[sdiy] lookup table as expo converter

[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.


>  No lookup tables (except for maintaining
> the expo-weighted values, which require 2^N storage words) and it's
> as accurate as your mixed-signal interface. Yep, it means regressing
> to non-monolithic SAR ADCs, but you can't have everything.

You could use a standard ADC and process the bits one by one after the 
conversion.

Ingo