[sdiy] Verniers for Patch Recording

[Ian Fritz]

Hi folks --

One of the most frustrating aspects (at least for me) of modular analog is 
the great difficulty of reproducing patches after they have been torn 
down.  A hybrid system where control parameters can be saved on disk drive 
solves this problem, but at the cost of requiring a fixed architecture plus 
interface construction and programming.

While thinking about this today, it occured to me that it might not be too 
difficult to read knob positions quite accurately using a vernier 
technique.  I'm talking not about mechanical gear-reduction verniers, but 
rather simple scale verniers.  I've seen these used on old radios and lab 
instruments but never on a synth.  They are also used on inexpensive 
(non-dial) calipers, which are surprisingly precise.

Has anybody ever thought about doing this?

For those not familiar with scale verniers, they incorporate two scales -- 
the usual one on the panel and a second one on the skirt of the knob.  The 
two scales have different spacings, and the dial reading is interpolated 
between dial marks by noting which marks on the two scales are closest to 
alignment.  I have put up a simple illustration of how this works on my site:
http://home.earthlink.net/~ijfritz/xfer.htm

By playing around with some paper cutouts I have convinced myself that 
7-bit resolution could be achieved this way, even with small (3/4 inch 
diam) knobs.  This technique will not work for parameters (filter 
frequency, e.g.) where coarse and fine control pots are used.  It might be 
necessary to use a switch for the coarse control in this case.

Thoughts?

   Ian