[sdiy] lookup table as expo converter

[Paul Maddox]

Tim,

> I've been giving this some thought. I think one could
> get away with having one entry per note (about 120)

This is what I did for my Monowave.

> and interpolating between the values for microtonal
> and modulation.

yep, additional maths, but at 16MIPs , shouldn't give a noticeable delay

> A 120 note table of values would feed into a
> Numeric-Controlled Oscillator. Another lookup table
> would convert the inherently sawtooth output of the
> NCO into other waveforms. Then an audio DAC provides
> the noise.

why bother with an audio DAC?
The DACs in the Monowave are AD7524's, 8bit voltage output DACs.
Unless you're going to be storing your waveforms as 24bit numbers (READ:
huge amounts of eprom/Ram) its wasted.

> I think the use of an oversampling DAC on the output
> would take care of the sharp edges you get with was
> and square waves.

This is only a problem if you're using a fixed sample rate, the Monowave
uses a variable sample rate technique.
The oscillators run upto around 1Mhz, with 256 samples per wave, thats 4Khz
of audio, but at a sample rate of 1Mhz..
if you play a lower note, the sample rate drops..
Why do it this way?
Easy, cheaper DACs, less concern about aliasing and simplicity (not anti
aliasing maths needed).

Paul

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