[Jack-Devel] JackTransport vs VST ppqPos

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

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