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");
}

Is there a way to get live frequency data for a visualizer, or is this strictly meant to control your media?

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.