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

Pavel Hofman pavel.hofman at ivitera.com
Mon Dec 26 21:07:23 CET 2011


I have been analyzing USB URBs in wireshark. Output stream, adaptive
mode, 16 bits, 48kHz, stereo. 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. This creates ripples in the data stream bitrate, possibly leading
to increased jitter of the PLL'd clock in the USB receiver.

When period size is aligned to multiples of samplerate/1000 * audio
frame bytes * nrpacks, all URBs carry constant number of bytes.

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.

Thank you for your expert opinion.

Best regards,


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