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

Thu Nov 8 14:41:55 UTC 2018

On Thu, Nov 8, 2018, at 14:00, pulseaudio-discuss-request at lists.freedesktop.org wrote:
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> Today's Topics:
> 
>    1. PulseAudio null sink monitor gives distorted audio randomly
>       (Mikael Nousiainen)
>    2. Re: PulseAudio null sink monitor gives distorted audio
>       randomly (Alexander E. Patrakov)
> 
> 
> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Thu, 08 Nov 2018 11:00:50 +0200
> From: Mikael Nousiainen <mikaelnousiainen at fastmail.com>
> To: pulseaudio-discuss at lists.freedesktop.org
> Subject: [pulseaudio-discuss] PulseAudio null sink monitor gives
> 	distorted audio randomly
> Message-ID:
> 	<1541667650.735106.1569789384.295560F1 at webmail.messagingengine.com>
> Content-Type: text/plain; charset="utf-8"
> 
> 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.
> 
> By looking at the waterfall display of WSJT-X when using "radio-output.monitor"
> as audio input, I can see some artifacts appearing randomly in the frequency
> domain data -- I would call this distortion, but it's hard to know as I cannot
> hear it. I can set up a loopback module to feed "radio-output.monitor" to the
> speakers and the audio sounds fine. Lowering the incoming volume
> radically, e.g. to 10-20% of the full volume, does not help. And occasionally,
> couple of times a minute -- and sometimes for a longer period of time --
> these artifacts disappear and WSJT-X is able to decode all data flawlessly.
> 
> Please see this screenshot for details: https://ibb.co/eiEo0A
> 
> I'm using a powerful PC (a quad-core Xeon with 48GB RAM) running Fedora 28
> with latest updates applied and CPU usage is quite low (20-30%) during
> decoding. PulseAudio version is 12.2-rebootstrapped packaged by Fedora 28.
> 
> Pulseaudio daemon.conf has default settings, except for the sample rate:
> 
> default-sample-format = s16le
> default-sample-rate = 48000
> 
> Also, I have the following null sink / loopback setup in default.pa file:
> 
> # Loopback devices
> load-module module-null-sink sink_name=radio-output 
> sink_properties=device.description="radio-output"
> load-module module-null-sink sink_name=radio-input 
> sink_properties=device.description="radio-input"
> load-module module-loopback source=radio-output.monitor latency_msec=100 
> adjust_time=0 rate=48000
> load-module module-loopback source=radio-input.monitor latency_msec=100 
> adjust_time=0 rate=48000
> 
> The latter loopback modules are there so that I can hear what is being fed into
> the null sinks, but I've confirmed the same behavior even *WITHOUT* the
> loopback modules.
> 
> I've also set up a fixed latency range for the hardware sound cards, but
> I believe this should not have any effect on the null sinks or loopback modules:
> 
> load-module module-udev-detect tsched=yes tsched_buffer_size=65536 
> fixed_latency_range=yes
> 
> I am not sure if these audio artifacts are caused by PulseAudio, WSJT-X
> or some specific configuration issue in my system. Any ideas?
> 
> 
> ------------------------------
> 
> Message: 2
> Date: Thu, 8 Nov 2018 14:56:23 +0500
> From: "Alexander E. Patrakov" <patrakov at gmail.com>
> To: General PulseAudio Discussion
> 	<pulseaudio-discuss at lists.freedesktop.org>
> Subject: Re: [pulseaudio-discuss] PulseAudio null sink monitor gives
> 	distorted audio randomly
> Message-ID:
> 	<CAN_LGv2FZC7ou+SYCPBiFY5AqXcKeLDrgk9gF6Pp8PMFQ_37+A at mail.gmail.com>
> Content-Type: text/plain; charset="UTF-8"
> 
> 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?
> 
> -- 
> Alexander E. Patrakov
> 

Ok, so I recorded audio with Audacity from both the null sink monitor and speaker monitor and the results are clear: null sink monitor audio contains distorted (buffer underrun-like?) artifacts, while speaker monitor audio does not.

I also recorded the samples using two sample rates (44.1 kHz and 48 kHz) just to see if it made a difference -- and the artifacts were certainly worse with 44.1 kHz capture, when PulseAudio has to do sample rate conversion (all devices are set to use 48 kHz). However, the artifacts are there also in the 48 kHz recording as you can hear.

One thing was interesting, though: I could not first reproduce the issue, but after restarting PulseAudio with "pulseaudio -k", the artifacts appeared again! I restarted WSJT-X after restarting PulseAudio, but did _not_ restart Firefox. After recording the samples, I restarted Firefox and WSJT-X both and I seem to be getting clean audio again. Not sure if NOT restarting Firefox while PulseAudio gets killed/restarted has something to do with this. I'll have to repeat these tests a couple of times at least to see if there is a pattern.

Here's a link to a ZIP file containing the recordings as WAV files: https://we.tl/t-QE6Cixchay
(one of them contains some quiet audio from microphone in the beginning, forgot to cut it out)

-Mikael