I'm trying to write 16-bit interleaved 2-channel data captured from a LiveSwitch audio source to a AVAudioFile. The buffer and file formats match but I get a bad parameter error from the API. Does this API not support the specified format or is there some other issue?
Here is the debugger output.
(lldb) po audioFile.url
▿ file:///private/var/mobile/Containers/Data/Application/1EB14379-0CF2-41B6-B742-4C9A80728DB3/tmp/Heart%20Sounds%201
- _url : file:///private/var/mobile/Containers/Data/Application/1EB14379-0CF2-41B6-B742-4C9A80728DB3/tmp/Heart%20Sounds%201
- _parseInfo : nil
- _baseParseInfo : nil
(lldb) po error
Error Domain=com.apple.coreaudio.avfaudio Code=-50 "(null)" UserInfo={failed call=ExtAudioFileWrite(_impl->_extAudioFile, buffer.frameLength, buffer.audioBufferList)}
(lldb) po buffer.format
<AVAudioFormat 0x302a12b20: 2 ch, 44100 Hz, Int16, interleaved>
(lldb) po audioFile.fileFormat
<AVAudioFormat 0x302a515e0: 2 ch, 44100 Hz, Int16, interleaved>
(lldb) po buffer.frameLength
882
(lldb) po buffer.audioBufferList
▿ 0x0000000300941e60
- pointerValue : 12894608992
This code handles the details of converting the Live Switch frame into an AVAudioPCMBuffer.
extension FMLiveSwitchAudioFrame {
func convertedToPCMBuffer() -> AVAudioPCMBuffer {
Self.convertToAVAudioPCMBuffer(from: self)!
}
static func convertToAVAudioPCMBuffer(from frame: FMLiveSwitchAudioFrame) -> AVAudioPCMBuffer? {
// Retrieve the audio buffer and format details from the FMLiveSwitchAudioFrame
guard
let buffer = frame.buffer(),
let format = buffer.format() as? FMLiveSwitchAudioFormat else { return nil }
// Extract PCM format details from FMLiveSwitchAudioFormat
let sampleRate = Double(format.clockRate())
let channelCount = AVAudioChannelCount(format.channelCount())
// Determine bytes per sample based on bit depth
let bitsPerSample = 16
let bytesPerSample = bitsPerSample / 8
let bytesPerFrame = bytesPerSample * Int(channelCount)
let frameLength = AVAudioFrameCount(Int(buffer.dataBuffer().length()) / bytesPerFrame)
// Create an AVAudioFormat from the FMLiveSwitchAudioFormat
guard let avAudioFormat = AVAudioFormat(commonFormat: .pcmFormatInt16, sampleRate: sampleRate, channels: channelCount, interleaved: true) else {
return nil
}
// Create an AudioBufferList to wrap the existing buffer
let audioBufferList = UnsafeMutablePointer<AudioBufferList>.allocate(capacity: 1)
audioBufferList.pointee.mNumberBuffers = 1
audioBufferList.pointee.mBuffers.mNumberChannels = channelCount
audioBufferList.pointee.mBuffers.mDataByteSize = UInt32(buffer.dataBuffer().length())
audioBufferList.pointee.mBuffers.mData = buffer.dataBuffer().data().mutableBytes // Directly use LiveSwitch buffer
// Transfer ownership of the buffer to AVAudioPCMBuffer
let pcmBuffer = AVAudioPCMBuffer(pcmFormat: avAudioFormat, bufferListNoCopy: audioBufferList) /* { buffer in
// Ensure the buffer is freed when AVAudioPCMBuffer is deallocated
buffer.deallocate() // Only call this if LiveSwitch allows manual deallocation
} */
pcmBuffer?.frameLength = frameLength
return pcmBuffer
}
}
This is the handler that is invoked with every frame in order to convert it for use with AVAudioFile and optionally update a scrolling signal display on the screen.
private func onRaisedFrame(obj: Any!) -> Void {
// Bail out early if no one is interested in the data.
guard isMonitoring else { return }
// Convert LS frame to AVAudioPCMBuffer (no-copy)
let frame = obj as! FMLiveSwitchAudioFrame
let buffer = frame.convertedToPCMBuffer()
// Hand subscribers a reference to the buffer for rendering to display.
bufferPublisher?.send(buffer)
// If we have and output file, store the data there, as well.
guard let audioFile = self.audioFile else { return }
do {
try audioFile.write(from: buffer) // FIXME: This call is throwing error -50
} catch {
FMLiveSwitchLog.error(withMessage: "Failed to write buffer to audio file at \(audioFile.url): \(error)")
self.audioFile = nil
}
}
This is how the audio file is being setup.
static var recordingFormat: AVAudioFormat = {
AVAudioFormat(commonFormat: .pcmFormatInt16, sampleRate: 44_100, channels: 2, interleaved: true)!
}()
let audioFile = try AVAudioFile(forWriting: outputURL, settings: Self.recordingFormat.settings)
