[sdiy] Basic core designs of commercial analogs

[Magnus Danielson]

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