Re: [Electronics_101] Thelephone line modulation with tv signal
>I want to send a video by telephone line, but i dont know how toWell, 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 aI'm sure it's possible to make..something..but don't count on high
>viedo sinal and at the other end it demodulate to see this imagen in
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
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..?
- The audio bandwidth over telephone connections is too low to support full-motion video. However, amateur radio operators have been sending color pictures over the airwaves using the small audio bandwidth for many years. This would work just as well over the telephone.
Read about SSTV (slow scan television) which sends pictures with audio modulation. It fits neatly within a 2.6 kHz bandwidth, and is suitable for telephones and all sorts of audio recordings. Search the Internet for SSTV and you'll get lots of hits, such as this one: http://www.ultranet.com/~sstv/
To make the pictures fit into audio, each frame takes a few seconds to a couple minutes, depending on the picture's resolution and number of colors. You give up motion, but the pictures are quite nice.
Equipment? SSTV can be built quite cheaply using a computer with a sound card. Free software is available from ham radio web sites.
There is another alternative, too. If you're going to use a computer anyway, then an ordinary modem connection is better optimized to send data and digital pictures across the phone lines.
Good luck, Barry
I want to send a video by telephone line, but i dont know how to
start. I'm looking for a circuit that modulate the telephone line with a
video sinal and at the other end it demodulate to see this image in
a tv. Please if any one can give me an idea or help, send me that.
- --- In Electronics_101@y..., ramiroi@e... wrote:
> I want to send a video by telephone line, but i dont know how toa
> start im lookin for a circuit that modulate the telephone line with
> viedo sinal and at the other end it demodulate to see this imagenin
> a tv. Please if any one can give me an idea or help, send me tahtIf you're not sending it over the public telephone system, maybe you
could consider using CAT5 cable instead of straight phone wire. It's
not that much more expensive and has a lot more bandwidth. Keith
Doxey at http://www.kat5.tv sells some modules for sending video and
audio over CAT5 cable. He sends it as an analog balanced line
signal. You could build one yourself using ICs from
http://www.analog.com if you live outside the UK (he only ships to
If you are sending it over the public phone system, you have to
consider that it's going to be messed around with a great deal
between the sender and receiver (converted to digital for one thing).