[asterisk-users] Choosing codecs

[A J Stiles]

On Sunday 05 Jul 2015, Luca Bertoncello wrote:
> Hi list!
> 
> I noticed that when the phone of my wife calls the gsm codec will be used,
> but if someone calls the phone, alaw will be used:

> Could someone explain me why?
> Second question: I think, ulaw/alaw are better then gsm, isn't it?
> If so, how can I change it?

GSM is the native codec used for calls to mobile phones; it uses lossy 
compression to achieve a low bit rate.

A-law is the native codec used by physical exchanges on the land line network  
(PSTN and ISDN).  It is non-lossy.  It works by arranging the "steps" closer 
together near the zero line, and further apart away from it; so the difference 
between the actual signal and the nearest digital representation is small in 
proportion to the signal.

To force the use of a-law, you need something like

disallow=all
allow=alaw

at the top of the configuration file for the calling technology in question  
(sip.conf for SIP, chan_dahdi.conf for DAHDI, &c.).  If you want to force a 
specific device to use a specific codec, then put an allow in the section for 
that device.

-- 
AJS

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