Improve MIDI SPP accuracy

Submitted by bluebell on
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qtractor

Unfortunately the MIDI SPP is only accurate on 1/16 positions. Qtractor's position have a much higher resolution. At the moment Qtractor rounds the SPP which leads to non-synchronized positions.

Based on my experiences with jack_midi_clock I propose the same or a similar approach:

When playback starts, then send immediately

  • MIDI CLOCK
  • MIDI SPP of a position in the future, e.g. the next but one beats or 1/8 or 1/16 position

When this position is reached then send

  • CONTINUE

Since SPP is no realtime message it's a robust solution to use the next but one beats (1/4) as the position in the future, e,g, when at 9.1.080 then choose 9.3.000.

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Submitted by rncbc on Tue, 28 Jan 2025 - 16:08

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When playback starts, then send immediately

MIDI CLOCK

this is already being done,... or isn't?

MIDI SPP of a position in the future, e.g. the next but one beats or 1/8 or 1/16 position

When this position is reached then send

CONTINUE

ok, might work as you say, just not sure about the exact future position: what exactly should it be? +1 16th note (aka beat/4), right?

we'll test this over during the next sprint :)

until then, cheers

Yes, MIDI CLOCK is already sent immediately.

The minimum or „nearest“ future is the next 1/16, that's right.
But SPP is no realtime message, so it can't hurt to position to the next but one 1/16, the next but one 1/8 or even the next but one 1/4. I'd try the next but one 1/8.

This looks good and works with start and stop.

Can you do the same after rewind/ff when playback is active? That would be nearly perfect.

If you could do it even when wildly positioning the playhead when playback is active it would be perfect and make jack_midi_clock obsolete.

Yes, now there are only one two cases missing:
1.) When playback is active then it doesn't apply to "backward".
2.) When playback is active then it doesn't apply when the playhead is repositioned with middle mouseclick

Maybe this could be generalized/centralized in the Qtractor code:

Whenever the playhead is repositioned (mouse click, backward, end of rewind/ff):

  • Calculate the "position in the future" and store it
  • Send STOP
  • Send SPP with the "position in the future"

Whenever the playhead reaches this position:

  • Send CONTINUE

Can it be that simple? Maybe.

what then?

(Transport/) "Backward" is an imperative command, just like moving the play-head around, in any case... same as "Forward"... and quite a lot else... :/

you're telling me that moving the play-head around, while playback is rolling, anytime, is subject to this SPP hack? hold up your horses! sorry, that is a deep move and one that probably will let the hell loose...

might it better off leaving to other nuptials (so to speak ;)) ?

thanks (for the time being)...

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