[sdiy] lookup table as expo converter

[Ingo Debus]

Am Freitag, 28.05.04 um 16:12 Uhr schrieb Ingo Debus:

> Am Freitag, 28.05.04 um 10:01 Uhr schrieb Colin Hinz:
>
>> (Semi)-seriously, imagine a mutant ADC with two accumulator registers:
>> one amasses the binary bits as usual, while the other one sums up
>> exponentially-weighted values.
>
> Don't these values have to be multiplied rather than summed? We're 
> talking an expo converter, i. e. something that can be put in front of 
> a (linear) mV/Hz VCO to make it a V/octave VCO, yes?
> So, at the input of this expo converter 1LSB (bit 0) would correspond 
> (say) to a semitone interval at the VCO output, the second but least 
> significant bit (bit 1) corresponds to a two-semitone interval, bit 2 
> corresponds to a four-semitone interval and so on. Since the bits 
> correspond to fixed intervals they correspond to frequency ratios. For 
> each set bit the frequency (and thus the output voltage of the expo 
> converter aka input voltage of the linear VCO) has to be multiplied 
> with some factor. Correct?
>
> But apart from this, I don't see any reason why it should not work. No 
> table, but a lot of multiplications instead.
>

Thinking a little further, the multiplications could also be done on 
the analog side. I'm thinking a string of switchable attenuators, each 
of them is controlled by an output bit of the ADC. The first attenuator 
is fed by a constant voltage, its output is connected to the input of 
the next attenuator and so on. The last attenuator's output is the 
linear (mV/Hz) CV source. Does this make sense? Does such a thing 
already exist? I reminds me of the keyboard of early Korg synths (MS-20 
for instance).

Ingo