[pulseaudio-discuss] PulseAudio null sink monitor gives distorted audio randomly

Tanu Kaskinen tanuk at iki.fi
Thu Nov 8 14:44:52 UTC 2018

On Thu, 2018-11-08 at 14:56 +0500, Alexander E. Patrakov wrote:
> Mikael Nousiainen <mikaelnousiainen at fastmail.com>:
> > I've got a very weird issue with PulseAudio when trying to route audio
> > from one application (Firefox 64.0b7 (64-bit)) to another one (WSJT-X v1.9.1).
> > I'm experiencing the same issue with different browsers (Chrome and Chromium too).
> > The browser is receiving audio from a radio transceiver through a WebRTC
> > connection and I'm feeding it to WSJT-X to decode the data in the audio signal.
> > 
> > I'm using two module-null-sink modules to transfer the audio between
> > the browser and WSJT-X. I use pavucontrol to make the browser play audio to
> > null sink called "radio-output" and then let WSJT-X listen to the audio
> > via "radio-output.monitor". The kind of setup exists for transmitted audio
> > where WSJT-X feeds audio to the browser through a null sink called "radio-input".
> > 
> > However, the issue her is that WSJT-X is mostly not able to decode the data
> > when it's using "radio-output.monitor" as audio input. Sometimes it works
> > and sometimes it does not. As a technical detail, I'm trying to decode FT8
> > digital mode traffic and I've confirmed that the reason is not related to
> > bad time sync (which FT8 requires), because I can even play the browser
> > audio through laptop speakers and let WSJT-X use the laptop microphone as
> > audio input and it decodes the data just fine -- the audio sounds clean
> > and strong with no audible artifacts.
> We need to figure out whether this is due to a monitor or due to a
> null sink. Could you please let the browser play to your speakers, and
> WSJT-X record from the monitor of the sound card that the browser is
> playing to? Does it work?
> Is the webrtc feed public? I.e., can I run WSJT-X locally in order to
> reproduce the issue?

The problem is most likely due to this bug regarding monitor sources:

Whenever the monitored sink has a rewind, a chunk of audio is
apparently duplicated in the monitor source. It would be great if
someone could figure out what goes wrong in the monitor rewinding code.



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