[alsa-devel] [PATCH v2] ALSA: usb: refine delay information with USB frame counter
clemens at ladisch.de
Sun Sep 11 17:49:12 CEST 2011
Pierre-Louis Bossart wrote:
> Existing code only updates the audio delay when URBs were
> submitted/retired. This can introduce an uncertainty of 8ms
> on the number of samples played out with the default settings,
> and a lot more when URBs convey more packets to reduce the
> interrupt rate and power consumption.
> This patch relies on the USB frame counter to reduce the
> uncertainty to less than 2ms worst-case. The delay information
> essentially becomes independent of the URB size and number of
> packets. This should help applications like PulseAudio which
> require accurate audio timing. Clemens Ladisch
> <clemens at ladisch.de> reported a decrease of mplayer's
> A-V difference from nrpacks down to at most 1ms.
> Thanks to Clemens for also pointing out that the implementation
> of frame counters varies between different HCDs. Only the
> 8 lowest-bits are used to estimate the delay.
> Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart at linux.intel.com>
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