Fur

Fur is a set of finite impulse response filters (FIR) for realtime use.

Minimal example

import fur

# fixed buffer size
var b = newSeq[float](64)

# steepish, sounds decent and only 16 samples latency
var f = initFur(32)

# scale to sample rate
let d = 1 / 48_000

# set coefficients to 220-240hz bandpass (you could turn the same filter into a lowpass later)
f.bandpass(220.0 * d, 240.0 * d)

# example data - a half buffer of slope
let nh = 1.0 / b.len.float
for i in 0 .. (b.len div 2) - 1:
  b[i] = i.float * nh

echo $s
for i in 0..s.len-1:
  s[i] = f.process(s[i])
echo $s

Realtime audio example

And here is how to use it realtime, using the jill wrapper for the jack audio connection kit. Since this works on sample level it can be used for any DSP, e.g. supercollider ugen or any DAW plugin.

note: This one sweeps the filter at every block (commonly called "control rate"). For a real system the coefficients should be precalculated and the sweep done at each sample.

import std/[math]
import jill, fur

# buffer size
let n = 64

# inverse of buffer size
let nh = 1.0 / n.float

#sample duration
let cs = 1 / 48000

# low latency but not completely distorted
var f = initFur(128)

# frequency init at 50hz
var c = 50 * cs

withJack output=out:
  c += 5 * cs
  if c > 2000 / 48000:
    c = 50 * cs
  #f.lopass(c)
  #f.hipass(c)
  f.bandpass(c, c+200*cs)
  #f.notch(c)
  for i in 0 .. (n div 2) - 1:
    out[i] = i.float * nh
    out[i] = f.process(out[i])

Notes

• The code is very simple and was designed for low end systems, but was not not painstakingly optimized (TODO!! SMD!!!)
• Using the filter should be pretty fast. You should use about 32 to 256 taps- any more than that and you are probably better off using a frequency domain FIR filter if you can tolerate that much latency.
• Usage note: These aren't perfect at 16-32 taps, I use them on purpose for musical quality. You may be better off using IIR filters. Listen for yourself! Personally I like it because it's a digital kind of distortion giving it character instead of all the analog emulation.
• Since these just operate on individual samples, you're pretty flexible.
• For realtime use, consider using the arc GC and keeping the initFur
• The name was supposed to mean "Futhark Fir" and I grew attached to it even though I ended up converting the whole thing into Nim instead of wrapping. Thanks, vfiksdal!!!