[wdmaudiodev] Re: GFX/LFX fullduplex support.

  • From: Tim Roberts <timr@xxxxxxxxx>
  • To: wdmaudiodev@xxxxxxxxxxxxx
  • Date: Thu, 04 Sep 2008 13:48:36 -0700

pete c wrote:
> in regards to the following:
> >> Yes, they drift, but you can detect that.  You will be getting
> blocks at regular (short)
> >> intervals, so you can detect changes in timing over time.  It's not
> easy, but I told you
> >> that already.
> How severe can this drift become?

Who knows?  Usually, the capture board and the render board are two
different pieces of hardware, with two different clocks.  Let's say the
quartz crystals on these boards have an accuracy of about 10 parts per
million.  In the worst case, where one was +10 and the other was -10, at
44.1 kHz you would be drifting apart by 1 sample every second or so.

> And if the GFX/LFX are running asynchronously the GFX(render) may fall
> behind the LFX(capture), in which case AEC would begin to fail. I
> guess the GFX could buffer 10ms frames for the LFX to use, however if
> the LFX process overtakes the GFX proccess(if this is even possible)
> then the GFX will always be behind the LFX stream. 

Right.  You have to compensate for that.  You have to detect starvation
in one side or the other, and decide what to do about it.

Tim Roberts, timr@xxxxxxxxx
Providenza & Boekelheide, Inc.

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