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

[rburnett at richieburnett.co.uk]

> 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)

It doesn't sound crazy to me Tom.  If you were programming a flanger or 
chorus effect on an audio DSP that used a sample rate of 50kHz, then you 
would generate the LFO waveform at that audio sample rate (50kHz), and 
you would update the pointers for the delay line at the full audio rate. 
  I would expect anything less to introduce audible artefacts.

-Richie,


On 2020-01-31 12:07, Tom Wiltshire wrote:
> 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 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
>> http://schmitzbits.de [1]
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> 
> 
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