[sdiy] tap tempo circuit

[WeAreAs1 at aol.com]

In a message dated 12/17/04 10:39:04 AM, tuj at fuse.net writes:

<< Anybody know how to build a tap-tempo circuit?  I know lots of gear these 
days has this type of feature.  Not sure if it just times the interval between 
the first two presses, or if its more complicated.  Most software versions 
I've seen do a least-squares method across the intervals to determine the tempo. 
  >>

"Touch Tempo Circuit"  US Patent #4046048  
Awarded to the Hammond Corporation on Sept. 7, 1977

This circuit was used with the built-in drum machine in the Hammond Concorde 
organ.  It even had a switchless brass "touch plate" for setting the tempo.  
I've never actually been able to play this particular organ, so I don't know 
how well the circuit works.  The same method they implemented with discrete 
logic chips could probably be duplicated with code in a PIC or AVR, with probably 
better accuracy and better tempo resolution.

Search for the original patent, which has a fairly detailed description and 
some decent block diagrams, if not complete schematics, at:   
http://www.uspto.gov/patft/index.html

And yes, the circuit works by a counter that times the interval between two 
presses.  After the timing interval is over, that counter outputs a binary word 
(six bits long, I think -- or maybe eight bits?).  That binary word is 
immediately used to preset another counter which outputs a medium-fast clock, which 
is then divided down to an appropriate clock for the drum machine (probably 6 
or 12 clocks per quarter note).  The entire system is clocked by a very fast 
(umm, relatively speaking) master clock -- somewhere in the hundreds of Hertz 
range.  It's pretty "Rube Goldberg", if you know what I mean, but that's kind 
of what I like about it.  It was a great idea for its time.

I'm just guessing, but I think an analog phase-locked loop based system would 
not be as accurate or reliable as the system that Hammond used.

Michael Bacich

P.S. -  The original Hammond tonewheel system was also quite a Rube Goldberg 
device, I think, and it's one of the greatest musical inventions of all time.

P.P.S. - Who, or what, is Rube Goldberg?  Gather around, kids, and be amazed: 
 http://www.rube-goldberg.com/