[sdiy] sync'ing BBD delays

[Don Tillman]

   From: jhaible at t-online.de
   Date: 11 May 2001 14:52 GMT

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

   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.)

Hi JH!

This appears be in the class of phase locked loop applications where
you really don't care about the phase.  The PLL works hard to keep the
phase accurate, not the frequency, but for a BBD clock we only care
about the frequency.  The worst case situation would be if the VCO
frequency was correct but the divided-down phase happened to be wrong,
the PLL would radically tweak the frequency until the phase came into
alignment.

So I think that resetting the divide-by-N counter on each sync
clock would improve things greatly.

   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).

These are physically the same thing, right?  They both directly set
the frequency of the VCO without waiting for the loop.

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com