How to use the audio equalizer in LibVLC

equalizer.c

/**
 * A simple program to test the audio equalizer API in LibVLC.
 *
 * An example build command:
 *
 * $gcc -std=c99 -I/home/linux/vlc/vlc/include -o equalizer equalizer.c `pkg-config --cflags --libs libvlc`
 *
 * You may also need to set PKG_CONFIG_PATH first, for example:
 *
 * $export PKG_CONFIG_PATH=/home/linux/vlc/install/lib/pkgconfig
 *
 * When running this test, you must pass via the command-line two separate audio filenames
 * to be played during the test - for example:
 *
 * ./equalizer /home/music/1.mp3 /home/music/2.mp3
 *
 * You may also need to set LD_LIBARRY_PATH, for example:
 *
 * $export LD_LIBRARY_PATH=/home/linux/vlc/install/lib
 *
 * mark dot lee at capricasofware.co.uk
 */

#include <unistd.h>
#include <vlc/vlc.h>
#include <vlc/libvlc.h>

void dump(libvlc_equalizer_t *p_equalizer);

int main(int argc, char *argv[]) {

    if(argc != 3) {
        printf("Specify two audio file names\n");
        return -1;
    }

    char *psz_file1 = argv[1];
    char *psz_file2 = argv[2];

    libvlc_instance_t *p_instance = libvlc_new(0, NULL);

    printf("Equalizer Tests\n\n");

    // Dump out the individual frequency bands
    printf("Bands...\n");
    unsigned u_bands = libvlc_audio_equalizer_get_band_count();
    for(int i = 0; i < u_bands; i++) {
        printf(" Band %2d -> %.1fHz\n", i, libvlc_audio_equalizer_get_band_frequency(i));
    }
    printf("\n");

    // Dump out each preset name
    printf("Presets...\n");
    unsigned u_presets = libvlc_audio_equalizer_get_preset_count();
    for(int i = 0; i < u_presets; i++) {
        printf(" Preset %2d -> %s\n", i, libvlc_audio_equalizer_get_preset_name(i));
    }
    printf("\n");

    // Dump out the settings for each preset
    for(int i = 0; i < u_presets; i++) {
        const char* psz_name = libvlc_audio_equalizer_get_preset_name(i);
        printf("%d %s\n", i, psz_name);
        libvlc_equalizer_t *p_equalizer = libvlc_audio_equalizer_new_from_preset(i);
        dump(p_equalizer);
        libvlc_audio_equalizer_release(p_equalizer);
    }

    // Now some tests with the equalizer and a media player...

    printf("Starting tests...\n\n");

    libvlc_media_player_t *p_media_player = libvlc_media_player_new(p_instance);

    libvlc_media_t *p_media = libvlc_media_new_path(p_instance, psz_file1);
    libvlc_media_player_set_media(p_media_player, p_media);
    libvlc_media_release(p_media);

    // sleep(i_delay) is used just because it is convenient for this test, of course you wouldn't do that normally
    int i_delay = 5; // seconds

    // Presets...

    libvlc_equalizer_t *p_equalizer_full_treble = libvlc_audio_equalizer_new_from_preset(6);
    libvlc_equalizer_t *p_equalizer_full_bass   = libvlc_audio_equalizer_new_from_preset(4);

    // It is possible to set an equalizer before playing media
    printf("Set full treble\n");
    libvlc_media_player_set_equalizer(p_media_player, p_equalizer_full_treble);

    // Now play...
    printf("Play\n");
    libvlc_media_player_play(p_media_player);
    sleep(i_delay);

    // Change equalizer while playing
    printf("Set full bass\n");
    libvlc_media_player_set_equalizer(p_media_player, p_equalizer_full_bass);
    sleep(i_delay);

    printf("Set full treble\n");
    libvlc_media_player_set_equalizer(p_media_player, p_equalizer_full_treble);
    sleep(i_delay);

    // Stop playback
    printf("Stop\n");
    libvlc_media_player_stop(p_media_player);
    sleep(1);

    // Start playback, settings should be remembered
    printf("Play full treble\n");
    libvlc_media_player_play(p_media_player);
    sleep(i_delay);

    // Change equalizer while playing
    printf("Set full bass\n");
    libvlc_media_player_set_equalizer(p_media_player, p_equalizer_full_bass);
    sleep(i_delay);

    printf("Set full treble\n");
    libvlc_media_player_set_equalizer(p_media_player, p_equalizer_full_treble);
    sleep(i_delay);

    // Change media
    printf("Change media\n");
    p_media = libvlc_media_new_path(p_instance, psz_file2);
    libvlc_media_player_set_media(p_media_player, p_media);
    libvlc_media_release(p_media);

    // Play new media, settings should be remembered
    libvlc_media_player_play(p_media_player);

    sleep(i_delay);

    // Change equalizer while playing
    printf("Set full bass\n");
    libvlc_media_player_set_equalizer(p_media_player, p_equalizer_full_bass);
    sleep(i_delay);

    printf("Set full treble\n");
    libvlc_media_player_set_equalizer(p_media_player, p_equalizer_full_treble);
    sleep(i_delay);

    printf("Disable equalizer\n");
    libvlc_media_player_set_equalizer(p_media_player, NULL);
    sleep(i_delay);

    printf("Set full treble\n");
    libvlc_media_player_set_equalizer(p_media_player, p_equalizer_full_treble);
    sleep(i_delay);

    printf("Disable equalizer\n");
    libvlc_media_player_set_equalizer(p_media_player, NULL);
    sleep(i_delay);

    // We're finished with the presets, they are no longer associated with any
    // media player, so release them
    libvlc_audio_equalizer_release(p_equalizer_full_bass);
    libvlc_audio_equalizer_release(p_equalizer_full_treble);

    libvlc_media_player_stop(p_media_player);

    // Custom...

    libvlc_equalizer_t *p_equalizer = libvlc_audio_equalizer_new();
    libvlc_audio_equalizer_set_preamp(p_equalizer, 6.5f);
    libvlc_audio_equalizer_set_amp_at_index(p_equalizer, 20.0f, 1);
    libvlc_audio_equalizer_set_amp_at_index(p_equalizer, 15.3f, 7);
    libvlc_audio_equalizer_set_amp_at_index(p_equalizer, 16.2f, 8);
    libvlc_audio_equalizer_set_amp_at_index(p_equalizer, 17.1f, 9);

    // Set a new custom equalizer before playback starts
    printf("Set custom\n");
    libvlc_media_player_set_equalizer(p_media_player, p_equalizer);

    printf("Play\n");
    libvlc_media_player_play(p_media_player);
    sleep(i_delay);

    // Change the custom equalizer while playing
    printf("Change custom\n");
    libvlc_audio_equalizer_set_preamp(p_equalizer, 8.75f);
    libvlc_audio_equalizer_set_amp_at_index(p_equalizer, 20.0f, 4);
    libvlc_audio_equalizer_set_amp_at_index(p_equalizer, 20.0f, 5);
    libvlc_media_player_set_equalizer(p_media_player, p_equalizer);
    sleep(i_delay);

    printf("Disable equalizer\n");
    libvlc_media_player_set_equalizer(p_media_player, NULL);
    sleep(i_delay);

    printf("End of test\n");

    libvlc_audio_equalizer_release(p_equalizer);

    libvlc_media_player_stop(p_media_player);

    libvlc_media_player_release(p_media_player);

    libvlc_release(p_instance);
    return 0;
}

void dump(libvlc_equalizer_t *p_equalizer) {
    if(p_equalizer) {
        unsigned u_bands = libvlc_audio_equalizer_get_band_count();
        printf(" Preamp: %18.2fdB\n", libvlc_audio_equalizer_get_preamp(p_equalizer));
        for(unsigned i = 0; i < u_bands; i++) {
           printf("   Band: %7.1fHz -> %5.2fdB\n", libvlc_audio_equalizer_get_band_frequency(i), libvlc_audio_equalizer_get_amp_at_index(p_equalizer, i));
        }
    }
    else {
        printf("No equalizer.\n");
    }
    printf("\n");
}

This gist doesn't really relate to your question.

What you're asking about can be done using various LibVLC language bindings that give you the raw audio samples via a callback. However, if you use those audio callbacks to do your frequency analysis you also have to play those samples yourself somehow - in this case VLC does not send the audio to any output.