Video Asylum

Posted by Joe Murphy Jr (A) on January 14, 2007 at 10:43:29

In Reply to: here's how to fix that posted by Joe Murphy Jr on January 13, 2007 at 18:44:12:

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.