Merging audio with AVAssetExportSession - ios

Question

I'd like to merge two audio files without too much overhead time. The following code successfully merges the audio but takes way too long (>30seconds for more than a couple minutes of audio), and I'm wondering if there is any way to expedite that process. I read in a couple places to use AVAssetExportPresetPassthrough, but I can't seem to get that preset to work with any file type. The only settings I've been able to get to work are using AVAssetExportPresetAppleM4A and exporting as a .m4a
Code to create AVAssetExportSession
if (audioHasBeenRecorded) {
// Merge recordings
let composition = AVMutableComposition()
let compositionAudioTrack = composition.addMutableTrack(withMediaType: AVMediaType.audio, preferredTrackID: kCMPersistentTrackID_Invalid)
guard let start = recorder.timeStart else { return }
compositionAudioTrack?.insert(originalRecording: FileManagerHelper.recordingLocalURL(secondRecording: false), insertedRecording: FileManagerHelper.recordingLocalURL(secondRecording: true), startTime: CMTime(seconds: start, preferredTimescale: 1000000))
if let assetExport = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetAppleM4A) {
print(assetExport.supportedFileTypes)
assetExport.outputFileType = AVFileType.m4a
assetExport.outputURL = FileManagerHelper.recordingLocalURL(secondRecording: false)
do { // Delete old audio
try FileManager.default.removeItem(at: FileManagerHelper.recordingLocalURL(secondRecording: false))
try FileManager.default.removeItem(at: FileManagerHelper.recordingLocalURL(secondRecording: true))
} catch { log(error.localizedDescription, msgType: .error) }
assetExport.exportAsynchronously(completionHandler: {
if let error = assetExport.error {
log(error, msgType: .error)
} else {
log("Successfully merged recordings!", msgType: .error)
self.idea.numberOfPoints = self.audioVisualizer.count
self.idea.save()
self.setupPlayer() // Prepare to play the recorded audio file
self.seekTo(TimeInterval((recorder.timeStart ?? 0) + (recorder.timeEnd ?? 0)))
DispatchQueue.main.async { [ weak self ] in
guard let self = self else { return }
self.audioVisualizer.visualize(self.idea)
}
}
})
}
}
Insert code:
extension AVMutableCompositionTrack {
func insert(originalRecording: URL, insertedRecording: URL, startTime: CMTime) {
let originalAsset = AVURLAsset(url: originalRecording)
let insertedAsset = AVURLAsset(url: insertedRecording)
let range1 = CMTimeRangeMake(start: CMTime.zero, duration: startTime)
let range2 = CMTimeRangeMake(start: CMTime.zero, duration: insertedAsset.duration)
let range3 = CMTimeRangeMake(start: startTime + insertedAsset.duration, duration: originalAsset.duration - startTime)
if let originalTrack = originalAsset.tracks(withMediaType: AVMediaType.audio).first,
let insertedTrack = insertedAsset.tracks(withMediaType: AVMediaType.audio).first {
try? insertTimeRange(range1, of: originalTrack, at: CMTime.zero)
try? insertTimeRange(range2, of: insertedTrack, at: startTime)
try? insertTimeRange(range3, of: originalTrack, at: startTime + insertedAsset.duration)
}
}
}