[linux-audio-dev] XAP and these <MEEP> timestamps...

[Paul Davis]

>A nice question to ask is 'what is time'. I suppose that there is a direct
>correlation between time and sample frequency; but what to do with
>non-constant sample frequency? (This is not a hypothetical situation, since
>a sampled system which is synchronised to an external source, could be
>facing variable sample rate - when slaved to a VCR for instance). I believe
>the answer lies in definition of which is your time master, and use that as
>such; so in case of the slowed down VCR, the notion of time will only
>progress slower, without causing any trouble to the system. If no house
>clock or word clock is availble, things might end up hairy...

i believe, though i am not certain, that you are confusing two
different kinds of synchronization. there is:

    * positional synchronization ("where are we?")
    * sample clock synchronization ("how long between samples?")

they are not related to each other in *any* way, AFAIK. synchronizing
position with a VCR via SMPTE (for example) has nothing to do with
sample clock sync. likewise, a word clock connection between two
digital devices has nothing to positional synchronization.

so, slaving to a positional reference has no effect on sample
rate. conversely, slaving to a sample clock source has no effect on
positional tracking. word clock does make variable sample rate
possible, and indeed some systems use it to implement sync'ed
varispeed. but its definitely not a consequence of using a positional
reference like SMPTE or MTC. when slaving to those signals, the sample
rate remains constant (all other things remaining the same), and all
that changes are the notions of "where we are?" and "how fast are we
moving?" and "what direction are we moving in?". the same number of
samples are processed every second, but what those samples contain
will vary with the positional reference.

a careful read of the last chapter of the protools manual is
recommended (the PDF is available online). its not superb, but its a
pretty good introduction to sync issues with DAWs and similar software.

--p