Noise and distortion in Swift implementation of SDL audio callback

SDL Development
1

Also posted on Stack Overflow.

I’m currently building a game music player in Swift for Mac OS, using GME for generating sound buffers and SDL for audio playback. I have previously (and successfully) used SDL_QueueAudio for playback, but I need more fine tuned control over the buffer handling in order to determine song progress, song end etc. So, that leaves me with SDL_AudioCallback in the SDL_AudioSpec API.

I CAN hear the song play, but it’s accompanied by noise and distortion (and sometimes echo depending on what I try).

Setup

var desiredSpec = SDL_AudioSpec()
desiredSpec.freq = Int32(44100)
desiredSpec.format = SDL_AudioFormat(AUDIO_U8)
desiredSpec.samples = 512
desiredSpec.channels = 2
desiredSpec.callback = { callback(userData: $0, stream: $1, length: $2) }

SDL_OpenAudio(&desiredSpec, nil)

Callback

func callback(userData: UnsafeMutableRawPointer?, stream: UnsafeMutablePointer<UInt8>?, length: Int32) {
    // Half sample length to compensate for double byte Int16.
    var output = Array<Int16>.init(repeating: 0, count: Int(length / 2))

    // Generates stereo signed Int16 audio samples.
    gme_play(emulator, Int32(output.count), &output)

    // Converts Int16 samples to UInt8 samples expected by the mixer below. Then adds to buffer.
    var buffer = [UInt8]()
    for i in 0 ..< (output.count) {
        let uIntVal = UInt16(bitPattern: output[i])
        buffer.append(UInt8(uIntVal & 0xff))
        buffer.append(UInt8((uIntVal >> 8) & 0xff))
    }

    SDL_MixAudio(stream, buffer, Uint32(length), SDL_MIX_MAXVOLUME)
}
2

There are several problems with this code:

  1. Don’t call SDL_OpenAudio(), it’s deprecated. Call SDL_OpenAudioDevice() instead.

  2. You should be specifying AUDIO_S16LSB (or possibly AUDIO_S16MSB if it’s big-endian), not AUDIO_U8.

  3. The code commented as “Converts Int16 samples to UInt8 samples” can be removed, it doesn’t achieve anything (if the source samples are little-endian the output will be identical to the input).

  4. I don’t see any reason to call SDL_MixAudio() (or SDL_MixAudioFormat) since you’re not doing any mixing, don’t need to change the format and are not using it to change the level.

3

Hi!

Thanks for the reply!

  1. Check.
    1. My original approach was AUDIO_S16LSB when using SDL_QueueAudio. However, since SDL_MixAudio requires UInt8 I needed the conversion (Swift is picky with type).
  3. I have tried simply copying the buffer to the stream via SDL_memcpy, but that gives me mostly static, very little actual melody.
4

So, I fiddled around with SDL_memcpy a little more and by halving the requested number of samples in the callback (the length param) it actually works! Not sure why though, since format is now set to AUDIO_S16… :thinking:

5

SOLVED!
In the end this was enough to make it work:

var buffer = Array<Int16>(repeating: 0, count: Int(length / 2))
gme_play(emulator, Int32(output.count), &output)
SDL_memcpy(stream, buffer, Int(length))