Play audio on iOS from a memory data stream

Question

I am porting an audio library to iOS allowing to play audio streams fed from callbacks. The user provides a callback returning raw PCM data, and I need to have this data be played. Moreover, the library must be able to play multiple streams at once.

I figured I would need to use AVFoundation, but it seems like AVAudioPlayer does not support streamed audio buffers, and all the streaming documentation I could find used data coming directly from the network. What is the API I should use here?

Thanks in advance!

By the way, I am not using the Apple libraries through Swift or Objective-C. However I assume everything is exposed still, so an example in Swift would be greatly appreciated anyway!

https://developer.apple.com/library/archive/documentation/MusicAudio/Conceptual/AudioQueueProgrammingGuide/Introduction/Introduction.html — nakano531, Jun 23 '18 at 09:02
`AVAudioEngine` + `AVAudioPlayerNode` let's you play audio from buffers. Here's an example that uses a single player, with audio buffers taken from the microphone: https://stackoverflow.com/a/43670810/22147 — Rhythmic Fistman, Jun 24 '18 at 07:00
Thank you to all of you, Audio Queues are exactly what I needed! — Moxinilian, Jun 26 '18 at 10:44

score 1 · Answer 1 · answered Feb 14 '19 at 08:39

You need to initialise:

The Audio Session to use input audio unit and output.

-(SInt32) audioSessionInitialization:(SInt32)preferred_sample_rate {

    // - - - - - - Audio Session initialization
    NSError *audioSessionError = nil;
    session = [AVAudioSession sharedInstance];

    // disable AVAudioSession
    [session setActive:NO error:&audioSessionError];

    // set category - (PlayAndRecord to use input and output session AudioUnits)
   [session setCategory:AVAudioSessionCategoryPlayAndRecord withOptions:AVAudioSessionCategoryOptionDefaultToSpeaker error:&audioSessionError];

   double preferredSampleRate = 441000;
   [session setPreferredSampleRate:preferredSampleRate error:&audioSessionError];

   // enable AVAudioSession
   [session setActive:YES error:&audioSessionError];


   // Configure notification for device output change (speakers/headphones)
   [[NSNotificationCenter defaultCenter] addObserver:self
                                     selector:@selector(routeChange:)
                                         name:AVAudioSessionRouteChangeNotification
                                       object:nil];


   // - - - - - - Create audio engine
   [self audioEngineInitialization];

   return [session sampleRate];
 }

The Audio Engine

-(void) audioEngineInitialization{

    engine = [[AVAudioEngine alloc] init];
    inputNode = [engine inputNode];
    outputNode = [engine outputNode];

    [engine connect:inputNode to:outputNode format:[inputNode inputFormatForBus:0]];


    AudioStreamBasicDescription asbd_player;
    asbd_player.mSampleRate        = session.sampleRate;
    asbd_player.mFormatID            = kAudioFormatLinearPCM;
    asbd_player.mFormatFlags        = kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked;
    asbd_player.mFramesPerPacket    = 1;
    asbd_player.mChannelsPerFrame    = 2;
    asbd_player.mBitsPerChannel    = 16;
    asbd_player.mBytesPerPacket    = 4;
    asbd_player.mBytesPerFrame        = 4;

    OSStatus status;
    status = AudioUnitSetProperty(inputNode.audioUnit,
                              kAudioUnitProperty_StreamFormat,
                              kAudioUnitScope_Input,
                              0,
                              &asbd_player,
                              sizeof(asbd_player));


    // Add the render callback for the ioUnit: for playing
    AURenderCallbackStruct callbackStruct;
    callbackStruct.inputProc = engineInputCallback; ///CALLBACK///
    callbackStruct.inputProcRefCon = (__bridge void *)(self);
    status = AudioUnitSetProperty(inputNode.audioUnit,
                              kAudioUnitProperty_SetRenderCallback,
                              kAudioUnitScope_Input,//Global
                              kOutputBus,
                              &callbackStruct,
                              sizeof(callbackStruct));

    [engine prepare];
}

The Audio Engine callback

static OSStatus engineInputCallback(void *inRefCon,
                             AudioUnitRenderActionFlags *ioActionFlags,
                             const AudioTimeStamp *inTimeStamp,
                             UInt32 inBusNumber,
                             UInt32 inNumberFrames,
                             AudioBufferList *ioData)
{

    // the reference to the audio controller where you get the stream data
    MyAudioController *ac = (__bridge MyAudioController *)(inRefCon);

    // in practice we will only ever have 1 buffer, since audio format is mono
    for (int i = 0; i < ioData->mNumberBuffers; i++) { 
        AudioBuffer buffer = ioData->mBuffers[i];

        // copy stream buffer data to output buffer
        UInt32 size = min(buffer.mDataByteSize, ac.playbackBuffer.mDataByteSize); 
        memcpy(buffer.mData, ac.streamBuffer.mData, size);
        buffer.mDataByteSize = size; // indicate how much data we wrote in the buffer
    }

    return noErr;
}

Play audio on iOS from a memory data stream

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