Full Version: Lossless channel inversion
I know you can losslessly trim files with various apps, but is it possibly to invert a channel with some similar process? I have a couple of rather nice mp3's from Rephlex, but one channel is the inverse of the other. Decoding and flipping it in an audio editor makes it sound much better as you'd expect - I'd like to be able to do this without having to re-encode to mp3 afterwards..
I'll take that as a no? 
No
No, there's (probably) no software available that is able to invert one channel of a stereo mp3 file.
But technically it IS possible if your mp3 doesn't contain intensity-stereo coded parts.
<more technical details>
In case of a dual channel or (L/R) stereo frame you only need to invert the samples' sign bits.
In case of a joint-stereo frame (M/S) swapping channel M and S (which is also losslessly possible) is equivalent to a sign-inversion of one channel (L or R - depends on the polarity of S which I currently can't recall).
The intensity-stereo coded part is already in-phase so you don't have to do anything in your case. (Inversion of just one channel for those parts is NOT possible since MP3 does not support 180° out-of-phase intensity stereo like AAC does.)
</more technical details>
SebastianG
But technically it IS possible if your mp3 doesn't contain intensity-stereo coded parts.
<more technical details>
In case of a dual channel or (L/R) stereo frame you only need to invert the samples' sign bits.
In case of a joint-stereo frame (M/S) swapping channel M and S (which is also losslessly possible) is equivalent to a sign-inversion of one channel (L or R - depends on the polarity of S which I currently can't recall).
The intensity-stereo coded part is already in-phase so you don't have to do anything in your case. (Inversion of just one channel for those parts is NOT possible since MP3 does not support 180° out-of-phase intensity stereo like AAC does.)
</more technical details>
SebastianG
I think that it will also be problematic to invert one channel in an M/S frame.
QUOTE(Gabriel @ Feb 19 2005, 06:46 PM)
I think that it will also be problematic to invert one channel in an M/S frame.
huh ?
MATLAB diary following...
CODE
> LR2MS = [1 1; 1 -1] .* sqrt(0.5);
> MS2LR = inv(LR2MS);
> SWAP = [0 1; 1 0];
> INVBOTH = [-1 0; 0 -1];
> MS2LR * (SWAP * LR2MS)
ans =
1.0000 0
0 -1.0000
> MS2LR * (INVBOTH * (SWAP * LR2MS))
ans =
-1.0000 0
0 1.0000
Thus, you can invert R in an M/S frame by swapping M and S and you can invert L by swapping M and S and also inverting both of'em.
...should be possible losslessly. Why not ?
SebastianG
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