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Baseband Video Testing with Digital Phosphor Oscilloscopes

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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 lumi- nance 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 stan- dards, 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 lumi- nance 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 com- posite 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 pro- cessing, 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 ver- tical 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-resolu- tion computer monitors dis- play 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 com- bined with one of the compo- nents (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 “horizon- tal 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.
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