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Just how good is Dolby Digital? (Cable TV)

Discussion in 'Playback Devices' started by Mark Hedges, Apr 7, 2003.

  1. Mark Hedges

    Mark Hedges Second Unit

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    I was wondering about the sound quality of dolby digital. For background, I have Time Warner digital cable. It is very obvious that some of the digital channels video is compressed more than others - some look pretty good, many have pretty severe artifacts and remind me of a VCD. Would it also stand to reason that the sound is extremely compressed as well? Does Dolby Digital have a minimum bitrate or can it be compressed as much as you want?

    Are there any opinions on the quality of the signal from the music-only channels? I think they sound OK but I think CDs sound better.

    Mark
     
  2. johnbr

    johnbr Stunt Coordinator

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    Mark dd do's have a minimum bitrate but is VERY low and at that rate it do's not sound very good.On a scale of one to 10 it is a 3 in sound.
     
  3. Michael Reuben

    Michael Reuben Studio Mogul

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    The DD signals on Time Warner Cable are not any more compressed than the signals commonly found on DVD. In fact, I've seen many DD 2.0 signals on TWC with a bitrate of 384kbps, which is twice what you typically find with DD 2.0 on DVD.

    The music channels usually run at 192kbps, with 2 channels. And yes, CDs do sound better, but it has nothing to do with any "extra" compression by TWC.

    M.
     
  4. Jeff Adams

    Jeff Adams Screenwriter

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    DD signals do not suffer the same kind of loss of quality that video signals do. I find that a 5.1 DD signal from DirecTV sounds almost as good as a dvd.
     
  5. Bob McElfresh

    Bob McElfresh Producer

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    Compared to a standard CD, Dolby Digital uses something called AC3 compression which is a 6:1 fixed compression.

    One minute of CD sound takes about 6 Megabytes of data per channel. That same channel compressed with AC3 is only 1 megabyte.

    (I used to build systems to get/distribute/archive these things.)

    So yes, compared to CD's, DVD movies are highly compressed.

    But consider the differences:

    - CD's have 2 channels pumping sound nearly 100% of the time. DVD's have 6 channels with lots of dead-time.

    - A CD tries to reproduce actual instruments & singers, including subtile flaws and details (in some ways, it's the 'flaws' that make things feel real). A DVD tries to reproduce a lot of artificial sounds added in post-processing. It's very controlled/sterile/without-flaws.

    - With a CD, the sound is the most important thing. With a DVD, the video is the main focus, followed by the dialog, then music, then special effects.

    - "Music is about accuracy, Movies are about impact" (Best advice I ever got about the differences between speakers for a music system vs a home theater system.)

    My point is: the sound from a CD and a DVD are fundamentially DIFFERENT. Just because it appears to play on identical equipment does not mean they are an apples-to-apples comparison.

    Hope this helps.
     
  6. Mark Hedges

    Mark Hedges Second Unit

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    So then why do CD's sound better than the music channels? When I listen to a song on one of the music channels the bass sounds weak compared to the same song from a CD (under the same reciever/decoder settings).

    I am using a converter to change the digital coax signal from the cable box to a toslink signal for my reciever. Could that have anything to do with it? I am using the midiman CO2 unit.

    Mark
     
  7. Michael Reuben

    Michael Reuben Studio Mogul

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  8. Stephen Tu

    Stephen Tu Screenwriter

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    Is decreasing dynamic range really what you want to do? I'd rather just turn the volume up. Yes many CDs are mastered with compressed dynamic range, but IMO that's a defect catering to listening in noisy environments like a car, not a feature.
     

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