[sdiy] Flangelicious noises - some queries about NCO jitter and resampling and similar

[Tom Wiltshire]

Ok, so the advice would be:

1) Use the highest NCO clock I can to minimise jitter
2) Increase the frequency increments as much as possible to use as much of the available increment resolution as I can
3) Divide the resulting frequency down if it’s too much high
4) Add a little bit of dither to break up audible cycles/tones
5) Sample the LFO at as high a frequency as possible

Is that a fair summary? Anything I’ve missed? (Noise shaping the dither, I suppose)

For (1) I can move the clock from 16MHz to 32MHz, so there’s a small improvement.
For (2) we can increase the frequency increment, assuming I can work out a way to get the chip to provide the division for (3). If I could run the NCO at (say) x8 of the frequency I need, that might offer some real benefit.
The dither (4) doesn’t really depend on anything else, so that should be do-able.
The LFO sample frequency should also be able to increase since the whole chip is running twice as fast as the previous generation (32 vs 16MHz again). So everything else being equal, I might be able to up the LFO sample rate to 50KHz or so. However, that seems a bit crazy (audio-rate sampling for an LFO with a max of 20Hz) and instead I might do better to add some interpolation of the frequency increment at the end of each cycle. E.g. We’re aiming for an increment of X, and we’ve got an increment of Y, so next cycle uses X+x to get a bit closer. The difficulty there is that we need to know how many NCO cycles there will be before our next LFO sample, and that sounds horribly like division! So maybe just whacking the LFO sample rate up is in fact the best you can do for that after all.

Thanks,
Tom

==================
       Electric Druid
Synth & Stompbox DIY
==================

> On 28 Jan 2020, at 13:07, René Schmitz <synth at schmitzbits.de> wrote:
> 
> Hi Richie,
> 
> On 28.01.2020 13:23, rburnett at richieburnett.co.uk <mailto:rburnett at richieburnett.co.uk> wrote:
> >>> What I don’t understand though is how this helps. Increasing the
> >>> output frequency is going to increase aliasing too, and dividing it
> >>> down again afterwards doesn’t seem to remove that to me. How does
> >>> this work please, René?
> >>
> >>  It's a standard trick. With higher DDS clock you get smaller
> >> time-errors, and then the divide down just removes transitions between
> >> the transitions. Higher synthesized frequency allows you to use more
> >> of the upper bits of the DDS, to achieve more effective bits useable
> >> in the DDS.
> >
> > Ok, so this allows you to make better use of the available DDS frequency
> > bit resolution, by synthesising a higher output frequency and then
> > dividing it down.  But it doesn't do anything to help with
> > jitter/aliasing at the top end.
> 
> Here are your original points:
> 1) Resampling of the LFO output by the NCO reset
> 2) Frequency stepping caused by the NCO minimum frequency step
> 3) Jitter cycles caused by the NCO
> 
> Well it helps updating your NCO more frequently, because each cycle is shorter now. I.e. higher sampling rate of your LFO.
> 
> The NCOs minimum frequency step is reduced. Because the tuning word effectively shifts right, bringing in room for more LSBs.
> 
> The cycle to cycle time variation (aka jitter) is still 1/f(clock) of your NCO. Not really an improvement there by the division, if you run the NCO at the same frequency as before. But if you can run the NCO clock higher then the variation is reduced.
> 
> Best,
> René
> 
> --
> synth at schmitzbits.de <mailto:synth at schmitzbits.de>
> http://schmitzbits.de <http://schmitzbits.de/>
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