Subject: Re: [linux-audio-dev] MTC, SMPTE, etc.
From: Paul Winkler (slinkp23_AT_yahoo.com)
Date: Sat Jul 22 2000 - 03:42:47 EEST
Paul Barton-Davis wrote:
>
> >> Note, interestingly, how bad this is for the idea of using audio
> >> devices as a timer for, say, the ALSA sequencer ...
> >
> >Well, only if you use one audio device for your timer and another
> >for audio at the same time.
>
> I don't think so! What we know is that 44,100 samples at a nominal
> rate of 44.1kHz is *not* guaranteed to correspond to 1 second of
> elapsed time. If you build a timer around this (e.g. the ALSA lowlevel
> timer interface), and then use it for sequencing, its not accurate.
>
> Obviously, this is all about "within a tolerance of X usecs".
Right. And if it's within close enough tolerance that recording an
audio stream sounds OK, then recording a midi stream will sound
equally OK (or equally bad!).
I believe that even a cheap soundcard with a bad timer could provide
a good enough timing reference to satisfy nearly all MIDI musicians,
but not good enough to assume it will stay in sync with another
soundcard. For example, with my two soundcards that had 1/4 sec. of
difference after 60 seconds, that's an 0.4% difference in soundcard
sampling rates. If you have a MIDI sequencer running 0.4% too fast,
that means that when you expect a tempo of 100 bpm you'll actually
get 100.4 bpm. I haven't actually tried it, but I think I'd be very
impressed with anyone who could reliably tell the difference between
100 and 100.4 bpm in a double-blind test. BUT, start playing two
parts simultaneously at 100 and 100.4 bpm and *anyone* will know
something's wrong pretty soon.
So, I think running a sequencer from the soundcard clock will
probably be very satisfactory until you try to use a different
soundcard for something else at the same time.
................. paul winkler ..................
slinkP arts: music, sound, illustration, design, etc.
web page: http://www.ulster.net/~abigoo
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