PLLs to replace TOS generators

[Haible_Juergen#Tel2743]

> The tonewheels sit on little axles, spring-coupled to black metal
> gears, driven by white nylon gears, which sit on other little axles
> that are linked together in line and driven by the motor.
>
> The Hammond "oscillators" are driven by gear ratios, which are
> required to be the ratio of two integral number of gear teeth.  So
> that M[1]/N[1] for C, M[2]/N[2] for C#, etc.
>
> So, perhaps a more accurate emulation of the Hammond tone generator
> would have individual Phase Locked Loop M[i]/N[i] Frequency multiplier
> circuits off (the equivalent of) the AC line.
>
> Add a hashing function off the N[i] counter to tweak the PLL VCO on a
> cycle-by-cycle basis to generate the wobble from individual teeth.
> And add something else to simulate the spring coupling.

 I also think PLLs would be the best way to go. There was an article in
an old "Funkschau" with a DIY project that replaced a TOS generator by
a PLL sytem with M/N dividers, just the way Don described it.
(I can dig it up, if someone is interested. It's in German, but it has 
complete
pcb layouts. There was a master PCB and many PLL cards, each card
containing a 4046, counters and some diode logic for programmable
division factors.)

Someone said PLLs would not work, because the phase is locked,
but this is not true !! PLL means that in a static (!) state phase would be 
locked,
and that in the dynamic case phase is the thing that is regulated. To make
things better, in a common 2nd order PLL (like 4046 + passive PI-filter) 
phase
is regulated with a 2pole-transfer function - and this is exactly the same 
thing
that would happen in a spring/mass system like tonewheels that are
spring-coupled to the gear !!

Disturbing individual stages would be one thing that is possible in a PLL 
system,
(you could disturb it by adding noise in the passive PI section, for 
example)
but there's another thing that is very similar to the original: Disturbing 
the
master (motor) frequency will excite the coupling transfer functions just 
the
way it would be in the original.

I have no idea how you would measure the transfer function / resonance
frequency of the individual Hammond tonewheel / spring stages. But I wanted
to back up that the PLL stuff would be a good way to model the system,
given you have all the parameters.

JH.