GSM codec corrupting Monitor

Asterisk Asterisk Support
1

Hi all,

We appear to have found an intermittent issue with the GSM codec when we use the Monitor( ) application, possibly when packet loss is encountered. We appear to have DC shift once it has occurred. I raised an issue via the ticketing system for this, but the version we are using is too old to support… which is fair enough, but we are not in a position to upgrade at present.

For reference the issue raised was here: issues.asterisk.org/view.php?id=18934

My question is, has anybody else seen this issue? Does anybody know of any workarounds?
At the moment, we are having to considering giving a small number of customers the ability to use Record( ) in a raw
format rather than have it compressed using the GSM codec.

Asterisk version is 1.4.11 with some back-ported code.

2

I find it strange that GSM is even capable of encoding a DC offset, although I can’t say for certain that it can’t.

My guess is that you are losing frames and you are seeing some artefact from the way the decoding codec interpolates. I’d suggest attacking it with wireshark, to see if the stream is complete and to get a second opinion on the decoding.