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Lossy compression - how much is really "lost"?

Posted by John C. - Aussie (A) on July 25, 2002 at 18:57:37

I have done an advanced Google search and so far have come up with -

Lossless Compression: A compression algorithm that does lose any of the original data because of the compression process. The original image or sound is completely preserved.

Lossy Compression: A compression algorithm that gets rid of some information for the sake of making a smaller copy of the original. There are elements of the original sound or picture that are lost when compressed.

At http://www.audiovideo101.com/dictionary/ac-3.asp
the following small article appears -

AC-3

AC-3 is a proprietary digital compression scheme developed and licensed by Dolby Laboratories. The Dolby Digital audio format uses AC-3 to compress multiple channels of information into the space normally occupied by one channel. In this effort, some of the information in a movie or audio soundtrack is essentially "thrown away." While this may sound offensive (and indeed some people do find this subject very touchy), the discarded information is carefully selected to ensure that the overall sound quality is not degraded or degraded to an absolute minimum.

The process of eliminating information is done by essentially discarding sounds the human ear cannot discern. For instance, in a movie scene where a large explosion occurs at the same time a cricket is chirping, we will not hear the cricket chirp (it is a small sound that is drown out by the much larger sound of the explosion - think how difficult it can be to hear someone speak when you're mowing the lawn or listening to loud music). The AC-3 decoding scheme (as with other similar "perceptual coding" schemes) discards the data of the cricket chirp since we cannot hear it over the explosion. In this manner, the overall signal size is decreased while there is no perceptible (or noticeable) degradation in quality. We don't miss the cricket chirp since we could not hear it even when it was there and thus the sound quality is not degraded. AC-3 does this over many spectrum bands in each channel, eliminating up to 80% of the information. However, the information eliminated is information unheard thus maintaining a high sound quality. Through careful coding, six channels of digital sound can occupy the space needed for one non-coded channel.

But so far I remain unclear about the bottom line degradation of the sound as it is heard by a critical listener. The term "lossy compression" raises images of MP3 type sound but is DD 5.1 and DTS really that bad?

And what about the video compression that is on DVDs? I'm gettings pics here which are brilliant on good software - quite as good as the best satellite (PAL) broadcasts which I presume are uncompressed.

So, anyone give an educated answer to just how much audio is subjectively lost with DD 5.1 & DTS? And, am I correct in saying the older pro logic is not compressed (although restriced in frequencies for the surround channels in PLI but not PLII)?

John


Peace at AA

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