracies in large picture of the field bar should be details. ignored. Distortions in that TEST SIGNALS region are not in the field time domain. Field time distortion is measured with a field square wave. In this signal, each line in one half of the field is a 0-volt pedestal, while each line in the other half is a 700-millivolt pedestal. The signal usually includes normal horizontal and vertical synchro- nization information.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Waveform Monitor Graticule. The define the centre of the line bar first step in making a field time (relative to blanking) as 100%. distortion measurement is to Remember to select the FAST normalize the gain. With the DC restorer setting. Then select a waveform monitor in a line-rate field-rate sweep and set the DC sweep mode, use the variable restorer to SLOW or OFF. Place gain control to set the centre of one cursor so that it intersects the line bar to 100% (1000 mV the top of the field bar in the or 500 mV). This can be done middle. Use the other cursor to most accurately with the wave- measure the peak positive and form monitor FAST DC restorer peak negative level deviation in on. The DC restorer will remove the top of the bar ignoring the the effects of field time distor- first and last 4 lines. The larger tion from the waveform monitor of the two numbers is the display and reduce the vertical amount of field time distortion blurring seen in the line rate in percent. display. Now select a field-rate Figure 33. A 2-field waveform monitor display showing field time distortion. sweep and either the SLOW or VM700T Automatic Measurement. OFF setting for the DC restorer. Select TWO FIELD in the Measure the peak positive and VM700T MEASURE mode to peak negative level change from obtain a field time distortion the centre of the field bar result (see Figure 35). Field time excluding the first and last 4 distortion can also be measured lines. The larger of these two in the AUTO mode. numbers, expressed as a percent- age of the line bar amplitude, is NOTES 12. Externally Introduced the amount of field time distor- Distortions. tion (see Figure 33). Externally introduced distortions such as mains hum 1781R Voltage Cursors. The 1781R are also considered field rate voltage cursors can be used in distortions. Be sure to turn the the RELATIVE mode to measure DC restorer OFF or select the field time distortion. Select a SLOW clamp speed when one-line or two-line sweep and measuring hum.

Figure 34. The 1781R voltage cursors can be used to measure field time distortion.

Figure 35. The VM700T Two Field display.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Long Time Distortion

DEFINITION MEASUREMENT METHODS Long time distortion is the low Long time distortions are mea- frequency transient resulting sured by examining the damped from a change in APL. This dis- low-frequency oscillation result- tortion usually appears as a very ing from a change in APL. low frequency damped oscilla- tion (see Figure 37). The affected Waveform Monitor. It is usually signal components range in necessary to use a storage oscil- duration from 20 milliseconds to loscope or a waveform monitor tens of seconds. in the SLOW SWEEP mode to measure long time distortion. A The peak overshoot that occurs waveform photograph can be as a result of an APL change, helpful in quantifying the distor- Figure 36. A flat field bounce signal. expressed as a percentage of the tion. Once a stable display is nominal luminance amplitude, obtained (or a photograph is generally quoted as the taken), measure overshoot and amount of distortion. Settling settling time (see Figure 37). time and occasionally the slope (in percent per second) at the VM700T Automatic Measurement. beginning of the phenomenon Select Bounce in the VM700T are also given. MEASURE mode to obtain a dis- play of long time distortion (see PICTURE EFFECTS Figure 38). Peak deviation and Long time distortions are slow settling time are given at the enough that they are often per- bottom of the screen. ceived as flicker in the picture.

TEST SIGNALS Figure 37. Long time distortion measurement parameters. Long time distortion is measured with a flat field test signal with variable APL. The signal should be "bounced'', or switched between 10% and 90% APL, at intervals no shorter than five times the settling time (see Figure 37).

Figure 38. The VM700T Bounce display.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Frequency Response

DEFINITION TEST SIGNALS Frequency response measure- Frequency response can be mea- ments evaluate a system's ability sured with a number of different to uniformly transfer signal com- test signals. Since there are ponents of different frequencies significant differences between without affecting their ampli- these signals, each one is discussed tudes. This parameter, also in some detail in this section. known as gain/frequency distor- Some test signals are available tion or amplitude versus fre- either as full-amplitude or quency response, evaluates the reduced-amplitude signals. It is system's amplitude response generally good practice to make Figure 39. A multiburst test signal. over the entire video spectrum. measurements with both as the The amplitude variation may be presence of amplitude nonlin- expressed in dB or percent. The earities in the system will have reference amplitude (0 dB, greater effect on measurements 100%) is typically the white made with full amplitude signals. bar or some low frequency. Frequency response numbers are Multiburst. The multiburst signal only meaningful if they contain typically includes six packets of three pieces of information: discrete frequencies that fall the measured amplitude, the within the TV passband. The frequency at which the measure- packet frequencies usually range ment was made, and the from 0.5 MHz to 5.8 MHz with Figure 40. The multipulse test signal. reference frequency. frequency increasing toward the right side of each line (see PICTURE EFFECTS Figure 39). This signal is useful Fr equency response problems can for a quick approximation of sys- cause a wide variety of aberra t i o n s tem frequency response and can in the picture, including all of be used on an in-service basis as the effects discussed in the sec- a vertical interval test signal. tions on short time, line time, field Multipulse. The multipulse signal time, and long time distortions. is made up of modulated 20T and 10T sine-squared pulses with high-frequency components at various frequencies of interest, generally from 0.5 MHz to 5.8 MHz (see Figure 40). This signal can also be inserted in the vertical interval.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Modulated sine-squared pulses, be used on an out-of-service basis. which are also used to measure chrominance-to-luminance gain (Sin x)/x. The (sin x)/x is a signal and delay errors, are discussed which has equal energy present in the Chrom i n a n c e - t o - L u m i n a n c e at all harmonics of the horizontal Gain and Delay section of this scan frequency up to its cutoff book. Although different high- frequency (see Figures 42 and frequency components are used 47). The (sin x)/x signal is pri- in the multipulse, the same prin- marily designed for use with a ciples apply. Bowing of the base- or an auto- line indicates an amplitude error matic measurement set such as between the low-frequency and the VM700T. Very little informa- high-frequency components of tion is discernible in a time that pulse. Unlike the multi- domain display. burst, the multipulse allows evaluation of group delay errors MEASUREMENT METHODS as well as amplitude errors. Since each signal requires a Figure 41. A 6 MHz field-rate sweep signal with markers (2-field display). different measurement method, Sweep Signal. It is sometimes separate discussions for the vari- recommended that line-rate or ous test signals are presented in field-rate sweep signals be used this section. The first three sig- for measuring frequency nals (multiburst, multipulse, and response. In a sweep signal, the sweep) can all be measured with frequency of the sine wave is a waveform monitor using either continuously increased over the graticule or the voltage the interval of a line or field. cursors to quantify the distor- An example of a sweep signal tion. Measurement results are is shown in Figure 41. The expressed in dB. markers indicate 1 MHz Figure 42. A time domain display of the (sin x)/x signal. frequency intervals. Waveform Monitor — Multiburst. A sweep signal allows examina- Frequency response measure- tion of frequency response con- ments are made with the multi- tinuously over the interval of burst signal by measuring the interest rather than only at the peak-to-peak amplitudes of the discrete frequencies of the multi - packets. The low-frequency burst and multipulse signals. square wave at the beginning of This can be important for the line should be used as the detailed characterization of a amplitude reference. system, but does not offer any significant advantages in routine testing. While the other signals discussed here can be used in the vertical interval and there- fore permit in-service testing, field-rate sweep signals can only

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Figures 43 and 44 show the When only gain distortion is 1781R voltage cursors being present, there will be a single used to measure a frequency peak in the pulse baseline. A response distortion of 3.59 dB at value of zero is therefore applied 5.8 MHz. The error in dB is to one axis of the nomograph. If calculated as follows: group delay distortion is also present, the baseline distortion 20 log10 (274/414) = -3.59 dB will be sinusoidal rather than a In the RELATIVE mode, the single peak. In this case, mea- 1781R's voltage cursors will sure both lobes and apply the provide results directly in dB. two numbers to the nomograph. This will yield correct frequency Waveform Monitor — Multipulse. response results as well as a Frequency response distortion group delay measurement. shows up in the multipulse sig- nal as bowing of the pulse base- The CHROMA/LUMA selection line (see Figure 45). Distortions in the 1781R MEASURE menu are quantified by measuring the can be used to make frequency Figure 43. The low-frequency square wave is defined as the reference. amount of baseline displacement response measurements with the in the pulse of interest. It is multipulse signal. Repeat the often easy to see which pulse cursor measurement procedure exhibits the largest gain inequal- for the pulse corresponding to ity so an overall result can be each frequency of interest. obtained by measuring that If the system is relatively free of pulse only. nonlinearity, it is also possible This measurement can be made to estimate the amplitude error by using a waveform monitor without using a nomograph. graticule to measure the baseline Normalize the white bar to distortion and then transferring 100% and then measure either the numbers for each pulse to a the amount of baseline bowing nomograph. The nomograph for or the displacement of the pulse chrominance-to-luminance gain top from the white bar (the two and delay measurements (see numbers will be equal in a linear Figure 23) can also be used for system). The amplitude error, in multipulse measurements. Be percent, is approximately equal sure to normalize each pulse to two times either value. This Fi g u r e 44. The peak-to-peak amplitude of the smallest packet is then measured . height to 100% before making a method yields valid results even measurement. Remember that in the presence of some delay the nomograph is intended for a error which is indicated by 20T pulse measurements. When asymmetrical baseline distortion. using a 10T pulse, the nomo- When delay error exceeds 150 graph delay number must be nanoseconds, this method is divided by two. not recommended.

Figure 45. The multipulse signal exhibiting frequency response distortion.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Waveform Monitor — Sweep Signal. Spectrum Analyzer — (Sin x)/x. Amplitude variations can be Frequency response testing with measured directly from a time- the (sin x)/x signal is done with domain display when a sweep a spectrum analyzer. Attenuation signal is used. Be sure to select a or peaking of the flat portion of field-rate display on the wave- the spectral display can be read form monitor when using a field directly from the analyzer sweep. Establish a reference at display in dB (see Figure 47). some low frequency and mea- In a time domain display, high sure the peak-to-peak amplitude frequency roll off will reduce the at other frequencies of interest pulse amplitude and the ampli- (see Figure 46). tude of the pulse lobes. It is dif- ficult, however, to quantify the error. The presence of amplitude nonlinearity in the system will cause asymmetrical distortion of Figure 46. A sweep signal showing frequency response distortion. the positive and negative pulses.

Figure 47. A spectrum analyzer display of a (sin x)/x signal with a cutoff frequency of 6 MHz.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com VM700T Automatic Measurement 14. More Information. Further The VM700T provides ampli- information on frequency tude versus frequency response response testing is available in information for either the multi- Tektronix application note burst or (sin x)/x signal. Select (25W-11149-0), “Frequency MULTIBURST or GROUP Response Testing Using a (Sin x)/x DELAY (SIN X)/X in the Test Signal and the VM700A/T MEASURE mode. Multiburst Video Measurement Set”. measurements are also available in the AUTO mode.

Notes 13. Multipulse and Nonlinear Distortions. When using the mul- tipulse signal, the system under test must be reasonably free of Figure 48. The VM700T Multiburst measurement. nonlinearity. Distortions such as differential phase and gain can cause erroneous readings of both frequency response and group delay.

Figure 49. The VM700T Group Delay & Gain measurement made with the (sin x)/x signal.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Group Delay

DEFINITION Waveform Monitor and Nomograph. Group delay distortion is present When making group delay mea- when some frequency compo- surements with the multipulse nents of a signal are delayed signal, the baseline distortion of more than others. Distortion is each pulse must be individually expressed in units of time. The measured and applied to a difference in delay between a nomograph. Normalize each reference low frequency and pulse height to 100% and mea- the highest frequency tested is sure the positive and negative typically quoted as the group peaks of the baseline distortion. delay error. Voltage cursors in the RELATIVE Figure 50. The multipulse test signal. mode can also be used for these PICTURE EFFECTS measurements. Apply the num- bers to the nomograph (in the Group delay problems can cause Chrominance-to-Luminance a lack of vertical line sharpness Gain and Delay section of this due to luminance pulse ringing, booklet) to obtain the delay overshoot, or undershoot. number. The largest delay mea- TEST SIGNAL sured in this way is typically quoted as the group delay error. The multipulse test signal, The first pulse in a multipulse described in the Frequency signal is generally a 20T pulse Response section, is used to and the others 10T pulses. The measure group delay. It is also nomograph works for any modu- possible to obtain a group delay lated 20T pulse regardless of the measurement from the (sin x)/x modulation frequency. For a 10T signal, but only with an auto- pulse, however, the delay num- matic measurement set such as ber from the nomograph must be the VM700T. divided by two. In practice, it is Figure 51. The multipulse signal exhibiting group delay distortion. Group delay differences between the high frequency and low frequency compo- MEASUREMENT METHODS often easy to see which of the nents of the pulse appear as sinusoidal distortion of the baseline. pulses exhibits the most delay Group delay is measured by ana- necessitating only one measure- lyzing the baseline distortion of ment when maximum delay is the modulated sine-squared the value of interest. pulses in the multipulse signal. As discussed earlier, delay errors 1781R Semi-Automatic Procedure. between the low frequency and Group delay can be measured high frequency components of with the CHROMA/LUMA selec- the pulse appear as sinusoidal tion in the 1781R MEASURE distortion of the baseline (see menu. Repeat the measurement Figure 51). The measurement procedure for each frequency methods for group delay are very of interest. similar to those used for chromi- nance-to-luminance delay differing only in the number of frequencies at which delay is measured.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Automatic Measurement — (Sin x)/x. If the phase versus frequency The VM700T uses the (sin x)/x response is not linear, then the signal to make group delay mea- derivative is not a constant and surements. This method offers group delay distortion is present. the advantage of providing delay The largest difference in d Ø/dw information for a large number that occurs over the frequency of frequencies rather than just at interval of interest is the amount the six discrete frequencies of of group delay (see Figure 54). the multipulse signal. Select GROUP DELAY (SIN X)/X in the 16. Envelope Delay. The term VM700T MEASURE mode “envelope delay” is often used (see Figure 52). interchangeably with group delay in television applications. NOTES Strictly speaking, envelope delay 15. Group Delay Definition. In is measured by passing an mathematical terms, group delay amplitude modulated signal Figure 52. The VM700T Group Delay & Gain measurement made with the is defined as the derivative of through the system and observ- (sin x)/x signal. phase with respect to frequency ing the modulation envelope. dØ/dw. In a distortion free sys- Group delay, on the other hand, tem, the phase versus frequency is measured directly by observi n g response is a linear slope and phase shift in the signal itself. the derivative is therefore a Since the two methods yield constant (see Figure 53). very nearly the same results in practice, it is safe to assume the two terms are synonymous.

Figure 53. Response of a distortion free system. Figure 54. Response of a system with amplitude and phase distortion.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com K Factor Ratings

DEFINITION There are also some definitions The K Factor rating system maps of Kpulse/bar that provide signed linear distortions of 2T pulses rather than absolute value and line bars onto subjectively results. Since there are several determined scales of picture different definitions in use, it is quality. The various distortions again recommended that the are weighted in terms of impair- definition be verified. ment to the picture. Kbar. A line bar (10 or 25 The usual K Factor measure- microseconds) is used to mea- ments are Kpulse/bar, K2T, Kbar, and sure Kbar. Locate the centre of sometimes K . The overall Figure 55. The 1781R external graticule includes a 5% K2T limit. 50 Hz the bar time, normalize that K Factor rating is the largest point to 100%, and measure the value obtained from all of these maximum amplitude deviation measurements. Special graticules for each half. Ignore the first and can be used to obtain the K fac- last 2.5% of the bar. The larger tor number or it can be calculat- deviation of the two, expressed ed from the appropriate formula. in percent, is generally taken as Definitions of the four K factor the Kbar rating. The peak-to-peak parameters are as follows: deviation is sometimes quoted, particularly if a 10 microsecond K . 2T K2T is a weighted function of bar is used. This is another case the amplitude and time of the where it is recommended the distortions occurring before and definition and test signal in use after the 2T pulse. In practice, a be verified and the information graticule is almost always used recorded along with the mea- to quantify this distortion. surement result. Different countries and stan-

dards use slightly different K50 Hz. A field-rate square wave is amplitude weighting factors. An used to measure this parameter. example is shown in Figure 55. Locate the centre of the field bar time, normalize that point to K . pulse/bar Calculation of this para- 100%, and measure the maxi- meter requires measurement of mum amplitude deviation for the pulse and bar amplitudes. each half. Ignore the first and Kpulse/bar is equal to: last 2.5%. The larger of the two 1/4 [ (pulse-bar)/pulse ] X 100%. tilt measurements, divided by It should be noted that some two, is the K50 Hz rating. documents, including CCIR 567-2, recommend that the (bar-pulse) quantity be divided by the bar amplitude rather than the pulse amplitude. The two definitions will yield very nearly the same answer for practical levels of distortion. Check for the definition recommended by the appropriate broadcast authority.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com PICTURE EFFECTS Then use the variable gain con- All types of linear distortions trol to set the top of the 2T pulse affect K Factor rating. Picture to the 1 volt graticule line (see effects may include any of the Figure 57). Set the horizontal aberrations discussed in the magnification to 0.20 microsec- sections on short time, line time, onds per division. Under these field time, and long time distor- conditions, the K2T graticule tions. Since overall K factor indicates a 5% limit. Enabling rating is the maximum value the X5 vertical gain, in addition obtained in the four measure- to the variable gain required to ments, the picture effects corre- normalize the pulse height, will change the graticule indication Figure 56. This signal contains the pulse and bar elements required for K sponding to a given K Factor Factor measurements. rating may vary widely. to a 1% limit. The 1781R is also equipped with

TEST SIGNAL an electronic K2T graticule. Select Any test signal containing a 2T K FACTOR in the MEASURE pulse and a white bar can be menu and make sure that the horizontal magnification is set to used to measure K2T, Kpulse/bar, 0.20 microseconds per division. and Kbar. A 50 Hz square wave is required for measurement Set the of the signal to overlay the dotted electronic of K50 Hz. graticule line and adjust the MEASUREMENT METHODS pulse amplitude until it reaches Waveform Monitor. The external the small cross drawn electroni- graticule provided with 1781R cally near the top of the screen. and 1481 waveform monitors Use the large front panel knob to includes special marks for adjust the graticule size until it making K Factor measurements. just touches the waveform at the

To make a K2T measurement, use point of greatest distortion. The readout will now indicate the the vertical position control to Figure 57. A 2T pulse properly positioned for a K2T measurement. This set the black level to coincide K2T distortion in percent. signal has a K2T distortion of slightly more than 5%. with the 0.3 volt graticule mark.

Figure 58. The 1781R electronic K Factor graticule measures a 2.5% K2T distortion for this signal.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com The external 1781R (and 1481) automatically track the wave-

graticule includes Kpulse/bar marks form or manually adjusted with in the centre near the top. To use the front panel knob. This dis-

this graticule, normalize the play also provides numeric K 2T pulse amplitude (or the bar and Kpulse/bar results (see Figure amplitude, depending on the 61). Measurements of these para- definition in use) to extend from meters are also available in the 0.3 to 1.0 volts. Then compare VM700T AUTO mode. The the other signal element to the VM700T provides a signed

Kpulse/bar scale to obtain a Kpulse/bar result that is negative K Factor reading in percent. when the pulse amplitude is smaller than the bar amplitude. There is also a 5% Kbar limit near the upper left-hand corner NOTES of the external graticule. This 17. Pulse-to-Bar Definitions. There limit is designed for use with a are several different methods of 10 microsecond bar when a 1H expressing the relationship Figure 59. With the pulse taken as the reference, the 1781R graticule sweep is selected. Position the between pulse amplitude and indicates that this signal has a Kpulse/bar distortion of 2%. waveform horizontally so that bar amplitude. It is important to the rising and falling edges of understand the difference and the bar pass through the gratic- know which method is speci- ule circles on the 0.65 volt line fied. Three of the most common (see Figure 60). The waveform definitions are given below. vertical gain should be adjusted so that the black level coincides PULSE-TO-BAR RATIO = with the 0.3 volt line and the (pulse/bar) X 100% centre of the bar passes through PULSE-BAR INEQUALITY = the cross in the centre of the (pulse-bar) X 100% K box. bar K PULSE-TO-BAR = VM700T Automatic Measurement. 1/4 [ (pulse-bar)/pulse ] X 100% Select K FACTOR in the VM700T MEASURE mode to

obtain a measurement of K 2T. The graticule can be set to

Figure 60. This signal is properly positioned for a Kbar measurement with the 1781R graticule.

Figure 61. The VM700T 2T Pulse K Factor measurement.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com III. NONLINEAR DISTORTIONS

Amplitude dependent waveform The first three distortions It is generally recommended that distortions are often referred to discussed in this section are nonlinear distortions be mea- as nonlinear distortions. This differential phase, differential sured at different average classification includes distor- gain, and luminance nonlinearity. picture levels. Some test si g n a l tions that are dependent on APL These are by far the most famil- generators provide variable APL (Average Picture Level) changes iar and most frequently mea- signals by combining the test and/or instantaneous signal sured nonlinear distortions. signal with a variable level level changes. These parameters are included pedestal. Since in-service Since and other elec- in the performance specifica- measurements cannot be made tronic circuits are linear over tions of most video equipment with these test signals, measure- only a limited range, they may and are regularly evaluated in ments requiring control of tend to compress or clip large television facilities. The other APL are often eliminated from signals. The result is nonlinear distortions are not as routinely routine testing. distortion of one type or another. tested, however, most measure- Nonlinear distortions may also ment standards and performance manifest themselves as crosstalk checks include them. and intermodulation effects between the luminance and chrominance portions of the signal.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Differential Phase

DEFINITION picture. A delay-line decoder Differential phase distortion, averages each two successive sometimes referred to as "diff lines in the field, and the resul- phase'', is present when chromi- tant information is displayed. nance phase is dependent on Chrominance phase shifts are luminance level. This phase dis - therefore cancelled out and do tortion is a result of a system's not result in a hue shift in the inability to uniformly process picture. (Differential phase is the high-frequency chrominance actually converted to differential information at all luminance gain in the resultant, but gain levels. errors are much less objection- able in the picture.) Figure 62. A modulated ramp test signal. The amount of differential phase distortion is expressed in degrees of subcarrier phase. Since both positive and negative TEST SIGNALS (lead and lag) phase errors may Differential phase is measured occur in the same signal, it is with a test signal that consists of important to specify whether the uniform-phase chrominance peak-to-peak phase error or peak superimposed on different lumi- deviation from the blanking nance levels. A modulated stair - level phase is being quoted. case (5 or 10 step) or a modulated ramp (see Figure 62) is typically PAL measurement standards used. A ramp is normally used most frequently refer to peak when performing measurements deviation differential phase mea- on devices and systems that con- surements. Two numbers are vert the signal from analogue to typically given to describe the digital and back to analogue. distortion: the peak positive phase deviation and the peak Some generators, such as the negative phase deviation from Tektronix TG2000, offer a phase- Figure 63. Vector display of the TG2000 phase-alternate modulated ramp. the subcarrier phase at blanking alternate modulated ramp test level. Sometimes the larger of signal. A vector display of this these two values is given as a signal is shown in Figure 63. single peak result. This signal can help detect dis- tortions that have affected the U Differential phase distortion and V components differently. should be measured different This is most likely to occur if average picture levels and the the signal has been demodulated worst error quoted. and the U and V components PICTURE EFFECTS passed through separate process- ing channels. If this signal is Since virtually all PAL receivers available, it may be desirable to now employ delay-line repeat the measurement proce- decoders, reasonable amounts of dures outlined below for both differential phase distortion signal vectors. cannot be readily detected in the

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com MEASUREMENT METHODS positive and peak negative When differential phase is pre- results from the vector display is sent, the chrominance phase will less straightforward but it is pos- be different on the different sible when the signal vector lies luminance levels of the test sig- along the 0 degree or 180 degree nal. This phase information can axis. In this case, align the bursts be conveniently displayed on a with the +135 and -135 degree vectorscope after the chromi- graticule marks and obtain an nance has been demodulated. approximate peak reading by Although a standard vector dis- noting how far positive or play can indicate the presence of negative the dots extend from large amounts of distortion, a the 0/180 degree axis. vectorscope equipped with a Demodulated R-Y Sweep. special differential phase mode Although or an automatic measurement set distortions show up in the vec- such as the VM700T is required torscope display, there are some advantages to be gained by for precision measurements. Figure 64. A vectorscope display showing a peak-to-peak differential phase examining the demodulated R-Y distortion of about 7 degrees. Differential gain distortion is also present. Vectorscope Display. In a vec- (V) signal in a voltage versus torscope display, the dots corre- time display. (Recall that the sponding to the various subcarri- weighted R-Y signal drives the er packets will spread out along vertical axis of a vectorscope, the circumference of the gratic- see Figure 65.) First of all, more ule circle when differential gain and therefore more mea- phase is present. When using a surement resolution is possible ramp signal, the dot will become in waveform displays. Secondly, elongated along the circumfer- the sweep display permits corre- ence. To make a measurement, lation of the demodulated R-Y first set the phase of the signal signal with the original test sig- vector to the reference 9 o'clock nal in the time dimension. This phase position. Use the vec- allows determination of exactly torscope variable gain control to how the effects of differential bring the signal vector out to the phase vary with luminance level graticule circle. or how they vary over a field. Figure 65. Differential phase distortion affects the R-Y (V) signal. Vectorscope graticules generally Precise measurements of differ- have special differential phase ential phase are therefore made and gain marks on the left-hand by examining a voltage versus side to help quantify the distor- time display of the demodulated tion. Peak-to-peak phase devia- R-Y information. Distortions tion can be directly from the manifest themselves as tilt or graticule. Obtaining peak level changes across the line.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Two different types of demodu- of the signal. To obtain peak lated R-Y displays, known as results, measure how far positive “single trace” and “double and negative the signal extends trace”, can be used to make this from the level that corresponds measurement. As described to blanking level subcarrier. below, different measurement techniques are used with the Double Trace Method. The double two displays. In the 1781R, trace method provides a more these modes are both accessed accurate way of measuring the by selecting DIFF PHASE in tilt in a one-line sweep of the the MEASURE menu. The R-Y information. Instead of com- SINGLE/DOUBLE touchscreen paring the waveform to a gratic- selection determines which of ule, the vectorscope calibrated the two displays will appear. phase shifter is used to quantify the amount of distortion. Single Trace Method. In the single The double trace display, which trace mode, distortions are quan- Figure 66. A single trace display indicating about 7 degrees of differential also appears on the waveform phase distortion. tified by comparing the R-Y screen in the 1781R, is produced waveform to a vertical graticule by displaying the single trace scale. To make a measurement, R-Y information non-inverted for first use the vectorscope display half the lines and inverted for to set the signal vector to the ref- the other half. Since phase erence 9 o'clock phase position. changes affect the amplitude of Use the vectorscope variable the R-Y signal, the inverted and gain control to bring the signal non-inverted traces can be vector out to the edge of the moved vertically with respect to vectorscope graticule circle. each other by shifting phase. Make sure the 1781R waveform Measurements can therefore be monitor gain is in the calibrated made by introducing calibrated (1 volt full scale) setting. amounts of phase shift with the The R-Y display appears on the vectorscope phase control. The waveform (right-hand) screen in basic technique involves nulling the 1781R. Each major division the blanking level part of the sig- (100 mV) on the vertical gratic- nal by bringing the inverted and ule scale corresponds to one non-inverted traces together at degree when the R-Y waveform that point. The amount of phase is being displayed. The amount shift that is then required to of peak-to-peak differential overlay the two traces at the phase can be determined by point of maximum level shift is measuring the largest vertical the amount of differential phase. deviation between any two parts

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Select DOUBLE in the 1781R VM700T Automatic Measurement. DIFF PHASE mode to make this To make an automatic measure- measurement. First look at the ment of differential phase with vectorscope screen and use the the VM700T, select DGDP in the phase shifter to set the signal MEASURE mode. Both differen- vector to the reference 9 o'clock tial phase and differential gain phase position. Neither vec- are shown on the same display torscope nor waveform monitor (the lower graph is differential gain is critical in this mode (see phase). Measurement results are Note 18), but setting the vector also available in the AUTO mode. to the graticule circle is a good starting point. Now refer to the NOTES waveform monitor (right-hand) 18. 1781R Waveform and Vector screen and use the phase shifter Gains. In the single trace mode, to overlay the blanking level the vector gain must be set so portions of the two waveforms. the signal vector extends to the Press REF SET to set the phase graticule circle. The waveform gain must be in the calibrated Figure 67. The 1781R double trace DIFF PHASE display with the readout to 0.00 degree (see phase readout zeroed. Figure 67). (1 volt full scale) position. The graticule is calibrated to one The next step is to use the phase degree per division only under shifter to overlay the point in the these conditions. R-Y waveforms that deviates most from blanking level. The With the double mode display, phase readout now indicates the however, more gain may be amount of differential phase introduced for greater resolution. distortion (see Figure 68). In this Additional vectorscope gain example the phase error is all in and/or waveform vertical gain one direction so peak and peak- can be selected without affecting to-peak results will be the same. the results. If the signal has both positive 19. Noise Reduction Filter. A digital and negative phase errors (the recursive filter is available in the R-Y signal extends both positive 1781R to facilitate differential and negative from blanking), phase and gain measurements in repeat the process for the largest the presence of noise. Select the positive and largest negative NOISE REDUCTION ON touch- signal excursions. Figure 68. The double trace DIFF PHASE display with the measure- screen selection in the DIFF ment results indicated on the readout. The double trace technique is PHASE or DIFF GAIN menu to similar when using a 521A enable this filter. The filter Vectorscope. Start by setting the removes about 15 dB of noise “calibrated phase” dial to zero. from the signal without any loss Use the A phase or B phase of bandwidth or horizontal reso- control to null the blanking lution. This mode is particularly level and then use the calibrated useful for VTR and transmitter phase shifter to null the largest measurements. excursion. The number above the calibrated phase dial will now give the amount of differential phase distortion.

Figure 69. The VM700T DG DP display.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Differential Gain

DEFINITION Differential gain should be mea- Differential gain, often referred sured at different average picture to as "diff gain''', is present when levels and the worst error quoted. chrominance gain is dependent on luminance level. These PICTURE EFFECTS amplitude errors are a result of When differential gain is pre- the system's inability to uniforml y sent, colour saturation is not cor- process the high-frequency rectly reproduced. Differential chrominance signal at all gain is generally most noticeable luminance levels. in reds and yellows. Differential gain distortion is TEST SIGNALS Figure 70. A modulated 5-step staircase test signal. expressed in percent. Since both attenuation and peaking can Differential gain is measured occur in the same signal, it is with a test signal that consists of important to specify whether the uniform-amplitude chrominance peak-to-peak amplitude differ- superimposed on different lumi- ence or the peak deviation is nance levels. A modulated stair- being quoted. The reference for case (5 or 10 step) or a modulated peak-to-peak results may be ramp is typically used. either the maximum chromi- Some generators, such as the nance amplitude or the ampli- Tektronix TG2000, offer a tude of the chrominance packet phase-alternate modulated ramp at blanking level. Peak deviation test signal. This signal can help measurements are generally ref- detect distortions that have erenced to the chrominance affected the U and V compo- amplitude at blanking level. nents differently. This is most PAL measurement standards likely to occur if the signal has generally refer to peak differen- been demodulated and the U tial gain measurements. Two and V components passed numbers are typically given to through separate processing describe the amount of distor- channels. If this signal is tion: the peak positive deviation available, it may be desirable to and the peak negative deviation repeat the measurement proce- in chrominance amplitude from dures outlined below for both the amplitude at blanking level. signal vectors. These numbers are expressed as a percentage of the blanking level chrominance amplitude. Sometimes the larger of the two values is given as a single peak result.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com MEASUREMENT METHODS Waveform Monitor/Chrominance Differential gain distortion can Filter. Differential gain measure- be quantified in a number of ments can also be made with a ways. Chrominance amplitudes waveform monitor. This process can be measured directly with a is facilitated by enabling the waveform monitor and large chrominance filter which passes distortions can be seen on a vec- only the high-frequency chromi- torscope display. For precision nance portion of the signal. measurements, however, a Peak-to-peak chrominance vectorscope with a special differ- amplitudes can be easily com- ential gain mode or an automatic pared in the resulting display. measurement set such as the To make a measurement, first VM700T is required. normalize the peak-to-peak amplitude of the first chromi- Vectorscope Display. In a vec- nance packet (the one at blanking torscope display, the dots corre- level) to 100 percent. Then mea- sponding to the various subcarri e r sure the peak-to-peak ampli- Figure 71. A vectorscope display of a signal with 10% peak-to-peak packets will spread out in the tudes of the smallest and largest differential gain. Differential phase distortion is also present. radial direction when differen- packets. The positive and nega- tial gain is present. When using tive peak differential gain results a ramp signal, the dot will are the differences between these become elongated in the hori- two measurements and the zontal direction. To make a mea- blanking level amplitude. surement, first set the phase of Equations are given below. the signal vector to the reference Peak dG (Negative) = position. Use the vectorscope -100 [ [Vp p(Blanking) - Vp p(Smallest Packet)] / Vp p(Blanking) ] % variable gain control to bring Peak dG (Positive) = the signal vector out to the +100 [ [Vp p(Blanking) - Vp p(L a r gest Packet)] / Vp p(Blanking) ] % graticule circle. This measurement can also be Vectorscope graticules generally made by using the 1781R voltage have special differential phase cursors in the RELATIVE mode. and gain marks on the left-hand Define the peak-to-peak ampli- side to help quantify the distor- tude of the blanking level packet tion. Peak-to-peak gain deviation as 100% and then move the can be read directly from the cursors to measure peak-to-peak graticule. A peak reading is more amplitude of the smallest and Figure 72. A chrominance filter display indicating about 6 % differential gain. difficult to obtain from this dis- largest packets. Use the equa- play since there is no convenient tions above to calculate results. method of establishing which amplitude corresponds to the amplitude at blanking level.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com B-Y Sweep. Some vectorscopes the graticule and measure the are equipped with a special largest deviation between the mode for making accurate differ- part of the signal that corre- ential gain measurements. Since sponds to blanking-level chromi- differential gain affects the B-Y nance and the largest and small- (U axis) signal (see Figure 73), a est packets. One major graticule line-rate sweep of demodulated division (100 mV) is equal to B-Y information can be used to one percent. measure the amount of distor- tion. Errors manifest themselves Double Trace Method. The double as tilt or level changes across the trace method in the 1781R pro- line. Like the R-Y display used vides a highly accurate way of to measure differential phase, measuring the amount of tilt or this display provides greater level shift in a one-line sweep of resolution and allows determina- the B-Y information. This tion of how the distortion varies method is very similar to the over a line. In the 1781R, both differential phase double trace “single trace” and “double trace” method described earlier, the versions of this display are avail- difference being a calibrated gain able. Both are accessed by control rather than a calibrated selecting DIFF GAIN in the phase control is used to null Figure 73. Differential gain distortion affects the B-Y (U) signal. MEASURE menu. the traces. Select DOUBLE in the 1781R Single Trace Method. The single DIFF GAIN menu to make this trace differential gain display is measurement. Use the phase familiar to users of the 521A shifter to set the signal vector to vectorscope and it is also avail- the reference phase position. able in the 1781R by selecting The vectorscope variable gain SINGLE in the DIFF GAIN must be adjusted so the signal menu. The amount of distortion vector reaches the graticule cir- is quantified by comparing the cle. The 1781R waveform moni- demodulated waveform to a tor gain setting is not critical in vertical graticule scale. this mode (see Note 21). The phase shifter should be used Now refer to the waveform to set the signal vector to the ref- (right-hand) display, and start erence (9 o'clock) position prior the measurement procedure by to making this measurement. using the large knob to overlay Adjust the vectorscope variable the blanking level portions of gain control so the signal vector the inverted and non-inverted Figure 74. A single trace DIFF GAIN display indicating a distortion of extends to the edge of the gratic- waveforms. Press REF SET to set about 3%. ule circle. Make sure the 1781R's the readout to 0.00 percent (see wa v e f o r m gain is in the calibrated Figure 75). Now use the large (1 volt full scale) setting. knob to bring together the largest In the 1781R, the differential positive and/or negative excur- gain display appears on the sions. The readout now indicates waveform screen. Compare the the amount of differential gain waveform to the vertical scale on distortion (see Figure 76).

Fi g u r e 75. The 1781R double trace DIFF GAIN display with the readout zeroe d .

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com VM700T Automatic Measurement. 21. 1781R Waveform and Vector To make an automatic measure- Gains. When using the single ment of differential gain with trace mode, the vector gain must the VM700T, select DGDP in the be set to the graticule circle and MEASURE mode. Both differen- the waveform gain must be in tial phase and differential gain the calibrated position. The are shown on the same display graticule is only calibrated to 1 (the upper graph is differential percent per division under gain). Measurements results are these conditions. also available in the AUTO mode. In the double mode display, more waveform vertical gain (X5 NOTES or VAR) may be introduced for 20. Demodulated “B-Y” Signal. It greater resolution. However, cor- should be noted that in instru- rect results will be obtained only ments such as the 521A when the vectorscope gain is set Vectorscope and the 1781R, the to the graticule circle. displayed signal is not simply the B-Y demodulator output of Figure 76. The 1781R double trace DIFF GAIN display showing measurement 22. Simultaneous Display of DP and results. With attenuation only, peak and peak-to-peak results are the same. the vectorscope. Rather, an DG. It is sometimes useful to envelope (square law) detector have a display that shows both scheme is used. The demodulat- differential phase and differen- ed signal is derived by multiply- tial gain, particularly when ing the signal by itself rather adjusting equipment for mini- than by a constant-phase CW mum distortion. A display subcarrier as in a synchronous which shows a one-line sweep demodulator. The primary of differential phase on the left advantage of this method is that and a one-line sweep of differen- in the presence of both differen- tial gain on the right can be tial phase and differential gain, accessed by selecting DP & DG synchronous detection yields a in the 1781R MEASURE menu phase-dependent term, but (see Figure 78). As noted above, square law detection does not. the VM700T DG DP display Thus the presence of differential also shows both distortions phase does not affect the differ- simultaneously. ential gain result. Figure 77. The VM700T DG DP display.

Figure 78. The 1781R DP & DG display.

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DEFINITION In colour pictures, luminance Luminance nonlinearity, or dif- nonlinearity is often more ferential luminance, is present noticeable. This is because when luminance gain is affected colour saturation, to which the by luminance level. In other eye is more sensitive, is affected. words, there is a nonlinear rela- tionship between the input and TEST SIGNALS output signals in the luminance Luminance nonlinearity should channel. This amplitude distor- be measured with a test signal tion is a result of the system's that consists of uniform- inability to uniformly process amplitude luminance steps. Figure 79. An unmodulated staircase signal. luminance information over the Unmodulated 5 step or 10 step entire amplitude range. st a i r case signals are typically used. The amount of luminance non- If an unmodulated signal is not linearity is expressed as a per- available, the measurement can centage. Measurements are made also be made with a modulated by comparing the amplitudes of staircase. This is generally not the individual steps in a stair- good practice, however, since case signal. The difference both differential gain and lumi- between the largest and smallest nance nonlinearity can have the steps, expressed as a percentage same net effect on the signal. of the largest step amplitude, is the amount of luminance nonlin- MEASUREMENT METHODS earity distortion. Measurements Luminance nonlinearities are should be made at different quantified by comparing the step average picture levels and the amplitudes of the test signal. worst error quoted. Since the steps were initially all of uniform height, any differ- PICTURE EFFECTS Figure 80. An example of luminance nonlinearity distortion. ences are a result of this distor- Luminance nonlinearity is not tion. The waveform in Figure 80 particularly noticeable in black exhibits luminance nonlinearity and white pictures. However, if distortion. Note that the top step large amounts of distortion are is shorter than the others. present, a loss of detail may be seen in the shadows and high- lights. These effects correspond to crushing or clipping of the black and white information.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Waveform Display. Luminance Either the waveform monitor nonlinearity can be made with a graticule or the voltage cursors waveform monitor by individu- can be used to measure the ally measuring each step in the spikes. Use the variable gain to test signal. It is most convenient normalize the largest spike to use the variable gain to nor- amplitude to 100% when using malize the largest step to 100% the graticule. The difference (500 mV or 1 Volt) so percentage between the largest and smallest can be read directly from the spikes, expressed as a percentage graticule. Voltage cursors can of the largest, is the amount of also be used to measure the luminance nonlinearity. steps. Although this method can The 1781R voltage cursors yield accurate results, it is very should be in the RELATIVE time consuming and is not mode for this measurement. frequently used in practice. Define the largest spike ampli- Waveform Monitor — Differentiated tude as 100%. Leave one cursor at the top of the largest spike Figure 81. This photograph shows a 5 step staircase after it has been passed Step Filter. Some waveform moni- and move the other cursor to the through a differentiated step filter. The 1781R voltage cursors indicate 9% tors are equipped with a special luminance nonlinearity. top of the smallest spike. The filter, usually called a “diff step” readout will indicate the amount filter, for measurement of lumi- of luminance nonlinearity nance nonlinearity. Since it pro- distortion (see Figure 81). vides an accurate and conve- nient method of evaluating this VM700T Automatic Measurement. distortion, it is generally recom- Select LUMINANCE NONLIN- mended practice to use such a EARITY in the VM700T filter for this measurement. MEASURE menu to obtain a External filters can be used if the display of this distortion. The waveform monitor is not VM700T uses an internal differ- equipped with the filter. entiated step filter to make this When the differentiated step fil- measurement. Measurement ter is enabled, each step transi- results are also available in the tion appears as a spike on the AUTO mode. display. As the amplitude of each spike is proportional to the corresponding step height, the Figure 82. The VM700T Luminance Nonlinearity display. amount of distortion can be determined by comparing the spike amplitudes.

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DEFINITION MEASUREMENT METHODS Chrominance nonlinear phase Chrominance nonlinear phase is distortion is present when quantified by measuring the chrominance phase is affected phase differences between the by chrominance amplitude. chrominance packets of the These phase errors are a result of modulated pedestal signal. the system's inability to uniform- ly process all amplitudes of Vectorscope. Since phase infor- chrominance information. mation is required, a vectorscope is used to measure chrominance Chrominance nonlinear phase nonlinear phase. Examine the Figure 83. A modulated pedestal test signal. distortion is expressed in three dots (which correspond to degrees of subcarrier phase. This the three chrominance levels) parameter should be measured at and measure the maximum different average picture levels phase difference between the and the worst error quoted. three signal vectors. This is easi- PICTURE EFFECTS est when the vectorscope vari- able gain is adjusted to bring the Like differential phase, the largest vector out to the graticule effects of chrominance nonlinear circle. When using a 1781R or a phase are averaged out in delay- 521A Vectorscope, the calibrated line PAL decoders. Hue shifts phase shifter can be used to therefore cannot be detected in obtain a precise reading. the picture. VM700T Automatic Measurement. TEST SIGNAL Select CHROMINANCE A modulated pedestal signal, NONLINEARITY in the VM700T sometimes called a three level MEASURE mode to obtain a chrominance bar, is used to mea- display of this distortion. The Figure 84. The 1781R vectorscope display showing a signal that suffers from sure this distortion. This signal chrominance nonlinear phase chrominance nonlinear phase distortion. consists of a single phase, three measurement is the middle level chrominance packet super- graph in the display (see Figure imposed on a constant lumi- 85). Measurement results are nance level. A typical modulated also available in the VM700T pedestal signal will have a 350 AUTO mode. mV luminance level and 140, 420, and 700 mV chrominance levels. This signal element is sometimes part of combination signals used as ITS.

Figure 85. The VM700T Chrominance Nonlinearity display.

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DEFINITION MEASUREMENT METHODS Chrominance nonlinear gain Chrominance nonlinear gain dis- distortion is present when tortion is quantified by measur- chrominance gain is affected by ing how much the amplitudes of chrominance amplitude. These the chrominance packets deviate amplitude errors are a result of from their nominal values. the system's inability to uniforml y process all amplitudes of Waveform Monitor. The waveform chrominance information. monitor graticule should be used for this measurement. First use Chrominance nonlinear gain the waveform monitor variable distortion is the amplitude devi- Figure 86. A modulated pedestal test signal. gain to normalize the middle ation expressed as a percentage subcarrier packet to its pre- of the nominal amplitude. This scribed value of 420 mV. The measurement is made on the amount of chrominance nonlin- lowest and highest chrominance ear gain distortion is the largest levels with the middle level deviation from nominal value for normalized to its nominal value. the other two packets expressed The larger of the two resulting as a percentage of the nominal numbers is generally taken as amplitude of the affected packet. the overall result. This distortion should be mea- VM700T Automatic Measurement. sured at different average picture Select CHROMINANCE levels and the worst distortion NONLINEARITY in the VM700T should be quoted. MEASURE menu to make this measurement. Chrominance PICTURE EFFECTS nonlinear gain is shown on the Chrominance nonlinear gain is upper graph. This parameter can often seen as attenuation of rela- also be measured in the VM700T tively high amplitude chromi- AUTO mode. Figure 87. This signal exhibits chrominance nonlinear gain distortion. Note that the amplitude of the largest packet is reduced. nance signals. It will appear in the TV picture as incorrect NOTES 23. Chroma Filter. colour saturation. It is sometimes recommended that waveform TEST SIGNAL monitor chroma filter be enabled when measuring chrominance A modulated pedestal signal, nonlinear gain. While the chro- sometimes called a three level ma filter will make the display chrominance bar, is used to mea- more symmetrical, the same sure this distortion. This signal results should be obtained either consists of a single phase, three way since it is the peak-to-peak level chrominance packet super- amplitudes being measured. imposed on a constant lumi- A possible exception is a case nance level. A typical modulated where chrominance harmonic pedestal signal will have a 350 distortion is present. The mV luminance level and 140, chrominance filter can remove 420, and 700 mV chrominance the effects of harmonic distor- levels. This signal element is tion which are likely to be differ- sometimes part of combination Figure 88. The VM700T Chrominance Nonlinearity display. ent for each chrominance level. signals used as ITS.

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DEFINITION MEASUREMENT METHODS Chrominance-to-luminance Chrominance-to-luminance intermodulation, also known as intermodulation is quantified by crosstalk or cross-modulation, is measuring the effects that present when luminance ampli- chrominance packets of different tude is affected by superimposed amplitudes have on the lumi- chrominance. The luminance nance level that they are super- change may be caused by clip- imposed on. This process is ping of high-amplitude chromi- facilitated by removing the nance peaks, quadrature distor- chrominance information from Figure 89. This combination ITS contains the Modulated Pedestal signal ele- tion, or various crosstalk and the display with a waveform ment (CCIR Line 331). intermodulation effects. monitor filter. The deviation in the pedestal Waveform Monitor. The chromi- level may be expressed: nance information can be • As a percentage of the filtered off with either the lumi- pedestal level nance or lowpass filter in the • As a percentage of the 1781R. The Y display of the measured white bar amplitude 521A Vectorscope also works well. • As a percentage of 700 Details of the measurement millivolts method will depend on the method chosen to express the These definitions will yield amount of distortion. In general, different measurement results the appropriate part of the signal under some conditions so it is must be normalized using the important to standardize on a waveform monitor variable gain single method of making inter- control. Then measure the modulation measurements. largest level shift in the top of the luminance pedestal. PICTURE EFFECTS Figure 90. A chrominance-to-luminance intermodulation distortion of 8.5% The 1781R voltage cursors can When intermodulation distortion referenced to the pedestal level. be used in the relative mode to is present, colour saturation will make this measurement. In not be accurately represented in Figure 90, the level shift is 8.5% affected pictures. of the pedestal level. TEST SIGNALS VM700T Automatic Measurement. A modulated pedestal signal, Select CHROMINANCE sometimes called a three level NONLINEARITY in the VM700T chrominance bar, is used to mea- MEASURE menu to measure sure this distortion. This signal chrominance-to-luminance consists of a single phase, three intermodulation. This parameter level chrominance packet super- is shown on the lower graph. imposed on a constant lumi- Measurement results are also nance level. A typical modulated available in the VM700T pedestal signal will have a 350 AUTO mode. mV luminance level and 140, 420, and 700 mV chrominance Figure 91. The VM700T Chrominance Nonlinearity display. levels. This signal element is sometimes part of combination signals used as ITS.

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DEFINITION Waveform Monitor. This distortion Transient sync gain distortion, is easiest to evaluate with the also referred to as transient non- test signal displayed on a wave- linearity, is present when abrupt form monitor with the differenti- changes in APL temporarily ated step filter selected. (Recall affect sync amplitude. The that this filter produces spikes amount of distortion is defined with amplitudes proportional to as the maximum transient depar- the step amplitudes). Be sure the ture in the amplitude of sync DC restorer is turned off for from the amplitude that existed this measurement. before the change in APL. It is Depending on the nature of the generally expressed as a percent- distortion, it may be possible to age of the original amplitude, observe it when the waveform Figure 92. A flat field bounce test signal. however, some standards specify monitor is in the field sweep the distortion as a percentage of mode. Otherwise it will be the largest amplitude. necessary to use the 1781R Measurement of this distortion SLOW SWEEP mode. (Some requires an out-of-service test. 1481 Waveform Monitors are Both low-to-high and high-to-low equipped with the SLOW APL changes should be evaluated. SWEEP option). A waveform photograph may make the PICTURE EFFECTS measurement easier. Sudden switches between high Adjust the waveform monitor APL and low APL pictures can variable gain to set the ampli- cause transient brightness or tude of the positive spike that saturation effects in the picture. corresponds to the trailing edge of sync equal to 100%. Switch TEST SIGNAL between APL extremes, typically Transient gain distortion is 12.5% and 87.5%. The resulting measured with a flat field signal envelope of the sync spikes (black burst with pedestal). represents the transient distor- A generator with a “bounce” tion. Measure the maximum feature can be used to make the departure from 100% to APL transitions if the time obtain the amount of transient interval between transitions is sync nonlinearity. considerably longer than any The 1781R voltage cursors can transient effect. also be used to make this mea- surement. In the relative mode, MEASUREMENT METHODS define the positive sync spike as Transient gain changes are mea- 100%. Then use the cursors to sured by abruptly changing APL measure the largest deviation and observing the transient from that amplitude. effects on a waveform monitor.

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DEFINITION TEST SIGNAL Steady state gain distortion of Any test signal with variable the sync signal is present when APL can be used to measure horizontal sync amplitude is steady-state sync gain. A 700 mV dependent on APL. This parame- signal element such as a white ter is evaluated by measuring bar is required for steady-state sync amplitude at high and low picture gain measurements. APL (typically 12.5% and 87.5%). The amount of distor- MEASUREMENT METHODS tion may be expressed as a per- Waveform Monitor. To make a centage of the amplitude at 50% measurement, first select 50% Figure 93. A staircase signal with variable APL. APL or as a percentage of the APL and use the waveform mon- maximum amplitude. This is an itor variable gain to set the sync out-of-service test. amplitude to 100%. Vary the APL of the signal to 12.5% and Steady-state gain distortion of then to 87.5%. At each APL the picture signal is also some- level, record the amplitude of times measured. In this case, the sync. The peak-to-peak variation effects of APL changes on peak for the three levels, expressed in white are evaluated. per-cent, is typically quoted as PICTURE EFFECTS the steady-state sync gain distor- tion. This measurement can be If only sync is affected, small made with the 1781R voltage amounts of static gain distortion cursors in the RELATIVE mode. will not be noticeable in the pic- Figures 94 and 95 illustrate the ture. Large amounts of distortion measurement procedure. may affect the ability of some equipment to derive synchro- nization information and/or to clamp the signal. If the picture Figure 94. The sync pulse measures 300 mV at 50% APL. signal is also affected, luminance levels will be APL dependent if this type of distortion is present.

Figure 95. At 87.5 APL, the sync pulse measures 260 mV. This indicates a steady-state distortion of about 13%.

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The electrical fluctuations that frequency components for mance over time. Tangential we refer to as noise form a very analysis. Each measurement noise measurements are made complex signal that does not standard typically calls for three with a specially equipped lend itself to straightforward or four measurements made waveform monitor. This feature amplitude measurements. A with various combinations of the is standard in the 1781R. number of special techniques filters. Note that specifications Specialized equipment is have therefore been developed for the filters vary from standard required to completely charac- for measuring noise. A compre- to standard. terize the noise performance of a hensive discussion of noise mea- The tangential method of noise system. Until recently, these surement is outside the scope of measurement, useful for making capabilities were available only this publication. However, some operational measurements of in dedicated noise measurement of the methods which apply to random noise, is the only instruments. The VM700T, television systems are discussed method discussed in detail in however, makes highly accurate in this section. this publication. While not the noise measurements using filters Special filters are generally most accurate technique, the implemented in software. The required for noise measure- tangential measurement can noise measurement features of ments. These filters are used to provide a quick way of keeping the VM700T are reviewed briefly separate the noise into its various track of system noise perfor- in this section.

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DEFINITION TEST SIGNALS Noise refers to the fluctuations The tangential method can be that are present in any electrical used on any video signal with system. Noise can be either ran- a constant luminance level with - dom or coherent and comes from out chrominance. The measure- a variety of natural and man ment can be made on a single made sources. Although there is line in the vertical interval always some noise present, an although full field measurements excessive amount is undesirable are more accurate and somewhat Figure 96. A red field test signal. since it tends to degrade or easier to make. obscure the signal of interest. Any line with a constant pedestal Signal amplitudes do not always level can be used to make remain constant as the video sig- VM700T Noise Spectrum mea- nal is processed and transmitted. surements. A quiet line in the An absolute measurement of ve r tical interval is typically used. noise is theref o r e not parti c u l a r l y The VM700T Chrominance relevant as a certain amount of AM/PM noise measurement noise will have very different requires a red field test signal effects on signals of different (see Figure 96). amplitudes. Since it is the amount of noise MEASUREMENT METHODS relative to the signal amplitude Tangential Method. Tangential rather than the absolute amount noise measurements can be of noise that tends to cause made with a 1781R. The method problems, measurements of is accurate to within 1 or 2 dB, signal-to-noise ratios, expressed down to noise levels of about 60 in dB, are made. dB. Filters can be inserted in the Figure 97. The 1781R tangential noise measurement mode showing exces- sive trace separation. AUX OUT/AUX IN path to PICTURE EFFECTS separate noise components of Noisy pictures often appear different frequencies. grainy or snowy and sparkles of colour may be noticeable. Extremely noisy signals may be difficult for equipment to syn- chronize to and the picture may suffer from blurriness and a general lack of resolution.

Figure 98. The 1781R tangential noise measurement mode with trace sepa- ration properly adjusted. This signal has a signal-to-noise ratio of 30 dB.

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Make sure the waveform monitor The rms signal-to-noise ratio of filter selection is set to FLAT the entire spectrum is always (unless using the auxiliary filter displayed in the upper right- capability) and the DC restorer hand corner of the display. A to OFF or FAST. Select NOISE in cursor can be used to select a the 1781R MEASURE menu. (In certain frequency for a peak-to- the 1481, use the WAVEFORM peak noise measurement. The COMPARISON mode to split the cursors can also be used to luminance level of interest in define a narrow range of fre- half and overlay the two parts). quencies for S/N measurements. The measurement is made by The CHROMINANCE AMPM adjusting the separation between selection in the VM700T MEA- the two traces until the dark area SURE mode, which requires a between them just disappears. red field test signal, provides When there is no perceptible dip information about the noise that in brightness between the two affects the chrominance portion Figure 99. The VM700T Noise Spectrum display. traces, the calibrated offset level of the signal. Since the chromi- (in dB) is the amount of noise. In nance signal is sensitive to both the 1781R, the large knob is used amplitude (AM) and phase (PM) to control the offset and the on components of noise, two sepa- screen readout provides the dB rate measurements are provided. reading. In the 1481, the offset A selection of filters is available function is performed by the two in this mode. dB NOISE controls in the lower Noise measurements are also right-hand corne r . The dB rea d i n g available in the VM700T is obtained from the knob settings. AUTO mode.

VM700T Automatic Measurement. NOTES Select NOISE SPECTRUM in the 24. Quiet Lines. "Quiet lines” in VM700T MEASURE menu to the vertical interval are some- make signal-to-noise measure- times used to evaluate the ments. A spectral display and amount of noise introduced in a numeric results are provided in certain part of the transmission Figure 100. The VM700T Chrominance AM PM display . this mode (see Figure 99). path. A line is reinserted (and is Several lowpass, highpass, and therefore relatively noise free) at weighting filters are available in the transmitting end of the path this mode. Measurement stan- of interest. This ensures that any dards typically require three or noise measured on that line at four measurements made with the receiving end was intro- various combinations of duced in that part of the path. these filters.

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In this section, we discuss two TV1350 or 1450 is required. Envelope detection is most parameters which should be These instruments provide enve- similar to the monitored and adjusted at the lope and synchronous detection used in most home receivers and transmitter — depth of modula- demodulation. Unlike envelope is also available in the TV1350 tion and ICPM. These two mea- detectors, synchronous detectors and 1450. surements are commonly made are not affected by the quadra- The TV1350 and 1450 produce with time domain instruments ture distortion inherent in the a zero carrier reference pulse such as waveform monitors or vestigial sideband transmission which provides the reference oscilloscopes. Most of the other system. For measurement pur- level required for depth of mod- tests for characterizing transmit- poses, the effects of quadrature ulation measurements. This ter performance are made with a distortion should be removed so pulse is created at the demodu- spectrum analyzer and are not as not obscure distortions from lator output by briefly reducing addressed in this publication. other sources. A quadrature the amplitude of the RF signal In order to make these measure- output is available when the to the zero carrier level prior ments, a high-quality demodula- instrument is operating in the to demodulation. tor such as the Tektronix synchronous detection mode.

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DEFINITION MEASUREMENT METHODS

ICPM (Incidental Carrier Phase ICPM is measured by examining

Modulation) is present when an XY plot of VIDEO OUT picture carrier phase is affected versus QUADRATURE OUT with by video signal level. ICPM dis- the demodulator operating in the

tortion is expressed in degrees synchronous detection mode. A using the following definition: phase error will produce an out-

ICPM = arctan (quadrature put from the quadrature detector. amplitude/video amplitude) If this phase error varies with amplitude, the result is a tilted PICTURE EFFECTS display. The demodulator zero carrier reference pulse must be The effects of ICPM will depend turned on and the detection on the type of demodulation mode set to synchronous. Select used to recover the baseband sig- Figure 101. How to set up the 1781R for ICPM measurements. the SLOW time constant when nal from the transmitted signal. using the 1450. ICPM shows up in synchronous- ly demodulated signals as differ- Waveform Monitor. To obtain an ential phase and many other ICPM display with a wavefor m types of distortions. With enve- monitor, connect the demodula- lope demodulation, the demodu- tor outputs to the waveform lation typically used in home monitor inputs as shown in receivers, the baseband signal is Figure 101. Select ICPM in the generally not as seriously affect- 1781R MEASURE menu or EXT ed and the effects of ICPM are HORIZ on the 1481 front panel. rarely seen in the picture. The sound, however, is another matter. Although it is not strictly neces- sary, it is generally recommend- ICPM may manifest itself as ed that the signals be lowpass- audio buzz in the home receiver. filtered to make the display In the intercarrier sound system, easier to interpret. With either the picture carrier is mixed with the 1781R or the 1481, this can the FM sound carrier to form a be accomplished in the vertical Figure 102. 1781R ICPM display with no distortion present. sound IF. Audio rate phase mod- channel by selecting the LOW- ulation in the picture carrier can PASS filter. Use an external therefore be transferred into the 250 kHz lowpass filter for the audio system and heard as a horizontal. Figure 101 shows a buzzing noise. typical measurement setup. TEST SIGNAL ICPM is measured with an unmodulated linearity signal. A staircase is generally used but a ramp signal may also be used.

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The display resulting from this VM700T Automatic Measurement. configuration, which appears on The ICPM selection in the the right-hand screen in the VM700T MEASURE mode 1781R, is shown in Figure 102. provides an ICPM display and The amount of tilt (deviation numeric results. An ICPM mea- from the vertical) is an indica- surement is also provided in the tion of ICPM. There is no ICPM AUTO mode. The quadrature in the signal shown in Figure output must be connected to 102, while distortion is present VM700T “C” input. in Figure 103. To adjust for min- imum ICPM, make the line as NOTES nearly vertical as possible. 25. Configuring the 1481. 1481 instruments are shipped with The 1781R has an electronic unblanking disabled in the graticule which can be used to EXTERNAL HORIZONTAL quantify the amount of tilt. The mode to prevent damage to the waveform should be positioned CRT. ICPM measurements can Figure 103. The 1781R electronic graticule indicating an ICPM distortion so the small dot corresponding be made in line select with the of 6 degrees. to the zero carrier reference instrument in this mode. For pulse is set on the cross at the full-field measurements, the top of the screen. The horizontal unblanking must be enabled. magnification will automatically Instructions on how to accom- be set to X25 when this mode is plish this can be found in the selected. X50 magnification can OPERATING CHANGES section be used for greater resolution. of the 1481 manual. Start with the two graticule lines widely separated and use the 26. Other XY Displays. Any XY la r ge knob to move them together display can be used to measure to the point where a graticule ICPM. Connect QUADRATURE line first contacts one of the OUT to the horizontal and dots. Disregard the “loops” in VIDEO OUT to the vertical and the display. These correspond to use the formula given on page the level transitions and are not 61 to calculate the amount of indicative of distortion. The distortion. For small errors, amount of ICPM distortion is some amount of gain will indicated on the screen (see be needed to improve the Figure 103). measurement resolution. Figure 104. The 1481 ICPM graticule. An external ICPM graticule is Lowpass filters in both channels available for the 1481. Position are recommended. the zero carrier reference pulse, which shows up as a small dot, 27. Phase Noise. Some demodula- on the cross at the top of the tors have large amounts of phase graticule. The graticule is cali - noise which makes it difficult to brated for 2 degrees per radial make ICPM measurements on division when the horizontal wa v e f o r m monitors. The VM700T magnifier is set to X25 or 1 AVERAGE mode can eliminate degree per division with 50X this effect. The Tektronix 1450 horizontal magnification. Read has sufficiently low phase noise the amount of ICPM from for measurements with a wave- the graticule at the point of form monitor, as do all TV1350 maximum distortion. units shipped after July, 1998. Older TV1350 units can be retrofitted to improve phase noise performance. Contact your Figure 105. The VM700T ICPM display. local Tek t r onix service departm e n t for information on how to update older instruments.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com Depth of Modulation

DEFINITION Waveform Monitor. Most waveform Depth of modulation (percentage monitors provide a depth of of modulation) measurements modulation scale on the gratic- indicate whether or not video ule. Use the variable gain to signal levels are properly position the zero carrier refer- represented in the RF signal. ence pulse at 1.25 volts and sync tip at 0 volts. Verify that The PAL System I modulation blanking level and white level scheme (see Note 29) yields an occur at the prescribed points RF signal that reaches its maxi- (0.3 and 1.0 volts respectively). mum peak-to-peak amplitude at The voltage cursors can also be sync tip (100%). In a properly used for this measurement. adjusted signal, blanking level corresponds to 76% and white NOTES to 20%. The zero carrier refer- 28. Envelope Detection Mode. ence level corresponds to 0% Depth of modulation measure- (see Figure 106). ments should be made with the demodulator in the envelope PICTURE EFFECTS detection mode to minimize Overmodulation often shows up effects of ICPM. (Quadrature as nonlinear distortions such as distortion will not affect differential phase and gain and modulation depth.) picture effects correspond to Figure 106. Depth of modulation levels for System I. those caused by the various 29. Depth of Modulation Numbers. distortions. ICPM or white The depth of modulation num- clipping may also result. bers used in this section are for Undermodulation often results System I PAL. For PAL Systems in degraded signal-to-noise B, G, D and K, the CCIR specifies performance. blanking level at 75% ±2.5% of peak carrier, and peak white at TEST SIGNAL 10% to 12.5%. To make mea- A signal with black and white surements that correspond to levels is required for depth of these specifications, use an over- modulation measurements. This all video amplitude of approxi- signal is used in conjunction mately 1.12 volts. Verify that the with the zero carrier reference white level is at about 11% of pulse, which the demodulator the overall amplitude, and that typically places on one line in blanking is at about 73%. Since different countries may use dif- the vertical interval. In the Figure 107. The zero carrier reference pulse as it appears in a baseband ferent RF levels, be sure to note composite signal the zero carrier signal (System I). pulse appears as a 0.95 volt the recommendations of your (above blanking) bar approxi- broadcast authority. mately 30 microseconds in duration (see Figure 107).

MEASUREMENT METHODS Modulation depth is measured at the output of a precision demodulator by verifying that the ratios between the parts of the signal are correct. Overall amplitude is not critical, but it Figure 108. A signal that extends to 700 mV, such as this staircase sig- should be adjusted in the system nal, is used in conjunction with the zero carrier pulse to verify modula- to be approximately 1.25 volts tion levels. from sync tip to zero carrier at 100% transmitter power. This will minimize the effects of nonlinearities in the measure- ment system.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com GLOSSARY OF TELEVISION TERMS

AC-COUPLED — A connection BLACK BURST — Also called CHROMINANCE — which removes the constant vo l t - “colour black”, black burst is a Chrominance refers to the colour age (DC component) on which the composite video signal consisting information in a television pic- signal (AC component) is riding. of all horizontal and vertical ture. Chrominance can be further Implemented by passing the synchronization information and broken down into two properties signal through a capacitor. burst. Typically used as the of colour, hue and saturation. AM — Amplitude Modulation house reference sychronisation CHROMINANCE SIGNAL — (AM) is the process by which the signal in television facilities. The high-frequency portion of amplitude of a high-frequency BLANKING LEVEL — Refers to the video signal, obtained by carrier is varied in proportion to the 0.3 volt level (with respect to quadrature amplitude modula- the signal of interest. In the PAL sync tip) which exists before and tion of a 4.43 MHz subcarrier television system, AM is used to after horizontal sync and during with R-Y and B-Y information. encode the colour information the vertical interval. COLOUR BLACK — See and to transmit the picture. BREEZEWAY — The portion Black Burst. Several different forms of AM of the video signal that lies COLOUR DIFFERENCE are differentiated by various between the trailing edge of the SIGNALS — Signals used by methods of sideband filtering horizontal sync pulse and the colour television systems to and carrier suppression. Double start of burst. Breezeway is part convey colour information in sideband suppressed carrier is of back porch. such a way that the signals go to used to encode the PAL colour BROAD PULSES — Another name zero when there is no colour in information, while the signal is for the vertical synchronizing the picture. U and V are colour transmitted with a large-carrier pulses in the center of the verti- difference signals. vestigial sideband scheme. cal inter-val. These pulses are COMPONENT VIDEO — Video APL — Average Picture Level. long enough to be distinguished which exists in the form of three The average signal level (with from all others, and are the part separate signals, all of which are respect to blanking) during of the signal actually detected by required in order to completely active picture time, expressed vertical sync separators. specify the colour picture. For as a percentage of the difference BRUCH BLANKING — A 4-field example: R, G and B or Y, R-Y between the blanking and burst blanking sequence employed and B-Y. reference white levels. in PAL signals to ensure that COMPOSITE VIDEO — A single BACK PORCH — The portion of burst phase is the same at the video signal containing all of the video signal that lies end of each vertical interval. the necessary information to between the trailing edge of the BURST — A small reference reproduce a colour picture. horizontal sync pulse and the packet of the subcarrier sine Created by adding quadrature start of the active picture time. wave sent during the horizontal amplitude modulated U and V Burst is located on back porch. blanking interval on every line to the luminance signal. BANDWIDTH — The range of of video. Since the carrier is CW — Continuous Wave. Refers frequencies over which signal suppressed, this phase and to a separate subcarrier sine amplitude remains constant frequency reference is required wave used for synchronization (within some limit) as it is for synchronous demodulation of chrominance information. passed through a system. of the colour difference signals in the receiver. dB (DECIBEL) — A decibel is a BASEBAND — Refers to the logarithmic unit used to describe composite video signal as it B-Y — One of the colour differ- signal ratios. For voltages, exists before modulating the ence signals used in the PAL dB = 20 Log (V /V ). picture carrier. Composite video system, obtained by subtracting 10 1 2 distributed through a studio and luminance (Y) from the blue DC-COUPLED — A connection used for rec o r ding is at baseband. camera signal (B). configured so that both the signal (AC component) and the constant voltage on which it is riding (DC component) are passed through.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com DC RESTORER — A circuit FRONT PORCH — The portion HUM — Hum refers to the unde- used in picture monitors and of the video signal between the sirable coupling of the 50 Hz waveform monitors to clamp one end of active picture time and the power sine wave into other point of the waveform to a fixed leading edge of horizontal sync. electrical circuits. DC level. GAMMA — Since picture moni- INTERCARRIER SOUND — DEMODULATOR — In general, tors have a non-linear relation- A method used to recover audio this term refers to any device ship between the input voltage information in the PAL system. which recovers the original and brightness, the signal must Sound is separated from video signal after it has modulated be correspondingly predistorted. by beating the sound carrier a high frequency carrier. In Gamma correction is always against the video carrier, television, it may refer to: done at the source (camera) in producing a 5.5 MHz IF that (1) An instrument, such as a television systems: the R, G and contains the sound information. 1 Tektronix TV1350 or 1450, B signals are converted to R /g, ITS — Insertion Test Signal. A 1 1 which takes video in its trans- G /g and B /g. Values for gamma test signal which is inserted in mitted form (modulated onto range from 2.2 to 2.8. one line of the vertical interval the picture carrier) and converts GENLOCK — The process of to facilitate in-service testing. it to baseband. locking both sync and burst of LINEAR DISTORTION — (2) The circuits that recover U one signal to sync and burst of Refers to distortions that are and V from the composite signal. another, making the two signals independent of signal amplitude. completely synchronous. EQUALIZING PULSE — The LUMINANCE — The signal pulses that occur before and HARMONIC DISTORTION — which represents brightness, or after the broad pulses in the If a sine wave of a single fre- the amount of light in the picture. vertical interval. quency is put into a system, and This is the only signal required harmonic content at multiples for black and white pictures, and ENVELOPE DETECTION — A of that frequency appears at demodulation process in which for colour systems it is obtained the output, there is harmonic as a weighted sum (Y = 0.3R + the shape of the RF envelope is distortion present in the system. sensed. This is the process used 0.59G + 0.11B) of the R, G and Harmonic distortion is caused by B signs. by a diode detector. non-linearities in the system. FIELD — In interlaced scan MODULATED — When referring HORIZONTAL BLANKING — to television test signals, this systems, the information for one Horizontal blanking is the entire picture is divided up into two term implies that chrominance time between the end of the information is present. (For fields. Each field contains half of active picture time of one line the lines required to produce the example, a modulated ramp has and the beginning of active sub-carrier on each step.) entire picture. Adjacent lines in picture time of the next line. It the picture are in alternate fields. extends from the start of front MODULATION — A process FM — porch to the end of back porch. which allows information to be moved around in the frequency (FM) is the process by which HORIZONTAL SYNC — the frequency of a carrier signal domain in order to facilitate Horizontal sync is the 300 mV transmission or frequency- is varied in proportion to the pulse occurring at the beginning signal of interest. In the PAL domain multiplexing. See AM of each line. This pulse tells the and FM for details. television system, audio infor- picture monitor to go back to mation is transmitted using FM. the left side of the screen and FRAME — A frame (sometimes trace another horizontal line of called a “picture”) contains all picture information. the information required for a HUE — Hue is the property of complete picture. For interlaced colour that allows us to distin- scan systems, there are two guish between colours such as fields in a frame. red, yellow, purple, etc.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com NON-LINEAR DISTORTION — R-Y — One of the colour differ- U — The B-Y signal after a Refers to distortions that are ence signals used in the PAL weighting factor of 0.493 has amplitude-dependent. system, obtained by subtracting been applied. The weighting is NTSC — National Television luminance (Y) from the red necessary to reduce peak modu- System Committee. The organi- camera signal (R). lation in the composite signal. zation that developed the televi- SATURATION — The property UNMODULATED — When sion standard currently in use in of colour which relates to the ref e r ring to television test signals, the United States, Canada and proportion of white light in the this term refers to pulses and Japan. Now generally used to colour. Highly saturated colours pedestals which do not have refer to that standard. are vivid, while less saturated high-frequency chrominance PAL — Phase Alternate Line. colours have more white mixed in-formation added to them. Refers to one of the television in and therefore appear pastel. V — The R-Y signal after a systems used in Europe and For example, red is highly satu- weighting factor of 0.877 has many other parts of the world. rated, while pink is the same been applied. The weighting is The phase of one of the colour hue but much less saturated. necessary to reduce peak modu- difference signals alternates from In signal terms, saturation is lation in the composite signal. line to line to help cancel out determined by the ratio between VECTORSCOPE — A specialized phase errors. luminance level and chromi- oscilloscope which demodulates QUADRATURE AM — A nance amplitude. It should be the video signal and presents a pr ocess which allows two signals noted that a vectorscope does display of V versus U. The angle to modulate a single carrier not display saturation: the length and magnitude of the displayed frequency. The two signals of of the vectors represents chromi- vectors are respectively related interest Amplitude Modulate nance amplitude. In order to to hue and saturation. carrier signals which are the verify that the saturation of the colours in a colour bar signal is VERTICAL INTERVAL — The same frequency but differ in synchronizing information that phase by 90 degrees (hence the correct, you must check lumi- nance amplitudes with a wave- appears between fields and tells Quadrature notation). The two the picture monitor to go back to resultant signals can be added form monitor in addition to observing the vectors. the top of the screen to begin together, and both signals recov- another vertical scan. ered at the other end, if they are SUBCARRIER — Refers to the also demodulated 90 degrees apart. high-frequency signal used for Y — Ab b r eviation for luminance. QU A D R A TURE DISTORTION — quadrature amplitude modula- ZERO CARRIER REFERENCE — Distortion resulting from the tion of the colour difference A pulse in the vertical interval asymmetry of sidebands used signals. For PAL, subcarrier which is produced by the in vestigial sideband television frequency is 4,433,618.75 Hz. demodulator to provide a transmission. Quadrature distor- SYNCHRONOUS DETECTION — reference for evaluating depth tion appears when envelope A demodulation process in of modulation. detection is used, but can be which the original signal is eliminated by using a synchro- recovered by multiplying the nous demodulator. modulated signal with the out - RF — . In tele- put of a synchronous oscillator vision applications, RF generally locked to the carrier. refers to the television signal TERMINATION — In order to after the picture carrier modula- accurately send a signal through tion process. a transmission line, there must RGB — Red, Green and Blue. be an impedance at the end The three primary colours used which matches the impedance of in colour television's additive the source and of the line itself. colour reproduction system. Amplitude errors and reflections These are the three colour com- will otherwise result. Video is a ponents generated by the camera 75 Ohm system, so a 75 Ohm and used by the picture monitor terminator must be put at the to produce a picture. end of the signal path.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com APPENDIX A: PAL COLOUR BARS

There are several varieties of bars, and EBU colour bars. In B signals are also sometimes PAL colour bars, three of which this case, the 100% and 95% used to describe the various are in common use. These three distinction refers to saturation, types of bars. (Recall from page 9 varieties, shown in Figure 109, however, this convention is that Tektronix vectorscopes use are frequently referred to as not universal. The maximum the 75%/100% designation to 100% colour bars, 95% colour amplitudes of the R, G and refer to amplitude.)

Figure 109. Waveforms and RGB voltages for three types of PAL colour bars.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com APPENDIX A: PAL COLOUR BARS

g Nomenclature. It is confusing to ER´, EG´ and EB´ are the three Saturation(%) = [1— (E min/Emax) ] x 100 use a single number to distin- colour signals. Each parameter is Thus 100.0.100.25 colour bars guish between the various types specified as a percentage of the have a saturation value of 95% of colour bars, particularly if it is maximum voltage excursion if a value of 2.2 is used for not clear which parameter that allowable for PAL colour signals, Gamma. However, CCIR stan- number describes. Furthermore, which is 700 millivolts. dards currently call for a Gamma a single number is inadequate to With this system of nomencla- value of 2.8 which yields a completely and uniquely define ture, the three common types of saturation value of 98% for a given signal. For these reasons, bars can be uniquely described 100.0.100.25 bars. Clearly, then, a four-parameter system of as 100.0.100.0 bars, 100.0.100.25 the saturation nomenclature is colour bar specification has been bars, and 100.0.75.0 bars. These best avoided altogether. developed. The following four numbers can readily be correl a t e d parameters are used to describe with the Red, Green and Blue the signal: signals corresponding to each (a) Maximum value of ER´, EG´ type of colour bars (see Figure 109). or EB´ for an uncoloured bar. Saturation. (b) Minimum value of ER´, EG´ Note that saturation or EB´ for an uncoloured bar. is not included in this list of parameters. Saturation is a (c)Maximum value of ER´, EG´ particularly difficult parameter or EB´ for a coloured bar. to use for uniquely specifying a (d)Minimum value of ER´, EG´ colour bar signal because it or EB´ for a coloured bar. depends on the value of Gamma. Saturation is calculated as follows:

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com APPENDIX B — SINE-SQUARED PULSES

Testing Bandlimited Systems. Fast T Steps. The rise times of transi - rise time square waves cannot tions to a constant luminance be used for testing bandlimited level (such as a white bar) are systems because attenuation and also specified in terms of T. A phase shift of high-frequency T step has a 10%-to-90% rise components cause ringing in the time of nominally 100 nanosec- output pulse. These out-of-band onds (see Figure 111). A 2T step distortions can obscure the has a rise time of nominally inband distortions of interest. 200 nanoseconds. Sine-squared pulses are them- Mathematically, a T step is selves bandwidth limited, and obtained by integrating a sine- are thus useful for testing band- squared pulse. (This is why the width limited systems. T step has a rise time that is only nominally equal to T. The Description of the Pulse. The Figure 110. 2T pulse and 1T pulses for PAL systems. integral actually yields a rise sine-squared pulse looks like time of 0.964T for a T step.) one cycle of a sine wave (see Physically, it is produced by Figure 110). passing a step through a sine- Mathematically, a sine-squared squared shaping filter. pulse is obtained by squaring a half-cycle of a sine wave. Energy Distribution. Sine-squared Ph y s i c a l l y , the pulse is generated pulses possess negligible energy by passing an impulse through a at frequencies above f = 1/HAD. sine-squared shaping filter. The amplitude of the envelope of the frequency spectrum at T Intervals. Sine-squared pulses 1/(2HAD) is one-half of the are specified in terms of half amplitude at zero frequency. amplitude duration (HAD), which Energy distributions for a T is the pulse width measured at pulse, 2T pulse, and T step are 50% of the pulse amplitude. shown in Figure 112. Bandwidth limited systems are tested with pulses having an Figure 111. T rise time step. HAD that is a multiple of the time interval T. T, 2T, 10T and 20T are common examples. T is the Nyquist interval, or 1/2fc, where fc is the cutoff frequency of the system to be measured. For PAL systems, fc is usually taken to be 5 MHz and T is therefore 100 nanoseconds. Most PAL test signals use this default value for T, even though the system under test may have a bandwidth of 5.5 or 6 MHz.

Figure 112. Frequency spectra of T pulse, 2T pulse, and T step.

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Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com For further information, contact Tektronix: World Wide Web: http://www.tektronix.com; ASEAN Countries (65) 356-3900; Australia & New Zealand 61 (2) 888-7066; Austria, Eastern Europe, & Middle East 43 (1) 70177-261; Belgium 32 (2) 725-96-10; Brazil and South America 55 (11) 3741 8360; Canada 1 (800) 661-5625; Denmark 445 (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) 765-6362; United Kingdom & Eire 44 (1628) 403300; USA 1 (800)426-2200 From other areas, contact: Tektronix, Inc. Export Sales, P.O. Box 500, M/S 50-255, Beaverton, Oregon 97077-0001, USA (503)627-1916

Copyright © 1999, 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. 9/99 FL3926/XBS 25W–7075-3

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