[sdiy] Re: Aliasing (or not) on Monowave

[Paul Maddox]

> Paul - on the monowave, do you have a constant-rate DAC, or are you 
> varying the DAC rate? If the latter, your aliases will be landing on top 
> of existing harmonics, so you won't get inharmonics and it won't sound 
> icky.

the DAC is in 'free run' mode, so basically the moment a value appears on 
the 8bit data port, it comes out the analogue side (excluding internal 
delay), so I'm actually not clocking the DAC at all, but the data being 
passed to the DAC is running at around 925Khz, which is a constant 'update 
rate'..

basically the 'oscillator' is an NCO using a 24bit word to control the 
pitch.
I use a small micro (at90s1200) which runs at 12Mhz,
it takes 13 instructions to generate a new value based on the control word..
each instruction takes 1 clock cycle, so 12Mhz/13 = 923Khz
(BTW, I'm not counting the IRQ that's called when the oscillator gets a new 
pitch value, which causes a little bit of jitter)

The output of this drives a 7bit counter, which gives you octave selection,
this then drives another counter,the original signal plus these extra 7bits 
for the LSByte of an address on an EPROM which holds the waveforms.
The output of the EPROM is passed directly to the DAC.

So as I say, I'm not actually clocking the DAC, the Write pin is kept low 
(ie. active) constantly, but the 'update rate' of the whole thing is 923Khz, 
a constant..

schematics (care of Laurie Biddulph) are here ;-
Oscillator - http://www.elby-designs.com/monowave/monowave-osc1.pdf
Wavegen - http://www.elby-designs.com/monowave/monowave-wavegen.pdf

hope this helps

Paul