[PD] Bandlimited oscils (was Re: anti-aliasing filtering)

[Orm Finnendahl]

Hi,

 concerning avoiding aliasing in the first place, here are
abstractions for generating bandlimited versions of the most popular
analog waveforms I developed for a seminar some time ago:

https://www.selma.hfmdk-frankfurt.de/selmafile/f/9eed9b01b9/

Open "bl-oscil-example.pd" to see how to use it. Note that the rect~
oscillator has an additional pulse-width input.

The patch uses wavetable lookup with 128 wavetables of an ideal
bandlimited sawtooth with partials 1 through 127 for each wavetable
respectively and the non-bandlimited phasor wavetable for the
128th. In the oscillator the wavetable is determined on each phasor
wraparound based on frequency.

At a samplerate of 44100 Hz this gives no aliasing above 173.6 Hz
which my ear considered good enough (for 173.6 Hz the aliasing happens
at about -42 dB for the first aliased partial relative to the first
partial, diminishing by 6 dB per Octave downwards). Increase the
number of wavetables for better signal/aliasing ratio.

As this method gives ideal results and is as efficient as it can get
it's strange I haven't found it on my web research of this topic, but
maybe I've looked at the wrong places. In times when wavetable lookup
synthesis has regained popularity in commercial soft-synths I could
well imagine companies like NI or Ableton also use this or simlar
methods under the hood of the oscils for their standard analogue
waveforms.

--
Orm

Am Samstag, den 28. Oktober 2017 um 19:24:00 Uhr (-0200) schrieb
Alexandre Torres Porres:
> 2017-10-28 18:30 GMT-02:00 cyrille henry <ch at chnry.net>: Its very
> different : a sawtooth have an infinite number of harmonics, but >
> not a signal distorted with tanh. And a band limited sawtooth is
> lot's > better (from sound and performance perspective) than an
> oversampled / > filtered phasor.
> >
> 
> yeah, I know it's got infinite harmonics, and that a band-limited
> oscillator is better. But this is also because I'm using this to measure
> the efficiency of the anti-aliasing filter, how good it works and all
> 
> cheers

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