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Joe Murphy Jr

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In Reply to: What's the point of Dolby Digital... posted by Tadlo on January 13, 2007 at 10:28:46:

Tell your DVD player to output those Dolby Digital 2.0 soundtracks as PCM, instead of bitstream. That way, your receiver will see it as a stereo PCM signal and do whatever processing you want (ie, you can turn that 2-channel signal into a multichannel output).
Just remember to change the DVD player's output back to bitstream after the 2.0 movie so your 5.1/6.1 digital soundtracks get decoded correctly, instead of 2-channel PCM.

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and to answer your question - Joe Murphy Jr 10:43:29 01/14/07 (1)
If the original soundtrack was only 2-channel, a studio may choose to distribute the movie this way. Not every studio will go through the trouble of making a multichannel mix for the DVD release. Thus, a 2-channel track is what gets encoded for the DVD. As to why they use Dolby Digital 2.0 instead of 2-channel LPCM, the logical reason is to allow more room for video bandwidth. *
Dolby Digital 2.0 will uses 192kb/s (384kb/s on a rare few discs), while a 2-channel LPCM track would use about 1.5Mb/s of bandwidth. Given these numbers, Dolby Digital 2.0 allows the studio to allocate the full 9.6Mb/s for video allowable on DVD. If they were to use PCM, then they would only have a bit over 8Mb/s total bandwidth at their disposal for the video.
* What I said in paragraph 1 only applies to logical reasoning: the studios are not logical. Here are 3 reasons why using Dolby Digital in its 2.0 form makes no sense. The majority of the movies I've seen with Dolby Digital 2.0 tracks do not have video bitrates running up to 9.6Mb/s -- nowhere near these rates. Most of them hover around 4.5 - 6.5Mb/s with an occassional push around 7.5 - 8.0Mb/s. The use of Dolby Digital anything requires royalties to be paid to Dolby Labs: using LPCM is free (it's not a codec). Dolby Digital anything (the lossy versions) is inferior in sound quality compared to LPCM.
- Thanks for the information. - Tadlo 15:30:45 01/14/07 (0)
  That makes sense.

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