Frame Grabbers

The final piece of equipment to discuss is the frame grabber. The frame grabber digitises each RS-170 or PAL frame and saves it to a location in computer memory, where the DVA program applies its thresholds and extracts the relevant data. With the memory and speeds of today's computers and frame grabbers, the DVA program can process nearly every frame in real time. Because RS-170 and PAL signals are a sequence of analogue scan lines, the frame grabber will effectively resize the pixels. For example, the video camera may have 780 detector pixels across the CCD. This row of pixels gets converted to an analogue signal, the scan line. The frame grabber then re-digitises the scan line into a row of, say, 256 pixels. Recall from the section on sensitivity that the sensitivity of the system is based on the number of detector pixels across the Airy disk. The frame grabber pixelation value chosen is simply a matter of processing speed.

The choice of frame grabber is a question of price and custom programming. At present, work is underway to port a DOS DVA program using an Imaging Technology Plus frame grabber (no longer available) to a Linux DVA program using the Matrox Meteor frame grabber (visit http://www.chara.gsu.edu/~nils/ dva.html). The Matrox Meteor costs about US$500, and a clone is available from Omnimedia Technology Inc. for about US$400. The Matrox Meteor is chosen because a device driver has been written for the Linux operating system. Matrox also makes a frame grabber called the Meteor II. The Meteor II is not yet supported under Linux.

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