Converting M4A file into Raw data - ios

Question

I am trying to read the raw values of a sound file. I am pretty new to IOS development. I am ultimately trying to take a fast fourier transform of the audio file. The output of the data looks like a sound wave, but when I take the fft of a beep sound provided hereenter link description here I do not get an obvious frequency from the fft, which leads me to believe I am not getting the real raw data. I have constructed the following code using a combination of several stack overflow posts. Am I reading the file incorrectly?
class AudioAnalyzer {
init(file_path: NSURL) {
var assetOptions = [
AVURLAssetPreferPreciseDurationAndTimingKey : 1,
AVFormatIDKey : kAudioFormatLinearPCM
]
println(file_path)
var videoAsset=AVURLAsset(URL: file_path, options: assetOptions)
var error:NSError?
var videoAssetReader=AVAssetReader(asset: videoAsset, error: &error)
if error != nil
{
println(error)
}
var tracksArray=videoAsset?.tracksWithMediaType(AVMediaTypeAudio)
var videotrack = tracksArray?[0] as! AVAssetTrack
var fps = videotrack.nominalFrameRate
var videoTrackOutput=AVAssetReaderTrackOutput(track:videotrack as AVAssetTrack , outputSettings: nil)
if videoAssetReader.canAddOutput(videoTrackOutput)
{
videoAssetReader.addOutput(videoTrackOutput)
videoAssetReader.startReading()
}
if videoAssetReader.status == AVAssetReaderStatus.Reading {
var sampleBuffer = videoTrackOutput.copyNextSampleBuffer()
var audioBuffer = CMSampleBufferGetDataBuffer(sampleBuffer)
let samplesInBuffer = CMSampleBufferGetNumSamples(sampleBuffer)
var currentZ = Double(samplesInBuffer)
let buffer: CMBlockBufferRef = CMSampleBufferGetDataBuffer(sampleBuffer)
var lengthAtOffset: size_t = 0
var totalLength: size_t = 0
var data: UnsafeMutablePointer<Int8> = nil
var output: Array<Float> = [];
if( CMBlockBufferGetDataPointer( buffer, 0, &lengthAtOffset, &totalLength, &data ) != noErr ) {
println("some sort of error happened")
} else {
for i in stride(from: 0, to: totalLength, by: 2) {
var myint = Int16(data[i]) << 8 | Int16(data[i+1])
var myFloat = Float(myint)
output.append(myFloat);
}
println(output)
}
}
}
}

Answer 1

Your AVAssetReaderTrackOutput is giving you raw packet data. For LPCM output, pass in some outputSettings:
var settings = [NSObject : AnyObject]()
settings[AVFormatIDKey] = kAudioFormatLinearPCM
settings[AVLinearPCMBitDepthKey] = 16
settings[AVLinearPCMIsFloatKey] = false
var videoTrackOutput=AVAssetReaderTrackOutput(track:videotrack as AVAssetTrack , outputSettings: settings)
p.s. I'd feel much better if you renamed videoTrackOutput to audioTrackOutput.