Class: SPCore::BiquadFilter

Inherits:
Object
  • Object
show all
Defined in:
lib/spcore/filters/iir/biquad_filter.rb

Overview

Based on the “simple implementation of Biquad filters” by Tom St Denis, which is based on the work “Cookbook formulae for audio EQ biquad filter coefficients” by Robert Bristow-Johnson, [email protected] a.k.a. [email protected]. Available on the web at www.smartelectronix.com/musicdsp/text/filters005.txt

Constant Summary collapse

LN_2 =

used in subclasses to calculate IIR filter coefficients

Math::log(2)

Instance Method Summary collapse

Constructor Details

#initialize(sample_rate) ⇒ BiquadFilter

A new instance of BiquadFilter.

Parameters:

  • sample_rate (Numeric) —

    The sample rate to use in calculating coefficients.



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# File 'lib/spcore/filters/iir/biquad_filter.rb', line 18

def initialize sample_rate
  @sample_rate = sample_rate
  @biquad = Struct::BiquadState.new(0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0)
  @critical_freq = 0.0
  @bandwidth = 0.0
end

Instance Method Details

#bandwidth=(bandwidth) ⇒ Object

Set the filter bandwidth.



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# File 'lib/spcore/filters/iir/biquad_filter.rb', line 36

def bandwidth= bandwidth
  set_critical_freq_and_bw(@critical_freq, bandwidth);
end

#critical_freq=(critical_freq) ⇒ Object

Set the filter critical frequency.



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# File 'lib/spcore/filters/iir/biquad_filter.rb', line 31

def critical_freq= critical_freq
  set_critical_freq_and_bw(critical_freq, @bandwidth);
end

#get_freq_magnitude_response(test_freq) ⇒ Object

Calculate the frequency magnitude response for the given frequency.

Parameters:

  • test_freq (Numeric) —

    The frequency to calculate magnitude response at.



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# File 'lib/spcore/filters/iir/biquad_filter.rb', line 66

def get_freq_magnitude_response test_freq
  # Method for determining freq magnitude response is from:
  # http://rs-met.com/documents/dsp/BasicDigitalFilters.pdf
  omega = 2.0 * Math::PI * test_freq / @sample_rate
  b0, b1, b2 = @biquad.b0, @biquad.b1, @biquad.b2
  a0, a1, a2 = 1, @biquad.a1, @biquad.a2
  b = (b0**2) + (b1**2) + (b2**2) + (2 * (b0 * b1 + b1 * b2) * Math::cos(omega)) + (2 * b0 * b2 * Math::cos(2 * omega))
  a = (a0**2) + (a1**2) + (a2**2) + (2 * (a0 * a1 + a1 * a2) * Math::cos(omega)) + (2 * a0 * a2 * Math::cos(2 * omega))
  return Math::sqrt(b/a)
end

#process_sample(sample) ⇒ Object

Calculate biquad output using Direct Form I:

y = (b0/a0)*x + (b1/a0)*x + (b2/a0)*x

- (a1/a0)*y[n-1] - (a2/a0)*y[n-2]

Note: coefficients are already divided by a0 when they are calculated. So that step is left out during processing.



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# File 'lib/spcore/filters/iir/biquad_filter.rb', line 48

def process_sample sample
  # compute result
  result = @biquad.b0 * sample + @biquad.b1 * @biquad.x1 + @biquad.b2 * @biquad.x2 -
      @biquad.a1 * @biquad.y1 - @biquad.a2 * @biquad.y2;

  # shift x1 to x2, sample to x1
  @biquad.x2 = @biquad.x1;
  @biquad.x1 = sample;

  # shift y1 to y2, result to y1
  @biquad.y2 = @biquad.y1;
  @biquad.y1 = result;

  return result
end

#set_critical_freq_and_bw(critical_freq, bandwidth) ⇒ Object

Set the filter critical frequency and bandwidth.

Raises:

  • (NotImplementedError) 


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# File 'lib/spcore/filters/iir/biquad_filter.rb', line 26

def set_critical_freq_and_bw critical_freq, bandwidth
  raise NotImplementedError, "set_critical_freq_and_bW should be implemented in the derived class!"
end