Re: iaxmodem connection failures | testing custom gain in Asterisk

Hello Nathan,

sox doesn't normalize the data... at least if you're using the suggested 
commands from the IAXmodem "how-to" guide.

If there is noise happening out there in the wild then increasing your 
TX gain may improve the SNR on the audio going out to the other end, but 
increasing your RX gain won't do anything other than make everything 
louder.  Now, if the noise is coming from within your system, then 
increasing RX gain before the noise is added may help, but since you 
believe that the problem is external, there's not much you can do to 
improve the SNR in the audio that you're receiving.

In the audio that you've recorded are you seeing jitter?  Or are you 
hearing actual noise?  The audio will give you a good idea about what 
side of the VoIP-PSTN fence the problem is on.

It's very rare for VoIP equipment to add noise other than echo normally 
fax-over-VoIP problems are jitter, jitter, and jitter. Noise (like pops 
and crackles) generally is a PSTN thing (and generally is representative 
of a timing problem).

The sad thing about using VoIP for fax is that it can work reliably well 
for some for a good deal of time... only to have its occasional moments 
of severe brokenness due to the difficulty in controlling, mitigating, 
or eliminating jitter.

Thanks,

Lee.


On 03/02/2018 07:43 PM, Nathan Scannell wrote:
>
> Hi All,
>
> I am hoping someone can help shed a little light here...
>
> I've been using IAXmodem happily for a long time now but recently 
> something changed out in the wild and my modems can no longer 
> correctly detect silence or train - particularly when going off-net to 
> PSTN and vice versa.
>
> So all of a sudden, a 3% failure rate became a 97% failure rate across 
> the board for all deployments in different locations throughout the 
> city. (I am in Adelaide South Australia)
>
> I've done a lot of investigation and what I suspect has happened is 
> that an interconnect hardware change has occurred at the wholesale 
> PSTN transition to on-net causing either noise on the lines OR low 
> gain and poor signal to noise.
>
> I've had a go at boosting the TX and RX in Asterisk using a custom 
> context but I'm not sure its actually doing anything... What I have 
> tried to do is use iaxmodem to record the modems conversation with and 
> without the boost and inspect the different waveforms but I can't see 
> any difference in the gain (had to use sox to convert the RAW to WAV 
> so i'm not sure if sox in Debian normalises by default.
>
> So...
> Can IAXmodem report gain for each side of the conversation?
> Is there a method external to Asterisk to improve SNR or up the tx/rx 
> gain?
>
>
> Cheers,
>
> Nathan
>
>
>
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