Re: how to schedule a timer in sync with audio
Re: how to schedule a timer in sync with audio
- Subject: Re: how to schedule a timer in sync with audio
- From: "Ross Bencina" <email@hidden>
- Date: Thu, 6 Jan 2011 12:20:22 +1100
Paul Davis wrote:
i think you need to read about DLLs, which are the correct way to link
two clocks.
I agree.
there is a good, somewhat canonical paper on it here:
http://www.kokkinizita.net/papers/usingdll.pdf
(canonical only because this is the sort of arcane hacker knowledge
that tends to remain inside the engineering departments of companies
that end up having to implement it, and rarely shows up in textbooks
or courses of any kind).
You can find it in standard control theory text books or read the Wikipedia
entry for PID controller, which is quite good:
http://en.wikipedia.org/wiki/PID_controller
We discussed this a bit on the music-dsp list recently too.
if you use a DLL correctly, you can always the question "given a time
T on clock 1, what is the time on clock 2". all other methods to try
to do this are a poorer solution, although they may initially appear
simpler to implement and use.
I agree that there are no good simpler methods, but a DLL is not always
sufficient, although it may suffice for the OP's needs.
I apologise if i'm jumping to conclusions here, I only took a brief look at
the paper, but it appears to be a digital version of a standard IIR PID
controller. For high-jitter scenarios, least squares regression methods can
provide a better estimate of the phase of each clock -- these can be
combined with a PID controller (as in the above linked paper) to actually
synchronise the clocks.
OLS regression may not be optimal for this kind of problem since it may make
the wrong assumptions about the jitter noise distribution -- therefore other
forms of regression may also be applicable.
For a good discussion of least-squares vs DLL/PLL clock recovery see Chapter
3 of
SYNCHRONIZATION OVER PACKET-SWITCHING NETWORKS: THEORY AND APPLICATIONS
Rafaele NORO
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.83.2811&rep=rep1&type=pdf
Cheers
Ross.
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