On Thu, Jun 21, 2012 at 3:09 AM, ocset <ocset@the800group.com> wrote:

Hi

I need some assistance in understanding the codec minefield.

When a stream of audio is sent to FreeSWITCH (FS) using a particular
codec, does FS just pass it on or is there some level of translation
occurring?

In some situations audio may be just passed thru, but most real life scenarios will involve some level of translation (transcoding).

I guess my question relates to a situation where an FXO gateway
(GXW410x) uses the G723 codec and the receiing phone does not have a
matching codec (the phone has G.722, G.711 a law, G.711 u law, G.726 and
G.729 as options). How is this managed by the phone, FreeSWITCH and the
FXO device? Should I be setting something in FS to handle this issue?

Whenever FreeSWITCH detects format mismatch between the parties it will perform transcoding (translation of one audio format to another). You have several configuration options to force a given list of codecs to be offered.

Now, in some shameless self-promotion, I suggest you to read or watch my presentation on FreeSWITCH media handling from here:

http://www.moythreads.com/congresos/cluecon2011/media-handling-freeswitch-final.pdf

http://www.youtube.com/watch?v=508jpSQk8qE

That should make things a bit clearer for you.

I have 2 cordless Siemens C610 IP phones an I am having sound quality
issues (incoming sound is soft) and I was wondering how big a part the
codec plays in this equation. I have desk phones that are from the same
manufacturer as the FXO gateway (Grandstream) and they have very good
sound quality.

I don't think the codec has a role in your issue, if the audio is soft or quiet it has more to do with the gain control. If you place calls to those Siemens phones without using the gateway (ie, calling in to play a file) is the audio still low?

Moises Silva
Manager, Software Engineering

msilva@sangoma.com

Sangoma Technologies

100 Renfrew Drive, Suite 100, Markham, ON L3R 9R6 Canada

t. +1 800 388 2475 (N. America)

t. +1 905 474 1990 x128

f. +1 905 474 9223