[sdiy] MIDI sync experiments

[Tim Heffield]

I have been experimenting with the Blacet MIDI-SYNC chip in preparation for
building a MIDI to analog CV sequencer.  I have observed the following
things if you are considering implementing a similar system.  I built up the
circuit noted in the Blacet docs with the CD4018 and CD4520 dividers and
connected it to my Boss DR-5 Rhythm Section.  I tapped into the LED beat
driver circuit on the DR-5 for a reference clock and compared the edges of
this clock with various points on the MIDI-SYNC circuit.  Here are the
results:

1.) The 24 ppqn coming out of the PIC MIDI-SYNC chip (pin 6) lags the DR-5
clock by ~440 uS.

2.) With a 120 bpm rhythm sequence playing on the DR-5 I noticed that the
edges of the pulses coming out of the 4520 divider do not match the DR-5
clock edges.  In fact, if you repeatedly press the DR-5 start/stop button,
the delta between the positive going DR-5 clock and the CD4520 negative
going (it's inverted) pulse can vary from 0 to over 200 mS.  It seems to
step in 42mS increments, the overall delta changes with different bpm
settings.

I expected that if I am playing a 4/4 rhythm sequence and use the 4 bpm
output on the 4520 divider (pin 4) that I would get a synced pulse for every
beat of the DR-5 LED.  Am I missing something here?  Is this because
pressing the start/stop button coincides with different 24 ppqn clocks and
the whole process starts in different places each time?  BTW I have tried
this same experiment using Cakewalk to drive the MIDI-SYNC and using the
audio metronome from the computer as a reference beat clock with similar
results.

Any suggestions?

Tim