Frank Klemm schrieb am Sam, 05 Aug 2000:
> We should support an option (-ma for Mode Auto) which switches between -a -mm
> for highly correlated channels (r > 0.98 => mono), -mj for a normal
> correlated signals (r = -1.00...-0.20, 0.20...0.91 => stereo) and -ms for nearly not
> correlated signals (dual channel audio with independent audio, i.e. movies
> with english/german audio track , r=-0.20...+0.20).
The joint stereo coding (-mj) in LAME switches automatically between Stereo and
Mid-Side Stereo. Uncorrelated signals will be LR Stereo coded and correlated
parts of your waves in MS stereo. Given L=left channel and R=right channel:
M = (L+R)/SQRT2
S = (L-R)/SQRT2
note: to get your left right channels back:
L = (M+S)/SQRT2
R = (M-S)/SQRT2
As you can see, if your input signal is mono (L=R), only the mid channel
carries information, the side channel is empty. The difference to a true
mono coding in this situation is, that we now need some bits for our empty
side channel which we could use in mono mode for the mid channel too.
My observations on old mono like sounds are, that it is a bad idea to
let LAME make the side channel really empty. If this happens, it is likely
to get an audible glitch.
> There are a lot of MP3s out there with mono recordings coded with -mj and
> also -ms.
>
> --
> Mit freundlichen Gr��en
> Frank Klemm
>
> PS: What's the difference between '-mm' and '-mm -a' ?
>
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Ciao Robert
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