Using the TDS3000 DPO for Video Maintenance and
Contents
1. Tektronix new TDS3000 Dig ital Phosphor Oscilloscopes DPOs are uniquely suited to video applications They offer the real time intensity graded display that s so important for video measure ments and they include the full feature set of a digitizing oscilloscope With a DPO the NTSC baseband waveform appears with the familiar brightened areas on the sig nal peaks and the sync burst When viewing a full frame test signal the DPO displays the color bar waveforms with smooth level tops undis torted by digital aliasing The TDS3000 Series DPOs deliver uncompromised analog style signal viewing for video mea Application Note Using the TDS3000 DPO for Video Maintenance and Service Measurements surements as well as advanced digital measure ment and analysis features The TDS3000 Series also offers the compelling advan tages of compact footprint combined with exceptional measurement performance Bandwidths from 100 MHz to 500 MHz are available The instruments are light only 3 2 kg and small Optional battery power provides com pletely untethered operation This makes the TDS3000 an ideal choice for video main tenance work in remote trucks and helicopters or even those behind the con sole video checks in the broadcast studio Signal Present Check is the Most Basic Undoubtedly the most com mon video measurement is the live video check It s mainly intended to confir2. surprisingly complex electro mechanical system A video recorder reads the signal from the video tape by means of a rotary scanner that houses two playback heads which contact the tape The scanner s rotation causes the two heads to read alternating inter leaved tracks of infor mation stored ina helical pattern on the tape Figure 4 Custom video trigger allows the TDS3000 to trigger on standards such as RS343 26 2 kHz scan rate Aligning aVTR is a matter of physically adjusting the tape path so that the tape passes across the scanning heads in perfect orientation Improper adjustment can cause line dropouts vertical striations in the video image In profes sional video equipment this may be corrected to some degree by a dropout compen sation circuit but in any event dropouts are undesirable Adjustments are made by positioning a series of tape guides while viewing the effect on an RF signal from the video heads Typically an exhaustive alignment is nec essary only when the VTR has been damaged or if the upper drum of the scanner assembly has been newly replaced The alignment procedure requires an oscilloscope and an alignment tape Some pro fessional VTR manufacturers offer an optional adjustment tool for setting the tape guide screws TIP It s wise to run a known good tape through the machine before starting the actual adjustments When a new scanner assem bly is installed the
3. tape path can be grossly out of alignment to the point where the first tape that goes through the machine gets wrinkled or scored Protect your costly alignment tapes start the alignment proce dure by running a tape you can afford to lose Make your initial coarse adjust ments with this tape Ch2 To set up the TDS3000 for head alignment start the cali bration tape and connect Channel 2 to the head switch ing pulse This becomes the trigger source Connect Chan nel 1 to the output of the high level RF amplifier receiving the signal from the video heads It s best to set the time base so that two and only two cycles of the signal are visible on screen The thicker portions of the waveform are the areas of head switching The screen image should be centered on one of these events Figure 5a shows the result In Figure 5a the heads are out of adjustment There are pinched areas in the wave form The ideal is to set the alignment screws so the waveform peaks are relatively consistent across the top and bottom There should be no gross variations in amplitude The alignment screws are interactive though so it s usually necessary to do the adjustment cycle several 100mvV A times making small changes each time Figure 5b shows the waveform after the proce dure Is finished Interestingly it s usually bet ter not to strive for perfectly flat waveform peaks with uniform amplitude
4. through out A small amount of varia tion actually improves the portability interchangeabi ity of tapes recorded on the machine The TDS3000 DPO makes an ideal oscilloscope platform for VTR head alignments Unlike any other type of digi tizing oscilloscope the TDS3000 s analog like real time intensity grading can display the familiar RF signal envelope seen in every VTR service manual Without intensity grading the wave form in Figures 5a and 5b would appear as a pair of indistinct lines almost use less for the alignment procedure The TDS3000 provides tactile feedback during the proce Ch2 100mvV dure The instrument s extraordinary waveform cap ture rate ensures that the dis play immediately tracks any changes as adjustments are made A continuous adjust ment therefore appears as a continuous change in the waveform not a series of abrupt jumps from one incre ment to the next Conclusion The TDS3000 DPO is a versa tile tool for all kinds of televi sion and video measure ments Its unique intensity graded display surpasses even analog oscilloscopes in legibility responsiveness and ease of interpretation Standard digital oscilloscope features as well as video specific functions included in the optional video mea surement module make the TDS3000 an ideal tool for video service and mainte nance applications B Figure 5 Digital Phosphor technology makes video head
5. ASURE button Triggering for Other Standards Obviously not every video system conforms to the NTSC PAL or SECAM for mats As a rule computer video monitors medical dis plays security cameras and other self contained systems aren t designed to interface directly with broadcast video equipment These therefore may not adhere to the normal 525 or 625 line standards Thus it is sometimes neces sary to use an oscilloscope to determine the line rate The TDS3000 offers an easy solution The first step is to capture a stable video wave form on the screen using the oscilloscope s Edge trigger not Video trigger since that function is set up for stan dard line rates Then use the Vertical Bars cursors accessed by pressing the CURSOR button to measure the time between sync pulses The Time Units menu button lets you select the reciprocal 1 time of the reading Next press the trig ger MENU button Select Type gt Video and then select Standard gt Custom as shown in Figure 4 When this selec tion is made the Scan Rate will appear as a menu option In this case select Rate 3 25 to 35 KHz as the scan rate Scan rates up to 65 KHz can be selected Video Recorder Tape Run Adjustments Helical scan video tape recorders VTRs are a cor nerstone of both professional and consumer video applica tions But they can be a main tenance challenge Even the humble VHS machine s front panel masks a
6. alignment as fast and easy as using an analog oscilloscope for the job a RFenvelope before adjust ment b After adjustment For further information contact Tektronix World Wide Web http www tektronix com ASEAN Countries 65 356 3900 Australia amp New Zealand 61 2 9888 0100 Austria Central Eastern Europe Greece Turkey Malta amp Cyprus 43 2236 8092 0 Belgium 32 2 715 89 70 Brazil and South America 55 11 3741 8360 Canada 1 800 661 5625 Denmark 45 44 850 700 Finland 358 9 4783 400 France amp North Africa 33 1 69 86 81 81 Germany 49 221 94 77 400 Hong Kong 852 2585 6688 India 91 80 2275577 Italy 39 2 25086 501 Japan Sony Tektronix Corporation 81 3 3448 3111 Mexico Central America amp Caribbean 52 5 666 6333 The Netherlands 31 23 56 95555 Norway 47 22 07 07 00 People s Republic of China 86 10 6235 1230 Republic of Korea 82 2 528 5299 South Africa 27 11 651 5222 Spain amp Portugal 34 91 372 6000 Sweden 46 8 477 65 00 Switzerland 41 41 729 36 40 Taiwan 886 2 2722 9622 United Kingdom amp Eire 44 0 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 1 503 627 6877 Copyright 1998 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
7. e optional Video Applica tion Module expands the TDS3000 s video triggering flexibility While the stan dard instrument can trigger on All Fields or on All Lines the video module adds menu selections to trigger on any specific line number in an NTSC PAL or SECAM system Line Number triggering isa first level QuickVideo menu selection on a video module equipped instrument Choosing Line Num ber sets up the general purpose knob to scroll through the line numbers 1 i 7 Time through 263 Odd source B Ch 112 3 4 Field Line Lines or 1 through 262 Even Lines for NTSC 1 through 625 for PAL and SECAM Automating Everyday Measurements The TDS3000 makes routine video amplitude and timing measurements easy Using the IRE graticule provided by the video module it s a sim ple matter to display any part of a video waveform and visually measure the ampli tude levels Unlike an analog oscilloscope there s no need to convert voltage readings to IRE levels Video cursors hasten the pro cess and provide improved accuracy The CURSOR but ton brings up a menu to select horizontal bars for amplitude measurements The general purpose knob positions the cursors individ ually to bracket the desired waveform points Whenever the active cursor moves the change in the amplitude reading appears automati cally on the screen When the IRE or mV graticuleis selected the amplitude read out is exp
8. m that the video signal is pre sent at a given test point say the output of a switcher But it can also reveal irregulari ties that affect picture quality The live video check isa basic horizontal rate display of the signal amplitude vs time Using a TDS3000 equipped with the optional video module the video check is as simple as a conti nuity test Both the IRE graticule and the video trig gering controls appear on the QuickVideo menu The Video Autoset function automati cally adjusts vertical hori zontal and trigger settings to bring up a waveform immedi ately if there is one Figure 1 shows an NTSC video waveform captured in this manner With the TDS3000 the whole process takes only a few seconds The TDS3000 s standard video trigger makes it easy to synchronize on Odd Even All Fields or All Lines of the signal NTSC PAL or SECAM These are standard selections under the TRIG GER TYPE menu Since the video check is mainly a sig nal present test it s often unnecessary to measure IRE or mV units Thus the con ventional oscilloscope gratic TRIGGER 7 A Trig Type video Holdoff ule is adequate for routine live video checks Line Count Triggering Sometimes it s necessary to view asingleline of the video waveform For exam ple the programming infor mation in the NTSC signal is sometimes found on Line 20 To see this data you must isolate that line Th
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10. ressed in the respective units Figure 3 shows the cursors and the readout A pair of vertical bars is also available for tim ing measurements Odd Even All Fields kaitka All Lines i OLine 17 Standard PETALING PASTA a SECAM Lastly the TDS3000 can per form automated measure ments This function is very useful for repetitive measure ments To measure sync width for example trigger on the video waveform then expand the vertical and hori zontal scales such that the In this example line 17 on an NTSC was chosen The line number is displayed prominently on the screen as shown in Figure 2 WAY ss i i A ee ie CRE a BTS 14BmV M10 0ps Chi All Lines MENU AUTOSET GRATICULE ACQUIRE CURSOR Quick video Lines Full Fast Trig ei Video Fields IRE mY Normal HBar VBar Figure 1 DPO technology brings out vivid waveform detail on the IRE gratic ule available with video module gt A 66 9IRE 14 41RE Cursor Function chi i M4 00pus 10 00 143mvV NM 10 0ps el Chi O 17 Time Units Seconds Function 10 00 M Pars Figure 2 Line count triggering finds line 17 on an NTSC broadcast video signal Figure 3 Cursor measurements allow accurate digital readout on both verti cal and horizontal signal details sync pulse fills most of the screen Select the automatic Negative Pulse Width mea surement in the measurement menu which is activated by the ME
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