[pulseaudio-discuss] Mixing audio from multiple sink-inputs to multiple sinks dynamically

[Lode Cools]

Thanks for the answer!

On Wed, Apr 6, 2016 at 1:39 PM, Tanu Kaskinen <tanuk at iki.fi> wrote:
> On Tue, 2016-04-05 at 15:13 +0200, Lode Cools wrote:
>> Hi,
>>
>> This topic has been touched before, but I did not find a conclusion:
>>
>> How does one create an end-user solution where it is possible to mix
>> (select & combine) multiple sink-inputs into multiple sinks and then change
>> this 'mix' selection dynamically (preferably glitch-free)?  The changes
>> would be performed using the D-Bus API (or any other API if that is
>> required).
>
> Only the C API is "supported". The D-Bus API can be used too, but if
> you have problems with it, you're on your own. The D-Bus API doesn't
> have any stability guarantees, but in practice it's pretty stable, like
> dead things tend to be.

Yes. So far it has worked fine.

>
>> A simple example is: I am playing sink-input SI1 on combine-sink C1 which
>> uses physical sink S1 as its slave.  Now I also want to play sink-input SI1
>> on physical sink S2.
>>
>> For the moment, I know 2 ways to accomplish this dynamically:
>>
>> 1) 'create new combines' - approach
>>    * I create a new combine-sink C2 which uses both sink S1 and sink S2 as
>> its slaves.  Then I move the sink input SI1 from old combine-sink C1 to new
>> combine-sink C2.  Afterwards, I delete the old combine-sink C1.
>>    * Disadvantages:
>>       ** not glitch-free (especially for listeners of sink S1 who do not
>> care about S2)
>>
>> 2) 'mute the unused links' - approach
>>    * I create from the beginning a 'full mesh': C1 uses both sink S1 and
>> sink S2 as its slaves.  'playing a sink-input on a sink' then just
>> translates to 'unmuting the sink-input from the combine-sink on the actual
>> sink'
>>    * Disadvantages:
>>       ** when dealing with a lot of sink-inputs and sinks, the amount of
>> 'links' is huge.  If a muted input is also increasing the resample load
>> (Can anyone confirm this statement?), this might be a resource-consuming
>> solution.
>
> Yes, muted streams are resampled too.
>
>>       ** all sinks need to be known upfront
>>
>>
>> Ideally, there would be a solution where you could change the slaves of a
>> combine-sink dynamically.  But, afaik, this does not exist and is not
>> planned either.
>
> It may not be planned, but patches implementing dynamic combine sink
> slave management would definitely be welcome.
>
> module-combine-sink already supports dynamic slave changes internally,
> because it has a mode where it plays to all sinks in the system, and
> that requires automatically adding and removing outputs as sinks come
> and go.

OK. Understood.  If it starts to become relevant for my intended
application, I will
implement this.  How would the API look like?  Is it enough to just
catch changes
on the sinks property list?

>
>> So the question is:  Are solution 1 and 2 really the only solutions for my
>> problem?
>
> I don't have any other solutions, apart from adding an API for the
> combine sink slave management.
>
>> Which one of the solutions would you favor over the other?
>
> They have different tradeoffs, so which one is better depends on the
> situation.
>
> --
> Tanu
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