[Vorbis] Mono encoding w/ a stereo source.

[Ian A. Underwood]

Michael Smith wrote:
> On 11/16/05, Ian A. Underwood <agentgrn at dcne.net> wrote:
> 
>>Crew,
>>
>>I've run into something a little odd with the vorbis encoder and would
>>like some input.
>>
>>I use SpacialAudio's SAM3 Broadcaster and am having problems with the
>>ogg encoder when I create a mono output stream.  It seems as if the
>>signal level of the left and right sides are being combined before the
>>encoding process, leaving the sound muddy and overdriven.  As such, if I
>>reduce the output level by 6db, the combined streams are at the right
>>level.  The settings I'm using are Quality -1, 32kbps/44kHz.
>>
>>SpacialAudio suggested that they are passing the audio info to the
>>library as required.
> 
> 
> If you're encoding in mono, then the data being passed to the encoder
> library is in mono - libvorbis does not do downmixing internally. It
> sounds, from your description, like SAM3 is internally doing
> downmixing to mono, but in a way that produces poor quality audio. Is
> that possible?
> 
> Mike

I'm not sure since this is what I got from Lou at SpacialAudio:

"We do not merge the channels for the encoding - this is all done by 
Ogg.  A simple method is usually to just add the left channel to the 
right to get the mono channel.  Although this is the correct way 
theoretically, in pratice it sounds too loud.  Thus I guess this is what 
they are doing in the Ogg encoder."

I'm not sure how the API to the library works, but from what I 
understand there is essentially a 16-bit/44.1kHz audio bus internal to 
the program that feeds all the encoders...so I'm assuming it's possible 
to tell the encoder to run in Mono while still sending it a stereo source.

-I