[pulseaudio-discuss] problem with module-role-ducking, ducked stream occasionally unmutes / switches / breaks through

[Chris Beat]

On 12/06/2022 08:43, Sean Greenslade wrote:
> On Wed, Jun 08, 2022 at 10:35:14AM +0200, Chris Beat wrote:
>> Hi,
>>
>> I am running Kodi on an old netbook to listen to http streamed radio from the internet. I also use
>> shairport-sync to cast audio from Apple devices to the same netbook. I use module-role-ducking to
>> mute the internet radio when a stream from shairport-sync comes in, this is my logic:
>>
>> load-module module-role-ducking volume=0 trigger_roles=ShairportRole,stream ducking_roles=KodiRole
>> PULSE_PROP='media.role=ShairportRole' /usr/bin/shairport-sync -uv
>> PULSE_PROP='media.role=KodiRole' /usr/bin/kodi
>>
>> This all works fine and as expected EXCEPT that occasionally (maybe once a minute) the ducked stream
>> from Kodi "breaks through" (I do not know a better word) - meaning that I can hear a short
>> (subsecond) "fragment" of the Kodi stream INSTEAD of the shairport-sync stream.
>>
>> I am not sure how this can be solved most effectively; maybe it is even better to debug this on the
>> shairport-sync side of things? Maybe there are better ways to mute a stream than ducking?
>
> Hi, Chris.
>
> I don't personally have any experience with Airplay or shairport-sync,
> but I took a quick glance at the shairport code and I have a potential
> theory. Shairport seems to cork the pulseaudio stream if it experiences
> an underflow. The module-role-ducking code interprets a corked or muted
> stream as inactive, which could be the cause of the momentary unducking.
>
> A once-a-minute interval makes me think it could be a wifi scan causing
> packet loss or latency. If the audio buffers are small, that could be
> enough to cause an underflow. I've read about similar issues happening
> with video conferencing calls done over wifi.
>
> If you're feeling up for some debugging, it looks like there's a debug
> message you could uncomment in the shairport code that would confirm if
> it is indeed an underflow-induced corking event:
>
> https://github.com/mikebrady/shairport-sync/blob/master/audio_pa.c#L370
>
> --Sean
>

Hi Sean,


thanks for the hint - you might be correct in regards to the corking and the underruns, because I
did notice a correlation of these ducking/corking glitches with shairport-sync and underrun
notifications in the log of the pulseaudio daemon itself. This is a very underpowered old netbook -
even loading a new picture in the kodi slideshow screensaver sometimes causes these underrun
messages to appear. If I find the time, I'll try to verify this with the debug line in the
shairport-sync code as suggested by you.

Kind regards & thanks again,

Chris.