AVAudioConverter.convertToBuffer throwing error code -50

Question

I need to convert a .wav file recorded with 2 audio channels to a .wav that has only 1 channel, as well as reduce the bit depth from 32 to 16. I've been trying to use AVAudioConverter.convertToBuffer However, the conversion is throwing an error: Error Domain=NSOSStatusErrorDomain Code=-50 "(null)"

Basically, the only thing that really needs to change is to strip the audio down to a single channel, and the bit depth. I'm getting these files from a different tool, so I can't just change the way the files are recorded.

I'm not that awesome at working with audio, and I'm a bit stumped. The code I'm working on is below - is there anything I'm missing?

let inAudioFileURL:NSURL = <url_to_wav_file>

var inAudioFile:AVAudioFile?
do
{
    inAudioFile = try AVAudioFile(forReading: inAudioFileURL)
}
catch let error
{
    print ("error: \(error)")
}

let inAudioFormat:AVAudioFormat = inAudioFile!.processingFormat
let inFrameCount:UInt32 = UInt32(inAudioFile!.length)

let inAudioBuffer:AVAudioPCMBuffer = AVAudioPCMBuffer(PCMFormat: inAudioFormat, frameCapacity: inFrameCount)

do
{
    try inAudioFile!.readIntoBuffer(inAudioBuffer)
}
catch let error
{
    print ("readError: \(error)")
}

let startFormat:AVAudioFormat = AVAudioFormat.init(settings: inAudioFile!.processingFormat.settings)
print ("startFormat: \(startFormat.settings)")

var endFormatSettings = startFormat.settings
endFormatSettings[AVLinearPCMBitDepthKey] = 16
endFormatSettings[AVNumberOfChannelsKey] = 1
endFormatSettings[AVEncoderAudioQualityKey] = AVAudioQuality.Medium.rawValue
print ("endFormatSettings: \(endFormatSettings)")


let endFormat:AVAudioFormat = AVAudioFormat.init(settings: endFormatSettings)
let outBuffer = AVAudioPCMBuffer(PCMFormat: endFormat, frameCapacity: inFrameCount)

let avConverter:AVAudioConverter = AVAudioConverter.init(fromFormat: startFormat, toFormat: endFormat)

do
{
    try avConverter.convertToBuffer(outBuffer, fromBuffer: inAudioBuffer)
}
catch let error
{
    print ("avconverterError: \(error)")
}

As for the output:

startFormat: 
  ["AVSampleRateKey": 16000,
  "AVLinearPCMBitDepthKey": 32,
  "AVLinearPCMIsFloatKey": 1,
  "AVNumberOfChannelsKey": 2,
  "AVFormatIDKey": 1819304813,
  "AVLinearPCMIsNonInterleaved": 0,
  "AVLinearPCMIsBigEndianKey": 0]

endFormatSettings:
["AVSampleRateKey": 16000,
"AVLinearPCMBitDepthKey": 16,
"AVLinearPCMIsFloatKey": 1,
"AVNumberOfChannelsKey": 1,
"AVFormatIDKey": 1819304813,
"AVLinearPCMIsNonInterleaved": 0,
"AVLinearPCMIsBigEndianKey": 0,
"AVEncoderQualityKey": 64]

avconverterError: Error Domain=NSOSStatusErrorDomain Code=-50 "(null)"

score 4 · Accepted Answer · answered Sep 15 '16 at 04:18

I'm not 100% sure why this is the case, but I found a solution that got this working for me, so here's how I understand the problem. I found this solution by trying to use the alternate convert(to:error:withInputFrom:) method. Using this was giving me a different error:

`ERROR:    AVAudioConverter.mm:526: FillComplexProc: required condition is false: [impl->_inputBufferReceived.format isEqual: impl->_inputFormat]`

The problem was caused in the line where I setup the AVAudioConverter:

let avConverter:AVAudioConverter = AVAudioConverter.init(fromFormat: startFormat, toFormat: endFormat)

It appears that the audio converter wants to use the same AVAudioFormat that the input buffer is using, instead of using a copy based on the original's settings. Once I swapped startFormat out for inAudioFormat, the convert(to:error:withInputFrom:) error was dismissed, and things worked as expected. I was then able to go back to using the simpler convert(to:fromBuffer:) method, and the original error I was dealing with also went away.

To recap, the line setting up the converter now looks like:

let avConverter:AVAudioConverter = AVAudioConverter.init(fromFormat: inAudioFormat, toFormat: endFormat)

As for the lack of docs on how to use AVAudioConverter, I have no idea why the API reference has next to nothing. Instead, in Xcode, CMD-click on AVAudioConverter in your code to go to it's header file. There's plenty of comments and info there. Not full sample code or anything, but it's at least something.

AVAudioConverter.convertToBuffer throwing error code -50

1 Answers1