[sdiy] TOGs and Vibrato/Pitch bend (was Vibrato range)

[Tim Ressel]

Hi,

Many thanks to everyone who joined the discussion and sharing your 
knowledge about vibrato.

It has been a frustrating journey trying to replace the venerable 
MK50240. My original take was to use a CPLD programmed with the same 
divider strings as the 50240. It worked well enough but the solution is 
big due to the size of the CPLD chip and the need for level shifters 
(the CPLD is a 3.3V device). Also the CPLD is a bit pricey at $7.25 and 
also a surface mount device (I have a kit in mind). On the positive 
side, the CPLD can be clocked at way higher rates to allow for 
downstream octave dividing (think waveform generators). Also doing 
vibrato and pitch bend is as possible as it is with the MK50240.

I have another solution that uses little processors to create the divide 
ratios. I can only do one note per chip so it takes a sea of processors 
to pull it off (if anyone knows of a small cheap proc in a dip package 
that has more than one 16 bit timer please let me know). The max clock 
rate is limited to 8x the 50240 which is enough for waveform generation. 
And it can do vibrato and pitch bend. Also this solution can do Just 
Intonation, in case anyone cares.

The third idea is to use the SI5351 clock generator IC. It has an 8 
output version so only 2 chips are needed at $2 each. It turns out they 
cannot go quite low enough to generate the top octave directly. 2x is 
the lowest they can go. Also I still do not know how fast they can be 
updated. They use I2C at 400 KHz, need several values per output 
updated, and all 8 outputs need to be written during a pitch change, so 
it seems unlikely. Still, it may be a good choice for a static TOG. It 
would not be a kit, but it would be pretty cheap. A board would need a 
small proc, the 5351s, level shifters (another 3.3v part), and possibly 
dividers to bring the output down to the top octave. So about $10 in parts.

Now that the windy prelude is complete, it is time for the reason I am 
gassing on: How to pitch bend and stay in tune.

It is easy enough to rig a 4046 as a VCO and use it to do pitch bends 
and vibrato. The hard part is this: how to keep it in tune? Is it enough 
to simply use low temco timing components and provide a fine tune knob? 
I have dreamed up a complicated method of using a PLL and a processor to 
keep the clock in tune, but is it necessary? I kinda hope not, because 
it is techically cool but hugely complicated (I tend to over think things).


-- 
--Tim Ressel
Circuit Abbey
timr at circuitabbey.com