[sdiy] String filters and divide down oscillator string synths

[Andrew Simper]

Hey JH!

Great work on the string filter.

I'm working on a digital emulation of a classic divide down string 
synthesizers at the moment, and as far as possible I am sticking to what 
is realistic to do in the analog world. I already have a divide down 
sawtooth oscillator with adjustable one pole high pass to generate the 
main buzz. I am now looking at the further processing involved to make 
it more string like.

I have checked several schematics for their formant filters to emulate 
the body of a string instrument or piano and so far have been pretty 
disappointed. I am going to construct my own filters based on whatever 
literature I can find and hand tuning the coefficients to get something 
that sounds as close to a sample of the instrument as I can get. I plan 
on approximating the lower resonances with four 2 pole filters, and 
currently I'm tossing up between a couple of designs:

* 4 parallel band pass filters with adjustable freq, res, and gain
* 1 high pass and low pass in series with freq, res and single gain in 
parallel with a band pass filter with adjustable freq, res, and gain, 
and then all that in series with a notch filter with freq and res.

Do you have any suggestions as to other possible designs? Do you know of 
any links to any pages that have formant shapes for instruments?

I have done some research in the Arp Omini (I have one sitting in front 
of me to test with) which others might be instereseted in (sorry if this 
is all obvious stuff that everyone already knows)

* To get a saw from a sqr divide down sqr wave you high pass filter the 
sqr then clip off the bottom with a diode then high pass again.
* The "amp" per note is achieved by varying a bias voltage to the diode 
so the clip point changes. This results in some "thumping" with note ons 
since the dc introduced has to be filtered away.
* The "hollow" or waveform enhancement mode is made by moving the bias 
voltage enough that the other spike of the high pass sqr waveform is 
present in the output signal.
* Since the amp per voice is made by varying the bias voltage for the 
clip you get timbral variations through the attack and release.
* The "string" sections simply put this waveform through a chorus.

All the best,

Andrew Simper