Linear PCM 2ch vs. "Dolby Digital" 2ch

Discussion in 'Archived Threads 2001-2004' started by Scott_AH, Oct 25, 2002.

  1. Scott_AH

    Scott_AH Stunt Coordinator

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    I subscribe to AT&T's digital cable service and am wondering about the audio quality. On the analog channels, my receiver displays "Linear PCM" as the audio mode while on the digital channels, it displays "Dolby Digital." However, the Dolby is only a two-channel feed according to the display so my question is which is better? What exactly is a Dolby digital feed that's only two channels? I didn't even know there was such a thing.

    Scott
     
  2. KeithH

    KeithH Lead Actor

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    Scott, Dolby Digital compression can manifest itself in a number of channel configurations. Some older movies on DVD are encoded in Dolby Digital mono. Others are just in Dolby Digital 2.0 (stereo), like the digital feed from your cable service. Some are in 4.0. In any event, linear PCM stereo is higher-resolution than Dolby Digital 2.0. The linear PCM stereo information from your cable service should be of CD resolution, which is 16-bit/44.1-kHz PCM. The Dolby Digital 2.0 stream is compressed audio.
     
  3. Scott_AH

    Scott_AH Stunt Coordinator

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    Which would explain why my digital channels sound like crap. I suppose I'll go back to RCA audio cables...

    Scott
     
  4. Chris Brown

    Chris Brown Stunt Coordinator

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    Well heck, I subscribe to AT&T digital cable and analog RCA cables are all I have! I *wish* there was a digital out on my box. Makes me even more mad when a movie comes on that says "Dolby Digital 5.1" on it [​IMG]
     
  5. Jeff Kleist

    Jeff Kleist Executive Producer

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  6. Vince Maskeeper

    Vince Maskeeper Producer

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    I would bet dollars to donuts that the feed is carried in DD 2.0 mode (basically using Dolby's lossy compression mode for 2 channel audio, similar to the lossy encoding of MP3)...

    In this case, like with DVD players, the cable box offers an internal conversion from compressed digital to PCM for digital output to devices which don't support DD (older receivers).

    So- bottom line is that all the audio is coming in compressed format- and you have 3 choices: 1) Pass it compressed format to receiver where the receiver decodes it. 2) Have the box internally convet the compressed stream to PCM- and pass the PCM to receiver (which offers zero advantage since it was compressed to begin with) 3) Have the box internally convet the compressed stream to Analog audio- and pass the analog to receiver (which offers zero advantage since it was compressed to begin with).

    -V
     
  7. Scott_AH

    Scott_AH Stunt Coordinator

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    OK, I wanted to be sure on this before I posted so I hooked up the RCA cables and compared them to the optical hookup on the same programming. What I found seems a little weird...
    With the optical connection, the lower (analog) channels seem clearer while the digital channels are extremely quiet (around 25% quieter) and sound like a bad MP3.
    With the RCA jacks, the analog channels are so-so as far as quality goes but the digital channels seem clearer and are much louder than the analog ones.
    It seems to me that the best quality I have found on any connection comes when I'm listening to optical on the analog channels (linear PCM).
    Now, according to your replies, shouldn't the optical connection sound better than the RCA's on the digital channels (Dolby digital) since my stereo receiver would be decoding the stream rather than the cable box? I don't have what you would exactly call a good receiver (JVC RX-6018V) but wouldn't even relatively "bad" receiver be better at decoding the audio stream than the cable box?
    Hopefully this post makes sense to everyone! [​IMG]
    Scott
     
  8. Jerome Grate

    Jerome Grate Cinematographer

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