[sdiy] pitch tracking

[harrybissell]

Pitch tracking might be one of the most difficult processes to make
work.
There are two types... Tachometer (like the MS-20) that always take a
large
number of pulses to get the correct result... and Ramp/Hold types that
usually
work in one, or two cycles.

The Tachometer circuit is probably the easiest. The MS-20 is a VERY good

example of how to do this. I would clone the circuit if you are
interested. It should
be easy, excpet for the four-gang potentiometer which will be hard to
find.

Three of the potentiometer stages form a low pass filter on the input,
the fourth is
a low pass filter on the output (lag).  You set the filter to the
expected input frequency
range. Higher frequencies can use less delay ... low frequencies need
more, much more.

The idea is a pulse is produced for every input cycle...and these charge
a capacitor.
The more pulses, the higher the voltage.  There will be ripple in the
output, something
that makes driving a VCO and tracking pitch almost impossible.

The ramp/hold types are much quicker, but MUCH harder to design and
build. I made
a board of this type some years ago, which was distributed by EFM. It
was based on a
design by Bob Moog (although he was not the first or last to use the
technique). This
board was intended for use with the Etherwave theremin, and could give a
V/oct output
that can be used to slave a VCO.  OTOH, the theremin has a continuous
wave output.

Your 'drum' is probably untrackable. The non-harmonic tones in a drum
would almost
guarantee that is IS not pitch to track. You might perceive a 'pitch' to
the drum but it proabably exists only in your mind :^P   A circuit is
unlikely to read that (oh how I wish !!!).  You will probably get
garbled gook out of the converter... or maybe you will read
a voltage proportional to the number of drum events (a drum roll would
be a higher voltage than a single hit).

Tom Gamble (retired of EFM) proposed a clone of the MS-20 using active
filter (OTA)
stages to replace the four-gang pot.  I don't know if he ever built it.
The MS-20 design is really very clever, and worth your effort in trying.

H^) harry

amokan wrote:

> Is there a modern version of the pitch tracking circuit in the MS20? I
> know the MS20 wasn't all that accurate, but I actually like the
> inaccuracies the most for things like drums and whatnot.
>
> Just wondered if there is a module out there that I'm overlooking.
>
> Thanks.