[sdiy] sync'ing BBD delays

[jhaible at t-online.de]

Just a thought:
Did anybody try to sync an analog (BBD) echo to a midi
or other BPM clock ?

I find myself using my TC-Electronic D-Two more and more because
I can sync it to a drum machine. But it lacks the warmth of
my various BBD-based echoes.

At first I thought sync for BBD is impossible, but it should work with
a PLL and large divider ratio.

The clock is in the 100kHz range, the sync is in the mHz (milli-Hz)
range, but so what, just need a long frequency divider chain to
get it running. (The response time will of course be in the sync
frequency range, and not in the kHz range, but that would have been
too good to be true.)

I think the standard passive PI loop filter of a CD4046 would
have to be slightly modified:

(1) include a CV adder to set a (manual) starting value.
(2) use the lower leg of the filter (where the cap normally
    is grounded) to bring in the LFO that slightly modulates
    th edelay time (common practise in BBD echoes.)

Maybe (1) could alternatively be done with a very small min / max
frequency rate (two resistors on 4046). The idea is to set the
delay time approximately by hand, and then let the PLL only do
slight corrections (to avoid long adjustment times).
The Q of the PLL equation must be set quite low, to prevent
overshoot in the clock tracking.

Is this just insane, or is it possible, what do you think ?

Anybody eager to try it ? (I know for sure that I cannot try it
myself in the next months.)

JH.