Okay, it's a little jazzy for an application note draft. But I wanted to make it fun to read. I can make things dull, honest. I just don't like putting that in the portfolio.
Love at First Sight:
Video Installation and Troubleshooting with the TDS3000B Digital Phosphor Oscilloscope
Automatic measurements, optional video modules, and battery-capable portability make the better-than-analog Tektronix TDS3000B the best choice for video measurement.
(Medium close up of technician and information packed waveform behind crowded rack or with ripped up floor tiles and cables.)
Seeing is Believing
A good eye for a waveform is a video technician’s first line of defense. That is why Tektronix, the leader in video measurement, developed the TDS3000B Digital Phosphor Oscilloscope. Its real-time, intensity-graded, full-color display gives technicians the intuitive edge of a legendary Tektronix analog oscilloscope and the full feature set of a digitizing oscilloscope.
Analog Heritage--Digital Perfection
Beauty begins deep within the TDS3000B. Tektronix dedicates unique ASIC hardware to the task of capturing, analyzing and portraying complex dynamic waveforms almost instantaneously. The result? An oscilloscope with the same information-rich display as an analog oscilloscope, but with much more detail. The TDS3000B is better at capturing those elusive anomalies that are invisible on an analog oscilloscope or poorly displayed on a DSO. Tektronix DPO technology gives you a live display that captures anomalies while providing the intensity graded vision you need to interpret the waveform. Both patterns and one-time events are easy to see and understand.
Just as attractive is the TDS3000B’s great performance. The TDS3054B, a favorite for video applications, brings four inputs, 500MHz of bandwidth, and a 5 GS/s sample rate to the measurement of fast video signals. With the TDS3054B’s high bandwidth and fast update rate, you can interpret waveforms with confidence.
To learn more about the DPO difference on the web, visit our Web site at www.stillhavenoideawhatImtalkingabout.com.
Creating the Virtual Test Bench
Some video challenges take a bench full of specialized gear to see and solve. Having to haul extra equipment, like picture monitors and vectorscopes, wherever you go doesn’t make your job any easier. The TDS3000B can lighten your load. Its two optional video modules, the TDS3VID and the TDS3SDI, give the TDS3000B a bench full of extra capabilities at an awesome power to weight ratio.
TDS3VID—The Tiny Video Expert
Assembling a capable video test bench is as easy as plugging in the TDS3VID video module. It transforms the TDS3000B into a troubleshooting vectorscope for standard and HDTV signals complete with graticules for 100% and 75% color bars. Measure chroma levels and correct white balance problems on the spot. The TDS3VID module also gives the TDS3000B picture monitor capabilities with on-screen line selection.
Other TDS3VID capabilities include an Individual Line trigger, IRE and mV graticlules, Custom line/scan rates, and Analog HDTV triggering for emerging standards like 1080i, 1080p, 720p and 480p. The module includes a simple to use Video QuickMenu that keeps all the sophisticated features in reach. Together the TDS3000B and the TDS3VID form a portable, battery-capable, all-in-one measurement tool that weighs less than 5 kilograms with the battery installed.
TDS3SDI—A Serial Hero
In the old movie serials, the hero had to solve impossible problems time after time. In today’s television facilities, that is the video technician’s role—and it gets harder each episode.
ITU-R BT.601 Serial Digital Video has become the digital transfer standard for in-studio use. This is great for the production crew, but a daily cliffhanger for the technical staff. The problem with serial digital video is that you can’t see it, as a familiar waveform at least. The soap opera, the commercial, the satellite sports feed, and the editing suite’s output all look the same—a stream of ones and zeroes.
Enter the TDS3SDI module. When plugged into a TDS3000B, this optional video module does everything a TDS3VID can—and adds a lot more. Feed serial video into the TDS3SDI and it converts the 601 signal into familiar analog output. Four separate cables carry composite and component signals (RGB, Y, Pb, and Pr) to the inputs of your four-channel TDS3000B. Make many standard video measurements, such as eye-diagram checks of the SMPTE 259M digital transport stream, without hauling around extra equipment. You can even work confidently up to 250 feet down from the source because of the TDS3SDI’s cable auto-equalization.
These capabilities make it the perfect module for troubleshooting standard or serial digital video installations, regular maintenance, and sophisticated troubleshooting. Combining the measurement capabilities of the TDS3SDI video module with the better-than-analog TDS3054B creates the smartest (and lightest) video measurement tool you’ve ever carried.
SideBar#1: "Analog Signal Present and Video Picture Checks"
The Signal Present Check is a quick test to confirm that an analog video signal is present at a given test point. It displays a classic analog video waveform showing the basic horizontal-rate of the signal amplitude vs. time. It is especially easy to do with a TDS3000B and a TDS3VID or TDS3SDI module.
Select the IRE graticule and the video triggering controls from the QuickVideo menu. Use the Video Autoset to automatically adjust vertical, horizontal, and trigger settings to bring up the waveform. Figure 1a shows an NTSC video waveform.
Figure 1a. DPO technology brings out vivid waveform detail on the IRE graticule (available with both video modules).
You know you have a signal present, but is it the right signal? Either video module lets you perform a Video Picture Check without running back for a monitor. Just select Video Picture Mode from the QuickMenu. Figure 1b shows you have the news broadcast not the cooking show you were searching for.
Figure 1b. The optional video module lets you see the signal as monochrome display with on-screen line select.
That bright line in the picture is the On-Screen Line Select. You can move it to check the waveform display of any line in the picture.
Side bar#2: "Line Count Triggering"
Sometimes, you need to see lines that aren’t even on the screen. The V-chip programming information, for example, is found on Line 20in the NTSC signal. You can use your TDS3000B and either video module to isolate single lines of video.
Figure 2. Line count triggering displays line 17 on an NTSC broadcast video signal
Both video modules expand the TDS3000B's video triggering capability. While the standard instrument can trigger on All Fields or on All Lines, the video modules add menu selections to trigger on any specific line number in an NTSC, PAL, SECAM, or Analog HD-TV system.
Line Number triggering is a first level QuickVideo menu selection. Choosing "Line Number" sets up the general-purpose knob to scroll through the line numbers. In Figure 2, we chose line 17 on an NTSC signal.
Side bar#3: "Automating Everyday Measurements"
The TDS3000B makes routine video amplitude and timing measurements faster and more accurate. Both video modules provide IRE graticules that allow you to display and to measure amplitude levels visually without converting voltage readings.
For higher accuracy, use video cursors. The CURSOR button brings up a menu to select horizontal bars for amplitude measurements. The general-purpose knob positions the cursors individually to bracket the desired waveform points. Whenever the active cursor moves, the change in the amplitude reading appears automatically on the screen. When the IRE or mV graticule is selected, the amplitude readout is expressed in the appropriate units. Figure 3 shows the cursors and the readout. For timing measurements, both video modules offer a pair of vertical bars.
Figure 3. Cursor measurements allow accurate digital readout on both vertical and horizontal signal details.
You can also use the TDS3000B to automate repetitive measurements. To measure sync width, for example, trigger on the video waveform, and then expand the vertical and horizontal scales until the sync pulse almost fills the screen. Press the MEASURE button to activate the measurement menu. Select the automatic Negative Pulse Width measurement to complete the test.
Side bar#4: "Triggering for Other Standards"
Often computer video monitors, medical displays, security cameras, and other self-contained systems don’t play by NTSC, PAL, or SECAM rules. You can count on the TDS3000B to keep you in the game.
Figure 4. Custom video trigger allows the TDS3000 to trigger on standards such as RS343 (26.2 kHz scan rate).
Start by capturing a stable video waveform on the screen using the oscilloscope's Edge trigger (not Video trigger, because that function is set up for standard 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 trigger MENU button. Select Type>Video and then select Standard>Custom as shown in Figure 4. Scan Rate appears as a menu option. For this signal select Rate 3 (25 to 35 kHz) as the scan rate.
Side bar#5: "Video Recorder "Tape Run" Adjustments"
The fast update rate of the TDS3000B speeds up many time-consuming video maintenance tasks. Helical-scan video tape recorders (VTRs), for example, can be a nightmare to adjust with ordinary oscilloscopes. Even humble VHS front panels hide surprisingly complex electromechanical systems.
A video recorder typically uses a rotary scanner with two playback heads to read the signal from the videotape. The two heads read alternating interleaved tracks of information stored in a helical pattern on the tape.
To align the VTR you need to physically adjust the tape path until it passes across the scanning heads in perfect orientation. Improper adjustment can cause line dropouts—vertical striations in the video image. Even if the VTR has a dropout compensation circuit, poor alignment can degrade overall quality. Moreover, if the VTR has been damaged or if the upper drum of the scanner assembly has been newly replaced, you will need to perform an exhaustive alignment.
Adjusting the tape path requires positioning a series of tape guides while viewing the effect on an RF signal from the video heads. You will need an oscilloscope and an alignment tape. Some 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 assembly is installed, the tape path can be grossly out of alignment—even wrinkling or scoring the first tape you try. Protect your costly alignment tapes by starting a tape you can afford to lose. You can use the "known good" tape for initial adjustments.
To set up the TDS3000B for head alignment, start the calibration tape and connect Channel 2 to the head switching pulse. This becomes the trigger source. Connect Channel 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.
Figure 5. Digital Phosphor technology makes video head alignment as fast and easy as using an analog oscilloscope for the job. a) RF envelope before adjustment; b) After adjustment.
In Figure 5a the heads are out of adjustment. There are "pinched" areas in the waveform. The ideal is to set the alignment screws so the waveform peaks are relatively consistent across the top and bottom. Avoid gross variations in amplitude. The alignment screws are interactive, so it's usually necessary to do the adjustment cycle several times, making small changes each time. Figure 5b shows the waveform after the procedure is finished.
Interestingly, it's usually better not to strive for perfectly flat waveform peaks with uniform amplitude throughout. A small amount of variation actually improves the portability (interchangeability) of tapes recorded on the machine.
The TDS3000B DPO is a great tool for VTR head alignments. Unlike a DSO, the TDS3000B's analog-like real-time intensity grading displays the familiar RF signal envelope seen in every VTR service manual. Without intensity grading, the waveform in Figures 5a and 5b would appear as a pair of indistinct lines, almost useless for the alignment procedure.
The TDS3000B also provides tactile feedback during the procedure. The instrument's extraordinary waveform capture and display update rates ensure that the display immediately tracks any changes as adjustments are made. A continuous adjustment, therefore, appears as a continuous change in the waveform - not a series of abrupt jumps from one increment to the next.
Side bar#6: "Typical Serial Digital Measurements"
The TDS3054B DPO and the TDS3SDI video module make a terrific combination for troubleshooting serial digital video installations. For example, verifying that a serial digital signal is present is as simple as plugging the source into the TDS3SDI module and checking to see if the "SDI" signal button lights.
Once you have confirmed that that you have a serial digital signal, it is easy to perform "eye diagram" measurements. Cable the source to Channel One on the oscilloscope, using one of the 75-Ohm terminators included with the SDI module. Capturing the 270 Mbit waveform requires a high bandwidth oscilloscope such as the TDS3054B. Use the automatic measurements to verify 800 mV peak to peak. The "X" crossings should show minimum jitter. See figure 6.
Figure 6. Eye diagram of a 601 serial digital bit stream.
Decoding the "eye" into its components requires the TDS3SDI video module. Plug the serial digital signal into the module. Then connect the four outputs (composite video and the three component signals) to the four channels of the TDS3054B. Press the QuickMenu and use the soft keys to display the three components. It is important to check the levels of the signals and their relative timing. See figure 7.
Figure 7. TDS3SDI decoding of the 601 bit stream showing luminance, B-Y, and R-Y signals.
The phase information among the three components creates the color information of a broadcast picture. The easiest way to adjust that relationship is on a vectorscope. The easiest way to carry a serial digital vectorscope with you is to combine the TDS3054B and the TDS3SDI video module.
Once you connect the TDS3SDI’s component output to the TDS3054, press the "Vectorscope" button on the QuickMenu soft key. That displays the three components in vectorscope format for easy adjustment. See figure 8.
Figure 8. TDS3SDI vectorscope display.
Go Ahead, You Can Take It with You
Together, your TDS3000B and its optional video application modules create a lightweight, battery capable, virtual test bench that can chase down trouble spots wherever they hide. Whether you need to measure eye diagrams, check color on a vector oscilloscope display, decode serial digital signals, or unravel video artifacts 250 feet down the cable, the TDS3000B with its TDS3VID and TDS3SDI modules is your best choice.
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