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

Orm Finnendahl orm.finnendahl at selma.hfmdk-frankfurt.de
Sun Oct 29 13:21:47 CET 2017


 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:


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


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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