[sdiy] pitch tracking / guitar synths

[Scott Gravenhorst]

David Moylan <dave at westphila.net> wrote:
>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?
>
>As for cleaning up the guitar signal the GR-300 uses a tracking filter 
>(you would know something about that, wouldn't you?), 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.  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.
>
>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?

Just for information sake, I'd like to know too.  I have GR-50 guitar
synth which uses a GK-2 pickup system.  Roland tells you to do the same
thing with that pickup and like you, I thought that would be the worse
thing you want.  Amazingly, the thing works quite well IMHO.


>
>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.
>>>>
>>>>
>>>>      
>>>>
>

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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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