[sdiy] How does a DCO work?

[Theo]

Hi Paul,
Yes sure for a variable samplerate system your right.
But example 3 was about a fixed sample rate DCO, different story there.

Have fun,
Theo

From: Paul Maddox <P.Maddox at signal.QinetiQ.com>

> Theo,
>
> > 3) A value is added to a modulo counter at a fixed (sample) rate.
> > Larger value => higher frequency. The counter output might feed directly
> > into a DA converter to produce a saw wave.
> > Or alternatively index a ROM/RAM containing a stored waveform. As the
> output
> > is always at full amplitude DA can be 8 bit (or less) but in modern
times
> > one would use 16 bit or better anyways??.
> > Downside, digital waveforms, therefore: quantizing noise, aliassing and
or
> > band-limiting (high frequency loss at lower pitches).
>
> you do *NOT* get aliasing (like DSPs) or have issues with band limiting of
> phase accumulator based oscillators.
> For example, the monowave uses these, 256 samples per wave (so yes, some
> quantisation noise), but if you press a 4Khz note your sample rate is over
> 1Mhz!
> So bandlimiting and aliasing do *NOT* happen with Digital phase
accumulator
> based oscillators.
>
> Aliasing *ONLY* occurs in fixed sample rate systems, not in variable
sampel
> rate systems.
>
> Sorry if that sounded like a rant, but its a myth that is so commonly held
> that is just plain wrong.
>
> Paul
>
>