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

Sebastian Arcus shop at open-t.co.uk
Thu Sep 13 17:24:30 CDT 2012

On 13/09/12 00:47, Vladimir Mikhelson wrote:
> 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
> Sebastian,
> I understand it is too late for your client, but for the sake of
> consistency.
> 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.
> Regards,
> Vladimir

Thanks for sharing Vladimir. It is interesting that somebody else is 
experiencing at least similar symptoms as I am. The hardware is 
generating the static noise (or at least it amplifies it greatly) - but 
I just couldn't find any way to reduce it sufficiently. After a chat 
with somebody working for a telecom company which installs proprietary 
PBX's - I reach the conclusion that it is not worth the effort. He said 
that whenever they encounter noise on analog lines, they don't tend to 
waste time tuning and adjusting things. They just recommend switching to 
ISDN - so I figured it's the only reasonable thing left to do. Of 
course, this won't make up of the many, many unchargeable hours spent 
troubleshooting this :-)

Also, I'm afraid it isn't much of a solution for your case, if you are 
using FXS.

Good luck,


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