SDL_Mixer SDL_AudioSpec and tracks

[TheCycoONE]

The basics:
SDL3 3.4.12
SDL_mixer 3.2.4

OS: Arch Linux
Arch: x86_64

I allow users to specify the frequency and number of channels in a settings file which I'm passing in like (abbreviated):

SDL_AudioSpec spec{SDL_AUDIO_F32, channels, freq};
MIX_CreateMixerDevice(SDL_AUDIO_DEVICE_DEFAULT_PLAYBACK, &spec));

Later on I prepare a raw audio track to stream audio too from ffmpeg with code like this (edited for brevity, remove c++isms):

 SDL_AudioSpec audio_spec;
 MIX_GetMixerFormat(mixer, &audio_spec); // <-- problem line
  mixer_frequency = audio_spec.freq;
  mixer_channels = audio_spec.channels;
  ...
  swr_alloc_set_opts2(&audio_resample_context, ch_layout.get(),
                      AV_SAMPLE_FMT_FLT, mixer_frequency,
                      &(audio_codec_context->ch_layout),
                      audio_codec_context->sample_fmt,
                      audio_codec_context->sample_rate, 0, nullptr);
 ...
 empty_audio_chunk = MIX_LoadRawAudioNoCopy(mixer, audio_chunk_buffer.data(),
                             audio_chunk_buffer.size(), &audio_spec, false);
 audio_track = MIX_CreateTrack(mixer);
 MIX_SetTrackAudio(audio_track, empty_audio_chunk);

  SDL_PropertiesID playProps = SDL_CreateProperties();
  SDL_SetNumberProperty(playProps, MIX_PROP_PLAY_LOOPS_NUMBER, -1);
  MIX_PlayTrack(audio_track, playProps);
  SDL_DestroyProperties(playProps);
  MIX_SetTrackCookedCallback(audio_track, movie_audio_callback, this);

Now that audio_spec I get from MIX_GetMixerFormat ignores the frequency I set and always returns 44100; which is not unexpected because that's what the system really supports. MIX_GetAudioFormat also returns 44100; but the spec that is included in the callback has my configured value e.g. 22050, and the number of samples is only enough for that frequency, resulting in the wrong playback speed.

I know from running the debugger that MIX_Mixer retains the 22050 and passes it to the track. What I haven't figured out is how I could query that, or better yet override the track's spec to match the hardware since I'm resampling the movie audio anyway.

[icculus]

Okay, I spent a bunch of time figuring out what this does, and deciding if this is what I intended it to do, and here's the deal:

MIX_CreateMixerDevice() is creating an audio stream internally, and no matter what the hardware decides to accept from the output end of that stream, the input end is always set to what you asked for. The intention, I think, was that no matter what the hardware wants, this lets you say "most of my input will be in this format, so let's operate there before final conversion to the hardware format" to avoid extra conversions on each track, etc.

So, at a minimum, this is a documentation issue, but it's possible MIX_GetMixerFormat should return the input format instead of the output format (which, if you passed a NULL spec when creating the mixer, would be the same, and would still fulfill the question "what format did I actually get?" in that case), or we should add a new API to change the input format after opening, or query it explicitly, etc.

I'm going to think on this a bit.

[TheCycoONE]

In my case I'll drop the user configurable frequency / channel settings, since they don't do what a user would expect. There's no reasons for users to configure the input format and these days there isn't much reason to hint the output format either.

Thanks for the consideration.