I want to send a video by telephone line, but i dont know how to
start
Well, first you should learn a bit about the nature of a video signal, and the frequency response of telephone systems. The audio in telephones is of a very limited band. Something like 500-3000 Hz (don't quote me on that, I don't actually know it). So what you need is something that can perhaps digitize your video signal. Real analog video signals are at a very high frequency which would never make it across a phone system. With a digital signal, you'd need the sampling rate to be within the audio range of the telephone system, but would otherwise probably not have a problem. The drawback of that is having such a low sampling rate leads to low quality and low frame rate. That's why nobody has a good looking video telephone yet.
im lookin for a circuit that modulate the telephone line with a
viedo sinal and at the other end it demodulate to see this imagen in
a tv.
I'm sure it's possible to make..something..but don't count on high quality. The phone system is one of the least reliable systems as far as quality goes. Nearest I can see is some sort of complicated audio range beat frequency type thing but it would probably be next to impossible to make, forgetting the distortion and signal loss over phone lines.
You'd be better off going digital.
I'm no expert on analog to digital stuff, but I know there are lots of analog to digital chips available. You'd just lose a whole lot of quality for the low sampling rate necessary.
What exactly were you planning on doing with this. Maybe there's a better way to transmit..?
-Tavys
