Since it is hard to find, and since I did not wish to send people on a wild goose chase, I took the liberty of pasting Harry's
initial comments from the document for the old PV-1 he alludes to, since they give a little more detail about functioning of
P2V units in general, and I wanted to save him the time of having to re-explain what he's already explained nicely.
I have omitted the content related to the actual design of the PV-1, given the shortcomings he notes.
A pitch to voltage converter takes a frequency input and converts it to a voltage output. There are several forms of this
circuit, both digital and analog. Most of these can be grouped into two basic classes. The Tachometer circuit, and the Ramp Sample/Hold.
The Tachometer circuit is the simplest. The frequency input is converted to pulses of a fixed width, and these pulses are
integrated, or averaged over time. There is a trade off between how fast you can get to a new level, and how much feed
throughof the input frequency you can accept. The less ripple you wantin the output, the slower the circuit responds to a
change ininput frequency. Tachometer circuits are best for frequencies that :
1) You want to average over a long time anyway
2) Frequencies that are continuous (do not start and stop...)
Note that things like motors etc... are the most common use for Tachometer circuits.
The best example of the Tachometer circuit as applied to Electronic Music is the Korg MS-20 external signal processor. It
features a filter at the input and output that are linked... so the user can set the tradeoff between ripple and acquisition time to
best fit the signal they are processing. The circuit also has an extended range Tachometer circuit... at very low frequencies
the pulse width is fixed. As frequency increases, these pulses get closer together. At even higher frequencies the pulses
overlap, and occasional gaps appear in between multiple pulses. This allows the circuit to perform at frequencies above
the normal range of the Tachometer circuit. There is still a very real delay in how fast the circuit can track a rapidly moving input
frequency.
The Ramp - Sample/Hold is another class of circuits used to solve the problem of slow signal acquisition. The input
frequency is divided in half, and then a linear ramp is generated for one cycle, and then sampled and held during the
following cycle, and the ramp is reset. This happens over and over again... so the output is always one cycle behind the
input. No matter how large the step in input frequency (within reason) the output will follow within two cycles of the
fundamental.
The advantage of the Ramp circuit is speed of acquisition, and lack of ripple. The drawback is that it is more complex, and
sensitive to noise in the input frequency. This approach was used in the 360 Systems "Slavedriver" Guitar to CV interface
(mid 1970's) with some success. The noise problems prevented the circuit from achieving satisfactory performance at that
time. Speed of response is still an issue.
Consider the lowest note of a Guitar = 80Hz, which has a period of 12mS. The Ramp circuit needs at least twice that time to
process the Pitch Voltage, or 24mS. This is a physical limit for converting the pitch. If you have a lot of noise in the signal
(harmonics etc.) it will take even longer.
The Pitch to Voltage converter must have an accurate reference to derive the pitch from. Harmonics will cause false
operation, so inputs from real instruments like guitar, voice, etc. usually need signal processing before the pitch conversion.
