[Freeswitch-users] Codec 101 - need help with the basics

[Moises Silva]

On Thu, Jun 21, 2012 at 3:09 AM, ocset <ocset at 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 at 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



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