MOTM

--- In motm@egroups.com, "Brousseau, Paul E (Paul)" <noise@A...> 
wrote:
> I'm somewhat confused; why does the mini-wave need an input 
driver?  What
> are you driving?  I thought the mini-wave was a sound source by 
itself...
> 
> --PBr

Negatory there, good buddy. The Mini-Wave makes no noise by itself 
without being driven externally. It's basically just a ROM and DAC, 
with some numbers stored in bins called waves. Each wave contains a 
set of 8 bit numbers which represent 1 cycle of an arbitrary 
waveform - sine, saw, arbitrary, whatever. Then 16 waves are 
organized into a bank. Then there are 16 banks for a total of 256 
waves. Think of these as sampled waves, because that's exactly what 
they are.

Now to make noise, something must step through the ROM addresses 
representing each wave. In the case of a typical sampler, you would 
clock the addresses with a system clock. In the world of analog 
control voltages, that's not a good idea. For instance, to play back 
a wave at the desired frequency, a clock would have to run however 
many samples each wave is long faster (maybe 256x?) So now you would 
require VCOs that would track very accurately at very high 
frequencies.

Instead, the Mini-Wave addresses each wave address with a control 
voltage. In normal mode, -5V addresses the first sample in the wave, 
0V the middle sample, and +5V addresses the last sample. Hence a 
rising CV from -5 to +5 plays the sample from start to finish in 
order. Hence, you drive a Mini-Wave with a rising sawtooth from a 
normal VCO to use it as a wavetable playback device. It's the same 
principle as some sequencers use, which allow you to use either a 
clock or a control voltage to select the stage.

This approach allows the controlling VCO to run at the same frequency 
as the desired wavetable wave, so no special high frequency 
gymnastics are required. Also, changing the driving waveform from 
rising saw to falling saw to something else allows you to mangle the 
sample playback order. The quantizer bank just maps the incoming 
control voltage to a stairstepped wave, whose step heights are 
calibrated at 1V/Oct to exact musical intervals. 

Once you think about it, it's actually an elegant approach.

Moe