Noise instead of silence

Dear all,

we have build an Asterisk solution (version 18.6, PJSIP2.10 and alaw codec used) and a couple of users complain that they hear noise at some specific parts of the call flows. Other users are not able to reproduce this error.
The investigation, I did, showed that at these specific parts, silence should be heard (e.g. waiting for user input).
We have also a tcpdump wireshark log file at the time of the error, which showed that the RTP sent to core network is clear and with no noise at all. Furthermore, analysis from core network did not reveal something unusual.

Is it valid to conclude that the devices of these users could generate the noise audio under some specific circumstances (e.g. at a specific silent case)? Have you ever heard of something similar? How could we proceed with troubleshooting, in order to find the root cause of this behavior?

Thank you in advance.

Some devices can produce comfort noise, as pure silence can be unsettling to some people and they can treat it as the line has “gone dead” and hang up.

OK, so the best approach would be to ask details regarding these devices and investigate if they have a setup for comport noise.
Thank you, I will give an update when I have news.

Dear all,

troubleshooting with the devices was not helpful, it seems that something else on the core network creates the noise at the endpoint/device.

Some remarks were made regarding the following signaling. It was not mentioned that it is false; however it may be the root cause of our error.

  • Asterisk sends RTP packets each 20msec
  • When silence should be played from Asterisk, Asterisk starts sending “empty” RTP packets containing as payload the value d5
  • After about 6-7 of these “empty” packets, Asterisk stops sending RTP packets
  • When “real” payload should be sent, Asterisk starts sending again RTP packets, marking the first of them also with ‘Marker: True’
  • The time interval that Asterisk does not send RTP packets at all could be 0.5sec or even >1sec. They suspect that at this time interval the core network produces at some cases noise and they would like to be eliminated.

The question is if we can somehow fill these gaps in RTP signaling with “empty packets”.

An experiment via using rtp_keepalive=1 in pjsip.conf created Comfort Noise every 1sec. However, this did not solve the issue.

What I would like to know is:

  • How can we fill these gaps in RTP signaling with empty RTP packets (payload=d5)? Is there any configuration in pjsip.conf that would cause such a behaviour?
  • Is it possible to create Comfort Noise every 0.2sec? The minimum value of rtp_keepalive is 1, which means 1sec, but we would like to test with smaller values.

Thank you in advance for your time.

Asterisk does not guarantee a constant stream of audio. There is the “transmit_silence” option in asterisk.conf which will transmit silence during certain operations, and certain things (such as ConfBridge) always provide a stream, but if you’re just bridged to something else Asterisk merely forwards back and forth.

No, you can’t create Comfort Noise every 0.2sec without potentially modifying code.

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