[sdiy] High frequency VCO designs

[rburnett at richieburnett.co.uk]

Here's some practical examples to help clarify quality concerns:

Top octave squarewaves from C10 (4186.01Hz) to B (7902.13Hz) generated 
by dividing down a MHz frequency master clock by fixed constants for 
each note.  i.e. The old fashioned TOG method....

http://www.richieburnett.co.uk/temp/TOGdiv.wav

The exact same top octave pitches generated using software DDS technique 
running at 96kHz:

http://www.richieburnett.co.uk/temp/TOG96k.wav

(You might have to cut and paste these URLs into your browser to keep my 
host's anti-leech thingy satisfied!)

Whilst the 96k DDS example isn't the several hundreds of kHz that Phil 
suggested, it's quite representative of the period jitter (aliasing) 
that you get trying to generate arbitrary-frequency square waves 
directly using this method.  The amplitude of the unwanted aliasing only 
drops by 6dB every time you double the DDS clocking rate, so you have to 
go up many times to get the aliasing right down.

I'll leave Tom to decide what sounds acceptable for his application and 
what doesn't :-)

-Richie,