[asterisk-users] Static noise on bridged calls to PSTN, although the trunk line is clean on its own

Vladimir Mikhelson vlad at mikhelson.com
Wed Sep 12 18:47:18 CDT 2012

On 9/12/2012 5:33 PM, Sebastian Arcus wrote:
> On 10/08/12 18:38, Chad Wallace wrote:
>> On Tue, 31 Jul 2012 09:44:26 +0100
>> Sebastian Arcus<shop at open-t.co.uk>  wrote:
>>> I have two setups with SIP hardware phones as extensions and POTS
>>> lines as trunks. Internal SIP to SIP calls are crystal clear, but all
>>> calls bridged to POTS have a significant amount of static noise. The
>>> problem is that if I plug a POTS phone directly into the line, there
>>> is almost no static noise - the line is clean. It's like Asterisk (or
>>> the hardware) amplifies the static noise. What I've tried so far:
>>> 1. Connect Asterisk with a short cable directly into the master phone
>>> socket, where it enters the building.
>>> 2. One of the lines carries ADSL - so I double filtered it.
>>> 3. Tried three different phone sets (one Grandstream, two Cisco
>>> models).
>>> 4. Tried an OpenVox A400P PCI card and a Sangoma U100 USB
>>> adapter as analogue-to-digital interfaces.
>> Have you run fxotune?  I remember doing that when we had analog
>> lines.  You'd have to look up how--maybe just in the fxotune man page.
> Thanks for replying Chad - and sorry for the delay in my reply. I
> should have mentioned that I ran fxotune and made no difference. I
> also checked the interrupts, and even changed motherboard, and tried a
> USB analog adapter (Sangoma U100) instead of the current OpenVox PCI
> adapter. None solved the problem.
> I have given in and asked the client to order ISDN lines I'm afraid.
> Sebastian


I understand it is too late for your client, but for the sake of

I am experiencing a similar issue with Digium TDM410 on FXS lines.  In
my case the static noise is always present on analog extensions provided
by TDM410.

I had a ticket opened with Digium, and they admitted the following, but
refused to make the findings public:

 1. The static noise is produced by Digium analog equipment on certain
    motherboards.  I specifically tried various Dell Dimension Pentium
    III machines with several power supplies.  They all consistently
    started producing the noise the moment DAHDI drivers loaded, even
    before Asterisk was loaded.  When I tried Dell Dimension Pentium IV
    machine the noise was not there.
 2. On a machine which produced noise as described in #1 switching to a
    non-Digium TDM card fixed the noise problem.  FXS daughter card
    stayed the same, just the base card was swapped.

I do understand your situation involved FXO, and switching to OpenVox or
Sangoma did not help.  But I feel the root cause well may be the same.


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