DIY box for adding shuffle to a Sync Signal

[The Old Crow]

On Wed, 29 Dec 1999, Dan Gendreau wrote:

> To do shuffle, we need to take a 24 pulse/quarternote pulse train and
> slow down 6 pulses by a certain amount and speed up the next 6 pulses
> by the same amount. While the average tempo remains the same, every
> 16th note is either "faster" or "slower" than average. How much
> "faster" and "slower" is controled by the amount of "shuffle" you
> want.

  It is PIC chip time.  The trick here is to keep every 24 ticks (12,
actually, if every 6 tick periods are alternating longer/shorter times) of
unadjusted tempo equal in time to 24 (12) ticks of shuffled tempo.
Otherwise, the incoming pulses would get "backed up" and at some point a
tick would get dropped.  Probably the most efficient way of keeping the
"true" and "shuffle" tick rates edge-coherent on every 12th/24th tick is
to input the 24 tick per quarter-note reference clock to a PIC or similar
micro and have the PIC measure the total period of all 24 ticks.  Then,
use a simple averaging algorithm to tally up 24 ticks of shuffle time of 6
ticks of period less than the reference, and 6 ticks of period more than
the ref. by the same amount such that the total period is equal to the
reference period. The only caveat is that it the in-to-out delay time is
24 ticks (the time needed to determine the initial 24-tick period).
This reduces to 12 ticks since the 2nd set of 12 if a 24 tick frame is
redundant.

  Hope this makes sense,

   Crow

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