“alaw = 1-2” in system.conf (dahdi) not working in UK

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Asterisk Users 4 Comments

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 – 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.

Thanks for any help,


4 thoughts on - “alaw = 1-2” in system.conf (dahdi) not working in UK

  • 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.

    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