[sdiy] pitch tracking / guitar synths

[harrybissell]

David Moylan wrote:   (inline)

> Thanks for clearing that up, Harry.  The patent does show diode
> clipping; I think the website explanation confused me.  So, you're
> saying the 'VCO' resets when it's internal ramp reaches the derived peak
> level of the ramp/hold, plus or minus some offset voltage for relative
> tuning?  And then to even out the height the clamp is used (as shown on
> the previously mentioned web site)?  So the actual VCO output is not
> pure ramp, but the clipped ramp shown?

Yes... it is the clipped ramp. I was amazed when I saw what my spice
simulations
of this showed.  At one of the AHMW gatherings... I put a GR-300 system to
the
'scope !  Its clipped.   Funny thing is... you really don't notice a timbre
change
on a single string.

I'm making a full sawtooth out of it. This way I can do PWM waves as well.
Its
MUCH harder to do that than the clipped circuit. Two points to Roland for a
good
simple method.  By varying the clip level, they get a VCA as well !!!

> As for cleaning up the guitar signal the GR-300 uses a tracking filter
> (you would know something about that, wouldn't you?),

uhhh... yeah I do :^P

> but I've had good
> luck in my PLL experiments with static LPF, plus as much compression as
> I can muster.  The compression helps increase sustain (or length of time
> that you have a trackable volume) but also compensates for higher notes
> having less amplitude due to the LPF.

Heavy compression is a mainstay of the EH Guitar Synthesizer, and the Gentle
Electric GE-101 P/V converter.  The Roland uses a switched (frequency) filter

that does not have a stable amplitude, but the changes do not affect the zero

cross anyway !    Give them another point for that.  The EH Guitar Synth has
a
tracking filter that works really well... and a kind of shitty compressor.
The Gentle
Electric has no filter (so, wider range) but a really well designed
compressor. About
half the circuit is compressor !

> I think it could work pretty well
> if I was using a hex pickup and only had to worry about 2 octaves for
> each string.  I think you're right that the gating is key.  The gate
> needs to be set high enough to hold pitch before the guitar signal drops
> out.

LOL. That still will not help you. If you mute the strings, the signal can
drop
really fast. The gate is not likely to warn you in time. You'll get a couple
of
cycles with an exponential decay... GOD help you if you lift your finger off
the
fret during that time, the held note will be flat if you do.  There is a Moog

patent that covers a way to improve the hold.

> One thing I could never figure out is why the hex pickups are always
> located so close to the bridge.  I would think that would accentuate the
> harmonics.  Anyone want to give me a lesson?

There is a really neat applet over at Don Tillman's site that shows the
effect of pickup
position.  One thing is... the pickup covers a narrow range of the string
length.  Picking
a note somewhere along the sting will create a series of nodes (zero
vibration) at
points related to where you picked.  If you get really near the bridge...
there is almost
no chance of there being a 'node' there... therefore a dead spot.  Yes... it
does accentuate the harmonics, but at least you GET a signal, for sure...

A piezo would probably work well also... that point HAS to be a node, but its
sensing
pressure at that point.  I have never tried a hex piezo pickup.

H^) harry



>
>
> Dave
>
> harrybissell wrote:
>
> >In the GR-300... the hex distortion is completely separate from the
> >tracking
> >section.  All the hex distortion is, is diode clipping of the hex
> >preamp.
> >
> >The ramp section 'is' a ramp/hold converter.  There are two possible
> >outputs...
> >one is a sawtooth shaped pulse that is directly generated from the
> >string pitch.
> >The other is a quasi-VCO... the reset level for the sawtooth it
> >generates is the
> >sample/hold peak voltage of the original ramp. This in effect, allows
> >you to tranpose
> >over a small range, without any dividers or expo converters.  Ergo (what
> >DOES that mean?) ... no temperature tracking problems and very little in
> >the way of calibration.
> >
> >otoh... if you do not do the correct filter before the ramp converter,
> >all is lost !!!
> >Its not the P/V conversion that is the problem... its
> >
> >(thanks, Scott)  Multiple zero crosses in a single cycle of guitar
> >
> >~and~  How to detect that the note is becomming untrackable, before it
> >really does !
> >
> >Roland cuts off the sustain when the note becomes untrackable.  In
> >all... probably
> >a real good choice.  Getting the fundamental out of the guitar is also
> >very, very hard.
> >
> >If you get a good fundamental, PLL tracking is a possibility...
> >
> >H^) harry
> >
> >David Moylan wrote:
> >
> >
> >
> >>As for the ramp/hold type roland used this in the GR-300 guitar synth
> >>(which some claim was the first playable guitar synth).  There's a bit
> >>of explanation about the ramp and it's reset at this link
> >>
> >>http://www.joness.com/gr300/patent.htm
> >>
> >>in the "square wave to sawtooth" section.  This is describing the hex
> >>distortion circuit, but I'm nearly positive the same ramp is used for
> >>the pitch CV.  It's split into two paths - one for the distortion
> >>circuit (clipper) and one to a sample and hold.  There's also a link
> >>from that page to the roland patent which gives more detail into the
> >>structure the GR-300.  The microsecond pulses shown at (c) in the
> >>diagram are actually triggered by another one shot.  So you have a
> >>chain.  The first one shot gates the sample/hold to catch the voltage
> >>at the top of the ramp, then the second triggers to reset the wave.
> >>The patent shows this pretty clearly.
> >>
> >>Dave
> >>
> >>amokan wrote:
> >>
> >>
> >>
> >>>Thanks. I'll dig up the schem for that and check it out.
> >>> On 12/18/05, harrybissell <harrybissell at prodigynet > 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.
> >>>
> >>>
> >>>
> >>>