將kAudioUnitProperty_StreamFormat設置爲AAC時，AudioUnitSetProperty會失敗，並顯示-10868

Question

我想使用該編碼AUGraph的輸出作爲AAC（在iOS 5.1上）。圖的基本拓撲結構如下所示：

inputs --> MixerUnit --> pcm2aac --> aac2pcm --> RemoteIO 

我要搶AAC編碼的數據，因爲它是通過PostRenderCallback打pcm2aac，然後通過UDP發送到我們的服務器應用程序。

據我所知，這個使用案例的AudioUnit是kAudioUnitType_FormatConverter，但是當我想爲aac2pcm設置ASBD輸入屬性時，AudioUnitSetProperty以-10868失敗（當我設置pcm2acc的輸出時它也失敗，同樣的錯誤）。

AudioComponentDescription converterUnitDescription; 
converterUnitDescription.componentType   = kAudioUnitType_FormatConverter; 
converterUnitDescription.componentSubType  = kAudioUnitSubType_AUConverter; 
converterUnitDescription.componentManufacturer = kAudioUnitManufacturer_Apple; 
converterUnitDescription.componentFlags   = 0; 
converterUnitDescription.componentFlagsMask  = 0; 
//... 
AUNode pcm2aacNode; 
AUNode aac2pcmNode; 
//... 
result = AUGraphAddNode (
          processingGraph, 
          &converterUnitDescription, 
          &aac2pcmNode 
          ); 

if (noErr != result) {[self printErrorMessage: @"AUGraphNewNode failed for Converter unit" withStatus: result]; return;} 
//... 
result = AUGraphOpen (processingGraph); 

if (noErr != result) {[self printErrorMessage: @"AUGraphOpen" withStatus: result]; return;} 

//... 
AudioUnit aac2pcmUnit; 
AudioUnit pcm2aacUnit; 
//... 
result = AUGraphNodeInfo (
          processingGraph, 
          aac2pcmNode, 
          NULL, 
          &aac2pcmUnit 
          ); 

if (noErr != result) {[self printErrorMessage: @"AUGraphNodeInfo (aac2pcm)" withStatus: result]; return;} 

//... 
NSLog (@"Setting stream format for input aac2pcm bus"); 
result = AudioUnitSetProperty (aac2pcmUnit, 
           kAudioUnitProperty_StreamFormat, 
           kAudioUnitScope_Input, 
           0, 
           &aacStreamFormat, 
           sizeof (aacStreamFormat) 
           ); 

if (noErr != result) {[self printErrorMessage: @"AudioUnitSetProperty (input aac2pcm stream format)" withStatus: result];return;} 
// returns -10868 

其中aacStreamFormat是

memset(&aacStreamFormat, 0, sizeof(aacStreamFormat));/ 
UInt32 size = sizeof(aacStreamFormat); 
aacStreamFormat.mFormatID   = kAudioFormatMPEG4AAC; 
aacStreamFormat.mChannelsPerFrame = 2; 
aacStreamFormat.mFormatFlags  = kMPEG4Object_AAC_Main; 
aacStreamFormat.mSampleRate  = graphSampleRate; //44100 
AudioFormatGetProperty(kAudioFormatProperty_FormatInfo, 0, NULL, &size, &aacStreamFormat); 

是這種與AUGraph轉換甚至有可能的還是我必須用其他方法進行動態轉換？

Answer 1

我不認爲你需要pcm2aac和aac2pcm節點。您只需在混音器節點上安裝渲染回調，然後使用ExtAudioFileWriteAsync將輸出保存爲AAC即可。

Answer 2

參考AUComponent.h

kAudioUnitSubType_AUConverter 
An audio unit that uses an AudioConverter to do Linear PCM conversions 
(sample rate, bit depth, interleaving). 

，所以你不能用kAudioUnitSubType_AUConverter PCM代碼轉換爲AAC格式。

您shoulde使用AudioConverter相反，這裏是參考網址 https://developer.apple.com/library/ios/samplecode/iPhoneACFileConvertTest/Introduction/Intro.html