[sdiy] Stupid Lookup Table Tricks - BLEPs when you can't divide (and can barely multiply)

rsdio at audiobanshee.com rsdio at audiobanshee.com
Mon Jul 2 07:59:52 CEST 2018


I didn’t look at any of this in detail. Does the Arduino sketch use interpolation of pre-computed table data? If so, wouldn’t that add distortion? ... and potentially aliasing?

Brian


On Jun 28, 2018, at 4:00 AM, rburnett at richieburnett.co.uk wrote:
> Neat!  The quality is not bad but there is still quite a bit of audible aliasing even with the corrections enabled.
> 
> I think the problem is that your LUT of reciprocal values really needs a lot more resolution.  It returns the same value for large ranges of input index.  This heavily quantizes the positioning of the polyBLEP corrections and ultimately limits their effectiveness for the anti-aliasing.
> 
> You're only seeing about 20dB reduction in aliasing, and should be able to do much better than this in practice.  But I think the implementation is being limited by the resolution of the reciprocal lookup.
> 
> The neat thing about polyBLEP is that the correction curves are simple functions, so you would ideally calculate these on a more capable platform, instead of using a pre-calculated lookup table.  Then you are only limited to the numerical precision of the calculations and not the number of entries in a LUT.  But I understand your motivations for doing it this way to see if it can be made to work :-)
> 
> FWIW, if you just want to generate anti-aliased sawtooths and pulse waveforms on a low-end 8-bit platform without oscillator-sync, then the best option is probably just to use wavetables.  You store a whole load of sawtooth renderings generated by additive synthesis with progressively less harmonics in a lookup table.  Every time you go up by one octave in pitch you switch to the next waveform from the table.  Square an pulse can be generated by taking the difference of two phase-shifted sawtooths.
> 
> -Richie,
> 
> On 2018-06-28 01:14, Gordonjcp wrote:
>> Why would you pick an avr8 to do bandlimited oscillators?
>> Because folk say it won't work, that's why!
>> https://github.com/ErroneousBosh/slttblep





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