[alsa-devel] safe support for rewind in ALSA

Raymond Yau superquad.vortex2 at gmail.com
Tue Feb 23 03:37:51 CET 2010

2010/2/21 Kai Vehmanen <kvehmanen at eca.cx>

> Hi,
> On Mon, 15 Feb 2010, Raymond Yau wrote:
>  Even using a high resolution timer , the application still cannot achieve
>> latency better than the configured period size
>> For USB case , the driver cannot give accurate hw pointer position , hw
>> pointer increase in steps for the current implementation, (i.e. the graph
>> is
> ... but that's a real hardware limitation for the USB-driver, right? And
> even in the USB case, hw pointer is incremented in steps of URB transfer
> size, so even in this case, latency of a highres timer based application is
> not limited by the set period-size.
I don't think so since the sound card still use interrupt driven , the
latency cannot be better than the period size

To achieve lower latency , you have to set a smaller period size even if you
are using high resoultion timer

The glitch is most likely underrun since the watermark may be too low on
machine with slow CPU since you assume all CPU have enough power and disk
I/O to write ( buttfer time - 20ms ) audio data in 20ms

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