[alsa-devel] USB Audio questions

Clemens Ladisch clemens at ladisch.de
Wed Aug 17 08:36:35 CEST 2011

Pavel Hofman wrote:
> Dne 16.8.2011 22:41, Torsten Schenk napsal(a):
> > - The device/driver sends per urb a constant number of packets (as
> > you already also discovered). The number of samples per packet differ
> > however (+- 1 sample per packet). I accuse the asynchronousity
> > between the urb interval and the sound card's internal clock for this
> > fact. The device expects the driver to send back an urb with exactly
> > the same structure. The device produced crackling noise if the
> > samples per packet were not equal.
> Could we attribute this to the adaptive mode and PLL striving to produce
> steady stream of well-timed samples to the DAC?

No; those devices are not adaptive, they have their own clock and
excpect the computer to derive the actual rate of the playback stream
from the actual rate of the capture stream.

> IMHO the lower variation in samples per packet, the lower jitter of
> the PLL-recovered clock.

For adaptive devices, the driver has complete control over sent packets,
and thus they already have as little variation as possible.


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