Re: [Jack-Devel] JackTransport vs VST ppqPos

On 05/16/2013 06:39 AM, Pawel wrote:
> Is there any good reason to not sync BBT with frames ?

Yes. Tempo maps.

> I made patch for jack_transport example client (actually it remove some unnecessary code ;-)
> 
> ... but I just realized that there is always one way for compute BBT 

That is only true if you have a single tempo and meter for the whole
timeline.

The tempo can change in the middle of the song (accelerando,
decellerando). The meter can change as well. You can also have multiple
songs (or movements of the same song) on the same timeline with BBT offsets.

- so Jack should do this by self, and master client should change these
values:
>                 pos->beats_per_bar = time_beats_per_bar;
>                 pos->beat_type = time_beat_type;
>                 pos->ticks_per_beat = time_ticks_per_beat;
>                 pos->beats_per_minute = time_beats_per_minute;
> 
> 
> What do you think ?

You'll need a tempo map.
N entries like above that can be positioned at arbitrary points on the
timeline. Furthermore, BPM can be hard or gradual.

It's not a trivial task for jackd to take care of. It is also against
design: jackd only passes information between jack-clients. The daemon
does nothing on its own (with few exceptions).

Now, if you like a challenge: tempo-ramps, flexible BBT offsets and
arbitrary tempo/meter changes mid song is a major item on the Ardour3
ToDo list :)

best,
robin

1368709457.32654_0.ltw:2,a <5194D8EA.10703 at gareus dot org>