14 Dec
2002
14 Dec
'02
8:10 p.m.
On Sunday 15 December 2002 00.26, Paul Davis wrote:
Well, they could be "related", I think... (Read on.)
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.
Good point. One could say that every sync source generates one of
these:
* timing data (tempo, sample clock,...)
* positional data (song position, SMPTE,...)
Positional data sort of implies that you can extract timing data as
well, provided you get a stream of positional data with sufficiently
accurate timing.
Anyway, in that other post, I think I said there *is* a relation
between all of these, but I forgot to explain why:
* Audio device syncs to wordclock
* Sequencer uses audio for timing (nominal sample rate assumed)
Note that both are just *sync* - not lock. If you wanted to sync with
a VRC, you would most probably be using SMPTE instead of wordclock -
and then, it would make a *lot* more sense to sync + lock the
sequencer to that, and just let the audio interface do 48 kHz, or
whatever you like.
Either way, the point I was trying to make still stands; by the time
the host starts calling all plugins to process one block of data,
*all* information the plugins will access must *static* - or you will
have plugins that run in "parallel" mysteriously fall out of sync.
So, the sequencer (which will obviously be one of the first plugins
in the graph, if not integrated in the host) will have to figure out
the SMTPE, audio, MIDI clock or whatever it's syncing or locking to
for the first sample of the block, construct the timeline info for
the block, and pass it to the plugins that want it.
At this point, all is decided. Sync and lock has been applied, and
for all practical matters, you may consider this block property of
the audio hardware; you just have to do some audio processing to
generate the actual data.
so, slaving to a positional reference has no effect on
sample
rate.
Well, it *could*, if you assumu that one SMPTE frame corresponds to N
samples... ;-)
Not much point, though - you would only end up modulating the tempo
of the still free running sequencer, as well as the pitch of any
audio you generate. :-)
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.
This is a great explanation, IMHO.
//David Olofson - Programmer, Composer, Open Source Advocate
.- The Return of Audiality! --------------------------------.
| Free/Open Source Audio Engine for use in Games or Studio. |
| RT and off-line synth. Scripting. Sample accurate timing. |
`---------------------------> http://olofson.net/audiality -'
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