How to convert period CV to pitch CV

[Paolo Predonzani]

> It's linear. The way the converter works is pretty cool. First the guitar
> string sound is filtered through a pretty elaborate adaptive filter whose
> response shape changes depending on where you play on the neck. This
> compensates for the fact that a guitar string's harmonic content is
> different as you go up the neck. This signal is converted to a square wave
> with a comparator. The square wave drives a monostable multivibrator to
> produce a short trigger pulse. This pulse drives another MM to produce
> another pulse slightly later. The second pulse is used to reset a
> constant-current integrator. The constant-current integrator produces a
> sawtooth whose slope is always the same, but whose height depends on the
> guitar string's frequency. Lower frequency-->more time for integrator to
> charge-->higher sawtooth wave.
> 
> The FIRST pulse is then used to trigger a sample and hold which samples the
> sawtooth wave right before it gets reset. The output of this sample and hold is the control voltage which is proprtional to the guitar string's frequency!!
>  Pretty cool! 
It seems that the output voltage is proportional to the string's period, right?

Now, what you need is to compute 1/T to obtain a frequency-proportional voltage.
The processing steps are:
log() -> negate() -> exp()

The explanation is: 1/T = exp(log(1/T)) = exp(-log(T))

An interesting circuit is (pseudo-SPICE notation):
vT 10 0 INPUT_VOLTAGE
r1 10 12
op-amp 12 0 14    (inverting, non-inverting, output)
q1 12 0 14 npn    (collector base emitter)

vtranspose 20 0    (see explanation)
q2 22 14 20 npn
op-amp 22 0 24    (inverting, non-inverting, output)
r2 22 24

The output voltage is at node 24.
The circuit has 1st order thermal compensation.
vtranspose is a NEGATIVE voltage between -0.2V and -1.0V and allows to
transpose (tune) the output voltage.
I didn't calculate the values of the resistors.

> 
> I know I've ranted about this on here before, but the GR300 has the most
> reliable tracking of any guitar synth I've ever played, and it's all done
> with analog! I think the secret is the pitch-tracking filter at the input.
> It consists of two band-pass filters whose cutoff frequencies move apart as
> you go up the neck. I don't really understand how that part works. In
> addition to that, there is extra circuitry which squelches the envelope
> follower's output when the second harmonic starts to become dominant in the
> guitar signal. This prevents the octave-jumping problem that so many guitar
> synths have. 
> 
Very interesting. Can you draw a schematic diagram of that part?


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| Paolo Predonzani  |  email: predo at dist.dist.unige.it |
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