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