The Fool(tm) wrote: >Also the only lossless audio compression I've come across is something >called 'Monkeys Audio Compressor', and I think These guys: >(http://www.melodymachine.com/sfark.htm) were making one called DaArk >(Digital Audio Arkive). > >Every other compression scheme is lossy.
The compression scheme used by Dolby Digital for distribution to movie theaters is lossless -- sort of. It looks at the info that's being sent to each of the speakers and calculates how it would all sound in a theater. Any parts of the sound from one speaker that would be masked by similar but louder sounds from another speaker are removed, since they are "useless" anyway. There is "lost" sound, but it's sound that would not have affected the sound quality, whereas other lossy schemes definitely degrade the sound. The reason they do that has to do with the way sound is printed on the sound track of the film. Previously, two tracks of sound were printed onto the film. If the film was mono or stereo, they were fine. Dolby analog surround sound used the phase relationship of those two track to create 4 tracks (front left, front right, front center (used mostly for dialog) and back, and the back signal was sent to multiple speakers wired in anti-phase pairs (if anyone is interested in more details on that process, let me know and I'll post it). When Dolby decided they wanted to use 5.1 separate audio tracks (front left, front right, center for dialog, and separate back left and back right, with the ".1" being a limited-bandwidth track to feed the subwoofer only), they had to figure out how to fit five tracks into the space allotted for 2 on the film. From what I understand, the amount of information in a track roughly corresponds to the width of the audio track on the film, and their compression scheme allows them to shrink the width of each of the tracks down enough that they can squeeze the 5.1 into place. At least, that's how it was explained to me. If I'm wrong or if anyone has a better description of the process, I promise I won't be offended if you post it. :-) Reggie Bautista -- who knew that his electronic music classes and recording studio classes would come in handy someday... _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp.
