Updated source code in question to Swift 4 and wrapped it in a class. Credit goes to Castles and Rythmic Fistman for source code and response. The rest of the author’s comments added a few assertion and print statements for debugging. Tested on iOS.
The transfer rate for the output file is hard-coded at 96 kbps, you can easily override this value. Most of the audio files that I convert are 320kb / s, so I use this class to compress files for offline storage. Compression results are at the bottom of this answer.
Using:
let inputFilePath = URL(fileURLWithPath: "/path/to/file.mp3") let outputFileURL = URL(fileURLWithPath: "/path/to/output/compressed.mp4") if let audioConverter = AVAudioFileConverter(inputFileURL: inputFilePath, outputFileURL: outputFileURL) { audioConverter.convert() }
Class
import AVFoundation final class AVAudioFileConverter { var rwAudioSerializationQueue: DispatchQueue! var asset:AVAsset! var assetReader:AVAssetReader! var assetReaderAudioOutput:AVAssetReaderTrackOutput! var assetWriter:AVAssetWriter! var assetWriterAudioInput:AVAssetWriterInput! var outputURL:URL var inputURL:URL init?(inputFileURL: URL, outputFileURL: URL) { inputURL = inputFileURL outputURL = outputFileURL if (FileManager.default.fileExists(atPath: inputURL.absoluteString)) { print("Input file does not exist at file path \(inputURL.absoluteString)") return nil } } func convert() { let rwAudioSerializationQueueDescription = " rw audio serialization queue" // Create the serialization queue to use for reading and writing the audio data. rwAudioSerializationQueue = DispatchQueue(label: rwAudioSerializationQueueDescription) assert(rwAudioSerializationQueue != nil, "Failed to initialize Dispatch Queue") asset = AVAsset(url: inputURL) assert(asset != nil, "Error creating AVAsset from input URL") print("Output file path -> ", outputURL.absoluteString) asset.loadValuesAsynchronously(forKeys: ["tracks"], completionHandler: { var success = true var localError:NSError? success = (self.asset.statusOfValue(forKey: "tracks", error: &localError) == AVKeyValueStatus.loaded) // Check for success of loading the assets tracks. if (success) { // If the tracks loaded successfully, make sure that no file exists at the output path for the asset writer. let fm = FileManager.default let localOutputPath = self.outputURL.path if (fm.fileExists(atPath: localOutputPath)) { do { try fm.removeItem(atPath: localOutputPath) success = true } catch { print("Error trying to remove output file at path -> \(localOutputPath)") } } } if (success) { success = self.setupAssetReaderAndAssetWriter() } else { print("Failed setting up Asset Reader and Writer") } if (success) { success = self.startAssetReaderAndWriter() return } else { print("Failed to start Asset Reader and Writer") } }) } func setupAssetReaderAndAssetWriter() -> Bool { do { assetReader = try AVAssetReader(asset: asset) } catch { print("Error Creating AVAssetReader") } do { assetWriter = try AVAssetWriter(outputURL: outputURL, fileType: AVFileType.m4a) } catch { print("Error Creating AVAssetWriter") } var assetAudioTrack:AVAssetTrack? = nil let audioTracks = asset.tracks(withMediaType: AVMediaType.audio) if (audioTracks.count > 0) { assetAudioTrack = audioTracks[0] } if (assetAudioTrack != nil) { let decompressionAudioSettings:[String : Any] = [ AVFormatIDKey:Int(kAudioFormatLinearPCM) ] assetReaderAudioOutput = AVAssetReaderTrackOutput(track: assetAudioTrack!, outputSettings: decompressionAudioSettings) assert(assetReaderAudioOutput != nil, "Failed to initialize AVAssetReaderTrackOutout") assetReader.add(assetReaderAudioOutput) var channelLayout = AudioChannelLayout() memset(&channelLayout, 0, MemoryLayout<AudioChannelLayout>.size); channelLayout.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo; let outputSettings:[String : Any] = [ AVFormatIDKey: Int(kAudioFormatMPEG4AAC), AVSampleRateKey: 44100, AVEncoderBitRateKey: 96000, AVNumberOfChannelsKey: 2, AVChannelLayoutKey: NSData(bytes:&channelLayout, length:MemoryLayout<AudioChannelLayout>.size)] assetWriterAudioInput = AVAssetWriterInput(mediaType: AVMediaType.audio, outputSettings: outputSettings) assert(rwAudioSerializationQueue != nil, "Failed to initialize AVAssetWriterInput") assetWriter.add(assetWriterAudioInput) } print("Finsihed Setup of AVAssetReader and AVAssetWriter") return true } func startAssetReaderAndWriter() -> Bool { print("STARTING ASSET WRITER") assetWriter.startWriting() assetReader.startReading() assetWriter.startSession(atSourceTime: kCMTimeZero) assetWriterAudioInput.requestMediaDataWhenReady(on: rwAudioSerializationQueue, using: { while(self.assetWriterAudioInput.isReadyForMoreMediaData ) { var sampleBuffer = self.assetReaderAudioOutput.copyNextSampleBuffer() if(sampleBuffer != nil) { self.assetWriterAudioInput.append(sampleBuffer!) sampleBuffer = nil } else { self.assetWriterAudioInput.markAsFinished() self.assetReader.cancelReading() self.assetWriter.finishWriting { print("Asset Writer Finished Writing") } break } } }) return true } }
Input file: 17.3 MB
// generated with afinfo on mac File: D290A73C37B777F1.mp3 File type ID: MPG3 Num Tracks: 1
Output file: 5.1 MB
// generated with afinfo on Mac File: compressed.m4a File type ID: m4af Num Tracks: 1