[asterisk-users] Audio distorted local side only

[cb]

I'm not sure where to check next, so I'm reaching out to those that  
know this stuff better than I.

I've got Asterisk up and running, but I've still got an occasional  
audio issue. Once in a while (maybe 1 out of every 20-30 calls), the  
audio becomes heavily distorted, but only on the local side. The party  
on the other end says the audio is fine. We can hear them, although on  
these calls the remote party is quieter than usual. They can hear us  
just fine. But when we speak, our side becomes heavily distorted. It  
only happens when we speak. As long as we are not talking, the audio  
is fine other than the remote party is not as loud as normal.

The distortion sounds a lot like what a speaker sounds like when it is  
blown. Not quite static, not quite feedback, but very crackly and  
metallic with pops and squeals. It drowns out all other sound on the  
call. We hear it very loud, almost painfully and you have to hold the  
handset away from your ear as you talk. The remote party says they  
hear nothing at all wrong. It happens on both handset and speaker  
phone, so it shouldn't be a speaker issue.

This never seems to happen on interoffice calls, only on calls going  
out over our POTS lines. Also, if we put the call on hold or transfer  
the call, usually the distortion is gone once the call is picked back  
up (although it may come back, but not as bad). Flip side, sometimes a  
good call that gets put on hold or transfered will have the issue when  
the call is picked up again.

I've done rxgain tuning, although I can't be 100% sure I've done it  
right. I followed the advice from <http://www.mattgwatson.ca/2008/05/howto-tune-zaptel-dahdi-fxo-interfaces-on-asterisk-pbx/ 
 >, but the levels he said to aim for cause me to need to raise my  
lines to between +12 and +20 and when listening to the tone on a phone  
sound as if they are starting to clip (although that could be correct  
for all I know). I had been doing txgain tuning, but in my testing, I  
found that either the values I was setting made no difference in the  
audio levels or topped out the audio levels around 9000 instead of the  
article's recommended 14844. As such, my txgain is currently set to 0  
for all lines. Also it is worth noting that I was not able to turn up  
a milliwatt test number for my local CO (I even asked a Verizon tech  
who claimed they didn't have one in my CO and he didn't know the  
number for one in the area, although he may have just been BSing me to  
not give it out). The number I used is several states away, so I don't  
know how much (if any) audio loss is present on the tone, thus it is  
possible my rxgain may be too high for my lines. If anyone knows of a  
milliwatt test number for northern NJ I'd be happy to repeat my tuning.

I am using Snom 370 phones (running the latest 7.3.23 firmware, but it  
was happening with the 7.3.14 firmware as well). My Analog lines are  
connected via a Sangoma A400DE. This is their 12 port card with  
hardware echo cancel and PCI-E. The network is a dedicated network,  
only the phones are on it, and all are connected to a netgear  
10/100/1000 POE switch. The phones are using POE. The only devices on  
the network that are not phones are the Asterisk server, a router to  
get the network access to the internet (currently outbound only, I've  
not yet allowed inbound access to the server) and my laptop for  
configuring the server (although the issue happens even without my  
laptop connected). So this likely is not caused by traffic killing the  
network (won't rule it out, but it doesn't seem like a first  guess).

Anyone know what is going on or can tell me where to go next to figure  
it out?

Thanks for any
-chris
<www.mythtech.net>