[sdiy] pitch tracking

[David Moylan]

Another note:  I built a circuit posted at generalguitargadgets.com

http://www.generalguitargadgets.com/index.php?option=content&task=view&id=175&Itemid=200

which is based on 4046 PLLs.  I was surprised how well and how fast it 
could track.  Yes, there's a little glitch at the start of a note, but 
you get a long of bang for the buck with the 4046.  The circuit 
basically LPFs the input then runs it into a opamp with positive 
feedback to get a nice square which feeds the PLL (same requirement of 
ramp/hold pitch trackers).  It also uses the square to gate a sample and 
hold (not filtered, gates at audio frequency) in the CV loop of the PLL 
- the idea is as long as there's signal the PLL will track, then hold 
when the signal stops.  In practice, this doesn't work to well on messy 
guitar signals; noise or glitchy decays set off the S/H and you lose 
your note.  Would probably work better with a separate gate circuit to 
hold when the input drops below a certain level.  The circuit works much 
better (int terms of note length) with a compressor/sustainer in front 
set to max sustain.

In general, the circuit sounds pretty dull, very akin to an octave pedal 
or bad organ, but could be made more interesting in a number of ways.   
VCF would help a lot.  Or have PLL number 2 not track, but use the CV 
from 1 plus a manual tuning voltage (would probably need to trim the 
scale. Often).  That would also free up the XOR in PLL2 which could be 
used to do psuedo ring mod.  Grant Richter's woggle bug schematic shows 
how to grab a sawtooth off the timing cap of a 4046 which would let you 
do PWM and all sorts of fun things.

Dave


amokan wrote:

> Thanks. I'll dig up the schem for that and check it out.
>
> On 12/18/05, *harrybissell* <harrybissell at prodigynet 
> <mailto:harrybissell at prodigy.net>> wrote:
>
>     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.
>
>

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