ios – 使用AVAudioConverter Swift将AAC解码为PCM格式

如何在 Swift上使用AVAudioConverter,AVAudioCompressedBuffer和AVAudioPCMBuffer将AAC转换为PCM？

在WWDC 2015上,507 Session被称为AVAudioConverter可以编码和解码PCM缓冲区,显示了编码示例,但未显示解码示例.
我试过解码,有些东西不起作用.我不知道:(

呼叫：

//buffer - it's AVAudioPCMBuffer from AVAudioInputNode(AVAudioEngine)
let aacBuffer = AudioBufferConverter.convertToAAC(from: buffer,error: nil) //has data
let data = Data(bytes: aacBuffer!.data,count: Int(aacBuffer!.byteLength)) //has data
let aacReverseBuffer = AudioBufferConverter.convertToAAC(from: data) //has data
let pcmReverseBuffer = AudioBufferConverter.convertToPCM(from: aacBuffer2!,error: nil) //zeros data. data object exist,but filled by zeros

这是转换代码：

class AudioBufferFormatHelper {

    static func PCMFormat() -> AVAudioFormat? {

        return AVAudioFormat(commonFormat: .pcmFormatFloat32,sampleRate: 44100,channels: 1,interleaved: false)
    }

    static func AACFormat() -> AVAudioFormat? {

        var outDesc = AudioStreamBasicDescription(
                mSampleRate: 44100,mFormatID: kAudioFormatMPEG4AAC,mFormatFlags: 0,mBytesPerPacket: 0,mFramesPerPacket: 0,mBytesPerFrame: 0,mChannelsPerFrame: 1,mBitsPerChannel: 0,mReserved: 0)
        let outFormat = AVAudioFormat(streamDescription: &outDesc)
        return outFormat
    }
}

class AudioBufferConverter {

    static func convertToAAC(from buffer: AVAudioBuffer,error outError: NSErrorPointer) -> AVAudioCompressedBuffer? {

        let outputFormat = AudioBufferFormatHelper.AACFormat()
        let outBuffer = AVAudioCompressedBuffer(format: outputFormat!,packetCapacity: 8,maximumPacketSize: 768)

        self.convert(from: buffer,to: outBuffer,error: outError)

        return outBuffer
    }

    static func convertToPCM(from buffer: AVAudioBuffer,error outError: NSErrorPointer) -> AVAudioPCMBuffer? {

        let outputFormat = AudioBufferFormatHelper.PCMFormat()
        guard let outBuffer = AVAudioPCMBuffer(pcmFormat: outputFormat!,frameCapacity: 4410) else {
            return nil
        }

        outBuffer.frameLength = 4410
        self.convert(from: buffer,error: outError)

        return outBuffer
    }

    static func convertToAAC(from data: Data) -> AVAudioCompressedBuffer? {

        let nsData = NSData(data: data)
        let inputFormat = AudioBufferFormatHelper.AACFormat()
        let buffer = AVAudioCompressedBuffer(format: inputFormat!,maximumPacketSize: 768)
        buffer.byteLength = UInt32(data.count)
        buffer.packetCount = 8

        buffer.data.copyMemory(from: nsData.bytes,byteCount: nsData.length)
        buffer.packetDescriptions!.pointee.mDataByteSize = 4

        return buffer
    }

    private static func convert(from sourceBuffer: AVAudioBuffer,to destinationBuffer: AVAudioBuffer,error outError: NSErrorPointer) {

        //init converter
        let inputFormat = sourceBuffer.format
        let outputFormat = destinationBuffer.format
        let converter = AVAudioConverter(from: inputFormat,to: outputFormat)

        converter!.bitRate = 32000

        let inputBlock : AVAudioConverterInputBlock = { inNumPackets,outStatus in

            outStatus.pointee = AVAudioConverterInputStatus.haveData
            return sourceBuffer
        }

        _ = converter!.convert(to: destinationBuffer,error: outError,withInputFrom: inputBlock)
    }
}

结果AVAudioPCMBuffer的数据为零.
在消息中我看到错误：

AACDecoder.cpp:192:Deserialize:  Unmatched number of channel elements in payload
AACDecoder.cpp:220:DecodeFrame:  Error deserializing packet
[ac] ACMP4AACBaseDecoder.cpp:1337:ProduceOutputBufferList: (0x14f81b840) Error decoding packet 1: err = -1,packet length: 0
AACDecoder.cpp:192:Deserialize:  Unmatched number of channel elements in payload
AACDecoder.cpp:220:DecodeFrame:  Error deserializing packet
[ac] ACMP4AACBaseDecoder.cpp:1337:ProduceOutputBufferList: (0x14f81b840) Error decoding packet 3: err = -1,packet length: 0
AACDecoder.cpp:192:Deserialize:  Unmatched number of channel elements in payload
AACDecoder.cpp:220:DecodeFrame:  Error deserializing packet
[ac] ACMP4AACBaseDecoder.cpp:1337:ProduceOutputBufferList: (0x14f81b840) Error decoding packet 5: err = -1,packet length: 0
AACDecoder.cpp:192:Deserialize:  Unmatched number of channel elements in payload
AACDecoder.cpp:220:DecodeFrame:  Error deserializing packet
[ac] ACMP4AACBaseDecoder.cpp:1337:ProduceOutputBufferList: (0x14f81b840) Error decoding packet 7: err = -1,packet length: 0

解决方法

你的尝试有一些问题：

>转换数据时,您没有设置多个数据包描述 – > AVAudioCompressedBuffer.您需要创建它们,因为AAC数据包的大小可变.您可以从原始AAC缓冲区复制它们,也可以手动(ouch)或使用AudioFileStream api从数据中解析它们.
>你一遍又一遍地重新创建你的AVAudioConverters – 每个缓冲区一次,扔掉他们的状态.例如AAC编码器出于个人原因需要增加2112帧的静音才能再现音频,因此重新创建转换器会让你沉默.
>您反复向AVAudioConverter的输入块提供相同的缓冲区.您应该只呈现每个缓冲区一次.
>比特率32000不起作用(对我来说)

这就是我现在所能想到的.请尝试对您的代码进行以下修改,您现在调用它们,如下所示：

(p.s.我将一些单声道改为立体声,所以我可以在我的mac上播放往返缓冲区,其麦克风输入奇怪的立体声 – 您可能需要将其更改回来)

(pps显然有一些往返/串行/反序列化尝试在这里进行,但你究竟想要做什么？你想将AAC音频从一个设备传输到另一个设备吗？因为它可能更容易让其他API像AVPlayer播放结果流而不是自己处理数据包)

let aacBuffer = AudioBufferConverter.convertToAAC(from: buffer,error: nil)!
let data = Data(bytes: aacBuffer.data,count: Int(aacBuffer.byteLength))
let packetDescriptions = Array(UnsafeBufferPointer(start: aacBuffer.packetDescriptions,count: Int(aacBuffer.packetCount)))
let aacReverseBuffer = AudioBufferConverter.convertToAAC(from: data,packetDescriptions: packetDescriptions)!
// was aacBuffer2
let pcmReverseBuffer = AudioBufferConverter.convertToPCM(from: aacReverseBuffer,error: nil)

class AudioBufferFormatHelper {

    static func PCMFormat() -> AVAudioFormat? {
        return AVAudioFormat(commonFormat: .pcmFormatFloat32,interleaved: false)
    }

    static func AACFormat() -> AVAudioFormat? {

        var outDesc = AudioStreamBasicDescription(
            mSampleRate: 44100,mReserved: 0)
        let outFormat = AVAudioFormat(streamDescription: &outDesc)
        return outFormat
    }
}

class AudioBufferConverter {
    static var lpcmToAACConverter: AVAudioConverter! = nil

    static func convertToAAC(from buffer: AVAudioBuffer,maximumPacketSize: 768)

        //init converter once
        if lpcmToAACConverter == nil {
            let inputFormat = buffer.format

            lpcmToAACConverter = AVAudioConverter(from: inputFormat,to: outputFormat!)
//            print("available rates \(lpcmToAACConverter.applicableEncodeBitRates)")
//          lpcmToAACConverter!.bitRate = 96000
            lpcmToAACConverter.bitRate = 32000    // have end of stream problems with this,not sure why
        }

        self.convert(withConverter:lpcmToAACConverter,from: buffer,error: outError)

        return outBuffer
    }

    static var aacToLPCMConverter: AVAudioConverter! = nil

    static func convertToPCM(from buffer: AVAudioBuffer,frameCapacity: 4410) else {
            return nil
        }

        //init converter once
        if aacToLPCMConverter == nil {
            let inputFormat = buffer.format

            aacToLPCMConverter = AVAudioConverter(from: inputFormat,to: outputFormat!)
        }

        self.convert(withConverter: aacToLPCMConverter,error: outError)

        return outBuffer
    }

    static func convertToAAC(from data: Data,packetDescriptions: [AudioStreamPacketDescription]) -> AVAudioCompressedBuffer? {

        let nsData = NSData(data: data)
        let inputFormat = AudioBufferFormatHelper.AACFormat()
        let maximumPacketSize = packetDescriptions.map { $0.mDataByteSize }.max()!
        let buffer = AVAudioCompressedBuffer(format: inputFormat!,packetCapacity: AVAudioPacketCount(packetDescriptions.count),maximumPacketSize: Int(maximumPacketSize))
        buffer.byteLength = UInt32(data.count)
        buffer.packetCount = AVAudioPacketCount(packetDescriptions.count)

        buffer.data.copyMemory(from: nsData.bytes,byteCount: nsData.length)
        buffer.packetDescriptions!.pointee.mDataByteSize = UInt32(data.count)
        buffer.packetDescriptions!.initialize(from: packetDescriptions,count: packetDescriptions.count)

        return buffer
    }


    private static func convert(withConverter: AVAudioConverter,from sourceBuffer: AVAudioBuffer,error outError: NSErrorPointer) {
        // input each buffer only once
        var newBufferAvailable = true

        let inputBlock : AVAudioConverterInputBlock = {
            inNumPackets,outStatus in
            if newBufferAvailable {
                outStatus.pointee = .haveData
                newBufferAvailable = false
                return sourceBuffer
            } else {
                outStatus.pointee = .noDatanow
                return nil
            }
        }

        let status = withConverter.convert(to: destinationBuffer,withInputFrom: inputBlock)
        print("status: \(status.rawValue)")
    }
}