Audio cracking with MIX_SetTrackFrequencyRatio below 1.0

[danilwhale]

Trying to play 44.1kHz mono(?) track produces noticeable crack in certain cases. Audio file I was able to reproduce this with:

wood2.zip

Minimal reproduction code:

#include <SDL3/SDL.h>
#include <SDL3_mixer/SDL_mixer.h>
#include <stdio.h>
#include <stdbool.h>

int main()
{
    if (!SDL_Init(SDL_INIT_AUDIO)) { printf("failed to init audio: %s", SDL_GetError()); return 1; }
    if (!MIX_Init()) { printf("failed to init mixer: %s", SDL_GetError()); return 1; }
    
    SDL_AudioSpec spec;
    if (!SDL_GetAudioDeviceFormat(SDL_AUDIO_DEVICE_DEFAULT_PLAYBACK, &spec, NULL)) { printf("failed to get format: %s", SDL_GetError()); return 1; }
    
    // enforce 44.1khz, which workarounds other bug with 44.1khz track cracking on 48khz or 96khz mixer
    spec.freq = 44100;
    MIX_Mixer* mixer = MIX_CreateMixerDevice(SDL_AUDIO_DEVICE_DEFAULT_PLAYBACK, &spec);
    if (!mixer) { printf("failed to create mixer: %s", SDL_GetError()); return 1; }
    
    MIX_Audio* audio = MIX_LoadAudio(mixer, "wood2.ogg", true);
    if (!audio) { printf("failed to load audio: %s", SDL_GetError()); return 1; }
    
    MIX_Track* track = MIX_CreateTrack(mixer);
    if (!track) { printf("failed to create track: %s", SDL_GetError()); return 1; }
    
    MIX_SetTrackAudio(track, audio);
    MIX_SetTrackFrequencyRatio(track, 0.8F); // <--- causes earlier mentioned crack when playing
    if (!MIX_PlayTrack(track, 0)) { printf("failed to play track: %s", SDL_GetError()); return 1; }
    
    while (MIX_TrackPlaying(track)) {}
    
    MIX_DestroyTrack(track);
    MIX_DestroyAudio(audio);    
    MIX_DestroyMixer(mixer);
    MIX_Quit();
    
    return 0; 
}

[icculus]

This doesn't reproduce for me:

Top is the (mono) Ogg Vorbis audio, bottom two is the audio converted to stereo.

I don't see anything that looks like a significant spike in the output, and can't hear it either.

Is it possible, on such a short sample, that your audio hardware crackles a little bit at startup, and this wouldn't persist in a longer playback?

[danilwhale]

I can very clearly hear the crack when creating new MIX_Track in a loop, though not when reusing the same one:

    while (true) {
        MIX_Track* track = MIX_CreateTrack(mixer);
        if (!track) { printf("failed to create track: %s", SDL_GetError()); return 1; }
        
        MIX_SetTrackAudio(track, audio);
        MIX_SetTrackFrequencyRatio(track, 0.8F); // <--- causes very noticeable crack when playing
        
        if (!MIX_PlayTrack(track, 0)) { printf("failed to play track: %s", SDL_GetError()); return 1; }
        while (MIX_TrackPlaying(track)) {}
        
        MIX_DestroyTrack(track);
    }
    

2026-07-09.23-01-18.mp4

I tried investigating this crack earlier by comparing recording audio track and original audio in Tenacity/Audacity, but I didn't see anything notable, too.