[sdiy] How does a DCO work?

[Paul Maddox]

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