The oscilloscope can be a very useful tool in analyzing video signals, particularly in situations where troubleshooting or detailed calibration is necessary. But just about anyone who has set up an oscilloscope for displaying video signals knows that the process is often frustrating, entailing rather extensive trial and error. In an attempt to yield a stable, usable display, you're constantly fumbling and experimenting with the scope controls. And if that's not enough, chances are you're also faced with the challenge of matching a specific spot of interest on the video image with what you're seeing on the oscilloscope.

Enter Scope-Trigger, a new Patented technology exclusive to Extron. Featured on the VTG 400D and VTG 400DVI reference A/V test generators, Scope-Trigger greatly simplifies display of video signals on an oscilloscope to a quick, easy, and painless "1-2-3" process.

Why Are Video Signals Difficult To Analyze On an Oscilloscope?

Video signal analysis using an oscilloscope is difficult because of the lack of a reliable trigger signal that the scope requires as a synchronization reference. As a result, the user attempts to subjectively control how and what part of the video signal is displayed, usually with a low degree of certainty. Oscilloscopes may be capable of triggering to the embedded sync in standard video signals, especially with the addition of an external device. However, display of HDTV and high resolution computer-video signals is troublesome.

Several challenges arise in successfully using an oscilloscope with video signals. Chief among them is achieving a stable display. This may be possible if the video image is a static test pattern. However, with dynamic video, stable scope display becomes much more difficult when the scope is relying on the video signal itself as its trigger. Similarly, a stable display cannot be guaranteed if you are analyzing various points within a circuit or a system. For RGB video, steady display may be possible by using the horizontal (H) or vertical (V) sync signals as the external trigger for the oscilloscope.

But more often than not, achieving stable display is just the beginning. Typically, your ultimate objective will be to analyze and troubleshoot some sort of localized visual glitch on the screen. Since you're feeding the same video signal for the display into the scope, you will need to somehow ensure that this glitch is being shown on the scope. To accomplish this, you'll first have to count, or estimate, from the top of the screen, the line where the glitch occurs. Then, with your scope, you've got to fumble with the trigger delay control, and try to sequence through the lines of video along the signal by counting the number of horizontal sync intervals. There are 240 lines to count within a field of interlaced standard definition video. But try keeping track among 1,024 lines in a frame of SXGA computer-video!

Scope-Trigger To the Rescue

With Scope-Trigger, you can now bypass all the hassle in setting up an oscilloscope for video signal analysis. In fact, all you really need to do with the scope is just ensure that the connections are correct! The VTG 400 then takes care of the rest.

Essentially, the way Scope-Trigger works is that you determine the location on the screen to analyze, and then the oscilloscope displays the signal at that exact location. This is possible because the VTG 400 generates a special trigger signal for the oscilloscope, with the triggering pulse timed precisely to the location on screen you have selected. Experimenting with the scope controls or counting lines of video is no longer necessary!

VTG 400DVI

Scope-Trigger in Three Easy Steps

1. Set-Up Set up the system as illustrated at left. A duplicate of one of the RGB video signals for the display is input to the oscilloscope. (Component video, S-video, and composite video signals can be analyzed as well.) Be sure to connect the Scope-Trigger output of the VTG 400D to the external trigger input of the oscilloscope. Set scope to reference an external trigger signal. Also, be sure that you have selected the desired video test pattern (the Checkerboard pattern is shown illustrated).

2. Identify the Location Engage Scope-Trigger by pressing any of the dedicated buttons on the main panel. An orange crosshatch cursor appears on screen, which you can also change to a single pixel cursor. Using the directional buttons, position this cursor at any desired location (pixel).

3. Oscilloscope Display Now, look at the oscilloscope. The trace that is displayed is centered precisely at the location of the Scope-Trigger cursor.

VTG 400: Looking Beyond the Visual Image

In addition to analyzing areas of the image on-screen, Scope-Trigger enables convenient examination of the horizontal and vertical sync intervals. This is useful when checking the polarity or integrity of the sync waveform, or for the presence of serration pulses. To display sync on your scope, simply position the cursor beyond the visible (active) image area, using the pixel counter for the total image area (shown on the VTG 400's LCD display) as a guide. If you're working with RGBHV, swap the video input to the scope for either the H or V sync cable, depending on whether you're analyzing horizontal or vertical sync.

The Windows® software that is included with the VTG 400 enhances and expedites the functionality of Scope-Trigger. Using your mouse or other pointing device, you can simply "click" anywhere within the image to locate the cursor. Analyzing sync intervals is much easier since all you'll need to do is click outside of the active image area.

A Convenient, Universal Tool for Video Signals

For measurement and analysis of video signals, professionals in broadcasting and video production use specialized oscilloscopes known as waveform monitors and vectorscopes. However, these devices are specialized for television signals including NTSC, PAL, and HDTV, and are not compatible with computer-video. The VTG 400 and Scope-Trigger, along with a conventional oscilloscope, allow for analysis of any video or computer-video signal, from composite video to HDTV, to ultra-high resolution computervideo (such as QXGA, or 2048 x 1536). Extron recommends the use of an oscilloscope with 200 MHz bandwidth to accommodate computer-video resolutions up to UXGA (1600 x 1200).