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Re: [ECS] More on Audio Data compression (was: [ECS] Help with Log States) Steven Wegman Tue, 27 Oct 1998 22:37:12 -0500

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At 11:02 AM 10/27/98 -0800, you wrote:
> Is AC3 a compression scheme?  I only know it as a fully
>seperate 5 channel plus sub system.  I have never heard of it as a
>compression scheme.  
>
>Dan Carrington


	AC3 is a "lossy" type compression scheme, meaning that it throws away bits
that can never be recovered.  The encoding rate for AC3 is 384kb for DVD
and laserdiscs (whereas DTS, which is also a lossy compression scheme is
1.44 Mb).  These schemes are based on the theory that certain sounds will
be masked (inaudible) when other sounds are present.  Phillips, SONY and
Microsoft  have been doing a large amount of research in this area.

	Many people (especially Dolby Labs) contend that the reproduced sound is
indistinguishable from the original; that is, that the throwing away of
bits does not degrade the sound quality.  However, most audiphiles maintain
that  such schemes affect the sonics, especially in the timbre and
harshness of the sound.  Many (most) AC3 movies sound shrill;  thus the
reason for THX's timbre equalization.

It should be noted that the DVD-Audio group (known as WG-4) just recently
selected a lossless compression scheme known as MLP, which was developed by
Meridian.  This scheme ensures that no bits are lost and that the
reproduced signal is the same as the original, in much the same manner that
a decompressed zip file is the same as the original.  This compression
scheme can handle up to 7 channels, with sampling rates up to 192 kHz and
up to 24 bits.  Initail reports is that the reproduced sounds are far
superior to the sound from CD's; most likely due to the larger number of
bits employed and higher sampling rate.

BTW, Meridian implements this compression scheme in software in their Model
800 and 565 processors; thus,  it may appear one day as an option for
compressing internet music files.

Steve Wegman

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