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  • arose
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AAC Dual-mono
Hello everyone,

I'm trying to encode a sample consisting of two independant channels to dual-mono AAC.
I've read somewhere on the forums that I could achieve separate encoding in a CPE if I disable all joint channel tools, but ideally I'd like to encode in two SCEs.
I've tried to achieve dual-SCE encoding using Fraunhofer's FDK-AAC and FAAC, but didn't succeed. I even tried to patch FDK-AAC to use one SCE per channel, and I think I kinda succeeded, but the quality is pretty bad (the right channel sounds heavily compressed) so something must be missing.

Can anyone recommend an ecnoder capable of encoding two channels as two SCEs ?
If there is no such encoder, do you know which  joint channel tools I should disable to get the most channel separation ?

Thanks in advance.

  • kode54
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  • Administrator
Re: AAC Dual-mono
Reply #1
Joint channel encoding will not impair the channel separation, and likely will not even encode the two channels as mid+side information if that would require more bits than encoding them as independent channels.

Have you actually tested a full stereo separation of two signals to see if any residue of either channel appears in the other?

  • DVDdoug
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Re: AAC Dual-mono
Reply #2
Quote
If there is no such encoder, do you know which  joint channel tools I should disable to get the most channel separation ?
I don't know that much about AAC, but LAME in Joint Stereo does the "right thing" depending on program material and I'm pretty sure there's no loss of stereo separation, and I would assume AAC does the right thing too.

  • naturfreak
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Re: AAC Dual-mono
Reply #3
In the specification of the aac format is a dual channel mode, but I don't know any aac encoder that make use of it.

  • arose
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Re: AAC Dual-mono
Reply #4
Hi,

Thank you for your replies. I have yet to try and examine such an AAC file (two unrelated channels encoded in stereo), but you (kode54, DVDdoug) seem confident it should not be a problem. After all, I'm using pretty low bitrates (<= 64 kbps) so having one channel "leaking" on the other is not my biggest problem :) .

naturfreak, yes and that is quite surprising. Someone at work found a commercial encoder (don't know the name) that seems to be able to encode two channels as two SCEs. I have some test files, and I managed to decode them successfully (although with some warnings in ffplay). This encoder is designed to run on a DSP though, so I can't use it.

  • nu774
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  • Developer
Re: AAC Dual-mono
Reply #5
Here in Japan, dual mono AAC containing two SCE is sometimes used for dual-language TV program. AFAIK, these TV stations are using dedicated hardware encoders.
IMHO,  having two separated stream is much more convenient than dual mono single stream.
TV-sets have proper interface for handling dual-mono audio (lets user select one of the two language and play only that, not both), but we can't expect it for software players.

  • arose
  • [*]
Re: AAC Dual-mono
Reply #6
Thank you nu774, that's very interesting to know ! I noticed someone added some code in ffmpeg to decode dual-mono properly, with a comment that reads "Dual-mono for japanese TV" or somethign similar.

IMHO,  having two separated stream is much more convenient than dual mono single stream.
I agree, but it also uses more bandwidth (you get the headers twice). It is almost negligible for most uses cases, though.