无法在格式转换器音频单元上设置格式

Question

我试图建立一个非常简单的AU图-只是一个RemoteIO附带一个varispeed用于输出。不幸的是，我似乎无法在变速装置上设置音频格式。我看到的其他示例似乎只是设置格式没有麻烦。我得到的确切错误是不支持格式。然而，RemoteIO单元内部的格式转换器将很高兴地采用我的格式，这是一个非常简单的16khz单声道16位signed int PCM格式。

这是我目前试图使工作的代码:

 // Describe the output unit.
AudioComponentDescription inputDescription = {0};   
inputDescription.componentType = kAudioUnitType_Output;
inputDescription.componentSubType = kAudioUnitSubType_RemoteIO;
inputDescription.componentManufacturer = kAudioUnitManufacturer_Apple;
AUNode ioNode;
CheckError(self, AUGraphAddNode(_graph, &inputDescription, &ioNode), "Couldn't add ioNode to AU graph");
AudioComponentDescription varispeedDescription = {0};
varispeedDescription.componentType = kAudioUnitType_FormatConverter;
varispeedDescription.componentSubType = kAudioUnitSubType_Varispeed;
varispeedDescription.componentManufacturer = kAudioUnitManufacturer_Apple;
AUNode varispeedNode;
CheckError(self, AUGraphAddNode(_graph, &varispeedDescription, &varispeedNode), "Couldn't add varispeed node to AU graph");
CheckError(self, AUGraphOpen(_graph), "Couldn't open AU graph");
AudioUnit varispeedUnit;
CheckError(self, AUGraphNodeInfo(_graph, varispeedNode, NULL, &varispeedUnit), "Couldn't get varispeed audio unit");
CheckError(self, AUGraphNodeInfo(_graph, ioNode, NULL, &_ioUnit), "Couldn't get io unit");
// Enable input
// TODO: Conditionally disable input if option has not been specified
UInt32 one = 1;
CheckError(self, AudioUnitSetProperty(_ioUnit,
                                 kAudioOutputUnitProperty_EnableIO,
                                 kAudioUnitScope_Input,
                                 kInputBus,
                                 &one,
                                 sizeof(one)), "Couldn't enable IO on the input scope of output unit");
// TODO: check this works on iOS!
_format.mBitsPerChannel = 16;
_format.mBytesPerFrame = 2;
_format.mBytesPerPacket = 2;
_format.mChannelsPerFrame = 1;
_format.mFramesPerPacket = 1;
_format.mReserved = 0;
_format.mSampleRate = 16000.0;
_format.mFormatFlags = kLinearPCMFormatFlagIsSignedInteger | kLinearPCMFormatFlagIsPacked;
_format.mFormatID = kAudioFormatLinearPCM;
self.samplingRate = _format.mSampleRate;
UInt32 size;
size = sizeof(AudioStreamBasicDescription);
CheckError(self, AudioUnitSetProperty(_ioUnit,
                                kAudioUnitProperty_StreamFormat,
                                kAudioUnitScope_Output,
                                kInputBus,
                                &_format,
                                size),
           "Couldn't set the ASBD on the audio unit (after setting its sampling rate)");
size = sizeof(AudioStreamBasicDescription);
CheckError(self, AudioUnitSetProperty(varispeedUnit,
                                      kAudioUnitProperty_StreamFormat,
                                      kAudioUnitScope_Output,
                                      0,
                                      &_format,
                                      size),
           "Couldn't set the ASBD on the audio unit (after setting its sampling rate)");
size = sizeof(AudioStreamBasicDescription);
CheckError(self, AudioUnitSetProperty(varispeedUnit,
                                      kAudioUnitProperty_StreamFormat,
                                      kAudioUnitScope_Input,
                                      0,
                                      &_format,
                                      size),
           "Couldn't set the ASBD on the audio unit (after setting its sampling rate)");

我还尝试简单地设置RemoteIO的输出总线的输入格式，它可以很好地用于转换器的输出格式，但它仍然不接受我的格式作为转换器的输入格式。

我如何将varispeed和RemoteIO与我想要的格式连接起来?

Answer 1

我在一个旧的邮件列表中发现varisspeed单元只是浮点数，实际上并不进行格式转换，尽管主要类型是FormatConverter。解决方案是使用另一个具有专用转换器子类型的转换器，将我自己的格式转换为Varispeed想要的FP格式。Varispeed的格式限制实际上并没有在我能告诉的任何地方记录，所以我不知道它实际上想要什么格式，除了扬声器输出格式对我有效，它应该是一个浮点线性PCM格式。

仅供参考，Varispeed单元不做任何音高校正，这在改变播放速率时听起来很荒谬。您可以使用NewTimePitch单元，它响应相同的varispeed参数，但结果听起来并不像废话。