[sdiy] Synthesizers...

[ASSI]

On Dienstag, 17. Februar 2009, Tom Wiltshire wrote:
> ...ones we built ourselves. Anyone fancy a chat about that?

Who'd do that?

> Myself, I'm still playing with the DCO design that I posted a
> problem with the other day. Now it's going properly, I've been
> sorting out the amplitude calibration, and looking at the way that
> the Rhodes Chroma uses a ramp plus a PWM pulse wave to mimic the
> effect of two detuned sawteeth. Apparently these two are
> equivalent, but I'm yet to prove it to myself to my satisfaction.

Not knowing what the Chroma does exactly, if is an accumulator based 
DCO design then things are comparatively easy: instead of having one 
reset pulse at the phase wrap you have two and both of them only take 
away half the amplitude.  Binary equivalencies can be used to 
suibstitute an addition ith (silent) overflow for substraction.  What 
you now need is a signal that tells you when to do each reset.  If the 
sawtooth were not detuned, they would always happen at the same time 
depending on the initial phase shift, or viewed as a waveform you'd 
get a pulse wave with varying duty cycle (the resets happen both on 
the up and the down flank of the pulse).  The only difference with 
detuned saw waves is that the phase shift will change each cycle by 
the amount of detune, so you need to modulate the PW (with a sine or 
something very similar at the beat frequency).


Achim.
-- 
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk]>+

Waldorf MIDI Implementation & additional documentation:
http://Synth.Stromeko.net/Downloads.html#WaldorfDocs