Speex dev - Apr 2011 - Why most AC97 soundcard has different sample rates of of capturing and rendering?

Hi all,

I remember someone (sorry I forgot his name) told me that AC97 capturing codec
has its own PLL clock generator to fit for S/PDIF input which has its own clock.
So the mismatch between clocks of capturing and rendering is unavoidable.

But after reading the Audio Codec 97 Revision 2.3 Revision 1.0 document
carefully, I can only find proofs that the mismatch should not exist.

In chapter 3.4, page 23, clocking for Multiple Codec Implementations, it is said
that "To keep the system synchronous, all Primary and Secondary Codec
clocking must be derived from the same clock source, so they are operating on
the same time base. In addition, all AC-link protocol timing must be based on
the BIT_CLK signal, to ensure that everything on the AC-link will be
synchronous.".

In Figure 8. Controller to Multiple Codec connections at page 21, you can also
find that AC97 controller and all Codecs share the same SYNC and BIT_CLK signal.

So anyone could give me more details for the reason of mismatch between sample
rates of of capturing and rendering? Which is a HUGE OBSTACLE for all LMS based
acoustic echo cancellers, such as speex AEC.

Maoquan
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