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I'm having trouble with the equalizer。I went through the equalizer api documentation and did not find a solution。Please help me find out the problem。The following is part of my code。

libvlc_audio_set_channel(vlc_player,1 );
libvlc_equalizer_t *equalizer = libvlc_audio_equalizer_new_from_preset(1);
int cc = libvlc_audio_equalizer_set_preamp(equalizer, 20.0);
int band_count = libvlc_audio_equalizer_get_band_count();
for (int i = 0; i < band_count; i++) {
cc = libvlc_audio_equalizer_set_amp_at_index(equalizer, 20.0, i);
cc = 2;
}
cc = libvlc_media_player_set_equalizer(vlc_player, equalizer);
libvlc_audio_equalizer_release(equalizer);

Viewing the return values is successful, but the volume does not increase. very funny！！！