[sdiy] NCO Jitter (was Large Numbers)

[Eric Brombaugh]

If you oversample your NCOs by a factor of 20 from a final output rate
of 48kHz that means they're running at approx 1MSPS. If the NCOs are
simply generating naive sawtooth waveforms that implies that the
audible aliased harmonics of a 1kHz note will be roughly the 90th
harmonic, at which point the amplitude is 1/90 which is about 40dB down
- not inaudible but certainly not in your face either.

Don't forget that when you oversample your NCOs by a factor of 20
you'll need to use bandlimited resampling to decimate them down to the
final 48kHz output rate. This will require a decent filter to prevent
the unwanted high frequency content from aliasing back into the audible
range. Ideally a FIR filter with flat response from DC to 20kHz or so,
but you can get away with simpler polynomial or CIC structures and
still have decent sounding results.

Eric

On Sat, 2018-07-14 at 08:41 -0700, Tim Ressel wrote:
> So how much overclocking on the an NCO is needed to reduce the jitter
> to 
> an acceptable level? I will be sampling at 48K. If I run the NCOs at
> 20x 
> that rate, is that good enough?  Keeping in mind I have 10 NCOs to 
> operate and a proc running at 180 MHz.
>