[sdiy] I suck at halfband filters and maths...

Gordonjcp gordonjcp at gjcp.net
Wed May 16 23:38:49 CEST 2018

... which is why I'd like some assumptions checked.

If I oversample something, say generating a waveform at twice the
desired frequency, I can filter it to 1/4 of the sample rate ensuring
that there's little-to-no energy above the "new" Nyquist frequency.  In
a microcontroller that's as arithmetic-impaired as I am, I might want
to do that using integers.  

If I don't mind the performance of the filter sucking a bit I can use a
biquad and set its Q to be 0.5 - rather more round-shouldered than
Butterworth - and by following RBJ's excellent cookbook something a bit
useful shows up:

w0 = 2 pi f0 / Fs = 6.28*12000/48000 = 1.57, give or take
alpha = sin(w0) / (2*Q) = 0.9999 (or so) / (2* 0.5)

and thus:
a0 = 1 + alpha = 2 or thereabouts,
a1 = -2 cos(w0) = 0, near as dammit
a2 = 1 - alpha = 0 again

b0 = (1-cos(w0))/2 = 0.5
b1 = 1-cos(w0) = 1

Normalising by dividing by a0, and we get

b0 = 0.25
b1 = 0.5
b2 = b0

so, using direct form 2, the equation for each sample would be:

y = 0.25x + 0.5xd + 0.25xdd

(where xd and xdd are the delay terms, because I can't figure out how to
type superscripted -1 and -2 to indicate the unit delays on this

This does actually appear to roll off not too badly with for certain
waveforms (like synth saw and squares that have been generated with a
bit of bandlimiting) an acceptable attenuation of out-of-band stuff.

Now, for the clever bit, that's a bit too clever for me.

Looking at that I reckon it ought to be possible to add a third delay
and then work out two input samples into a single output sample at the
same time.  I need to sit down properly and have a play and try and
remember enough high-school algebra to make it work.

Does this make enough sense to anyone else that they can either tell me
if I'm along the right lines, or if I've made some horrible fatal flaw
in it somewhere?


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