[asterisk-users] meetme conference using g729?

[Tim Panton]

On 3 Oct 2007, at 10:16, Mark Quitoriano wrote:

>
>
> On 10/3/07, Tilghman Lesher <tilghman at mail.jeffandtilghman.com>  
> wrote: On Tuesday 02 October 2007 16:55:52 Brian West wrote:
> > On Oct 2, 2007, at 4:42 PM, Mark Quitoriano wrote:
> > > anyway still if there's a hack for meetme to work with g729 codec
> > > this won't be an issue. So is there a hack or patch that i can use
> > > any codec for meetme? tnx
> >
> > You still do not understand.  It doesn't matter if the call  
> coming in
> > is g729 you must transcode it to signed linear, mix the frames and
> > then code it back into g729 you end up with quality loss doing that.
>
> Or, in other words, you cannot mix compressed data.  You must first
> decompress the data for mixing, then recompress it for transmission.
> During both operations, there is a potential for signal degradation.
>
>
> yeah i still don't understand.  this is what i want to do. I want  
> asterisk not to compress and decompress codecs. so either i can use  
> SLIN as my codec for my SIP or IAX. or i can remove SLIN codec in  
> meetme and change it to g729a so there's is no compression and  
> decompression.
>
> do you get what i want to do? Thanks!

Not exactly.
Here are the facts:
	meetme mixes in SLIN.
	Any data arriving in anything other than slin will get transcoded  
twice,
		once on the way in and again on the way out.

Now some opinions:
	The more efficient the compression of the codec, the less well it  
copes with
	decoding and re-encoding. Ulaw and Alaw are simple and not that  
efficient,
	but you don't lose any more by re-encoding than you did by decoding  
in the first place.
	Tighter codecs like 729 and GSM you will definitely hear the  
difference.


Theory:
	If you have a conference where there is only _ever_ one speaker
	at a time, you could (in theory) optimize meetme to do without  
mixing, and if all
	the participants were using the same codec, you could get away with  
not re-encoding
	by sending out the appropriate incomming packet to all (other) members.
	I'm guessing that isn't the case for you.

Advice:
	use Ulaw - it's a decent tradeoff for this sort of thing.

Tim.