[vorbis-dev] Re: Hi

[Ralph Giles]

On Fri, 17 Nov 2000, sreenivas  karanam wrote:

> Hello,
> Iam working on MPEG-2 AAC Decoder and in need of some basic theoritical 
> information regarding CHANNEL COUPLING.Can u please send me some technical 
> papers on this subject or web sites where i can get some information.

Unfortunately, I don't have a bibliography on compression techniques.

A quick search on google.com ("channel coupling audio") turned up some
basic information, but you'll probably need to search a technical library
to find real technical documentation.

My vague understanding is that by applying a matrix transform to the
sample vectors it is usually possible to achieve better compression. The
entropy can be re-arranged so that the encoder is more successful: if A
and B are correllated, A+B,A-B is easier to compress than A,B. It's also a
handy way to perform perceptual filtering, since our senses can follow the
same coherence: for example, low frequency sounds are hard to localize, so
projecting those signals onto a single channel isn't noticable.

I've forwarded your question to the vorbis-dev list, in case anyone there
has some pointers. You might also try the lame list (q.v. sulaco.org/mp3)
I know they've experimented with several encoding techniques and can
probably answer specific questions.

Hope that helps,
 -ralph

> Expecting an early reply.
> Thanks in advance.
> 
> -Sreenivas
> DSP Engineer,
> Wipro Technologies
> Bangalore

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