Re: [Jack-Devel] JackTransport vs VST ppqPos

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DateSun, 12 May 2013 11:45:21 +0200
From Stéphane Letz <[hidden] at grame dot fr>
ToPawel <[hidden] at wp dot pl>
Ccjack-devel <[hidden] at lists dot jackaudio dot org>
In-Reply-ToPawel [Jack-Devel] JackTransport vs VST ppqPos 
Le 9 mai 2013 à 03:22, Pawel <[hidden]> a écrit :

> Hi,
> 
> I discover strange problem while improving JackTransport support in FSTHost.
> 
> Most VST plugins while trying to synchronize with host usually ask for such things:
> 1) tempo ( == jack_pos.beats_per_minute)
> 
> 2) ppqPos - which I understand like "current quarter note" == current jack_pos.beat.
> 
> 3) barStartPos .. ok, this one is not a subject in this case ;-)
> 
> 4) samplePos == jack_pos.frame
> 
> 5) sampleRate == jack_pos.frame_rate
> 
> The problem is which pppPos. There are two ideas how to compute this:
> 
> 1) Using BBT (which should be a preferred option here, right ?)
> double ppqBar = (jp.bar - 1) * jp.beats_per_bar;
> double ppqBeat = jp.beat - 1;
> double ppqTick = (double) jp.tick / jp.ticks_per_beat;
> 
> ppqPos = ppqBar + ppqBeat + ppqTick;
> 
> 2) Using sample rate / current frame / tempo - this is a standard way in VST world. We count how much samples is in quarter note (beat) ..
> 
> double ppq = jp.frame_rate * 60 / jp.beats_per_minute;
> ppqPos = jp.frame / ppq;
> 
> The problem is that .. this second counter "hurry" (sorry can't find better word for this issue in english ;-)
> I made really simple , example app for observe this issue - it is included in FSTHost repository:
> http://sourceforge.net/p/fsthost/code/HEAD/tree/trunk/jt-test.c
> 
> Frame: 0013314048 | Bar: 0068 | Beat: 3 | Tick: 1680 | CBFF: 277.38 | CBFBBT: 270.88
> Frame: 0013410304 | Bar: 0069 | Beat: 1 | Tick: 1600 | CBFF: 279.38 | CBFBBT: 272.83
> Frame: 0013506048 | Bar: 0069 | Beat: 3 | Tick: 1500 | CBFF: 281.38 | CBFBBT: 274.78
> Frame: 0013602304 | Bar: 0070 | Beat: 1 | Tick: 1420 | CBFF: 283.38 | CBFBBT: 276.74
> Frame: 0013698560 | Bar: 0070 | Beat: 3 | Tick: 1340 | CBFF: 285.39 | CBFBBT: 278.70
> Frame: 0013794304 | Bar: 0071 | Beat: 1 | Tick: 1240 | CBFF: 287.38 | CBFBBT: 280.65
> Frame: 0013890560 | Bar: 0071 | Beat: 3 | Tick: 1160 | CBFF: 289.39 | CBFBBT: 282.60
> 
> Please look that after a while CBFF > CBFBBT, and trend is that difference will grow. I tested this with either Hydrogen and jack_transport as master. The funny thing is that - when I skip location to for example in jack_transport:
> jack_transport> stop
> jack_transport> locate 0
> jack_transport> locate 51111111
> jack_transport> play
> 
> CBFF and CBFBBT back to normal (almost) .. aha - so we know that formulas are good - and problem is in Jack.
> Frame: 0051936967 | Bar: 0271 | Beat: 2 | Tick: 1184 | CBFF: 1082.02 | CBFBBT: 1081.62
> Frame: 0052033223 | Bar: 0271 | Beat: 4 | Tick: 1104 | CBFF: 1084.03 | CBFBBT: 1083.58
> 
> This was tested only with Jack2 (1.9.8 and 1.9.9.5).
> 
> Any ideas ?
> 
> Best Regards
> Pawel / Xj
> 


JACK does not do anything regarding those values: the timebase master *client* is responsible to update extended position information, counting beats, timecode, etc. (see : http://jackaudio.org/files/docs/html/transport-design.html)

I don't see any specific issues here on OSX testing either with jack_transport or Ardour as timebase master (well I see CBFBBT value staying the same while CBFF moving with jack_transport  at very low tempo like 1 or 2)

Can you try with Ardour also?

Stéphane
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