Adventures with the Tilman modulator

[Tony Allgood]

>AFAIK the Tilman proposal is a post processor, i.e.. rides on a saw
given by an independend vco. Thus I can not understand why you hear a
pitch change, ... ahh, say measure a frequency change.

Sorry Martin, perhaps I didn't make myself clear. Don's waveshaper
design appears to introduce a phase change. Modulation of the duty cycle
produces a vibrato type of effect, much in the same way as PWM alters
the phase of a signal. If the duty cycle is fixed then of course no
frequency shift is heard, but a changing duty cycle produces a changing
pitch. This is because each sucessive zero crossing point is nearer or
further away to its neighbour.

>And what happens if you cross the 10% (or 90%) border? I guess the wave
form gets confused then...

I haven't tried this... I will do that soon. Actually, one of the limits
of my design is offsets of the 13700. At extremes of the duty cycle
range, the offset of one OTA adds to the waveform. This leads to small
steps before a change in slope. One can trim out the offsets over a
small range... but I know from experience once Iabc exceeds 1mA the
offset becomes increasingly and disproportionally important. A better
VCA would be good here. Perhaps one of those AD parts, but they get
discontinued all too often...

Incidently, on my new VCO design, I have made the PWM comparator take
its input from either the saw or the triangle output. A switch selects
the chosen input. This does produce noticeable affects, especially at
audio rate modulation frequencies.

Regards,

Tony Allgood  Penrith, Cumbria, UK

synth bits....

http://www.techrepairs.freeserve.co.uk/