RTCM Choppy Audio

The RTCM voter protocol is UDP which of course means packets can be dropped causing holes in the audio. Here are a couple of things to look for if the audio is choppy.

Network Problems

The server (chan_voter) sends a keep-alive packet to the RTCM once a second. Likewise, the RTCM sends a keep-alive packet to chan_voter once a second. Timeouts will occur if excessive packets are dropped.

  • Start the asterisk console with verbose set to 3 (asterisk -rvvv) and watch for disconnect messages.
    -- Voter client nameOfClient disconnect (timeout)

    This means the chan_voter has missed 3 keep-alive packets in a row, or said another way, 3 seconds has passed since the last keep-alive was received.

  • If the RTCM off-line fail over message is heard over the air (perhaps quickly followed by the on-line message) the RTCM has missed 6 keep-alive packets in a row. In other words 6 seconds has passed since the last keep-alive was received.

Either of these indicate a network problem that can not be compensated for with RTCM or voter.conf settings. The problem must be fixed using traditional network trouble shooting techniques, ie traceroute and ping. Try running a ping with a count of 100 or more to insure there is no packet loss.

Buffer Size

Voter ping is useful for end-to-end network evaluation when ICMP ping is turned off and/or the RTCM is behind a firewall and is not ICMP reachable. It can help with finding the correct RTCM and voter.conf buffer settings. See Setting Voter Buffers and look at oos (out of sequence) and packet loss. Both should be zero. If not, adjusting the buffer size may reduce oos or packet loss.