Video Overlay

Video Overlay


OK, sportscasters do it on TV ... how do I superimpose data from my computer screen on a video image from a camera in the lab? Subtitle: the Grand Ultraview Pro.

See if this sounds useful. You're running a test. You've got a nifty little computer, equipped with a nice 16-channel A/D card, monitoring a dozen instruments connected to a device under test. You've also got a video camera pointed at the device, hoping to catch something interesting. Well, sure enough, something interesting happens. The device shoots out a few sparks, and then catches fire (don't laugh, I used to run tests like this routinely, safety-testing consumer products).

Reviewing the data collected by the computer, you see several anomolous jumps in power consumption, possibly arcing or some other malfunction, followed by a steep rise in temperature, then a huge changes in all parameters. However, you have no way of knowing exactly which of these events correspond with the sparks seen on the video. Did the temperature increase preceed the sparks? Who knows? Durn, if only there were some way to superimpose some of the computer data on the video screen, especially elapsed time, so the computer record and videotape could be linked in time.

There is a way, and it is not prohibitively expensive. Several devices are on the market, under $1000, which will do the job. The unit I've used, the Grand Ultraview Pro, is priced around $400. The class of product you need is a "scan converter with genlock adapter."

It is very important that you do not confuse the device you need with a simple scan converter. If all you need to do is prepare a low-resolution videotape of your computer's screen, a scan converter will do just fine. There are great numbers of these available, some incorporated into computer video cards, for under $100. They're commonly used for presentations. However, they won't combine your computer screen data with a video image, unless you happen to be using software that brings the video image into a window on your computer - something I've never seen in data acquisition software, and which would almost certainly slow the computer down severely.

What the genlock adapter does is synchronize the two video images. The device you need not only converts the computer screen image to the NTSC format (or PAL, for some non-US systems) used by your VCR, it stores it, and flashes the frames out synchronized with the signal from the camera. Furthermore, it puts the camera's image only in selected areas of the screen. The professional broadcast systems use something called chroma-keying, in which one image is substituted for only a specific color (you probably see this every night on the weather segment of your local news, the meterologist standing in front of a blue or green backdrop, which you see as a weather map). The Grand Ultraview Pro keys to black: the camera's image will substitute for any area of the computer's screen which is black.

Connection of this device is fairly simple, and there's only one twist: you can't use a camcorder. Actually, you can, but you can't record the merged image on the camcorder, it must be used only as a camera. You simply connect a video cable between the output of the camera and the camera input of the Grand, connect a cable from the Grand to the VGA connector of your computer, and plug the computer's monitor into the Grand. A last cable then goes from the Grand to the video input of an ordinary VCR. The camera image will not show up on the computer's screen, but it will show up on a TV monitor connected to the VCR. You can also patch in area microphones, or even the computer's sound card, to put sound on the videotape.

Special software is not required. The Grand will work with any computer that uses a standard VGA connection. All that is required is that you have control over the appearance of the screen, and can make the needed data large enough to show up on the videotape. You need to establish a large black area on the computer's screen, where you wish the video image to show up. You can display the computer data in any form you wish, but I find the best is light-colored blocks containing large numerals. These criteria are easy to produce with off-the-shelf data acquisition packages such as LabTech and Labview, and also are easy to achieve in DOS, or any other system you can program yourself.

One final bit of advice: avoid color saturation. Broadcasters are not supposed to transmit colors more than 70% saturated, i.e., no pure red, green, or blue at full strength. Colors that intense and pure don't record well on a VCR and may bloom on a TV monitor. Choose blended colors when designing your computer screen layout, and limit their intensity.

I don't sell video equipment. I'm just a consultant, looking for work, who happens to be pretty darned good at computer data acquisition. If you think one of these little gems is just what you need to round out your data acquisition system, or to overlay computer graphics on your videotapes, you'd like some help setting one up, and you live close to Northern Virginia, I'd be happy to come over and sell you a little of my time.