[sdiy] MIDI Clock sync advice

[Spiros Makris]

On Fri, 15 Mar 2024 at 17:47, Roman Sowa via Synth-diy <
synth-diy at synth-diy.org> wrote:

>
> If I may just add - maybe it's just my impression, but I think that new
> generations of programmers educated today have absolutely no idea how
> microprocessor works.
>
>
That feels true, but we should keep in mind that "programmer" has been a
continuously expanding group of people, and you can now be a programmer
starting from a wild variety of disciplines, most of which don't start from
the bottom, as is customary in electrical engineering and (sometimes)
computer science degrees. Most programs I know of have computers and
operating systems as a standard class, but microcontrollers and embedded
systems as an elective. That is, you can't become an embedded programmer
"by accident", unlike python, java and others, which any STEM major will
have some knowledge of.

I gave it a quick try on a teensy 4. I used USB-MIDI and fed a clock from
Ableton, without using interrupts. I made all the wrong choices, I know.
So my strategy was to create something that resembles a phase frequency
detector: I created an internal clock that will run at double the speed of
the incoming clock, and implemented a /2 divider. I am listening for clocks
from either USB or the internal clock, and switch between decreasing and
increasing a count variable based on the lead-lag relationship of the
incoming messages. The result resembles the functionality of a PFD coupled
with a charge pump/integrator filter, and it does lock to the incoming
tempo. It will follow abrupt tempo changes reasonably fast, but then sync
between beats is lost. Furthermore, I can see the PFD values fluctuating,
which does happen in such a configuration, but I would expect less of a
ripple under a good lock. I suspect Ableton clocks are fairly jittery
through USB, so I'll give it another go using a more stable clock and
figure out if the ripple is system instability or Ableton acting up.
Another approach I intend to try is simply measuring the incoming frequency
using a reference timer and then assigning the value (multiplied by a
factor) to a second timer to act as a clock. I know it will more or less
work, but doesn't really have any syncing mechanism; I expect the resulting
beats will be out of sync.
I am contemplating adding a syncing mechanism that keeps track of the
number of incoming beats and introducing a secondary feedback mechanism
following the quarter notes. It feels like a wrong approach though.


On Fri, 15 Mar 2024 at 17:47, Roman Sowa via Synth-diy <
synth-diy at synth-diy.org> wrote:

>
>
> W dniu 2024-03-15 o 13:57, René Schmitz pisze:
> >
> >
> > The speed of processors roughly keeps up with programmers ability to
> > waste it.
>
> Yes, that's my feeling exactly
>
> >
> > My gripe is that, for beginner programmer, it does not necessarily teach
> > good practices, and even for a seasoned one it proliferates some bad
> > habits.  (How often is everything in loop()...)
> >
> > It's like giveing you a nice sugar coated piece of cake, so you might
> > never be motivated to learn how to bake a better one.
>
> hell yeah!
>
> If I may just add - maybe it's just my impression, but I think that new
> generations of programmers educated today have absolutely no idea how
> microprocessor works.
>
>
> > Back to MIDI: generally, when using the serial interrupt you have two
> > choices:
> >
> > Either make the ISR short, and for example just throw the data into a
> > buffer.
> >
> > Or do all the processing (which could take some time) there. Risk is
> > that you might block other ISRs, such as timers, which is of course bad.
>
> If I need a buffer, I usually process only MIDI clock inside ISR and put
> all other messages into the buffer. Less than 20 cycles worst case. But
> for example in my MIDI note decoder there's no buffer at all. The work
> is done before next byte comes in, so there's no chance of cluttering or
> missing a message.
>
> Roman
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