Baseband Video Testing with Digital Phosphor Oscilloscopes

Application Note Baseband Video Testing With Digital Phosphor Oscilloscopes

Video signals are complex pose instrument that can pro- This application note demon- waveforms comprised of sig- vide accurate information – strates the use of a Tektronix nals representing a picture as quickly and easily. Finally, to TDS 700D-series Digital well as the timing informa- display all of the video wave- Phosphor Oscilloscope to tion needed to display the form details, a fast acquisi- make a variety of common picture. To capture and mea- tion technology teamed with baseband video measure- sure these complex signals, an intensity-graded display ments and examines some of you need powerful instru- give the confidence and the critical measurement ments tailored for this appli- insight needed to detect and issues. cation. But, because of the diagnose problems with the variety of video standards, signal. you also need a general-pur-

Copyright © 1998 Tektronix, Inc. All rights reserved. Video Basics Video signals come from a for SMPTE systems, etc. The active portion of the video number of sources, including three derived component sig- signal. Finally, the synchro- cameras, scanners, and nals can then be distributed nization information is graphics terminals. Typically, for processing. added. Although complex, the baseband video signal Processing this composite signal is a sin- begins as three component gle signal that can be carried analog or digital signals rep- In the processing stage, video on a single coaxial cable. resenting the three primary component signals may be combined to form a single Component Video Signals. color elements – the Red, Component signals have an Green, and Blue (RGB) com- composite video signal (as in NTSC or PAL systems), advantage of simplicity in ponent signals. Baseband generation, recording, and video signals are the signals divided into separate luminance and chrominance sig- processing where many com- that are not modulated on an binations of switching, mix- RF carrier, such as in analog nals (as in Y/C systems: S-VHS or Hi-8), or main- ing, special effects, color cor- terrestrial or cable transmis- rection, noise reduction, and sion systems. tained separately as discrete component signals (as in RGB other functions may be Figure 1 shows a typical graphics and HDTV systems). applied to the signals. Since video system block diagram. there is no encoding/decod- Notice that in the video sig- Composite Video Signals. For ing process as in composite nal path shown, the signal analog broadcast and cable video, signal integrity is more changes formats between TV applications, the most easily maintained in compo- source and destination. To common signals are compos- nent video systems and design and debug such sys- ite signals which contain equipment, resulting in a tems, test equipment must be more than one signal compo- higher quality image. How- able to examine signals in a nent. In North America and ever, the signals are carried variety of formats. Japan, for example, the NTSC on separate cables. In prac- defines the way that lumi- Conversion tice, this limits the distances nance (black and white infor- over which the signals can be The next step, conversion, is mation), chrominance (color transmitted and requires where the real differences in information), and synchro- careful matching of signal video standards begin. The nization (timing information) paths. RGB signal is converted into are encoded into the compos- three component signals: ite video signal. In Europe, Y/C Video Signals. A com- promise solution, imple- • Luminance signal, Y the PAL standards provide the same function. In the case mented in systems such as • Two color-difference sig- S-VHS and Betacam, modu- nals, often B-Y and R-Y of the NTSC and PAL standards, the chrominance sig- lates the chrominance signals The color difference signals nals are modulated on a pair on a pair of color subcarriers, may be modified, depending of color subcarriers. The but keeps the chrominance on the standard or format modulated chrominance sig- signal separate from the lumi- used. For example, I and Q nal is then added to the luminance signal. This minimizes for NTSC systems, U and V nance signal to form the the luminance/chrominance for PAL systems, PB and PR artifacts of composite systems while simplifying the inter- channel timing issues of component systems. This pair of signals can be carried on a single special cable. Display After transmission, the objec- tive is to faithfully reproduce the processed image. In composite systems, the signal is decoded to component form and then translated to RGB format for display on the monitor. Component video signals go through less processing, being converted directly to an RGB signal for display. Figure 1. Typical video system block diagram.

page 2 longer than the horizontal retrace, so a longer synchro- nizing interval – the “vertical blanking interval” – is employed. No information is written on the video screen during the horizontal or vertical blanking intervals. Each video standard defines a series of synchronization signals that control how the video signal is displayed. PAL signals display a video frame 25 times a second, where a frame contains 625 video lines. NTSC signals display a video frame 30 times a second, but with only 525 lines. Some high-resolution computer monitors display more than 1000 lines with a frame rate of 72 times a second. Note that component signals need timing signals too. The synchronization is often combined with one of the components (such as the green channel). Serial Digital Interface For digital video applica- Figure 2. The synchronization signals in an analog composite baseband video signal provide the timing signals nec- tions, the SMPTE and ITU essary to reproduce a video signal on a display. specify the way that the Analog Video Synchronization Both the camera and receiver video signal is represented Signals must be synchronized to scan and formed into a serial data stream. For example, the Let's take a closer look at an the same part of the image at the same time. This synchro- most common serial compos- actual analog baseband video ite signal is an NTSC signal signal. To reproduce an nization is handled by the horizontal sync pulse, which that is sampled at 14.3 MS/s image, both the camera and with 8 to 10 bits of resolu- the video display are scanned starts a horizontal trace. Dur- ing the horizontal blanking tion. The resulting bit stream horizontally and vertically (143 Mb/s) is encoded with (see Figure 2a). The horizon- interval, the beam returns to the left side of the screen and Non-Return-to-Zero-Inverted, tal lines on the screen might or NRZI coding and scram- be scanned alternately – odd waits for the horizontal sync pulse before tracing another bled so it can be sent over numbered lines first, then 75 Ω coaxial cable. For stu- even numbered lines – as in line. This is called “horizontal retrace” (see Figure 2b). dios, the most common stan- “interlaced” scanning sys- dard samples component sig- tems, or they might be When the beam reaches the nals (Y, PR, and PB) at scanned sequentially, one bottom of the screen, it must 13.5 MS/s with 8 to 10 bits of after another, as in “progres- return to the top to begin the resolution. This bit stream sive” scanning systems. Each next field. This is called the (270 Mb/s) is also encoded vertical scan is called a field. “vertical retrace” and is sig- and scrambled and can be Two interlaced fields make naled by the vertical sync sent over 75 Ω coaxial cable. up a frame. pulse (see Figure 2c). The vertical retrace takes much

page 3 Test Requirements Before discussing measure- than 15,000 waveforms a polation, these samples can ments on video signals, let’s second. be connected to create a con- review the requirements for The record length of a digital tinuous waveform display. the test setup. These require- oscilloscope indicates how However, a scope can also ments include the required many sample points the digitally process the signal oscilloscope specifications oscilloscope acquires in a before it is displayed, and capabilities, signal con- waveform record. The result enabling complex measure- ditioning, and triggering. is a trade-off between detail ments to be made easily. Oscilloscope Requirements and record length, or between For example, you can use the Most oscilloscopes are sample rate and time dura- scope’s Average mode to described by a few basic tion acquired. You can remove the effects of random specifications. The first is acquire either a detailed pic- noise to enable you to make usually bandwidth. A good ture of a signal for a short precise amplitude measure- rule of thumb is to use an period of time (the oscillo- ments. The averaging func- oscilloscope with an analog scope “fills up” on waveform tion, found in the ACQUIRE bandwidth at least five times points quickly) or a less MENU, smoothes the wave- the bandwidth of the signal detailed picture for a longer form by averaging multiple to assure accurate representa- period of time. waveforms together. tion of the signal. (A way to Acquisition and Display Modes HiRes mode filters the sam- estimate the bandwidth of The most critical display issue ples taken during an acquisi- your signal is to divide the for many video engineers is tion to create a higher-resolu- number 0.35 by the 10 to the intensity-graded display. tion, lower-bandwidth signal. 90% risetime of the fastest This display, a familiar char- On the other hand, you may signal component.) acteristic of analog scopes and want to see and measure a The sample rate dictates waveform monitors, shows relatively small noise riding how fast the signal is sam- the signal’s statistical behav- on a relatively large video pled. In theory, the sample ior by varying the intensities signal. For such problems, rate must be at least twice the of the displayed samples. the TDS Zoom Preview bandwidth of the signal. In (The result is that frequently mode allows detailed signal practice, the sample rate on occurring signals are bright, examination and waveform each scope channel should be and relatively infrequent expansion. You can expand 4 to 5 times the bandwidth of details are proportionately and position the waveform in the signal for accurately cap- dim.) The TDS 700D-series both the horizontal and verti- turing signals in a single Digital Phosphor Oscillo- cal direction for precise com- acquisition and displaying scopes provide this intensity- parison of fine waveform them with sin(x)/x interpola- graded display, providing you detail without affecting tion. insight through qualitative on-going acquisitions. Often you will want to intensity information and Other acquisition functions acquire signals repetitively to enabling your eyes to assimi- can make it easy to see noise monitor changes over time. late the subtle details and anywhere in the video wave- Unfortunately, traditional variations of the signal. Since form. The Peak Detect mode digital storage oscilloscopes many digital storage oscillo- captures and displays the actually capture signals at a scopes are not capable of minimum and maximum val- much lower repetition rate acquiring enough data to ues of a waveform, which than analog oscilloscopes. To accurately represent the video shows its worst-case ampli- be sure you have a lively dis- signal, special acquisition and tude excursions. Choosing play of the signal, you will display modes are made avail- the envelope mode causes want to look at the oscillo- able in DSOs to compensate. the scope to accumulate and scope’s waveform capture The basic acquisition mode display the minimum and rate which specifies the rate of a digitizing oscilloscope is maximum values of a series at which signals are acquired the Sample mode, where the of waveforms over time. (in waveforms/second). For waveform is sampled in time Measurement Features example, if you’re looking at and the amplitude of each all lines of NTSC or PAL sig- sample is digitized and dis- If you’re working with NTSC nals, you expect to see more played. With the use of inter- or PAL signals, the TDS

page 4 video graticules help display the CURSOR menu. Horizon- automatically measure a the signal in a familiar for- tal cursors allow you to mea- number of signal parameters. mat. Graticules for NTSC and sure signal amplitudes, with For example, measurements PAL signals are available the readouts available in such as peak-to-peak ampli- from the DISPLAY menu. units of volts or IRE (for tude, sync-pulse width, and When either of these software NTSC signals). Vertical cur- inter-channel timing can be graticules are selected, the sors allow measurement of easily made. Automated mea- oscilloscope automatically signal timing, with readouts surements are selected and scales the video signal to the in seconds, Hertz, or video controlled through the graticule you’ve chosen, line numbers. Paired cursors MEASURE menu. allowing you to quickly allow you to simultaneously assess the captured signal. measure relative amplitude Manual on-screen measure- and timing parameters. ments can be easily made The processing power of the using the cursors. Controls Digital Phosphor Oscillo- for the cursors are found in scope can also be used to

page 5 Signal Conditioning Termination standing wave ratio). An TDS 700D video trigger option Most video systems are example of such a terminator includes a video clamp that designed to deliver a known is the Tektronix AMT75, effectively removes AC hum, amplitude signal into a speci- which is specified to 1 GHz. as well as any DC offset on the fied impedance. Therefore at Improper termination can signal. If the signal has been low frequencies, the measure- result in degraded frequency AC-coupled, the clamp also ment accuracy depends on the response. removes low-frequency varia- signal being terminated in a Video Clamping tions which result as the aver- precise resistance, usually age picture level changes. The Ω A common signal anomaly clamp pod attaches to the 75 . At higher frequencies, encountered in analog video the termination must match input BNC connector and measurements is the low- serves as a pre-processor of the impedance of the trans- frequency hum produced by mission line (usually coaxial the video signal. It provides AC line voltage. This hum, “back-porch” clamping on all cable). In this case, the termi- when not removed from the nation impedance must have a standard video signals. The video signal, causes the signal video clamp also provides flat precise resistance with negli- to drift up and down in the gible reactance (also known as frequency response, allowing display and can cause the trig- accurate video measurements. maximizing the return loss ger point to vary. The and minimizing the voltage

page 6 Triggering The first step toward measur- even, or numeric video fields you can specify the timing of ing video waveforms is get- on the side menu. customized tri-level sync ting a stable waveform. To Since much of the informa- pulses (see Figure 4), select enable you to capture and tion of interest in a video sig- any field rate between 20 and analyze the signal, you must nal is on specific video lines, 200 Hz with up to two digit first trigger the oscilloscope you can choose which partic- resolution, and define the on the signal. There are a ular line to display. Select number of lines and fields in number of advanced trigger the “Line” option in the side your customized format. modes in the TDS oscillo- menu and turn the general- Single-Pixel Triggering scopes to make your job eas- purpose knob or use the key- With more of the video moni- ier. pad to specify the line of tor market moving to flat- interest. The line number Analog Composite Video panel displays, design and appears on the screen to help Triggering debug applications need sin- you keep track. The TDS video trigger is gle pixel triggering and analy- selected by pressing the FlexFormat Triggering sis capabilities. A TDS scope TRIGGER button on the front There are a variety of high- with the video trigger and the panel and choosing “Video” definition video systems “Delay By Events” trigger from the on-screen trigger under development around allows you to define each type menu. By default, this the world. These include the pulse of the device-under- selection automatically sets 787.5/60, 1050/60, 1125/60, test’s system clock as an the scope to trigger on 525- and 1250/50 formats. How- event. Each event then corre- line, 60 Hz NTSC video sig- ever, new formats are still sponds to a pixel, and succes- nals. It also directs the instru- being experimented with. sive events equate to succes- ment to lock on the inter- Certain markets have created sive pixels. laced color field 1 using neg- their own high-definition for- First, connect the video sig- ative sync pulse polarity (see mats and established their nal of interest into Channel 1. Figure 3). own standards. For example, Set up Channel 1, main trig- Use the menus to alter these the medical imaging market ger, to trigger on the video default settings. Using the and the military have devel- signal. Press the TRIGGER “Standard” option, you can oped HDTV standards to fit MENU button on the front also direct the scope to trig- their immediate needs. This panel and select VIDEO trigger on PAL/SECAM, HDTV, can add to the confusion ger. Select appropriate stan- and a variety of custom video when searching for video test dard and parameters to trig- signals. Or select “Sync and measurement instrumen- ger on the interesting section Polarity” and change to posi- tation. of the signal. tive sync if the portion of the The TDS video trigger option Connect the system reference circuit you are debugging has provides a solution for cus- clock to Channel 2. Set the inverted the video signal. tomized HDTV triggering delay trigger to use Channel 2 Select “Field” in the main needs. With the Flex- as its source by pressing the menu and choose all, odd, Format™ triggering mode, SHIFT and TRIGGER MENU

Figure 3. The TDS video trigger allows convenient selection of video standard, channel, sync polarity, and field and line. Figure 4. The FlexFormat triggering mode allows you to define the start and stop times of tri-level sync pulses for both odd and even fields.

page 7 buttons on the front panel laid upon another, to form a correlated. Or, the oscillo- and select Channel 2 as the consolidated image of the scope may trigger on the data source of the delay trigger. data pulses which resembles itself, wait for a few unit Now select Delay by Events. an eye. In general, the larger intervals, and then acquire Turn on the Delay Trigger by the opening of the center of enough waveforms to build a going to the Horizontal menu the eye, the better the perfor- display. This can be done and selecting the Delayed mance of the system under with a delayed timebase with Only time base. test. A wider vertical opening delay by time or events. shows a greater noise toler- Now, you can go back to the An easier method is to use an ance, while a wider horizon- Delay Trigger menu and dial eye diagram trigger. Select tal opening indicates more in the event you want to see, the COMM trigger type from jitter tolerance. In other or enter the appropriate num- the TDS 700D TRIGGER type words, excessive amplitude ber on the keypad (see menu and NRZ from the code noise or timing jitter will Figure 5). menu. Then when you select tend to close the eye. the serial digital video stan- Serial Digital (NRZ) Triggering The oscilloscope may trigger dard from the list, the oscillo- The most common way to on the rising edge of the scope is automatically set up characterize a serial digital serial system clock and cap- to display an eye diagram of signal is by examining an eye ture the data that coincides the signal (see Figure 6). diagram. This display is a with the clock edge. This composite display of many method requires that the waveform acquisitions, over- clock and the data signals be

Figure 5. The system clock (bottom waveform) serves as the Delay Trigger Figure 6. Setting up an eye diagram is easy using the NRZ communication for the video signal (top waveform). With Delayed by Events, each event cor- signal trigger. responding to a pixel, you can observe the video signal at each pixel.

page 8 Video Signal Measurements Video Signal Monitoring H-rate Intensity-graded Displays and select the XY mode. If a Whether you are monitoring of Live Video B-Y signal is connected to analog or digital video sig- The most basic analog video Channel 1 and an R-Y signal nals, an oscilloscope with an display is the horizontal-rate is connected to Channel 2, intensity-graded display display of the signal ampli- the scope will imitate a famil- which is tailored for video tude vs. time. This can be iar vectorscope display. Also, applications can be your done most easily by edge- the intensity-graded display most valuable debug tool. triggering on the leading edge shows details in the signal Subtle variations in the sig- of sync. As shown in Figure which are not visible on ordi- nal, which are not visible on 7, a Digital Phosphor Oscillo- nary DSOs. a DSO display, can spell the scope with an intensity- Intensity-graded Displays of difference between a video graded display (and a wave- Digital Video Eye Diagrams system that works and one form capture rate high Intensity-grading is also that doesn’t. enough to capture every line) important for monitoring eye provides the familiar wave- diagram displays, where you form monitor H-rate want to qualitatively examine display. the signal variations over XY Displays of time, whether the variations Chrominance are due to noise or timing jit- The Digital Phosphor ter. Intensity-graded displays, Oscilloscope’s XY available with analog oscillo- display mode allows scopes and Digital Phosphor you to display one Oscilloscopes, combined signal against another with a high waveform cap- in a manner similar to ture rate, give you the best a vectorscope. Press method of capturing and FORMAT selection in identifying infrequent the DISPLAY menu anomalies.

Figure 7. A horizontal-rate waveform monitor display, showing the effect of an intensity-graded display on the oscilloscope.

page 9 Analog Signal Measurements Amplitude Measurements TDS 700D’s video cursors to mon signal and adjust the Amplitude measurements make the same measurement. channel deskew with the gen- can be made a number of Finally, if you want to ana- eral-purpose knob until the ways with an oscilloscope. lyze variations over time, the traces line up on the display. For example, to measure the scope can make a number of Now, connect the signals of peak-to-peak amplitude of measurements automatically, interest to the scope channels the NTSC burst signal, you and accumulate the measure- and adjust the channel timing can simply compare the sig- ment statistics. controls to match the signals nal to the TDS 700D’s IRE Timing Measurements (see Figure 9). video graticule (see Figure 8.) Timing measurements are The oscilloscope can also You can also use the especially critical for compo- make timing measurements nent analog systems automatically and accumu- because they require late statistics on those mea- precise inter-channel surements. For example, to timing. The most measure sync width, trigger important use of a on the leading edge of sync, multi-channel oscillo- turn on HiRes acquisition scope can be to dis- mode, and adjust the hori- play the relative tim- zontal and vertical controls ing differences so the sync pulse fills most of between channels. the display. This optimizes the accuracy of the measure- Before you can accu- ment system. Now turn on rately display the the negative pulse width multiple channels, measurement in the MEA- you need to match SURE menu. To monitor the the probe path delays. mean (µ) and standard devia- This can be done tion (σ) of the pulse width with the deskew fea- measurement, enable the ture, found in the measurement statistics (see TDS 700D’s VERTI- Figure 8. An example of amplitude measurements on an NTSC signal. The Figure 10). peak-to-peak amplitude of the burst packet can either be measured visually CAL menu. Connect with the graticule, or with the video cursors (note cursor readout in upper both probes to a com- right corner).

Figure 9. Inter-channel timing is of critical importance in component analog Figure 10. Automatic timing measurements provide an easy and accurate video systems. The display shows the relative timing of the luminance and method of repetitively measuring basic signal parameters. one of the color-difference signals (after the cable delays were equalized with the channel deskew controls).

page 10 Serial Digital Video Measurements Jitter Measurements by statistically analyzing the To determine if a serial digi- Timing jitter on a signal can waveform, using the Digital tal video signal complies affect a receiver’s ability to Phosphor Oscilloscope’s his- with the standard, all rele- decode a video data stream. togram capability. Display vant parameters must be The effects are readily seen and draw a histogram box examined to see whether they on an eye diagram because around the rising edge, are within specifications. jitter shrinks the opening of falling edge, or eye crossing Measuring the parameters the eye. As the jitter where the jitter is to be mea- individually would be a increases, the data transition sured, and then have the tedious business and could points move closer and closer oscilloscope draw a his- easily result in errors. To to the decision point of the togram of the delay of the simplify the verification task, receiver, eventually increas- edge from the trigger point. If the video standards specify ing the bit-error rate of the the histogram of the place- the shape of compliant sig- system. ment of the signal edge is a nals by defining a mask. You normally distributed curve, simply overlay the mask on Jitter comes in two types: the standard deviation is the eye diagram and can deterministic and random. equal to the RMS jitter of the immediately see if the signal Deterministic, or data-depen- waveform. You can also turn complies by fitting into the dent, jitter is caused by the on the observed RMS jitter allotted areas of the mask (see pattern of data bits preceding (standard deviation) or other Figure 12). the current bit in the data histogram measurements to stream. By triggering on Advanced communication further characterize the jitter oscilloscopes have built-in repetitive data patterns and (see Figure 11). measuring the variation in standard masks, which you edge placement, you can Mask Testing can select from a menu. characterize deterministic jit- As discussed before, an eye These oscilloscopes also pro- ter components. Such an diagram reveals a lot about a vide calibrated, variable time analysis can be time-consum- serial digital signal, espe- delay and voltage scales, can ing but useful for detecting cially about the relative mar- automatically adjust the sig- problems early in the design gin available for noise and jit- nal to fit the mask, and can process. ter. It represents the most even count the number of important time-domain signal waveforms acquired and the Random jitter, on the other number of mask violations, hand, is due to random noise characteristics in one display: rise time and fall time, pulse or “hits,” for faster and more in a system and is not corre- accurate testing. lated to the data. It can be overshoot and undershoot, characterized and measured ringing, duty cycle, jitter, and noise.

Figure 11. Characterize random jitter on a digital video signal with a his- Figure 12. Mask testing provides a convenient and reliable method for verify- togram. Notice the bi-modal nature of the histogram. Also, measurements on ing the compliance of serial video signals to industry standards. In this the histogram are shown at the right of the screen, indicating such character- example, a minimum of 100 waveforms was compared to the mask, with no istics as the observed peak-to-peak jitter. errors (0 “hits”).

page 11 Conclusion make a variety of common ture rate, and abundance of In this application note, baseband video measure- waveform data, this general- we’ve demonstrated the use ments on a variety of com- purpose instrument is the of a Tektronix TDS 700D- plex video signals. With the tool of choice to debug, char- series Digital Phosphor Oscil- power of the intensity-graded acterize, and verify your loscope to quickly and easily display, high waveform cap- video circuits and systems.

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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 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. 4/98 TD/XBS 55W–12113–0