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Cable TV Signal

  1. May 10, 2005 #1
    Does anyone know why on some cable channels I didn't subscribe, I can still see something (with a lot of distortion and noise), while I can't see anything at all in some others?
    How are the signal modulated and encoded? Are they encrypted? How come some channels are encoded that I can still "see" even I am not a subscriber? What does a subscriber have that enable him/her to decode the signal? Is it just an encryption code?
     
  2. jcsd
  3. May 10, 2005 #2

    chroot

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    Cable companies sometimes use notch filters on your line to block out your access to pay channels -- these block out the signal entirely, preventing your TV from displaying an image at all (you may get a blue screen, for example). Other channels are not filtered, but are scrambled at the cable plant, and are descrambled by a set-top box. Older scramblers just used analog filters with nonlinear phase to modify the signal in complementary ways at both ends. Newer scramblers use a wide variety of digital methods to acheive better security.

    - Warren
     
  4. May 10, 2005 #3
    Do you know if the cable industry use a standard for set-top boxes? I want to mod mine so that I can get all the channels. I've seen those analog scramblers (they are small and cylindrical in shape right), anyway what does it take to make one of those?
     
  5. May 10, 2005 #4

    chroot

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    There are no industry-wide standards. There are a number of different set-top box manufacturers, each with different capabilities, and different cable operators choose to use different boxes and different scrambling mechanisms.

    The analog filter scramblers are almost unheard of these days, by the way. They have been supplanted with much more sophisticated scrambling.

    I should mention that scouting to obtain cable-theft equipment is a violation of PF site guidelines -- if you keep this topic to the electrical engineering itself, it will be okay, however.

    - Warren
     
  6. May 11, 2005 #5
    I was expecting this, it makes perfect sense though.

    So whether it is digital or analog it would be safe to say that their foundation is at the frequencies at which they allow. So what exactly does the descrambler do? Does it just increase the frequency range? Is there some special encryption that uses an algorithm?
     
  7. May 11, 2005 #6

    chroot

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    Standard-resolution NTSC television signals have a bandwidth of 6 MHz. Scrambling and descrambling should not increase the bandwidth of the signal, as it would then interfere with neighboring channels.

    The simplest analog scramblers are just analog filters with non-linear phase. Different parts of the signal (sync pulses, luminance data, color data, etc.) have different characteristic frequencies, and thus are each delayed differently by a non-linear filter. The result is that your TV cannot lock to the scrambled signal, and you can't watch it.

    Another analog filter (the descrambler) with the opposite phase non-linearity restores the signal to nearly its original form.

    Digital scrambling can take a wide variety of paths, anything from simple coding schemes to full-blown encryption.

    - Warren
     
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