[Flac-dev] multi-channel / ambisonics

[Curt Sampson]

On Sat, 23 Aug 2003, smoerk wrote:

> ... The left and right
> channels are converted to center and side channels through the following
> transformation: mid = (left + right) / 2, side = left - right. ...
> For normal CD audio this can result in significant extra compression.

> is this only relevant for stereo files or also for multi-channel (more
> than 2 channels)?

I would guess that it's relevant only to stereo files, since knowing
which tracks are likely to have less or more information in a recording
with more than two channels would depend entirely on the contents and
arrangement of the channels.

> i wonder how well a 16 channel 3rd order ambisonics file could be
> compressed.

The mid-side encoding of a stereo signal increases compression because
typical signals have a lot of common information between the left and
right and speakers. For example, a kick drum or a bass guitar is often
panned dead centre in the stereo field, and thus the exact same signal
for it will be set to both channels. In an M-S encoding, the mid channel
will carry that signal, and the side channel will carry nothing, since
there's no difference between the two channels.

Whether you can use tricks such as this on an ambisonics recording
will depend on the material and what speaker positions the channels
are encoding. If you're going to experiment with this sort of stuff,
you'd probably want to add the ability to specify various encoding
transformations to the encoder (e.g., "(1, 2) <-> ((1+2)/2, 2-1)"), so
that you could hand it a set of transformations that are likely to be
helpful for the given format and source material.

cjs
-- 
Curt Sampson  <cjs at cynic.net>   +81 90 7737 2974   http://www.NetBSD.org
    Don't you know, in this new Dark Age, we're all light.  --XTC