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