Best way to encode mono CDs Options

[2Bdecided]

Gabriel has suggested ABX results are probably not very useful, since whatever is happening in the code needs to be understood, audible or not.

Unfortunately, I couldn't resist playing. My inability to easily ABx any of the mono samples lead me to create a kind of torture test for mp3 (attached) though lame 3.97 does very well with it.

I managed a kind of a result:
lame397 -V 9 --vbr-new
gives an audible difference. It's the mono version that sounds close to the stereo version - the dual mono version sounds different (pre-echo slightly more controlled). It's not fair to judge on one sample though.

However, the really strange thing is that
lame397 -V 6 --vbr-new
Reverses the bitrate difference! The mono version comes out at 284.6kbps, while the dual mono version (less efficient up until now!) come out at 193.8kbps. (The stereo version is 314.9kbps). I can't really hear a significant difference between them (the two mono encodings, that is - obviously the stereo one sounds different).

So it's not a hard and fast rule that mono gives a lower bitrate than dual-mono, though this is an extreme example.

Cheers,
David.

Attached File(s)

 impulses.zip ( 4.61K ) Number of downloads: 86

 

[The Sheep of DEA...]

However, the really strange thing is that
lame397 -V 6 --vbr-new
Reverses the bitrate difference! The mono version comes out at 284.6kbps, while the dual mono version (less efficient up until now!) come out at 193.8kbps.

That's astounding. What happened to the "safety margin?" Do the files analyse in the same way? Are you sure both channels in the original were identical (and that one wasn't, say, silent)?

[2Bdecided]

I'm sure, but at 4.61kB zipped anyone can easily download it and check.

Cheers,
David.

[Gabriel]

Just wanted to add that the bitrate increase of dual mono is not because of the side channel "safety belt". In those cases the side channel is allocated 0 bits.
Something else to find...

[2Bdecided]

From this thread...

http://www.hydrogenaudio.org/forums/index....showtopic=54421

...it seems that dual mono encoded as mono is (more or less) identical to dual mono encoded as discrete stereo.

In other words, it's dual mono encoded as joint stereo which is the "odd one out", not the mono encoding.

I can understand why the "M" channel of M/S might be given slightly more bits than the "M" channel of mono, or the L or R channels of L/R: The "M" channel of M/S will be added to "S" to create two channels, and as such there might be some unmasking which needs to be compensated for.

However, if the S channel = 0 then there can't be any unmasking, so it seems sensible to make the "M" channel of M/S identical to the M channel of mono in this specific circumstance. Also, if the "M" channel is intentionally "over coded", it's not by very much, so maybe it is not intentional but accidental.

Cheers,
David.

[grombulk]

Hi all! :-)

CODE

-V 0 --vbr-new -m m

This way, the encoder downmixes the stereo input and outputs a mono mp3 file.

Just to add to the confusion.
Foobar2000 doesn't play mono mp3's if you use Kernel Streaming or Asio as output.
At least that is what I discovered.
ymmv

[Gabriel]

I think that David put me on the right track with his last post:

In mp3, mid channel is defined as (l+r)/sqrt(2).
Lame has no specific provision to overcode mid channel, but consider mid in the same way as a regular (l/r) channel. However, this sqrt(2) factor provides an implicit overcoding of the mid channel.

In the case of dual-mono input, we have l channel identiqual to r channel. It means that we have mid channel equal to l*sqrt(2), this the noticed overcoding.

As pointed by David, although this overcoding is usefull on regular stereo, it's useless if there is no side channel.

Side note: if I had time I woulc check the mid channel overcoding, as it seems to me that right now the overcoding is only provided by a shift against the ATH, while it would be better the adjust the whole masking values.

[drbeachboy]

I just ordered "Jan & Dean - The Complete Liberty Singles" in which the entire CD is Mono. After reading this thread I am not sure whether to encode my MP3's as mono, dual-mono or joint stereo. I would appreciate some opinions on how to proceed. Thanks!

[Slipstreem]

I'd be inclined to encode in VBR with the default settings personally. If the content is genuinely mono, the encoding should happen in M/S as opposed to L/R and the bitrate should drop accordingly with no drop in quality. Note the usage of the word "should". Any accidental stereo will still be captured and will increase the bitrate slightly over that of a plain mono encoding.

The term "suck it and see" springs to mind.

Cheers, Slipstreem.

EDIT: Or maybe not taking into account the last few posts above yours. It may still end up bigger than encoding in mono in VBR for the same quality. I'm confused now.

This post has been edited by Slipstreem: Jan 13 2009, 22:20

[drbeachboy]

Thank you.

Also, if someone would be so kind; please explain exactly what "dual-mono" is or does, vs. "mono"? The thread is a bit confusing.

[greynol]

Dual mono is just a stereo file that contains identical data in both left and right channels. It isn't a setting in Lame or anything like that.

[drbeachboy]

In LAME 3.98.2 in "Operational options" there is -m d switch for dual-mono. There is no explanation in --longhelp as to why it is there.

[greynol]

That's dual channel, not dual mono. In this setting, the amount of data is divided evenly between the two channels as opposed to allowing the encoder to allocate a different amount to each channel depending on the relative complexity in stereo mode.

You definitely don't want to use that setting.

http://lame.cvs.sourceforge.net/*checkout*.../switchs.html#m

[drbeachboy]

So, then the option is mis-worded? Thank you for the explanation.

[greynol]

I don't think it's mis-worded, just that the wording closely resembles the description of one of the types of source files being encoded using Lame 3.97 --vbr-new.

[2Bdecided]

EDIT: Or maybe not taking into account the last few posts above yours. It may still end up bigger than encoding in mono in VBR for the same quality. I'm confused now.

I haven't checked this issue with latest lame, so I don't know if the bitrate differential remains.

However, at least in Kernal Streaming mode, foobar2k plays back single channel mono mp3s through one channel only. That issue (and the chance of other similar issues elsewhere) is probably enough to convince most people to use the standard "two identical channels" kind of mono that comes off the CD, rather than forcing genuine single channel mono.

You could argue that this approach is over-cautious - "most" podcasts are single channel mono, and any set-up that doesn't direct this to both speakers is arguably broken.

Cheers,
David.

This post has been edited by 2Bdecided: Jan 14 2009, 10:41

[Dynamic]

You could argue that this approach is over-cautious - "most" podcasts are single channel mono, and any set-up that doesn't direct this to both speakers is arguably broken.

There's always the standard fb2k DSP "Convert Mono to Stereo", which won't impair stereo files at all, but will solve mono problems for these sorts of playback modes or for burning mono files to conventional red book CD (which must be stereo).

I'm not sure how it handles surround recordings, but I'd be surprised if it does anything at all when the channel count exceeds 1, except to pass all input channels unaltered to the next DSP in the chain.

This post has been edited by Dynamic: Jan 14 2009, 18:39

[ameyer17]

I think I've also seen a mp3 player that had a few issues with mono source material, playing it twice as fast as it should have.
You really should stick with joint-stereo for compatibility reasons,