Variable ramp generator

[Scott Gravenhorst]

"Curtin, Steven D (Steven)" <sdcurtin at lucent.com> wrote:
>> > My idea was to generate a stable saw or triangle and shape it
>> > using a switched gain circuit. I must say I tried to design such 
>> > a circuit, but either I am too narrow minded or blind - I just
>> > cannot come up with anything. I hope somebody invented this before
>> > and would not mind to share. 
>> 
>> It's cool, sounds fm-ish. You can just x-fade the saw/tri out. If you've
>> never 
>> seen this on a scope, too bad. It's cool to look at. Freq won't change,
>> just 
>> the wave morphing saw/tri. This can be simulated by grabbing your saw and
>> tri mixer knobs and twisting like mad in opposite directions.....:-)
>> 
>> A more reasonable approach is to use Juergen's pnp/ota x-fade scheme,
>> two trannys and two otas and you're in business.Alternately the Serge wave
>> shaper/multi also works well and it's even more simple. 
>> 
>I remember from the Gnome I built long ago that the transition from the saw
>to the ramp sounded very much like pulse width modulation.  This was very
>different than just crossfading from a triangle to a ramp.  One side-effect
>of the circuit on the Gnome (at least on mine) was that the pitch would also
>change a little bit when the shape knob was twiddled.  The Serge Dual
>Universal Slope Generator could also do this, since it operates at both
>audio and subaudio ranges.   Grant's Richter/Wiard's Envolator module might
>also do this (Grant, does it operate at audio range?).  This would be a cool
>circuit to add to the Fatman, since its 555 outputs a square wave. 

Not so.  The 555 logic output (pin 3) delivers a very narrow pulse.  Pin 3
is "on" for only the discharge part of the saw cycle.  "Off" the for the
ramp.

-- Scott Gravenhorst
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