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diff --git a/src/3rdparty/resonance-audio/resonance_audio/dsp/filter_coefficient_generators.h b/src/3rdparty/resonance-audio/resonance_audio/dsp/filter_coefficient_generators.h new file mode 100644 index 000000000..4c7659a7a --- /dev/null +++ b/src/3rdparty/resonance-audio/resonance_audio/dsp/filter_coefficient_generators.h @@ -0,0 +1,177 @@ +/* +Copyright 2018 Google Inc. All Rights Reserved. + +Licensed under the Apache License, Version 2.0 (the "License"); +you may not use this file except in compliance with the License. +You may obtain a copy of the License at + + http://www.apache.org/licenses/LICENSE-2.0 + +Unless required by applicable law or agreed to in writing, software +distributed under the License is distributed on an "AS-IS" BASIS, +WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +See the License for the specific language governing permissions and +limitations under the License. +*/ + +#ifndef RESONANCE_AUDIO_DSP_FILTER_COEFFICIENT_GENERATORS_H_ +#define RESONANCE_AUDIO_DSP_FILTER_COEFFICIENT_GENERATORS_H_ + +#include "dsp/biquad_filter.h" + +// Functions for the generation of filter coefficients for various common tasks. +// Currently supports the following filter types: +// Low pass first-order filter. +// Low pass biquad filter. +// Band pass biquad filter. +// Dual band, matched phase biquad shelf filters. +namespace vraudio { + +// Computes corresponding biquad coefficients for band pass filter with respect +// to the centre frequency and the bandwidth between -3 dB frequencies. +// +// b0 + b1*z^-1 + b2*z^-2 +// H(z) = ------------------------ +// a0 + a1*z^-1 + a2*z^-2 +// +// where: +// w0 = 2*pi*center_frequency/sample_rate +// +// alpha = sin(w0)*sinh( ln(2)/2 * bandwidth * w0/sin(w0) ) +// +// b0 = alpha +// b1 = 0 +// b2 = -alpha +// a0 = 1 + alpha +// a1 = -2*cos(w0) +// a2 = 1 - alpha +// +// @param sample_rate Sampling rate in Hz. +// @param centre_frequency Centre frequency of passband. +// @param bandwidth Bandwidth in octaves between -3 dB frequencies. +// @return Output structure of band-pass BiquadCoefficients. +BiquadCoefficients ComputeBandPassBiquadCoefficients(int sample_rate, + float centre_frequency, + int bandwidth); + +// Computes two sets of transfer function coefficients to be used with a +// pair of generic bi-quad filters. The coefficients are used to implement a +// phase-matched low-pass |low_pass_state| and high-pass |high_pass_state| +// filter pair with a cross-over frequency defined as |crossover_frequency|. +// +// Implementation of the matched bi-quad filter pair as described in: +// http://www.ai.sri.com/ajh/ambisonics/BLaH3.pdf +// +// b0 + b1*z^-1 + b2*z^-2 +// H(z) = ------------------------ +// a0 + a1*z^-1 + a2*z^-2 +// +// where: +// +// a0 = 1 +// +// 2(k^2 − 1) +// a1 = -------------- +// k^2 + 2k + 1 +// +// k^2 - 2k + 1 +// a2 = -------------- +// k^2 + 2k + 1 +// +// low-pass: high-pass: +// +// k^2 1 +// b0 = ------------- b0 = -------------- +// k^2 + 2k + 1 k^2 + 2k + 1 +// +// b1 = 2b0 b1 = -2b0 +// +// b2 = b0 b2 = b0 +// +// and +// +// pi * crossover_frequency +// k = tan -------------------------- +// sample_frequency +// +// @param sample_rate Sampling rate in [Hz] +// @param crossover_frequency Cross-over frequency in [Hz] +// @param low_pass_coefficients Output structure of low-pass bi-quad +// coefficients +// @param high_pass_coefficients Output structure of high-pass bi-quad +// coefficients. +void ComputeDualBandBiquadCoefficients( + int sample_rate, float crossover_frequency, + BiquadCoefficients* low_pass_coefficients, + BiquadCoefficients* high_pass_coefficients); + +// Computes biquad coefficients for low pass filter with respect to the +// specification frequency and the attenuation value at that frequency. +// +// b0 + b1*z^-1 + b2*z^-2 +// H(z) = ------------------------ +// a0 + a1*z^-1 + a2*z^-2 +// +// where: +// Q = 2.5273e-06*attenuation^4 + 0.00018737*attenuation^3 + +// 0.0061882*attenuation^2 + 0.11395*attenuation + 0.99905 +// +// w0 = 2*pi*specification_frequency/sample_rate; +// +// alpha = sin(w0)/(2*Q); +// +// a0 = 1 + alpha; +// a1 = -2*cos(w0); +// a2 = 1 - alpha; +// b0 = (1 - cos(w0))/2; +// b1 = 1 - cos(w0); +// b2 = (1 - cos(w0)/2; +// +// These coefficients were generated after a bilinerar transform of an +// analogue filter H(s) = 1 / (s^2 + s/Q + 1), from: +// www.analog.com/library/analogdialogue/archives/43-09/edch%208%20filter.pdf. +// +// Please note Q has been calculated by fitting a 4th order polynomial to the +// average of Q vs attenuation curves at different f0's from 10kHz to 19kHz. +// Please note that at differing frequencies these graphs had identical shape +// and differed only by an overall offset of at most 0.1 (attenuation) dB from +// the mean. This script can be found at + +// +// @param sample_rate Sampling rate in Hz. +// @param specification_frequency Frequency at which attenuation applies in Hz. +// @param attenuation Attenuation at specification_frequency in negative dB. +// @return low_pass Output structure of low-pass BiquadCoefficients. +BiquadCoefficients ComputeLowPassBiquadCoefficients( + int sample_rate, float specification_frequency, float attenuation); + +// Generates a coefficient for the |MonoPoleFilterClass| based on a 3dB +// bandwidth. +// +// The Laplace transfer function of a first order low pass system is: +// +// 1 +// ------------ where tau is the RC time constant of the system. +// 1 + tau * s +// +// For a discrete moving average filter with input x[n] and output y[n], +// the difference equation is: +// y[n] = a * y[n - 1] + (1 - a) * x[n] +// tau +// a = ---------- where T is the sample period. +// tau + T +// since the -3dB bandwith of a first order system can be +// 1 related to its time constant by +// f3 = -------------- +// 2 * pi * tau we can obtain 'a' from cuttoff_frequency and +// sample rate. +// +// @param cuttoff_frequency The -3dB frequency in Hz of the low pass lobe. +// @param sample_rate System sampling rate. +// @return A |MonoPoleFilterClass| coefficient for the specified bandwidth. +float ComputeLowPassMonoPoleCoefficient(float cuttoff_frequency, + int sample_rate); + +} // namespace vraudio + +#endif // RESONANCE_AUDIO_DSP_FILTER_COEFFICIENT_GENERATORS_H_ |