使用AVAudioConverter Swift将AAC解码为PCM格式

Question

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

在WWDC 2015，507 Session上，AVAudioConverter可以对PCM缓冲区进行编码和解码，展示了编码示例，但没有展示解码示例。我试着解码，但有些东西不起作用。我不知道是什么：(

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, to: outBuffer, 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!, packetCapacity: 8, 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

Answer 1

您的尝试出现了一些问题：

1. 在转换数据->AVAudioCompressedBuffer时，您没有设置多个数据包描述。您需要创建它们，因为AAC数据包的大小是可变的。您可以从原始AAC缓冲区复制它们，也可以手动(ouch(或使用AudioFileStreamapi从数据中解析它们。

2. 您可以一次又一次地重新创建AVAudioConverter——每个缓冲区一次，丢弃它们的状态。例如，AAC编码器由于其自身的个人原因，需要添加2112帧静音，然后才能再现您的音频，因此重新创建转换器会让您获得大量静音。

3. 您向AVAudioConverter的输入块一次又一次地呈现相同的缓冲区。每个缓冲区只能显示一次。

4. 32000的比特率(对我来说(不起作用

这就是我现在能想到的。尝试对您的代码进行以下修改，您现在这样调用：

(附言：我把一些单声道改成了立体声，这样我就可以在mac上播放往返缓冲区，mac的麦克风输入是奇怪的立体声——你可能需要把它改回来(

(很明显，这里正在进行某种往返/串行化/去串行化尝试，但你到底想做什么？你想把AAC音频从一台设备流到另一台设备吗？因为让AVPlayer这样的另一个API播放结果流可能更容易，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, 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 var lpcmToAACConverter: AVAudioConverter! = nil
static func convertToAAC(from buffer: AVAudioBuffer, error outError: NSErrorPointer) -> AVAudioCompressedBuffer? {
let outputFormat = AudioBufferFormatHelper.AACFormat()
let outBuffer = AVAudioCompressedBuffer(format: outputFormat!, packetCapacity: 8, 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, to: outBuffer, error: outError)
return outBuffer
}
static var aacToLPCMConverter: AVAudioConverter! = nil
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
}
//init converter once
if aacToLPCMConverter == nil {
let inputFormat = buffer.format
aacToLPCMConverter = AVAudioConverter(from: inputFormat, to: outputFormat!)
}
self.convert(withConverter: aacToLPCMConverter, from: buffer, to: outBuffer, 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, to destinationBuffer: 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, error: outError, withInputFrom: inputBlock)
print("status: (status.rawValue)")
}
}