Class: Antialias

Inherits:
Object
  • Object
show all
Defined in:
lib/antialias.rb

Class Method Summary collapse

Class Method Details

.antialias_for_fundamental(sample_rate, fundamental, samples) ⇒ Object

sample_rate, Hz, e.g. 44100 fundamental, Hz, e.g. 440 samples: -1..1 The sample_rate gives us the Nyquist limit, and the fundamental tells us how to calculate the partials to find out which ones fall above the Nyquist limit. With this knowledge we dampen those higher partials for the purpose of preventing aliases at that fundamental.



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# File 'lib/antialias.rb', line 11

def self.antialias_for_fundamental(sample_rate, fundamental, samples)
  fft = FFTW3.fft(NArray[samples]).to_a.flatten
  dampened = dampen_higher_partials(sample_rate, fundamental, fft)
  (FFTW3.ifft(NArray[dampened]) / samples.count).real.to_a.flatten
end

.dampen_higher_partials(sample_rate, fundamental, fft) ⇒ Object

kill everything higher than a scaled nyquist limit ease in/out everything else to minimize partials near nyquist



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# File 'lib/antialias.rb', line 19

def self.dampen_higher_partials(sample_rate, fundamental, fft)
  nyquist = sample_rate.to_f / 2
  sample_fundamental = sample_rate.to_f / fft.count
  scaled_nyquist = nyquist / fundamental * sample_fundamental
  sample_duration = fft.count.to_f / sample_rate
  sub_nyquist_sample_count = scaled_nyquist * sample_duration
  fft.each_with_index.map {|power, i|
    hz = i.to_f / fft.count * sample_rate.to_f
    if hz < scaled_nyquist
      scale_partial(i, sub_nyquist_sample_count, power)
    else
      0+0i
    end
  }
end

.scale_partial(partial_index, partial_count, partial_value) ⇒ Object

dampen partials higher than a certain frequency using a smooth “ease-in-out” shape



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# File 'lib/antialias.rb', line 37

def self.scale_partial(partial_index, partial_count, partial_value)
  partial_value * (Math.cos(partial_index.to_f*Math::PI/2/partial_count)**2)
end