[vorbis-dev] M/S encoding ?

[David Balazic]

Hi!

I'm sending this message to both lame and vorbis developers, since
my concerns apply to both ( and in case of vorbis it probably applies
to the more advanced ambisonic modes the X, Y and Z parts )

Recently I did some thinking about M/S encoding and wondered if the same
psychoaccoustic model is applied to the S ( and M ) channel as to normal
L and R channels. My concern is that different maskings might result
because of the mixing of the channels.

Consider this example :
L channel has a signal at 5 kHz with amplitude 10,
R channel has a signal at 5.2 kHz with amplitude 5

In L/R mode there is no masking and both signals are present in the
encoded material.

But in M/S mode the signals are :
( assuming that the formulas are M = (L+R)/sqrt(2) , S = (L-R)/sqrt(2),
saw this in a message on the LAME list )
M : 5 kHz with A = 7.07 + 5.2 kHz at A = 4.24
S : 5 kHz with A = 7.07 + 5.2 kHz at A = -4.24

now if this channels are processed the same way as L/R then the louder
signal (5kHz) might mask the quieter one ( 5.2 kHz ) in both M and S
channels ( provided there is a bitrate pressure ), thereby eliminating
the 5.2 kHz signal.

In short : M/S mode might consider parts of music masked, that would
not be masked in regular stereo mode.

This seems wrong to me, alltough is is possible that the brain would do
this masking itself.

So what is the comment of experts here ?

Forgive me if I posted non-sense :-)

David Balazic

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