How do you save generated audio to a file in iOS?

Question

I have successfully generated a tone using iOS with the following code. After that, I want to save the generated tone to an audio file. How can I do this?

 - (void)createToneUnit
    {
        // Configure the search parameters to find the default playback output unit
        // (called the kAudioUnitSubType_RemoteIO on iOS but
        // kAudioUnitSubType_DefaultOutput on Mac OS X)
        AudioComponentDescription defaultOutputDescription;
        defaultOutputDescription.componentType = kAudioUnitType_Output;
        defaultOutputDescription.componentSubType = kAudioUnitSubType_RemoteIO;
        defaultOutputDescription.componentManufacturer = kAudioUnitManufacturer_Apple;
        defaultOutputDescription.componentFlags = 0;
        defaultOutputDescription.componentFlagsMask = 0;

    // Get the default playback output unit
    AudioComponent defaultOutput = AudioComponentFindNext(NULL, &defaultOutputDescription);
    NSAssert(defaultOutput, @"Can't find default output");

    // Create a new unit based on this that we'll use for output
    OSErr err = AudioComponentInstanceNew(defaultOutput, &toneUnit);
    NSAssert1(toneUnit, @"Error creating unit: %ld", err);

    // Set our tone rendering function on the unit
    AURenderCallbackStruct input;
    input.inputProc = RenderTone;
    input.inputProcRefCon = (__bridge void *)(self);
    err = AudioUnitSetProperty(toneUnit,
                               kAudioUnitProperty_SetRenderCallback,
                               kAudioUnitScope_Input,
                               0,
                               &input,
                               sizeof(input));
    NSAssert1(err == noErr, @"Error setting callback: %ld", err);

    // Set the format to 32 bit, single channel, floating point, linear PCM
    const int four_bytes_per_float = 4;
    const int eight_bits_per_byte = 8;
    AudioStreamBasicDescription streamFormat;
    streamFormat.mSampleRate = sampleRate;
    streamFormat.mFormatID = kAudioFormatLinearPCM;
    streamFormat.mFormatFlags =
    kAudioFormatFlagsNativeFloatPacked | kAudioFormatFlagIsNonInterleaved;
    streamFormat.mBytesPerPacket = four_bytes_per_float;
    streamFormat.mFramesPerPacket = 1;
    streamFormat.mBytesPerFrame = four_bytes_per_float;
    streamFormat.mChannelsPerFrame = 1;
    streamFormat.mBitsPerChannel = four_bytes_per_float * eight_bits_per_byte;


    err = AudioUnitSetProperty (toneUnit,
                                kAudioUnitProperty_StreamFormat,
                                kAudioUnitScope_Input,
                                0,
                                &streamFormat,
                                sizeof(AudioStreamBasicDescription));



    NSAssert1(err == noErr, @"Error setting stream format: %ld", err);
}

The Render Code :

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

{
    // Fixed amplitude is good enough for our purposes
    const double amplitude = 0.25;

    // Get the tone parameters out of the view controller
    ViewController *viewController =
    (__bridge ViewController *)inRefCon;
    double theta = viewController->theta;
    double theta_increment = 2.0 * M_PI * viewController->frequency / viewController->sampleRate;

    // This is a mono tone generator so we only need the first buffer
    const int channel = 0;
    Float32 *buffer = (Float32 *)ioData->mBuffers[channel].mData;

    // Generate the samples
    for (UInt32 frame = 0; frame < inNumberFrames; frame++)
    {
        buffer[frame] = sin(theta) * amplitude;

        theta += theta_increment;
        if (theta > 2.0 * M_PI)
        {
            theta -= 2.0 * M_PI;
        }
    }

    // Store the theta back in the view controller
    viewController->theta = theta;

    return noErr;
}

And to play the generated tone, I just :

OSErr err = AudioUnitInitialize(toneUnit);
err = AudioOutputUnitStart(toneUnit);

Answer 1

The extended audio file api provides an easy way to write audio files to disk.