Re: [linux-audio-dev] timebases and sync in LAAGA

New Message Reply About this list Date view Thread view Subject view Author view Other groups

Subject: Re: [linux-audio-dev] timebases and sync in LAAGA
From: Tom Pincince (stillone_AT_snowcrest.net)
Date: Mon Jun 18 2001 - 20:58:18 EEST


I did some of this stuff about 15 years ago, when computers were only
used for midi. Since switching to computer for hd recording, I have
only done pure audio recording, no sound for picture.

Basically this is a hardware issue. A video tape striped with smpte
must be played back through a timecode vcr. The vcr outputs the smpte
timecode to a sync box that converts smpte to midi time code. The sync
box connects to whatever physical port the computer is using as a midi
port. The midi sequencer is set to external sync so that the incoming
mtc becomes the timeline. Transport controls are handled possibly by
midi machine control (my memory is vague on this one), so that the vcr
will respond to start and stop commands issued by the midi sequencer. I
imagine that the addition of digital audio to this equation is also
handled by the sync box. It would output a digital clock, like
wordclock, that would be received by a digital in on the soundcard. The
soundcard would be set to external sync. The mtc and wordclock signals
would automatically be in sync with each other as a function of the sync
box, which does all of the adjusting to stay in sync with the incoming
smpte code, so there is no need for the computer to adjust rates, drop
samples, interpolate, or whatever. Everything is still driven by the
soundcard interrupts. The only special issue here is that the midi
sequencer must be able to sync to external mtc. Clients like a
softsynth would be driven by note-on note-off commands from the midi
sequencer.

Any reasonable attempt to do computer audio with smpte sync must be done
with an external sync box and a soundcard that has digital i/o and can
perform external sync, afaik.

Tom


New Message Reply About this list Date view Thread view Subject view Author view Other groups

This archive was generated by hypermail 2b28 : Mon Jun 18 2001 - 20:54:55 EEST