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Flac - Mono Vs. Stereo
Azevedo
post Oct 20 2012, 13:59
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Hey everyone,

I have this audio source in stereo FLAC+CUE and used CUETools (libflake max compression) to split the CUE/FLAC into single tracks
Then, AS THE ORIGINAL CD IS MONO I converted the STEREO FLAC to MONO FLAC using sox:

sox.exe stereo.flac -C 8 mono.flac channels 1

The results:

Stereo:

CODE
Format                                   : FLAC
Format/Info                              : Free Lossless Audio Codec
Duration                                 : 3mn 22s
Bit rate mode                            : Variable
Bit rate                                 : 507 Kbps
Channel(s)                               : 2 channels
Sampling rate                            : 44.1 KHz
Bit depth                                : 16 bits
Stream size                              : 12.2 MiB (99%)
Writing library                          : Flake#0.1


Mono:

CODE
Format                                   : FLAC
Format/Info                              : Free Lossless Audio Codec
Duration                                 : 3mn 22s
Bit rate mode                            : Variable
Bit rate                                 : 353 Kbps
Channel(s)                               : 1 channel
Sampling rate                            : 44.1 KHz
Bit depth                                : 16 bits
Stream size                              : 8.51 MiB (100%)
Writing library                          : libFLAC 1.2.1 (UTC 2007-09-17)



Now I have these questions for you folks:

1. After these operations, the mono Flac audio still losslessly 'original'?
2. Why didn't I get 50% space saving when using -C 8 compression level?
3. Is it recommend to scan replay gain and store int on the flac's tag?
4. Is it recommended to encode flac audio in mono out of DUAL MONO?
I mean, dual mono isn't it such a waste?



Thanks

This post has been edited by Azevedo: Oct 20 2012, 14:48
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chi
post Oct 20 2012, 15:22
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QUOTE (Azevedo @ Oct 20 2012, 14:59) *
Then, AS THE ORIGINAL CD IS MONO I converted the STEREO FLAC to MONO FLAC using sox:
sox.exe stereo.flac -C 8 mono.flac channels 1

1. After these operations, the mono Flac audio still losslessly 'original'?


If the original CD really is mono (left channel = right channel), then more or less: SoX will have re-dithered the output. It had been better to use remix 1 instead of channels 1.

QUOTE
2. Why didn't I get 50% space saving when using -C 8 compression level?
4. Is it recommended to encode flac audio in mono out of DUAL MONO?
I mean, dual mono isn't it such a waste?


FLAC exploits similarities between stereo channels. So, when you hand it a “stereo” file that’s actually dual mono, it will essentially compress the second channel down to nothing.
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Azevedo
post Oct 20 2012, 16:03
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QUOTE (chi @ Oct 20 2012, 12:22) *
FLAC exploits similarities between stereo channels. So, when you hand it a “stereo” file that’s actually dual mono, it will essentially compress the second channel down to nothing.


Thanks chi. So let's see if i get it. Flac compress stereo like "joint stereo"? It has a mono stream with stereo differences L/R to produce the stereo from the mono?
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tuffy
post Oct 20 2012, 16:15
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QUOTE (Azevedo @ Oct 20 2012, 10:03) *
Thanks chi. So let's see if i get it. Flac compress stereo like "joint stereo"? It has a mono stream with stereo differences L/R to produce the stereo from the mono?

That's pretty much it. For 2 channel streams, FLAC encoders will try left/difference, difference/right, average/difference and independent, and will use the combination that makes the smallest frame.
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Azevedo
post Oct 20 2012, 16:37
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QUOTE (tuffy @ Oct 20 2012, 13:15) *
QUOTE (Azevedo @ Oct 20 2012, 10:03) *
Thanks chi. So let's see if i get it. Flac compress stereo like "joint stereo"? It has a mono stream with stereo differences L/R to produce the stereo from the mono?

That's pretty much it. For 2 channel streams, FLAC encoders will try left/difference, difference/right, average/difference and independent, and will use the combination that makes the smallest frame.

Hmmm so that's why i didn't get 50% (coparing to the stereo flac) compression in MONO

Thanks!
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mjb2006
post Oct 20 2012, 16:58
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Actually it depends on the encoder and settings chosen... libFLAC / flac / flac.exe will not try too hard to figure out the best stereo coding when using the -0, -1, -3, or -4 presets.

I've never seen a CD with mono content that was truly mono (both channels identical). They always ran the mono signal through stereo media or gear on its way into digital, or they dither, so there's always some stereo background noise and slight variations in the channels. This is normally handled very well by mid-side coding. But if the signal is panned the least bit off-center, mid-side coding stops being efficient real quick, so you may find your converted-to-true-mono content compresses better in those situations.

To answer the original question, no, changing a near-mono stereo signal into true single-channel mono is not lossless. It's only lossless if the two channels are identical. However it may actually make the sound more pleasant when you make it true mono. Sometimes the most appealing option is to just take one channel, whichever sounds better to you. Other times you may find it better to combine the channels; this tends to reduce the signal to noise ratio, but may result in some unappealing sound if the non-noise part of the audio is ever out of phase. And sometimes you may prefer not to make things mono at all... I find that when listening with headphones, true mono tends to sound "distant" to me, especially when it's played right after something in stereo. Some stereo noise, however unintentional it may be, actually helps...

This post has been edited by mjb2006: Oct 20 2012, 17:00
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Azevedo
post Oct 20 2012, 17:21
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QUOTE (mjb2006 @ Oct 20 2012, 13:58) *
Actually it depends on the encoder and settings chosen... libFLAC / flac / flac.exe will not try too hard to figure out the best stereo coding when using the -0, -1, -3, or -4 presets.

I've never seen a CD with mono content that was truly mono (both channels identical). They always ran the mono signal through stereo media or gear on its way into digital, or they dither, so there's always some stereo background noise and slight variations in the channels. This is normally handled very well by mid-side coding. But if the signal is panned the least bit off-center, mid-side coding stops being efficient real quick, so you may find your converted-to-true-mono content compresses better in those situations.

To answer the original question, no, changing a near-mono stereo signal into true single-channel mono is not lossless. It's only lossless if the two channels are identical. However it may actually make the sound more pleasant when you make it true mono. Sometimes the most appealing option is to just take one channel, whichever sounds better to you. Other times you may find it better to combine the channels; this tends to reduce the signal to noise ratio, but may result in some unappealing sound if the non-noise part of the audio is ever out of phase. And sometimes you may prefer not to make things mono at all... I find that when listening with headphones, true mono tends to sound "distant" to me, especially when it's played right after something in stereo. Some stereo noise, however unintentional it may be, actually helps...


Well it is music from 1930s & 40s. Then only mono recordings were made, right?
We could say that there is a loss of 0.001% or so... i don't really know...
This method with Audacity confirmed it is REALLY mono:
http://forum.videohelp.com/threads/307834-...ual-Mono-Stereo

I think when the CDs are produced from mono sources they 'upmix' it in 2 channels as if it was an industry standard.
I got 70% compression ratio over stereo flac using mono.

HOWEVER you're right, it applys differently on each situation. Some poor hardware may play only L or R for mono songs.



This post has been edited by Azevedo: Oct 20 2012, 17:35
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chi
post Oct 21 2012, 04:58
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QUOTE (Azevedo @ Oct 20 2012, 18:21) *
This method with Audacity confirmed it is REALLY mono:
http://forum.videohelp.com/threads/307834-...ual-Mono-Stereo


As you are already using SoX, you can use that too (I find it simpler than Audacity, but that is personal preference of course): remix -m 1,2i will calculate the difference (side channel) of stereo input, so
CODE
sox.exe file.flac -n remix -m 1,2i stats
will show you how much of a difference there is. Output will look like this for true dual mono (silent side channel):
CODE
Min level   0.000000
Max level   0.000000
Pk lev dB       -inf
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pdq
post Oct 21 2012, 12:22
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The "AAD" means that the source material was analog (obviously), and it is distributed in digital (obviously), but the mastering was performed in analog, so noise was introduced into the two channels, making them slightly different. By encoding losslessly you are capturing every detail of this added noise, which takes up bits.
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