[sdiy] String filters and divide down oscillator string synths

[Andrew Simper]

Hey David,

Thanks for you post. I did consider using a top octave saw then adding 
sqr sub octaves to give me my sub saw as a possible excuse for just 
doing saws in dsp, so I'm glad someone actually did that in analog :-) 
I'll check the PS series schematics for further inspiration.

In dsp every edge to band-limit requires more cpu, so since a saw has 
only one edge per cycle and a simple phase += inc otherwise then it's 
very efficient. For the divide down part of this you get the edges all 
aligning at the edge in the top octave saw, so this means you get many 
divide down octaves for very little extra cpu. Most of the extra cpu for 
more octaves comes from the extra envelopes and high pass filters needed.

Pulse width is possible by using two sawtooth oscillators, and add a 
phase flip button, so you can get a square with variable pulse width by 
adjusting the phase of the oscillator - something which is very easy in 
dsp. Without the phase flip you get a double saw that you can detune 
instead to get a korg lambda type double saw divide down section.

Andy

David Moylan wrote:
> 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
>> --------------------------------------------
>> check out various dan music at:
>>
>> http://www.myspace.com/lossnyc
>>
>>
>> http://www.soundclick.com/lossnyc.htm
>>
>>
>> http://www.indie911.com/dan-snazelle
>> (or for techno) http://www.myspace.com/snazelle
>>
>>
>>> 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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>>> Synth-diy at dropmix.xs4all.nl
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>>
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