[pulseaudio-discuss] Virtual audio cable - high cpu usage

[Sean Greenslade]

On Thu, Feb 04, 2021 at 06:29:47PM +0100, Renaud GHIA wrote:
> >> Thank you for the tip.
> >> Now I am sure that resampling does not apply (see below).
> >> But unfortunately pulseaudio always consumes 30% of one CPU core!
> >>
> >
> > The reason is that you are using module-loopback. Due to a potential
> > difference between the clocks on the null sink and on the real sound card
> > (even though both report 44100 Hz), it always has to resample in order to
> > correct for the clock skew. If you don't understand, here is an analogy:
> > you have two mechanical watches. Even though both claim that their hour
> > hand makes one full circle every 12 hours, in fact, if left unattended,
> > they will diverge over time. There is no way to avoid that, except by
> > moving to Jack or PipeWire which simply don't introduce a null sink with an
> > independent clock.
> >
> > If you decide to stay on PulseAudio, you can tweak the resampling method.
> > The default, speex-float-1, is light on the CPU resources and should
> > produce no distortions detectable by human ear on typical speech and music.
> > It does produce easily detectable distortions on specifically crafted
> > signals. If you want to make sure that the resampler is transparent no
> > matter what is thrown at it, use speex-float-5. There is no point in going
> > higher than that.
>
> Thanks for your response.
> I understand the problem of clock drift when we have several audio hardware
> devices (with different qwartz).
> But here, it's a pity that the null video driver has an independent clock...
> Concerning pipewire, for now it doesn't work with our particular
> application (no sound), but it seems to evolve quickly.
> I haven't tried jack for now, but if it depends on pulseaudio, won't I have
> the same problem?

Jack doesn't depend on pulseaudio, it's an independent sound server.
Though there are ways to make the two interconnect.

As far as I know, jack solves this problem by only allowing one hardware
sound device and locking all clocks to that. See:

https://jackaudio.org/faq/multiple_devices.html

One thing that just occured to me was to ask whether you acutally need
to use the remap module at all. I just did a quick test on a very weak
machine of mine: a null sink, a music player outputting to that sink,
and ffmpeg recording the null sink's monitor-source, all with matching
44.1 kHz sample rates. This resulted in no resamplers in the chain, and
only 9% CPU usage on a 2010 Atom CPU.

--Sean