[sdiy] String filters and divide down oscillator string synths

[Andrew Simper]

Thanks for the reply Tom.

I am making a vst as part of a synth pack for fxpansion. There are three 
synths in the pack, Strobe the mono synth, Cypher the triple osc audio 
rate mod synth, and Amber the string synth.

The first two synths are done. Strobe is a simple synth with 1 osc, 1 
filter, 1 lfo, 1 mod env, 1 amp env based on sh101, sci pro one, moog 
prodigy etc. Cypher is an audio rate mod synth with 3 osc, 2 shaper, 2 
filter, 2 lfo, 2 env, 1 amp env inspired by arp 2600, moog voyager, 
oberheim x-pander etc.

Cypher has a fairly unique osc section that features variable depth 
sync, sample on hold between the osc, variable waveform similar to the 
voyager from tri through saw through pulse, audio rate wave mod, audio 
rate through zero linear frequency fm and simple ring mod. Each osc also 
features beat detune and control rate phase modulation with the ability 
to set the start phase - which is really useful for osc beating effects 
and getting a repeatable pattern with key on.

Some early audio examples are here:
http://www.vellocet.com/dsp/strobe/
http://www.vellocet.com/dsp/cypher/

Sorry for all the lack of variation in the examples, I do them 
sporadically. If there is anything in particular you would like an 
example of I'll do one specifically.

I agree completely with your comments about the limitations of circuits 
becoming the holy grail. To band-limit the diode clip of a high pass 
filtered sqr is extremely difficult in dsp, but to make a divide down 
saw, which is what the omni was setting out to do, is very efficient - 
so that's what I'm doing :-)

All the best,

Andrew

Tom Wiltshire wrote:
> Andrew,
>
> On 21 Jun 2008, at 13:36, Andrew Simper wrote:
>
>> I'm working on a digital emulation of a classic divide down string 
>> synthesizers at the moment,
>
>
> Sounds interesting. What are you using for that? Is this a VST or do 
> you have particular hardware in mind?
>
>> * 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'd probably try bandpass, like you suggest, implemented as 
> state-variable filters.
> I looked into formant shapes (a set of formant filters is definitely 
> on my "one day..." list) but found precious little good information. 
> It seems measuring the frequency responses of musical instruments 
> isn't straightforward, and that few people have bothered.
> Still, in the end I decided that all acoustic instruments have *some* 
> resonances and dead spots and so forth, so having a section in your 
> instrument that copies that effect is going to improve things, even if 
> I don't know enough to realistically copy an existing instrument.
> Looked at the other way around, one of the things that makes synths 
> sound "Synthy" is the dead flat frequency response from 20-20KHz.
>
>> 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.
>
> This is another great example of the limitations of some very basic 
> circuitry becoming the holy grail that more sophisticated circuitry 
> strives to emulate (See also: Hammond organ, Minimoog filter, 
> envelopes, Fender tube amplifiers). I always try and remember this 
> when I'm designing something, and worrying too much about making the 
> inevitable compromises. Instead, I should just get on and compromise! 
> If it's successful, in twenty years time, someone will be using a 25 
> Ziggahertz DSP to copy the exact quality of audio grunge my compromise 
> caused! It's a funny old world!
>
> Sorry, not a lot of useful information in that post - just some thoughts.
>
> Regards,
> Tom
>
>