[asterisk-users] Line noise/hiss on Openvox A400P card on FXO

Sebastian Arcus shop at open-t.co.uk
Tue Mar 13 12:48:55 CDT 2012

On 01/03/12 10:05, Sebastian Arcus wrote:
> I have a server with an OpenVox A400P card with 2 FXO modules on it. The
> internal extensions are SIP Grandstream phones. When making or receiving
> external calls through PSTN, there is an interrupted hissing like high
> pitch noise - which might go away for few seconds then start again.

Just a follow up to my own post. After taking apart the * server, 
replacing motherboard, replacing analog card with a usb FXO card from 
Sangoma - none of the above have helped with the problem.


1. I tried disabling echo cancellation on the FXO ports in DAHDI and all 
of a sudden the line is much clearer. The noise is still there, but 
because the conversation is clearer, I could drop the gains on the FXO 
ports to -2db and -5db. This has reduced the background hiss to a 
certain extent. This was really an important lesson for me - as I would 
normally set the default echo cancellation (128 I think) and just leave 
it on - unless more was required. What I really should have done is 
started with no echo cancellation and just add a bit at a time - up to 
minimum necessary. I never realised how much harm too much echo 
cancellation does to the sound quality on the line.

2. I have tested the setup with an old Cisco 7940 phone. To my surprise, 
although the noise is still there somewhere, it is nowhere near as 
noticeable on the Grandstream GXP280 phone. It looks like in great part 
the Grandstream GXP 280 is just too sensitive to line noise - or it 
picks up / amplifies too much the wrong frequencies.

3. Again, using Ekiga, there is virtually no line noise. They must be 
using some really good algorithms which clean up the line.

Maybe the above will help someone. I just have to decide now if I scrap 
the Grandstreams and replace them with Cisco phones - or just live with 
the line quality.


> 1. The noise is not present when calling in between internal extensions
> (SIP only).
> 2. The noise is the same on both PSTN lines.
> 3. The noise is NOT present when I tried two different phones directly
> in the PSTN line(s) (a Philips DECT phone and a BT Converse phone)
> Is the noise interference actually on the line, which the phones filter
> out because of their better electronic design (then the OpenVox card) -
> or is it generated somewhere in the server or on the OpenVox card?
> I have tried:
> 1. Checking the interrupts and making sure the OpenVox card has its own
> IRQ.
> 2. Moving the card around on different PCI slots.
> 3. Changing the second network card with a different model (the first
> one is integrated in the motherboard).
> 4. Changing the motherboard, CPU and RAM (one motherboard AMD with Sis
> chipset, the other one Intel).
> 5. Placing ferrite cores on the phone cables.
> 6. Checking to see if the OpenVox card gets 1000 interrupts per second
> and it does.
> 7. Upgrading the kernel from 2.6.29 to 2.6.37
> 8. Ran FXO tune and made sure it starts with DAHDI
> 9. Disabled and enable software echo cancellation - it makes no difference.
> The server is virtually under no load during the tests. It does have IDE
> hard-drives (which apparently can cause problems) - but there is not
> much I can do about that.
> I also have a Sangoma USB FXO adapter - which I'm about to install and
> configure to see if it makes a difference.
> I would really like to figure out where is the noise coming from - as
> I'm going a bit in circles. If I can find out for sure that the OpenVox
> card is either broken or low quality - I'll just have to replace it. But
> I can't even figure that out for sure.
> The specs are:
> CPU: Celeron 2.4GHz
> Asterisk 10.1.2
> Dahdi 2.6.0
> Hard-drives: IDE
> OpenVox A400P analog card
> Many thanks for any advice.
> --
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