[sdiy] Hammond Organ Query

[Don Tillman]

   > Date: Fri, 25 Oct 2002 10:25:24 -0700 (PDT)
   > From: Tim Ressel <madhun2001 at yahoo.com>
   > 
   > On the Hammond organ the tone generators were gear-tooth wheels
   > ... so like, does it use just-intonation? Seems like it would
   > have to so the ratios come out even. Equal temperment (an ironic
   > term to use with this crowd :-) generates all these nasty
   > irrational numbers that would be difficult to make with gear
   > ratios.

The number of teeth involved in the gear ratios gives you a really
close approximation to equal temperment.  Other scales are of course
possible, but that would involve fabricating a lot of gears.

For details, get a copy of the Hammond Service Manual, or check out
Laurens Hammond's 1934 (!!!) patent US 1,956,350 "Electrical Musical
Instrument".  You can get pdf scans of this and all other patents at
the European Patent Office's esp at cenet site:
  http://ep.espacenet.com/

The Hammond patent includes a large table of all the gear ratios used.

   > From: Tim Parkhurst <tparkhurst at siliconbandwidth.com>
   > Date: Fri, 25 Oct 2002 11:00:15 -0700
   > 
   > Remember that the Hammond used a bunch of "gears" (really just
   > toothed wheels) all mounted on one common shaft. No gear ratios
   > are involved since the gears don't mesh together or drive
   > anything. They just made each wheel smaller or larger to give it
   > the right number of teeth.

Not true.  If you open up a Hammond organ you'll see that the gears
are not on a common shaft at all.

   > Date: Fri, 25 Oct 2002 11:54:55 -0700
   > From: patchell <patchell at silcom.com>
   > 
   > It would be just like making a top octave generator with digital
   > logic.  You would have a set of magic division ratios that you would use
   > to approximate equal temperment.

The cool thing about the Hammond is that it has superior accuracy to
the top octave generators because it has control of both the numerator
and denominator while the top octave generators just do a divide-by-N.

On the Hammond, the driving gear has between 102 and 124 teeth, the
driven gear has between 41 and 104 teeth, And the sound-making gear
has 2,4,8,16,32,64 or 128 teeth.

  -- Don

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