Video capture card used as scope.

A video card looks for vertical and horizontal sync pulses to sync the capture memory.

Sound like cheap is out the window.

hamilton

There is no way to bypass (part of the) input circuitry and feed a voltage directly to the adc?

On a sunny day (Thu, 10 Jun 2010 14:57:16 -0700 (PDT)) it happened GiveMeL wrote in :

Not wit ha video card, but many years ago (> 35 I think), I devised a very clever circuit to use normal video display as a scope, so that should also work via your frame grabber, worked fine for me. I will try to explain how to do that:

Some who are familiar with old trick effect generator may quickly see this.

Anyways, start with generating a white dot on the screen.

You create a H frequency ramp, and a V frequency ramp, and a composite sync pulse first.

Now you take 2 window comparators, (window comparators are circuits that give a logical 1 if the signal is in a given range, you can make one out of two normal comparators and an AND gate), anyways you feed the H ramp into one widow comparator and the V ramp into the other. On the other input of the first window comparators you put your vertical scope input, and on the other input of the other one your horizontal timebase. Then you AND the output of the 2 window comparators together and sum that with the composite sync to make a white pulse. With all signals in midrange you get a horizontal white line. The spot diameter is set by the window aperture. It works only slowly. Nice for a heart monitor or such. IIRC I could drop the horizontal timebase and use one window comparator, then it would scan at H rate, I had a 560 kHz signal displayed on a normal color set that way, used fast transistors to make the comparators and diode AND gates. Was for sure 5 MHz bandwidth :-)

By this statement you have no idea of what can or can not be done.

good luck

hamilton

"By this statement you have no idea of what can or can not be done. "

I don't - that's why I'm asking. Looks like you don't know much about it either?

On Jun 10, 6:59=A0pm, GiveMeL wrote: > "By this statement you have no idea of what can or can not be done. " >

He knows enough to know you can't make an apple pie using tomatoes. What he's trying to tell you is the sample rate is based on sync pulses, a specific number of samples per line, lines per frame, frames per second. To just see the video card as a high speed A-D converter ignores all the other tasks that neeed to be taken care of.

G=B2

Thanks G2

Take an old (20 kHz) stereo audio PCM adapter sampling at 44.05594 kHz and it will generate a valid looking video signal that can be recorded on a standard video (cassette) recorder. Each video line contains three samples from the left channel and three samples from the right channel.

Instead of the VCR, you can connect the PCM adapter video output to your video card, which should handle it properly, since there are proper synch pulses :-).

Of course, the bandwidth is only 20 kHz and you have to dig out the PCM bit stream from the video stream and then convert the bit stream to integer values after parity checks.

This brings back memories of trying to record video on an audio cassette recorder when I was a kid. On playback the T.V. actually sync'ed up with what was left of the vertical sync, but the horizontal was a complete mess. The only way to discern the "image" was to watch the original program and then convince yourself that the maelstrom on the screen bore some resemblance. Kind of like a Rorschach test.

Then I learned something about electronics.

-- Joe

Try recording audio on a VTR without a sync signal from the video. The recorder will try to lock onto the missing sync, and the recorded audio will have high flutter and wow. Some promo feeds I used to take from a private HBO feed and copy to cassette tapes for radio stations was missing the sync one day. Since I would just start the recorder and walk away to do other work i didn't know the feed was bad till I had to make the dubs.

Thanks for the answers everybody. I'm off to buy a vcr and and a bw tv.