[alsa-devel] Period time/size in adaptive USB - alignment to nrpacks?

[Clemens Ladisch]

Pavel Hofman wrote:
>Each URB is issued always at nrpacks
>milliseconds. Each URB transfers 48 samples * 2 bytes * 2 channels *
>nrpacks milliseconds of bytes. The last URB of period keeps the same
>pace (nrpacks milliseconds), however holds only the remaining number of
>bytes.

No, the period's last URB uses only as many frames as needed.

>This creates ripples in the data stream bitrate, possibly
>leading
>to increased jitter of the PLL'd clock in the USB receiver.

Did you actually observe this?

>I am wondering if for the xx.000 Hz samplerates it made sense to offer
>only these aligned multiples in snd_pcm_hw_params_set_period_time_near.

Only for adaptive playback. And the URB length is also
derived from the period size, so I'd be interested in seeing
how this constraint would be implemented.


Regards,
Clemens