[sdiy] String filters and divide down oscillator string synths

[David Moylan]

Well, in the analog realm there's always the korg PS series' method (and 
many organ's) of a single oscillator per note in the top octave then 
divide down.  What's nice about the PS series is that its a sawtooth 
oscillator on top and if you add the saw and it's square wave sub octave 
you end up with a sawtooth suboctave.  I always thought that would be 
interesting because you could then have the saw and very easily a 
variable width pulse, though it would require one comparator per key. 
Or look at the PS schematics from Juergen's site to get an idea of how 
the PS did wave conversion with one transistor (albeit imperfect 
conversion).

If you went for four octaves (which is what an Omni has) you could 
probably do all keys for each note with the main oscillator components, 
2 dual D flip flops, and 1 quad opamp or comparator.  There's a huge 
range of sounds in variable width pulse waves in my opinion that would 
take you past the average string synth.  Plus, you're then open for PWM.

In the realm of string imitations the Paia Stringz n' Thingz used narrow 
pulses rather than sawtooths.  I think the article on the SnT available 
at Marvin Jones' site describes the idea behind using narrow pulses. 
Interesting reading.

Dave


Dan Snazelle wrote:
> 
> when you say divide down, i assume you mean an top octave generator. what are you using to acheive this?? as far as i can tell, these chips are now impossible to buy
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>> Date: Sat, 21 Jun 2008 13:36:56 +0100
>> From: andylist at vellocet.com
>> To: jhaible at debitel.net; synth-diy at dropmix.xs4all.nl
>> CC:
>> Subject: [sdiy] String filters and divide down oscillator string synths
>>
>> 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
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