Convert AVAudioPCMBuffer to NSData and back

问题:

How to convert AVAudioPCMBuffer to NSData? If it should be done as

let data = NSData(bytes: buffer.floatChannelData, length: bufferLength)

then how to calculate bufferLength?

And how to convert NSData to AVAudioPCMBuffer?

回答1:

Buffer length is frameCapacity * bytesPerFrame. Here are functions that can do conversion between NSData and AVAudioPCMBuffer.

func toNSData(PCMBuffer: AVAudioPCMBuffer) -> NSData {
    let channelCount = 1  // given PCMBuffer channel count is 1
    var channels = UnsafeBufferPointer(start: PCMBuffer.floatChannelData, count: channelCount)
    var ch0Data = NSData(bytes: channels[0], length:Int(PCMBuffer.frameCapacity * PCMBuffer.format.streamDescription.memory.mBytesPerFrame))
    return ch0Data
}       

func toPCMBuffer(data: NSData) -> AVAudioPCMBuffer {
    let audioFormat = AVAudioFormat(commonFormat: AVAudioCommonFormat.PCMFormatFloat32, sampleRate: 8000, channels: 1, interleaved: false)  // given NSData audio format
    var PCMBuffer = AVAudioPCMBuffer(PCMFormat: audioFormat, frameCapacity: UInt32(data.length) / audioFormat.streamDescription.memory.mBytesPerFrame)
    PCMBuffer.frameLength = PCMBuffer.frameCapacity
    let channels = UnsafeBufferPointer(start: PCMBuffer.floatChannelData, count: Int(PCMBuffer.format.channelCount))
    data.getBytes(UnsafeMutablePointer<Void>(channels[0]) , length: data.length)
    return PCMBuffer
}

回答2:

Copying the buffers is much easier if you copy via the audioBufferList API's. This also works no matter what the format is of the actual buffer.

extension Data {
    init(buffer: AVAudioPCMBuffer, time: AVAudioTime) {
        let audioBuffer = buffer.audioBufferList.pointee.mBuffers
        self.init(bytes: audioBuffer.mData!, count: Int(audioBuffer.mDataByteSize))
    }

    func makePCMBuffer(format: AVAudioFormat) -> AVAudioPCMBuffer? {
        let streamDesc = format.streamDescription.pointee
        let frameCapacity = UInt32(count) / streamDesc.mBytesPerFrame
        guard let buffer = AVAudioPCMBuffer(pcmFormat: format, frameCapacity: frameCapacity) else { return nil }

        buffer.frameLength = buffer.frameCapacity
        let audioBuffer = buffer.audioBufferList.pointee.mBuffers

        withUnsafeBytes { addr in
            audioBuffer.mData?.copyMemory(from: addr, byteCount: Int(audioBuffer.mDataByteSize))
        }

        return buffer
    }
}