[Freeswitch-users] Recording G729

[Steven Ayre]

>
> Does transcoding (for example GSM<>PCMU) negatively affect the sound
> quality, even when the CPU seems to be ok with it?



> If yes, does recording 2 legs using the same codec (say G729) to a
> different sound format (say mp3) have the same (negative) effect?


Any conversion between lossy codecs will cause a drop in sound quality.

That'll be amplified if you use  >2 codecs such as when you receive a GSM
call, record in MP3 and playback to a G729 call.

Not only are you throwing away stuff, but they're not throwing away the
same thing so the result'll sound worse than either codec on its own. That
said some throw away more than others and G.711 (PCMU) shouldn't be too bad.

Using something that doesn't throw away anything for the recording will
minimise that. WAV files are good for that and also use barely any CPU, but
will increase storage space.


When you use G729 on both legs (using mod_com_g729), it is even possible to
> record to mp3?


If you're using mod_com_g729 you can transcode so providing you have a
license available, yes.

Everything gets decoded to the raw audio by the core then encodes to the
desired codec (to send to the other leg, recording, etc). So all
combinations are possible providing you can transcode the codec.


-Steve


On 26 November 2013 02:39, Kees Varekamp <kees at mroffice.org> wrote:

> Hi all,
>
> I'm suffering from poor sound quality on my setup, using GSM on the a-leg
> and PCMU on the b-leg. I'm thinking it's mostly bandwidth/network related,
> so I'm considering switching to G729 on both legs, as that is the only
> compressed codec that the b-leg supports (it's a sip gateway).
>
> But a complicating factor is that the calls need to be recorded in mp3.
>
> So I have some questions that I hope someone here can answer:
>
>
>    - Does transcoding (for example GSM<>PCMU) negatively affect the sound
>    quality, even when the CPU seems to be ok with it?
>    - If yes, does recording 2 legs using the same codec (say G729) to a
>    different sound format (say mp3) have the same (negative) effect?
>    - When you use G729 on both legs (using mod_com_g729), it is even
>    possible to record to mp3?
>
>
> Thank you,
>
> Kees
>
>
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