Static Noise On Bridged Calls To PSTN, Although The Trunk Line Is Clean On Its Own

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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.
5. Reduced the software echo canceller in chan_dahdi.conf to 32 and even
16 – until I could actually start to hear echo. Still no difference.
6. Reduced the rxgain and txgain in chan_dahdi.conf to 0 – but the static noise is still there.
7. Tried different phone cables for the pots line.
8. Tried a different motherboard on the computer with Asterisk and checked there is no IRQ sharing. Tried when there was no other load on the Asterisk computer.
9. Tried Asterisk 1.6, 1.8 and 10

Is there anything else I can do – or should I just give in to the static noise? Is that how other hybrid setups work – do you get static noise on the line – more than if plugged directly? The client is adamant that the noise on the line is too high – by comparison with the quality on mobile phone calls (which are digital, incidentally) – so if I don’t find a solution, I suppose I will just have to rip it all out and let one of the companies with proprietary phone systems install one.

Any hints appreciated.


5 thoughts on - Static Noise On Bridged Calls To PSTN, Although The Trunk Line Is Clean On Its Own

  • 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 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,


  • Sebastian,

    There is a major difference between a noisy analog line your TELCO
    friend was talking about and a computer component generating the noise on it its own under certain circumstances.

    Switching to ISDN will help with one leg. If another leg stays analog it will be noisy in my scenario. I think your problem may have been of the same nature since you mentioned connecting an analog phone to that line eliminated the noise.

    As far as wasted hours go, I am with you – 100%.

    Regards, Vladimir