[sdiy] Digital Waveshape Generator.

[Richie Burnett]

> I wanted to avoid frequencies so high as to necessitate using ECL. The 
> design document I mentioned details how I solved that problem. You will 
> find that here: http://www.xavax.com/alvitar/DWG_Design.pdf

I think you might have made a mistake calculating your divider ratios... 
You need to divide the master clock frequency down to a frequency that is 
256 times that of the note you want to produce, because you still have to 
cycle through 256 samples in one period of the waveform in memory as you 
play it out to the DAC.  That would put your minimum master clock frequency 
somewhere up in the hundreds of MHz if I'm not mistaken?  For example, to 
achieve the same pitch resolution as the Roland Juno 106's DCOs (just 
acceptable in my opinion) you would have to have a master clock frequency 
that was 256 times higher than whatever Roland chose (2MHz?) Their 
programmable timer was generating one clock edge per pitch period, but yours 
needs to generate 256 edges over the same pitch period if I understood your 
document correctly?

That's why going down the DDS route is a better option in my opinion.  You 
can directly synthesise the clock rate you need at 256 times the pitch 
frequency, and to sub Hz resolution.  You don't need to deal with any 
frequency higher than about 12MHz to clock the DDS and you can get sub 
millihertz pitch resolution out of the DWG across the full MIDI pitch range 
without any aliasing doing it this way.

-Richie,



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