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04-07-2003, 04:25 PM
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#1 of 10 
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Just how good is Dolby Digital? (Cable TV)
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
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04-07-2003, 05:49 PM
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#2 of 10
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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.
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04-07-2003, 06:25 PM
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#3 of 10
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Michael Reuben
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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.
"Most people never have to face the fact that, at the right time and the right place, they're capable of anything." -- Chinatown
"What kind of movies would there be if everyone in them had to do what we thought they should do?" -- Roger Ebert
HTF Beginner's Primer and FAQ
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04-08-2003, 10:09 AM
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#4 of 10
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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.
\"I\'m your Huckleberry\"
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04-08-2003, 10:59 AM
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#5 of 10
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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.
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04-11-2003, 07:27 AM
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#6 of 10
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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
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04-11-2003, 08:30 AM
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#7 of 10
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Michael Reuben
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Quote:
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So then why do CD's sound better than the music channels?
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One possible reason: Because the Dolby Digital 2.0 signal on the music channels is compressed, whereas the PCM signal on the CD is not.
Another possible reason: You say that you're using "the same receiver/decoder settings". Does that include the volume level? CDs generally play back louder than DD sources. Louder is almost always perceived as better.
M.
"Most people never have to face the fact that, at the right time and the right place, they're capable of anything." -- Chinatown
"What kind of movies would there be if everyone in them had to do what we thought they should do?" -- Roger Ebert
HTF Beginner's Primer and FAQ
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04-11-2003, 01:12 PM
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#9 of 10
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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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