Audio filters for a stereo widening effect and for simple bass/treble eq

bass_and_treble.h

/*
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          Licensing information can be found at the end of the file.
------------------------------------------------------------------------------

bass_and_treble.h - v1.0 - Simple audio filter to boost bass and treble.

Do this:
    #define BASS_AND_TREBLE_IMPLEMENTATION
before you include this file in *one* C/C++ file to create the implementation.
*/

#ifndef bass_and_treble_h
#define bass_and_treble_h

typedef struct {    
    float bass_alpha; // filter smoothing factors
    float treble_alpha;   
    float bass_gain; // linear gains (converted from db)
    float treble_gain;
    float bass_prev; // internal filter memory
    float treble_prev;
} bass_and_treble_t;


void bass_and_treble_init( bass_and_treble_t *eq, float sample_rate, 
    float bass_freq, float bass_gain_db, float treble_freq, float treble_gain_db );

float bass_and_treble_process( bass_and_treble_t *eq, float sample );

#endif /* bass_and_treble_h */


#ifdef BASS_AND_TREBLE_IMPLEMENTATION
#undef BASS_AND_TREBLE_IMPLEMENTATION

#include <math.h>

void bass_and_treble_init( bass_and_treble_t *eq, float sample_rate, 
    float bass_freq, float bass_gain_db, float treble_freq, float treble_gain_db ) {

    const float PI = 3.14159265358979323846f;
    float dt = 1.0f / sample_rate;

    // compute time constants for bass (low-pass) and treble (high-pass)
    float rc_bass = 1.0f / ( 2.0f * PI * bass_freq );
    float rc_treble = 1.0f / ( 2.0f * PI * treble_freq );

    // convert cutoff frequencies into smoothing (alpha) factors
    eq->bass_alpha = dt / ( rc_bass + dt );
    eq->treble_alpha = dt / ( rc_treble + dt );

    // convert dB gains to linear
    eq->bass_gain = powf( 10.0f, bass_gain_db / 20.0f );
    eq->treble_gain = powf( 10.0f, treble_gain_db / 20.0f );

    // reset filter history
    eq->bass_prev = 0.0f;
    eq->treble_prev = 0.0f;
}


float bass_and_treble_process( bass_and_treble_t *eq, float sample ) {
    // bass (low frequencies) boost via one-pole low-pass
    eq->bass_prev += eq->bass_alpha * ( sample - eq->bass_prev );
    float bass_boosted = ( eq->bass_prev * ( eq->bass_gain - 1.0f ) );

    // treble (high frequencies) boost via simple high-pass
    float high_part = sample - eq->treble_prev;
    eq->treble_prev += eq->treble_alpha * high_part;
    float treble_boosted = ( high_part * ( eq->treble_gain - 1.0f ) );

    // mix original signal + boosted bass + boosted treble
    return sample + bass_boosted + treble_boosted;
}

#endif /* BASS_AND_TREBLE_IMPLEMENTATION */

stereo_widening.h

/*
------------------------------------------------------------------------------
          Licensing information can be found at the end of the file.
------------------------------------------------------------------------------

stereo_widening.h - v1.0 - Audio filter for a stereo widening effect.

Do this:
    #define STEREO_WIDENING_IMPLEMENTATION
before you include this file in *one* C/C++ file to create the implementation.
*/

#ifndef stereo_widening_h
#define stereo_widening_h

typedef struct stereo_widening_t {
    float width; // controls amount of widening

    // haas delay (left)
    float delay_left_buffer[ 441 ]; // ~10ms at 44.1 kHz
    int   delay_left_index;

    // haas delay (right)
    float delay_right_buffer[ 661 ]; // ~15ms at 44.1 kHz
    int   delay_right_index;

    // 4 allpass filters (left)
    float ap_left_buffer[ 4 ][ 484 ];
    int   ap_left_index[ 4 ];

    // 4 allpass filters (right)
    float ap_right_buffer[ 4 ][ 484 ];
    int   ap_right_index[ 4 ];
} stereo_widening_t;


void stereo_widening_init( stereo_widening_t* widen, float width );

void stereo_widening_process( stereo_widening_t* widen, float *left, float *right );


#endif /* stereo_widening_h */


#ifdef STEREO_WIDENING_IMPLEMENTATION
#undef STEREO_WIDENING_IMPLEMENTATION

#include <string.h>

void stereo_widening_init( stereo_widening_t* widen, float width ) {
    memset( widen, 0, sizeof( *widen ) );
    widen->width = width;
}


void stereo_widening_process( stereo_widening_t* widen, float *left, float *right ) {
    float in_left  = *left;
    float in_right = *right;

    // haas delay left
    float out_left = widen->delay_left_buffer[ widen->delay_left_index ];
    widen->delay_left_buffer[ widen->delay_left_index ] = in_left;
    widen->delay_left_index = ( widen->delay_left_index + 1 ) % 441;

    // haas delay right
    float out_right = widen->delay_right_buffer[ widen->delay_right_index ];
    widen->delay_right_buffer[ widen->delay_right_index ] = in_right;
    widen->delay_right_index = ( widen->delay_right_index + 1 ) % 661;

    // allpass chain left
    for( int i = 0; i < 4; ++i ) {
        float d = widen->ap_left_buffer[ i ][ widen->ap_left_index[ i ] ];
        float o = -0.6f * out_left + d;
        widen->ap_left_buffer[ i ][ widen->ap_left_index[ i ] ] = out_left + 0.6f * o;
        widen->ap_left_index[ i ] = ( widen->ap_left_index[ i ] + 1 ) % ( 220 + i * 88 );
        out_left = o;
    }

    // allpass chain right
    for( int i = 0; i < 4; ++i ) {
        float d = widen->ap_right_buffer[ i ][ widen->ap_right_index[ i ] ];
        float o = -0.6f * out_right + d;
        widen->ap_right_buffer[ i ][ widen->ap_right_index[ i ] ] = out_right + 0.6f * o;
        widen->ap_right_index[ i ] = ( widen->ap_right_index[ i ] + 1 ) % ( 220 + i * 88 );
        out_right = o;
    }

    float w = widen->width;
    *left  = in_left + w * out_left;
    *right = in_right + w * out_right;
}

#endif /* STEREO_WIDENING_IMPLEMENTATION */

/*
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This software is available under 2 licenses - you may choose the one you like.

------------------------------------------------------------------------------

ALTERNATIVE A - MIT License

Copyright (c) 2025 Mattias Gustavsson

Permission is hereby granted, free of charge, to any person obtaining a copy of
this software and associated documentation files (the "Software"), to deal in
the Software without restriction, including without limitation the rights to
use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
of the Software, and to permit persons to whom the Software is furnished to do
so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

------------------------------------------------------------------------------

ALTERNATIVE B - Public Domain (www.unlicense.org)

This is free and unencumbered software released into the public domain.

Anyone is free to copy, modify, publish, use, compile, sell, or distribute this
software, either in source code form or as a compiled binary, for any purpose,
commercial or non-commercial, and by any means.

In jurisdictions that recognize copyright laws, the author or authors of this
software dedicate any and all copyright interest in the software to the public
domain. We make this dedication for the benefit of the public at large and to
the detriment of our heirs and successors. We intend this dedication to be an
overt act of relinquishment in perpetuity of all present and future rights to
this software under copyright law.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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*/