More ramblings about VCDO...

[Tim Ressel]

Sean,

The oscillator I am proposing is called an NCO, or Numerically-Controlled
Oscillator. NCOs clock at a fixed rate much higher than the highest frequency
needed. For audio, I figure about 2 MHz. The oscillator itself is just an adder
and accumulator. The output of the accumulator is fed back to one input of the
adder, and a frequency determining number is fed into the other adder input. As
the NCO clocks, the freq-number is added to the accumulator each clock cycle.
The accumulator rolls over to zero when it maxes out. If you look at the
accumulator output, you will see a sawtooth wave being generated. To change the
frequency, you simply change the input number, which is mathematically related
to the frequency and the clock rate. The accumulator output can be fed to a
(fast) D/A converter to generate a sawtooth for further processing (my plan), or
it can feed a wave table then a DAC. Since the NCO adds no noise of its own, the
frequency is as stable as the clock oscillator being used.

For a visual to go along with my verbal hand-waving, check out:

http://www.analog.com/pdf/ad7008.pdf

--Timster

> ----------
> From: 	Sean Costello[SMTP:costello at seanet.com]
> Sent: 	Sunday, July 11, 1999 12:25 PM
> To: 	Tim Ressel
> Cc: 	synth-diy at mailhost.bpa.nl; 'mbartkow at ET.PUT.Poznan.PL'
> Subject: 	Re: More ramblings about VCDO...
> 
> Please tell me which digital oscillators you are talking about, as I have
> worked
> with many that could use some of this "ultimate stability." Many of the
> digital
> oscillators I have programmed either die out in a matter of minutes, or blow
> up
> almost instantly. Trying to get those poles exactly on the unit circle, with
> finite precision math, can be a pain sometimes. OK, table lookup oscillators
> don't have that problem, but I like to do things the hard way sometimes...
> 
>