[asterisk-users] "alaw = 1-2" in system.conf (dahdi) not working in UK

Shaun Ruffell sruffell at digium.com
Sun Mar 11 13:57:26 CDT 2012

On Sun, Mar 11, 2012 at 06:49:01PM +0000, Sebastian Arcus wrote:
> On 11/03/12 14:07, Tzafrir Cohen wrote:
> >On Sun, Mar 11, 2012 at 12:05:48PM +0000, Sebastian Arcus wrote:
> >>Hi all,
> >>
> >>I've tried to explicitly set my two PSTN trunks/FXO lines to alaw with:
> >>
> >>alaw = 1-2
> >>
> >>in /etc/dahdi/system.conf. However, when I do this, all I get is
> >>loud intense noise on the line and nothing else - can't dial, can't
> >>make calls, can't receive calls. If I omit it altogether, everything
> >>works fine.
> >>
> >>As I'm in the UK - I thought Europe uses alaw on phone lines
> >
> >No, they don't. Phone lines are analog. ulaw / alaw settings here are
> >internal to DAHDI. The digital PCM gets converted to analog "audio"
> >(voltage level) on the adapter.
> >
> >>- how
> >>come Asterisk seems to be using only ulaw - and alaw won't even
> >>work? Config examples on the Internet seem to suggest that it should
> >>work.
> >
> >Though it is, indeed, a bug.
> Thank you Tzafrir. I initially though that the signal being analog,
> it can't be encoded with a digital compression algorithm - but as
> config examples online kept on suggesting the setting - I thought I
> would try it.

Hi Sebastien,

If for some reason you do need it to be in alaw, i.e. sometimes you
can get bettwe results when bridging to a digital line in alaw mode
if the audio is originally companded to alaw on the analog card, you
can use the 'companding' or 'alawoverride' module parameter
depending on the analog card that you're using.

There is an old open bug for this, DAHLIN-58 [1], which will require
a board driver / core of dahdi interface change to allow the board
drivers to be notified when the companding mode on a channel is

[1] https://issues.asterisk.org/jira/browse/DAHLIN-58?focusedCommentId=177136&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-177136


Shaun Ruffell
Digium, Inc. | Linux Kernel Developer
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
Check us out at: www.digium.com & www.asterisk.org

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