[pulseaudio-discuss] Pulseaudio latency

Georg Chini georg at chini.tk
Tue Jul 31 18:56:55 UTC 2018

On 31.07.2018 10:22, Harish Gaddameedi wrote:
> On Tue, Jul 31, 2018 at 12:42 PM Harish Gaddameedi 
> <harish.gaddameedi at smartron.com 
> <mailto:harish.gaddameedi at smartron.com>> wrote:
>     On Tue, Jul 31, 2018 at 12:11 PM Harish Gaddameedi
>     <harish.gaddameedi at smartron.com
>     <mailto:harish.gaddameedi at smartron.com>> wrote:
>             /Please do not top-post.
>             To me it looks like that is an issue in the ALSA driver
>             and not related to pulseaudio.
>             The driver must be reporting the wrong latency. Did you
>             set the loopback latency to/
>             /300 ms? Default is 200 ms./
>         /
>         /
>         Sorry, I use default settings, gmail is doing top posting.
>         No, i didn't set any loopback latency. I'll check with the
>         alsa driver and get back to you.
>         --
>         Thanks,
>         Harish Gaddameedi
> Hi Georg,
> There is one more important point i wanted to discuss, this is which 
> we have capture from your reply of clock synchronisation. Can you 
> conform whether the system clock and audio clock both are same or 
> different?

System clock and audio clock need not be equal. Each sound card has its own
clock which might not be synchronized with system clock or wall clock.
Basically, if your sound card claims to run on 44100 Hz, it may be slightly
more or less if measured in "real" (wall clock) time.
module-loopback is normally capable of detecting the clock difference and
adjusts the sink-input sample rate so that the latency remains constant. 
is why you see in the log, that the module is not using 44100 Hz but in fact
some other (in your case lower) sample rate. PA then does re-sampling
from that rate to your sound card rate.

For the A2DP sink (bluetooth headset or speaker) the system clock is used
for timing, so for this special case the audio clock matches system clock.
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