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Dolby prologic via TV

Discussion in 'Televisions' started by scooby do, Mar 18, 2003.

  1. scooby do

    scooby do
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    Please help. I have a Sony TV with audio out phono plugs I have connected these to my amp and was expecting to get Dolby prologic for terrestrial TV via this connection. Alas I am not only 2 channel stereo. If I connect the same input to my SAT decoder then hey presto Dolby prologic works so Amp is alright. How on earth do you get prologic for terrestrial.???????????
     
  2. LV426

    LV426
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    To get ProLogic you only need 2 things.
    1: A stereo sound source
    2: A prologic decoder.

    ALL truly stereo sound sources can be decoded by a prologic decoder and will produce some surround presence. If the producer of the programme mixes the stereo sound source 'properly' then you get significant surround presence.

    The only reasons why you might get absolutely no rear channel presence is either
    a) the TV is receiving mono only - it isn't picking up NICAM, but is defaulting to the FM mono soundtrack.
    ** Note that this is ALWAYS the case when the source is a VCR or Set Top box (etc) connected to the TV via RF (aerial cable) and being watched via the TV's tuner (rather than via a SCART or other connection).
    ** For analog broadcasts coming down the aerial, it indicates that either NICAM is disabled in the TV's menu or that the TV is tuned incorrectly
    OR
    b) your amp/decoder etc, is set to 'stereo' for this source, rather than 'prologic'.
     
  3. scooby do

    scooby do
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    Cheers Nigel,

    I have done some more testing this morning and it looks like it may be down to an intermittant phono plug coming from the back of the set to the amp, I have substituted for a cheapo and it sounds like all is ok. I will have to do some more testing as time was short this morning but looks like we might of cracked it. So much for the so called experts at my work saying that you can't get it via the Nicam that the Tv outputs. I was sure it was encoded within the Nicam source and it sounds like you are confirming that.
     
  4. LV426

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    Yes. A prologic decoder simply looks for differences in the waveforms between the two channels (of a stereo signal) to 'decide' which speaker to route a given sound to. So, so long as there IS a difference, a prologic decoder will find it and try to use it.

    DPL encoded signals are still actually just stereo. It's just that the sound mix is engineered so that these differences occur predictably and are therefore decoded predictably.

    So, so long as you drive your amp with two channels that are different the DPL encoder will generate at least some surround effect. Only mono signals (eg where left and right are identical) produce absolutely no surround presence. In fact, a properly used DPL decoder should route a mono signal to the front centre, only.
     
  5. scooby do

    scooby do
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    Thanks again Nigel, well explained when I get home again tonight I will hopefully still have DPL, my faulty lead had a 12 ohm short between centre and shield thus causing the problem.
     

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