Swift 2 : AVAssetReader and NSInputStream Audio Graph

Question

I'm trying to convert an example from Bob McCune's Learning AVFoundation book and having some issues using AVAssetReader and NSInputStream. The graph should be a pure sine wave but the values seem reflected on the X-axis somehow.

I've tried every iteration of byte swapping I could think of and that didn't work.

Playground posted to github here: https://github.com/justinlevi/AVAssetReader

//: Playground - noun: a place where people can play

import UIKit
import AVFoundation
import XCPlayground

func plotArrayInPlayground<T>(arrayToPlot:Array<T>, title:String) {
  for currentValue in arrayToPlot {
    XCPCaptureValue(title, value: currentValue)
  }
}

class SSSampleDataFilter {
  var sampleData:NSData?

  init(data:NSData) {
    sampleData = data
  }

  func filteredSamplesForSize(size:CGSize) -> [Int]{
    var filterSamples = [UInt16]()

    if let sampleData = sampleData {
      let sampleCount = sampleData.length
      let binSize = CGFloat(sampleCount) / size.width

      let stream = NSInputStream(data: sampleData)
      stream.open()

      var readBuffer = Array<UInt8>(count: 16 * 1024, repeatedValue: 0)
      var totalBytesRead = 0

      let size = sizeof(UInt16)
      while (totalBytesRead < sampleData.length) {
        let numberOfBytesRead = stream.read(&readBuffer, maxLength: size)
        let u16: UInt16 = UnsafePointer<UInt16>(readBuffer).memory

        var sampleBin = [UInt16]()
        for _ in 0..<Int(binSize) {
          sampleBin.append(u16)
        }

        filterSamples.append(sampleBin.maxElement()!)
        totalBytesRead += numberOfBytesRead
      }

      //plotArrayInPlayground(filterSamples, title: "Samples")
    }

    return [0]

  }
}

let sineURL = NSBundle.mainBundle().URLForResource("440.0-sine", withExtension: "aif")!
let asset = AVAsset(URL: sineURL)
var assetReader:AVAssetReader

do{
  assetReader = try AVAssetReader(asset: asset)
}catch{
  fatalError("Unable to read Asset: \(error) : \(__FUNCTION__).")
}

let track = asset.tracksWithMediaType(AVMediaTypeAudio).first
let outputSettings: [String:Int] =
  [ AVFormatIDKey: Int(kAudioFormatLinearPCM),
    AVLinearPCMIsBigEndianKey: 0,
    AVLinearPCMIsFloatKey: 0,
    AVLinearPCMBitDepthKey: 16,
    AVLinearPCMIsNonInterleaved: 0]

let trackOutput = AVAssetReaderTrackOutput(track: track!, outputSettings: outputSettings)

assetReader.addOutput(trackOutput)
assetReader.startReading()

var sampleData = NSMutableData()

while assetReader.status == AVAssetReaderStatus.Reading {
  if let sampleBufferRef = trackOutput.copyNextSampleBuffer() {
    if let blockBufferRef = CMSampleBufferGetDataBuffer(sampleBufferRef) {
      let bufferLength = CMBlockBufferGetDataLength(blockBufferRef)
      var data = NSMutableData(length: bufferLength)
      CMBlockBufferCopyDataBytes(blockBufferRef, 0, bufferLength, data!.mutableBytes)
      var samples = UnsafeMutablePointer<Int16>(data!.mutableBytes)
      sampleData.appendBytes(samples, length: bufferLength)
      CMSampleBufferInvalidate(sampleBufferRef)
    }
  }
}

let view = UIView(frame: CGRectMake(0, 0, 375.0, 667.0))
//view.backgroundColor = UIColor.lightGrayColor()

if assetReader.status == AVAssetReaderStatus.Completed {
  print("complete")

  let filter = SSSampleDataFilter(data: sampleData)
  let filteredSamples = filter.filteredSamplesForSize(view.bounds.size)
}

//XCPShowView("Bezier Path", view: view)
XCPSetExecutionShouldContinueIndefinitely(true)

Here's what the graph should look like (taken from Audacity)

Here's what the graph looks like in the playground

Answer 1

Unfortunately your playground doesn't render anything for me in Xcode7b5, however you're asking the AVAssetReaderTrackOutput to give you signed 16bit ints, yet your code treats them as unsigned UInt16s (and your Audacity file uses floats).

Changing all instances of UInt16 to Int16 in your playground seems to print sensible looking sinusoidal data.