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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
Follow Ups:
My experience is that AC3 is not even close in reproducing the original soundtrack to uncompressed analog or digital (based on mono movie soundtrack comparisons on LD vs DVD). Not close at all.There's an immediately noticeable and consistent loss of high end ambient and transient energy (not sine wave response) with AC3 and modification of timbres throughout the audio band (worse at higher frequencies). The excerpt you quoted has huge, huge problems with veracity, claiming time and again as givens that what is discarded is 'inaudible' when, even under controlled test conditions with limited audio reference material, many subjects found the losses audible in some tests with such severe compression as AC3 has, and with the acoustical response variationsand much wider available range of material in typical playback at home, many of the sonic lacunae of AC3 are perceptually magnified in terms of lack of similitude.
That's what I hear with identical soundtracks compressed with AC3 on DVD vs uncompressed LD audio. End of story.
AC3 can only be regarded as 'close' in the same way as, say, an amplifier which has 5% THD at all levels and frequencies is 'close' to accurately reproducing its input.
Not sure if I can help here, but I do knwo there is a book called
"The COmpression Book" (boring title!) that I have used. It covers the general topic of compression, lossy and non lossy, and goes into great detail on the history of compresison methods, up until when the book was latest revised. It's a great primer if you really want to understand the subject. I don't know if the latest version covers AC-3, but there are alot of books that do, if you look hard enough.ciao
Doug
... what I'm really after is an opinion on the subjective appraisal of the loss. I'm guessing it might be a bit like the watermarking on DVD-A's. Everyone hates it yet few seem to be able to hear it!My point is we are living happily with lossy stuff on video - the eye rarely detects any loss. The question is can the ear really detect the loss in AC3 & DTS and if it can what makes the loss obvious?
John
Peace at AA
My opinion is that while Dolby Digital works fine for action movies and stuff like that, it doesn't work as well for music intensive material as does PCM. I find that music sounds less warm and musical on Dolby Digital compared to PCM, but the advantage of DD's 5.1 channels is viewed as so much of an advantage over Pro-Logic PCM that its audio shortcomings wind up never being discussed much anymore. But perhaps I'm just an old crank.Todd
Dolby Digital typically packs the audio for an entire two-plus hour movie on DVD in one-quarter of the space used to master an uncompressed PCM CD. The loss varies according to program content, but can be be as great as 75% in many places. The DD bitrate is slightly higher on LD than DVD - try listening to the same material in both formats, you should hear a significant improvement with LD, especially in respect to "venue" or the ambient sound of the recording.Meridian has been pushing for some time to implement their propriatary "MLP" lossless packing codec which can handle playback at rates of 16.44 24/96 on DVD with zero data loss and can go as high as 24/96 lossless (with some program time limitations) on dual-layer discs. It's a far better system in both theory and practice but so far Meridian has not be able to topple the Dolby Labs juggernaut.
If you have a processor with a pure analog pass-through, try playing a DVD encoded in pure PCM - it should sound superior to even the very best LD AC-3 replay. Ditto for dedicated Dolby 2.0 surround audio tracks (just be sure it isn't a 5.1 mixdown) - the bit rate only has to handle two channels instead of 5.1 and thus the signal is more robust.
BTW, if you really want to have fun, try cueing up a 5.1 DVD-Audio soundtrack with the DVD video - it's a mind-blowing experience. High-end HT rules!
The DD bitrate is slightly higher on LD than DVD - try listening to the same material in both formats, you should hear a significant improvement with LD, especially in respect to "venue" or the ambient sound of the recording.Actually it's the other way 'round. The maximum DD bitrate on DVD is slightly higher than LD (448K vs. 384K)
Though DVD can have a higher rate than LD, it virtually never does. In general, I find the sound of DTS on Laser much better (and I do mean much) than DVD.As for DD, on many Lasers I find myself choosing the Dolby Surround track, simply because it has less irritating timbre modifications.
---> The maximum DD bitrate on DVD is slightly higher than LD (448K vs. 384K)Technically correct, however, only a few music-oriented DVDs like James Taylor Live at the Beacon are mastered at 448K bit rate - almost all 5.1 movies are 384K bit rate. Mono movies on DVD can be mastered as low as 91K bit rate (Gary Reber's "thin, compressed mono") and 5.1 downmixes are typically 256Kb. LDs always use the full 384K bit rate and are processed internally at a full 20 bits with 16/44 output instead of DVD's 18 bit internal -16/48 output which gives subjectively better sound.
Numbers aside and all things being equal, LDs sound better than DVDs when replaying 5.1 DD surround.
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