[sdiy] Approximating sine with plain integer math

Olivier Gillet ol.gillet at gmail.com
Thu Apr 7 21:51:31 CEST 2016


Here's what I'm talking about in terms of ugliness:

"Naive" sawtooth (modulo counter):
https://dl.dropboxusercontent.com/u/612135/sawtooth_naive.wav

Band-limited sawtooth:
https://dl.dropboxusercontent.com/u/612135/sawtooth_bandlimited.wav

Plots:
http://i.imgur.com/K45ljR6.png

We are talking about all harmonics of the sawtooth wave above half your
sample rate being folded back, to the point that they might very audible
beat against the fundamental or add partials under it. It's aliasing
inherent to the modulo counter itself, which is there as soon as your
frequency does not divide the sample rate.

This cannot be addressed outside of the oscillator by low-pass filtering,
because the offenders are not in a specific frequency range - they are
inbetween the harmonics of the signal. High-pass filtering can alleviate
things a bit, but it's just better to use a band-limited synthesis method
in the first place instead of using a naive generator.

On Thu, Apr 7, 2016 at 9:27 PM, John Ames <commodorejohn at gmail.com> wrote:

> Hm. Yes, that is the goal. I'm just not sure I understand what "ugly"
> is here - are we talking aliasing noise, or just an unpleasant amount
> of high-frequency components? Either way, it's something I have to
> think you'd address outside of the oscillator proper, with some kind
> of filtering, which is also going to be involved in the softsynth
> total...
>
> Maybe this is just something I'm not understanding due to being a
> programmer with an interest in synthesis rather than a signal analyst
> with a knowledge of programming?
>
> On 4/7/16, Olivier Gillet <ol.gillet at gmail.com> wrote:
> >>
> >> As for bandlimiting the input...I guess I'm not even sure why you'd do
> >> that?
> >
> >
> > I thought your goal was to design an audio oscillator with a sawtooth
> > waveform, and then to get the sine waveform "for free" just by
> manipulating
> > your sawtooth signal.
> >
> > Above a few hundred Hertz a naive sawtooth without band-limiting starts
> > sounding ugly...
> >
>
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