Grungy Digital Tonewheels (was Re[3]: What means

[Bob.Schrum at harpercollins.com]

     I'm not sure anyone on the Hammond list would really know, since the 
     tonewheel generator, to most, is just that thing that makes the 
     organ's sound unique, but are not sure why.   I'm sure the gear ratios 
     are exactly equal to the frequency ratios between semitones in an 
     equal tempered scale.  The MK50240 dividers are definitely just 
     approximations. 
     
     Let's make this thread really DIY...
      
     If your goal is to replace the MK50240, you might be able to rig your 
     own dividers running off a much faster clock than the MK50240.  That 
     would give a finer resolution, allowing for truer ratios.  If you want 
     some instability, throw in a few strategically-placed gates, triggered 
     by short, random pulses to knock out a clock cycle every now and then. 
     That would simulate wheel slippage.  You might want to FM the master 
     clock a tiny amount, as despite the flywheel, the motor shaft rotation 
     had a little 60Hz flutter.  At what frequency this affected the acual 
     tones, I'm not sure, but it would be fun to play with.
     
     Another factor in the Hammond sound was the fact that the generator 
     was wired to the rest of the organ through a fat bundle of unshielded 
     wire.  This caused a little of all 91 tonewheel's signals to leak into 
     the audio, even though one key may have been pressed.  (The sound 
     going through a Leslie just before and while the sequencer starts at 
     the beginning of "On The Run" on Pink Floyd's "Dark Side of the Moon.) 
     I have not heard a CX3 in years, but many subsequent organs (even 
     Hammond's later stuff) do not have this to the degree the original B3 
     did.  Any organs with Top Octave generators and flipflop dividers 
     shoud be modifiable to emulate this.  All you would need is an array 
     of identical megohm-range resistors, one for each output of the 
     generator board summing all the tones together.  This would also 
     provide a tiny amount of crosstalk between adjacent keys--something 
     else the old beasts had.  Feed the summed output into a variable gain 
     stage to vary the degree of effect, and finally through a noise gate 
     triggered off the organ's dry signal.  Make the noise gate a little 
     leaky as well.  Mix the gate's output with the dry signal, then send 
     it off to the Leslie!  Glorious grunge!
     
               1-10Meg?
      C0 -----/\/\/\/------*
                           |     Variable
      C#0 ----/\/\/\/------*     Gain
                           |    |\           ______
      D0  ----/\/\/\/------*----|  \_________|Noyz|
                           |    |  /         |Gate|      
      D#0 ----/\/\/\/------*    |/           |    |----|\ Mixer
                           |                 ------    |  \
      E0  ----/\/\/\/------*                    |      |    \_____ To
                           .                    |      |    /    Leslie
                           .   Organ -----------*------|  /    & effects
                continued  .   Signal                  |/
              for all gen  .
                outputs    .


     (BTW The Voce V3 does this--in software, I think!)

______________________________ Reply Separator _________________________________
Subject: Re[2]: What means 'phaselocked'?  (was: Re[3]: Fourier-Analy
Author:  gstopp at fibermux.com at Internet-Server
Date:    5/30/96 4:59 PM


     I think the Hammond list is the only place to try to get the answer 
     about gear ratios.
     
     As for the randomized phasor, it's hard to say if it's worth the 
     trouble - there's really only one way to find out! Probably I'd make 
     it LFO controlled with the random thing as an option so if it does 
     turn out to be a wild goose chase then at least you have a cool 
     phasor.
     
     - Gene
     
     
______________________________ Reply Separator _________________________________
Subject: Re: What means 'phaselocked'?  (was: Re[3]: Fourier-Analysis
Author:  Robert May <wfrb at miworld1.miworld.net> at ccrelayout 
Date:    5/30/96 3:10 PM
     
>I play a Korg CX-3 organ, which uses the MK50240 top octave chip, and the 
equal tempered approximations do not seem quite the same or as accurate as 
those in the Hammond organ. I have a data sheet with divisors used by the 
50240, but would like to know the various gear ratios used in the Hammond 
tonewheel organs. I posted this question to the Hammond list, but got no 
response.
   On a related note to this thread, would be worth the trouble to take
several stages of all-pass (phase shifter) filtering and modulate each stage 
with filtered and uncorrelated noise to simulate the slight unsteadiness in 
the tonewheel organs?
     
Rob