[sdiy] Roland DCOs

Eric Brombaugh ebrombaugh at earthlink.net
Tue Feb 26 00:08:03 CET 2008


rob wrote:
> For a DCO I am planning to use a Phase Accumulator chip (Analog Devices
> AD9833) to drive a simple wave table, with a PIC reading control voltages
> and programming the Phase Accumulator frequency..in fact the AD9833 will
> create 10-bit triangle, sine and sqaure, but I'm not sure about it running
> slow at 20k rather than 12.5MHz...more fun to run through a wavtable and
> having all that waveform fun...

Hi Rob,

(I'm not Tom, but since you're posting to the list I'll jump in anyway)

Your oscillator idea sounds interesting. I've used the Analog Devices 
AD98xx series parts for many years (usually in test equipment) and 
they're fun parts. I expect you won't have any trouble with it.

A few things you might consider from someone who has done a lot of 
digital oscillators:

* What you're proposing isn't exactly a DCO in the traditional 
synthesizer terms. Yes it is digital, and yes it is an oscillator, but 
in the world of synths, a DCO has a specific meaning: an analog sawtooth 
integrator driven by a digital divider (see Tom's webpage for a nice 
drawing of the signal flow). The differences between this structure and 
an ordinary NCO are critical in that a DCO won't suffer from the type of 
aliasing that an NCO does, due to the fact that the digital divider's 
output edges don't jitter.

* Since the AD9833 is an NCO-based device, it will suffer from 
jitter/phase noise. Keeping the NCO clock rate as high as possible will 
mitigate that somewhat, so even though the chip may run at 20kHz (which 
it probably will) you may find that doing so will result in fairly dirty 
sounding waveforms. Of course, some folks like that sound and if that's 
your goal then you'll be happy with the results. Just be aware of the 
impact which NCO clock rate will have on your oscillator performance. If 
pitch accuracy is a concern, note that the AD9833 gives you 0.047Hz 
resolution when clocked at 12.5MHz - pretty good even in the low registers.

Eric


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