[alsa-devel] [PATCH 0/2] RFC: support for audio wall clock

Raymond Yau superquad.vortex2 at gmail.com
Sat Jun 30 03:03:49 CEST 2012

2012/6/29 Pierre-Louis Bossart <pierre-louis.bossart at linux.intel.com>:
> V2 of patches to show how the audio wall clock (eg. the
> HDAudio one) can be used to provide precise audio timestamps and track
> the drift between audio time and system time. On my laptop I am able
> to track a 7ppm delta.
> I will explain in more details how this can be
> used during LPC in San Diego, but with the summer coming I'd like to
> get feedback as there are changes to the core/alsa-lib that haven't
> been modified in years.
> - 64-bit alignment, verification of pcm_compat.c changes
> - handling of digital inputs, need to disable wall-clock timestamps in that case (feedback from Clemens)
> - check differences in protocol
> - change timecounter to avoid accumulation of rounding errors in cycle->ns conversions
> - check overflows
> - rework the timecounter init on trigger

Do suspsend/resume affect the usage of the wall clock ?

Those au88x0 also have a 32 bit register which return the ticks from a
12.288 Mz oscillator (i.e. 256 x 48000Hz) , this register wrap around
in around 349.5 seconds

Can this clock be used to determine the drift too ?

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