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Re: [Electronics_101] Thelephone line modulation with tv signal

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  • Tavys Ashcroft
    ... 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
    Message 1 of 6 , May 18, 2001
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      >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
    • Barry Hansen
      The audio bandwidth over telephone connections is too low to support full-motion video. However, amateur radio operators have been sending color pictures over
      Message 2 of 6 , May 18, 2001
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        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
        http://www.oz.net/~coilgun/home.htm

        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.
      • Rehan Khwaja
        ... a ... in ... If 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
        Message 3 of 6 , Jul 27, 2001
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          --- In Electronics_101@y..., ramiroi@e... wrote:
          > I want to send a video by telephone line, but i dont know how to
          > start 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. Please if any one can give me an idea or help, send me taht

          If 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
          the UK).

          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).
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