[sdiy] Basic core designs of commercial analogs

[Grant Richter]

Here is my thinking,

If you accept, that modulating the current to the transconductor is a form
of multiplication.

For polarity of multiplication: In the sawtooth ramp, modulation positive
and negative slope can only be multiplied by a positive slope from the core.

In a triangle core, modulation positive and negative slope can be multiplied
by a positive or negative slope in the core.

If we take a sine modulating a sawtooth core vs. a sine modulating a
triangle core, the effect of both will be to bend the slopes of the waveform
at the output.

But if you take the modulated sawtooth and waveshape it to get a triangle,
the negative modulation bend will point out the other side than if you
modulate a triangle core directly?

> From: Magnus Danielson <cfmd at swipnet.se>
> Date: Mon, 23 Dec 2002 21:04:42 +0100 (CET)
> To: grichter at asapnet.net
> Cc: jhaible at debitel.net, synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] Basic core designs of commercial analogs
> 
> From: Grant Richter <grichter at asapnet.net>
> Subject: Re: [sdiy] Basic core designs of commercial analogs
> Date: Mon, 23 Dec 2002 12:58:39 -0600
> 
>> Merry Christmas Jurgen,
> 
> Merry Christmas Grant,
> 
>> Some more crackpot speculation on my part.
> 
> You just can't get enought, can you?
> 
>> I was thinking about the effect of linear FM on the two different
>> cores.
>> 
>> In the sawtooth core, you have a continuously increasing ramp for the
>> entire cycle. For a triangle core, you reverse the slope during the
>> cycle.
>> 
>> Now if you consider linear FM of the two cores, with a frequency near
>> the VCO frequency, would it produce the same waveform?
>> 
>> To rephrase it. Would a sawtooth core with a triangle waveshaper,
>> produce the same waveform under linear FM as a triangle core alone?
> 
> Depends on how you actually apply the linear FM! If you do it correctly,
> no, you should not expect any difference other than that to expect from
> the different methods of generating saw and triangle waveforms as such.
> 
> Also, I hate to say linear FM since there is indeed two forms of linear
> FM in action, the unscaled linear FM and the scaled linear FM.
> 
> The unscaled linear FM modulates the same amount of Hz regardless of
> which tone is played.
> 
> The scaled linear FM modulates the different amount of Hz depending of
> which tone is played. If you play one octave up then the modulation in Hz
> would become twice as much. Many synths provide scaled linear FM
> modulation since it comes almost for free for the exponential curcuit
> being in use.
> 
> Jürgen is the promotor for the unscaled linear FM. I have not had the
> oppertunity to test it out (OK - blame me for being lazy not to modify
> my stuff up to provide it as an additional feature).
> 
>> The reason I ask is because of the relative polarities of the waveform
>> slopes. In the sawtooth case, the FM always modulates a positive slope
>> (in the core itself), in the triangle case, the waveform modulates both
>> a positive and negative slope (in the core itself).
> 
> Now, think of how a triangle oscillator works. You have yourself an
> additional transistor pair and a current mirror. The transistor pair is
> there to control the current from the exponent curcuit (and also the
> unscaled linear FM is you have one) and will direct it either directly
> into the current-controlled oscillator or into the current mirror, which
> will mirror the current and toss it into the current-controlled
> oscillator, but in the opposite direction. Thus, the linear FM (scalled
> or unscalled) will affect the slopes correctly in relation to how it
> works on a normal sawtooth oscillator.
> 
>> Thank you!
> 
> Oh... there's the Buchla 259!
> 
> Cheers,
> Magnus
>