[sdiy] sync conditioning/simple triangle oscillator without linear voltage control

Richie Burnett rburnett at richieburnett.co.uk
Sun Feb 24 21:55:25 CET 2019

Beautifully made instrument.  I love the photographs on your website, even 
if the first picture does have an element of "hiding from firing squad" to 
it!  :-O


-----Original Message----- 
From: ulfur hansson
Sent: Friday, February 22, 2019 4:56 PM
To: Richie Burnett
Cc: synth-diy mailing list
Subject: Re: [sdiy] sync conditioning/simple triangle oscillator without 
linear voltage control

hello richie,

I already have constructed an instrument that works quite well, using high 
powered class D amplifiers and custom wound electromagnets to induce 
feedback in an acoustic set of strings. here is a brief description of my 
harp, along with demo recordings; 
https://ulfurhansson.com/SEGULHARPA-ELECTROMAGNETIC-HARP (kind of an 
acoustic synthesizer/organ?)

the internal circuitry has patchpoints for experimenting with additional 
processing/signal conditioning. right now I am using waveshapers to change 
timbre/harmonics as you press harder/lighter on the capacitive touch sensor 
keyboard on the front panel.

it works great, but i'd still like to explore other avenues for varying 
timbre control. I feel my idea of a synced oscillator tethered to each 
string is exciting as the sync artifacts will provide a much richer harmonic 
response, that is if i can figure out how to implement it with limited space 
and solve the challenge of signal conditioning before SYNC input.

it doesn't have to be perfect, and sync errors as the signal grows stronger 
could actually be an interesting addition to the instrument timbre. I would 
probably add a VCA to the oscillator output to control the level of output 
mixed into the feedback signal depending on how hard you press against the 
keyboard (aftertouch).

thank you for your thoughts on this!
all the best,

fös., 22. feb. 2019 kl. 11:43 skrifaði Richie Burnett 
<rburnett at richieburnett.co.uk>:
Synchronising an oscillator to a vibrating guitar string waveform might be 
quite challenging. Different frequencies travel along tensioned strings at 
different speeds leading to a dynamic waveform with likely many 
zero-crossings per cycle. This is particularly so just after plucking where 
string tension is highest and high-frequency partials are loudest.

Attempts to extract pitch periods from harmonically complex waveforms based 
on zero-crossings are usually of limited success unless you can either 
heavily filter the input waveform to remove harmonics, or use something like 
a PLL that will tend to inherently reject harmonic energy in its reference 
signal if you slug the loop filter enough.

Are you trying to make something like the e-bow?


Sent from my Xperia SP on O2

---- ulfur hansson wrote ----

hello list,

I need to find a triangle oscillator design that meets two requirements;

1 - it needs to have a sync input that can sync to a signal of varying 
amplitude ( it is actually a single guitar string)

2 - it has to be tiny! I plan to design it using 0805 caps/resistors and SOT 
transistors etc...

they idea is to use this oscillator to reinforce a feedbacking signal 
induced in the string using electromagnetic pickups and actuators. I have 
already constructed an instrument that works great, but I'd like to use this 
addtional circuitry to accentuate different harmonics of the resulting tone 
by mixing the synced oscillator in with the feedbacking string signal.

since the sync input signal is not a steady pure waveform,i will probably 
need to do some signal conditioning - any ideas regarding comparator designs 
for converting varying amplitude signal to steady square wave would be 
greatly appreciated! perhaps a simple diode compressor would be helpful 

I hope this topic doesn't come across as too nebulous, but really any ideas 
or helpful thoughts comments would be greatly appreciated!!

all the best,

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