[pulseaudio-discuss] How does automatic output resampling in Pulseaudio work?
erich at uruk.org
Mon Feb 18 13:19:43 PST 2008
Tanu Kaskinen <tanuk at iki.fi> wrote:
> On Mon, Feb 18, 2008 at 11:56:05AM -0800, Erich Boleyn wrote:
> > When Pulseaudio converts from one format to another at the sound output
> > (say for an ostensibly 16-bit/48-khz DAC) *and* performs the automatic
> > output resampling (due to the normal output hardware clock imperfection
> > issues), which one more accurately represents the real chain of events:
> > A) 24-bit/96-khz PCM --(resampled)--> 16-bit/48-khz
> > --(resampled)--> 16-bit/near-48-khz-real-freq
> > B) 24-bit/96-khz PCM --(resampled)--> 16-bit/near-48-khz-real-freq
> > I.e. is it resampled once to get to the "output format" and again at
> > the final hardware matching stage, or is it resampled once directly to
> > the hardware matching frequency?
> If I understood you correctly, then the answer is that there
> is no "final hardware matching stage". In your example the
> sound card reports that it uses sampling rate of 48000 Hz,
> and pulseaudio trusts that it's correct information. If it's
> not, then the sound is played slightly too fast or too
> slowly. The small deviation is taken into account only when
> there are multiple clocks, for example two sound cards, and
> even then the correction is not based on actual time, but
> just the observed difference in the clocks.
> So resampling is always done only once (afaik).
Is this also true for sources received from hard input sample rates?
In particular, my example is that I am sending audio via RTP to multiple
computers, each possibly more or less in hearing distance of the other.
(though this would apply possibly to input from a digital or other source
on the local machine as well, since it has a fixed input rate)
So, you have 2 problems:
-- The source was sent out at a particular frequency over the network,
and you'd get bufferring problems if you didn't resample at the
-- The deviations between the different computers will be noticable
after a while.
I thought pulseaudio accounted for that difference, perhaps I interpreted
the "ability to resample the outputs to be synchronized" comments in the
Erich Stefan Boleyn <erich at uruk.org> http://www.uruk.org/
"Reality is truly stranger than fiction; Probably why fiction is so popular"
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