AVAudioConverter float32/48khz -> int16/16khz conversion failure

Question

This method handles the callback from installTap on the input node from an AVAudioEngine. I have confirmed that I'm getting mono float32/48000hz buffer data, and I'd like to convert it to mono int16/16000hz.

var converter: AVAudioConverter? = nil
var convertBuffer: AVAudioPCMBuffer? = nil
let targetFormat = AVAudioFormat(commonFormat: AVAudioCommonFormat.pcmFormatInt16, sampleRate: 16000, channels: 1, interleaved: false)

func recordCallback(buffer: AVAudioPCMBuffer, time: AVAudioTime) {
    if converter == nil {
        convertBuffer = AVAudioPCMBuffer(pcmFormat: targetFormat, frameCapacity: buffer.frameCapacity)
        convertBuffer?.frameLength = convertBuffer!.frameCapacity
        converter = AVAudioConverter(from: buffer.format, to: convertBuffer!.format)
        converter?.sampleRateConverterAlgorithm = AVSampleRateConverterAlgorithm_Normal
        converter?.sampleRateConverterQuality = .max
    }

    guard let convertBuffer = convertBuffer else { return }

    log.info("Converter: \(self.converter!)")
    log.info("Converter buffer: \(self.convertBuffer!)")
    log.info("Converter buffer format: \(self.convertBuffer!.format)")
    log.info("Source buffer: \(buffer)")
    log.info("Source buffer format: \(buffer.format)")

    do {
        try converter!.convert(to: convertBuffer, from: buffer)
    } catch let error {
        log.error("Conversion error: \(error)")
        observer?.onError(EngineRecordTaskError.ConversionError(error: error))
        return
    }

    …
}

This gives me the helpful error:

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

If I use the alternative method .convert(to:error:withInputFrom:), it does not populate the NSError, and does not fill the target buffer - fails silently.

Logged info:

Converter: <AVAudioConverter: 0x17001cd40>
Converter buffer: <AVAudioPCMBuffer@0x17021e310: 9600/9600 bytes>
Converter buffer format: <AVAudioFormat 0x17448ba90:  1 ch,  16000 Hz, Int16>
Source buffer: <AVAudioPCMBuffer@0x17021e1b0: 19200/19200 bytes>
Source buffer format: <AVAudioFormat 0x174480910:  1 ch,  48000 Hz, Float32>
Conversion error: Error Domain=NSOSStatusErrorDomain Code=-50 "(null)"

Additional info from debugger:

(lldb) po buffer.frameCapacity
4800

(lldb) po convertBuffer.frameCapacity
4800

(lldb) po buffer.frameLength
4800

(lldb) po convertBuffer.frameLength
4800

Here's the alternative code for conversion I tried:

    var conversionError: NSError? = nil
    converter!.convert(to: convertBuffer, error: &conversionError, withInputFrom: { (_, _) in
        return buffer
    })

    if let conversionError = conversionError {
        log.error("Conversion error: \(conversionError)")
        observer?.onError(EngineRecordTaskError.ConversionError(error: conversionError))
        return
    }

Answer 1

From the discussion of convert in the header file:

The output buffer's frameCapacity should be at least at large as the inputBuffer's frameLength. If the conversion involves a codec or sample rate conversion, you instead must use convertToBuffer:error:withInputFromBlock:.

You're converting from 48kHz to 16kHz, which counts as a rate conversion, so you have to use convertToBuffer:

let inputBlock: AVAudioConverterInputBlock = { inNumPackets, outStatus in
    outStatus.pointee = AVAudioConverterInputStatus.haveData
    return buffer
}

var error: NSError? = nil
let status: AVAudioConverterOutputStatus = converter!.convert(to: convertBuffer, error: &error, withInputFrom: inputBlock)
// TODO: check status

p.s. it must be really expensive to create new NSError codes'n'strings over at apple - their budget just couldn't cover a "rate conversion requires convertToBuffer" message.