Subject: Re: [linux-audio-dev] Sequencer Sync Woes
From: Stephane Letz (letz_AT_grame.fr)
Date: Fri Feb 09 2001 - 20:35:17 EET
>Billy Biggs wrote:
>
>> Do you calculate your position in the song based on the current time?
>>That's no good for me. In my app, you need to be able to turn looping
>>on and off, and move the loop mid-song. Otherwise, how can you play
>>live?
>>
>> I usually have to sync to an external source: I have a TR-707 which
>>can output DIN sync to my TR-606, but only if it's the sync master. So,
>>I usually don't care too much about keeping track of the tempo myself.
>
>i did some more improvements to the time-based clock, and now i am
>thinking about how to implement sync to external q/24. not so hard
>to do with the current design, but one thing keeps bugging me, and
>i'd like to know how you would (or do) handle it: in the interval
>that passes between receiveing the first two 0xF8 signals, one cannot
>afaik have any idea of what the tempo will be. do you say "oh well, i
>don't know anything about the current tempo, so i'll do nothing til
>the second signal comes in" or do you make an assumption on the tempo?
>(or the third way: all MIDI messages are aligned to q/24 anyway so you
>don't have to care about this at all ...)
>
>the first, lazy assumption would mean you'd possibly delay some MIDI
>events by a couple of ticks; the second could mean that if you do
>too fast a tempo, you'd have to rewind the 'MIDI tape'. i'm reluctant
>to apply the first, do not like the second for doing something arbitrary
>and cannot do the third because it isn't appropriate for complex
>patterns in time.
>
>any ideas?
>
For my own sequencer library, i'm using the second solution, assuming a
default tempo to play events between the first clock and the second clock.
The general idea is that when a clock is received, the sequencer starts
playing (using the current tempo) but only events which dates_in_ticks
are inferior to the
date_in_ticks of the next expected clock.
When the next clock arrives, there are 2 cases:
- estimated tempo was to high, events are already played
- estimated tempo was to low, events are still not played, the new tempo is
computed, and remaining events are played (in only one "clock slice"),
actually at the same date (in millisecond).
So with this method, there is no problem of "rewinding the MIDI tape". The
tempo between the first and second clock is set to a default value.
Stephane Letz
Grame: Centre National de creation musicale
9, Rue du Garet
69001 Lyon
Tel: 04-72-07-37-00
Fax: 04-72-07-37-01
Web: www.grame.fr
This archive was generated by hypermail 2b28 : Fri Feb 09 2001 - 20:49:20 EET