I am trying to convert an example from Bob McCune Learning AVFoundation and have some problems using AVAssetReader and NSInputStream. The graph should be a pure sine wave, but the values are somehow reflected on the X axis.
I tried every iteration of exchanging bytes that 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)
This should look like a graph (taken from Audacity) 
Here's what the chart looks like on the playground
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