:: 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
::
That's clear.
:: As you can see, if your input signal is mono (L=R), only the mid channel
:: carries information, the side channel is empty.
::
Take some mono recordings and prove this. It's very seldom that L=R.
See also alt.binaries.sounds.mp3.* .
FM radio mono recordings (tuner set to mono):
* differences between the channels from the MPX decoder -> AD converter
FM radio mono recordings (tuner set to stereo, p.e. News):
* differences between the channels from the MPX decoder -> AD converter
* additional noise, distortion, whistle, ... in the X signal
Mono CDs:
* Both channels are converted by different AD converters with different
parameters (offset, amplification).
Records:
* a lot of noise and rumble
:: 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.
::
I've never seen a True Mono Coding. It is mono (historic reasons),
it sounds like mono, statistic says it's mono, but L != R.
Example:
CD: Jazz - Lyrik - Prosa
No: (Amiga 74321326192)
Title: My Bonnie is over the Ocean
Interpreter: Jazz-Optimisten Berlin
Length: 11239032 samples [19114 CD frames, 4:14.64]
Correlation: r = 0,99879
Coder:
Lame: 3.86 alpha
Options: -V0 -d -q1 --cwlimit 11.5 -X6
Results:
-mm 3642584 bytes 114.3 kbps
-mm -a 3642584 bytes 114.3 kbps (bitwise identically with -mm)
-mj 5637588 bytes 177.0 kbps (+55%)
-ms 7223049 bytes 226.7 kbps (+98%)
::
:: 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.
::
-mm Use Mono
-mi Use Intensity Stereo, MS-Stereo and LR-Stereo
-mj Use MS-Stereo and LR-Stereo
-ms Use LR-Stereo
-ma Analyze FIle before any converting, select -mm, -mj or -ms
Another question:
Is there any tool to analyze the number of SI, MS and LR frames in a MP3?
--
Mit freundlichen Gr��en
Frank Klemm
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