121 lines
4.0 KiB
C
121 lines
4.0 KiB
C
#ifndef ALC_FILTER_H
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#define ALC_FILTER_H
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#include "AL/al.h"
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#include "math_defs.h"
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#ifdef __cplusplus
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extern "C" {
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#endif
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/* Filters implementation is based on the "Cookbook formulae for audio
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* EQ biquad filter coefficients" by Robert Bristow-Johnson
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* http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt
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*/
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/* Implementation note: For the shelf filters, the specified gain is for the
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* reference frequency, which is the centerpoint of the transition band. This
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* better matches EFX filter design. To set the gain for the shelf itself, use
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* the square root of the desired linear gain (or halve the dB gain).
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*/
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typedef enum BiquadType {
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/** EFX-style low-pass filter, specifying a gain and reference frequency. */
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BiquadType_HighShelf,
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/** EFX-style high-pass filter, specifying a gain and reference frequency. */
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BiquadType_LowShelf,
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/** Peaking filter, specifying a gain and reference frequency. */
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BiquadType_Peaking,
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/** Low-pass cut-off filter, specifying a cut-off frequency. */
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BiquadType_LowPass,
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/** High-pass cut-off filter, specifying a cut-off frequency. */
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BiquadType_HighPass,
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/** Band-pass filter, specifying a center frequency. */
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BiquadType_BandPass,
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} BiquadType;
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typedef struct BiquadFilter {
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ALfloat z1, z2; /* Last two delayed components for direct form II. */
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ALfloat b0, b1, b2; /* Transfer function coefficients "b" (numerator) */
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ALfloat a1, a2; /* Transfer function coefficients "a" (denominator; a0 is
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* pre-applied). */
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} BiquadFilter;
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/* Currently only a C-based filter process method is implemented. */
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#define BiquadFilter_process BiquadFilter_processC
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/**
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* Calculates the rcpQ (i.e. 1/Q) coefficient for shelving filters, using the
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* reference gain and shelf slope parameter.
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* \param gain 0 < gain
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* \param slope 0 < slope <= 1
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*/
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inline ALfloat calc_rcpQ_from_slope(ALfloat gain, ALfloat slope)
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{
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return sqrtf((gain + 1.0f/gain)*(1.0f/slope - 1.0f) + 2.0f);
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}
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/**
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* Calculates the rcpQ (i.e. 1/Q) coefficient for filters, using the normalized
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* reference frequency and bandwidth.
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* \param f0norm 0 < f0norm < 0.5.
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* \param bandwidth 0 < bandwidth
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*/
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inline ALfloat calc_rcpQ_from_bandwidth(ALfloat f0norm, ALfloat bandwidth)
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{
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ALfloat w0 = F_TAU * f0norm;
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return 2.0f*sinhf(logf(2.0f)/2.0f*bandwidth*w0/sinf(w0));
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}
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inline void BiquadFilter_clear(BiquadFilter *filter)
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{
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filter->z1 = 0.0f;
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filter->z2 = 0.0f;
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}
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/**
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* Sets up the filter state for the specified filter type and its parameters.
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*
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* \param filter The filter object to prepare.
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* \param type The type of filter for the object to apply.
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* \param gain The gain for the reference frequency response. Only used by the
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* Shelf and Peaking filter types.
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* \param f0norm The normalized reference frequency (ref_freq / sample_rate).
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* This is the center point for the Shelf, Peaking, and BandPass
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* filter types, or the cutoff frequency for the LowPass and
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* HighPass filter types.
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* \param rcpQ The reciprocal of the Q coefficient for the filter's transition
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* band. Can be generated from calc_rcpQ_from_slope or
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* calc_rcpQ_from_bandwidth depending on the available data.
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*/
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void BiquadFilter_setParams(BiquadFilter *filter, BiquadType type, ALfloat gain, ALfloat f0norm, ALfloat rcpQ);
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inline void BiquadFilter_copyParams(BiquadFilter *RESTRICT dst, const BiquadFilter *RESTRICT src)
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{
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dst->b0 = src->b0;
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dst->b1 = src->b1;
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dst->b2 = src->b2;
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dst->a1 = src->a1;
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dst->a2 = src->a2;
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}
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void BiquadFilter_processC(BiquadFilter *filter, ALfloat *RESTRICT dst, const ALfloat *RESTRICT src, ALsizei numsamples);
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inline void BiquadFilter_passthru(BiquadFilter *filter, ALsizei numsamples)
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{
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if(LIKELY(numsamples >= 2))
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{
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filter->z1 = 0.0f;
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filter->z2 = 0.0f;
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}
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else if(numsamples == 1)
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{
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filter->z1 = filter->z2;
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filter->z2 = 0.0f;
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}
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}
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#ifdef __cplusplus
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} // extern "C"
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#endif
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#endif /* ALC_FILTER_H */
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