>There are also some products which hook to a PC parallel port and
>can grab a "frame" after several passes of full rate video have
>gone by.  This is a bummer if your image is not relatively static
>but OK for something like a security camera or some such. Unfortunately,
>I can't think of any published designs for one of these "cheapo"
>circuits.  Anyone else ?

These are composed of a standard slow ADC with a sample-and-hold at the
input.  The sample and hold is triggered by a delayed horizontal sync pulse.
As each frame comes in, the grabber takes and digitizes typically one sample
from each line.  (i.e. the samples form a vertical line down the picture).
The delay is adjusted for the next frame (field if not interlaced) so the
samples are taken one pixel later, so for a given horizontal resolution of n
pixels, n frames of video are required.  This works out to about 8 seconds
for a 512 x 256 image (for the "el cheapo" system, interlace isn't used; the
two fields are used interchangeably, giving a picture repetition rate of 60
Hz rather than 30)  The ADC on a PIC 16C71 is probably suitable for this.
It may even be fast enough to take two samples per line, sampling the whole
picture in 4 seconds.

One or several PIC chips could do most of the timing functions in a
full-rate frame grabber, (maybe not the lowest few bits of the address
pointer) as well as interface to the ultimate destination for the data.  Of
course, a fairly large external RAM is required, and for full rate a fast
ADC as well, in addition to the usual sync extraction and synchronization.

It's suprising no one has asked how to grab video in *color* yet.

-Mike