c++ - 核心音频指定要解码的音轨

Question

我能够使用Core Audio API成功获取音频文件的解码PCM数据。下面是简化后的代码，显示了如何执行此操作:

CFStringRef urlStr = CFStringCreateWithCString(kCFAllocatorDefault, "file.m4a", kCFStringEncodingUTF8);
CFURLRef urlRef = CFURLCreateWithFileSystemPath(NULL, urlStr, kCFURLPOSIXPathStyle, false);    

ExtAudioFileOpenURL(urlRef, &m_audioFile);

bzero(&m_outputFormat, sizeof(AudioStreamBasicDescription));
m_outputFormat.mFormatFlags = kAudioFormatFlagIsSignedInteger | kAudioFormatFlagsNativeEndian | kAudioFormatFlagIsPacked;
m_outputFormat.mSampleRate = m_inputFormat.mSampleRate;
m_outputFormat.mFormatID = kAudioFormatLinearPCM;
m_outputFormat.mChannelsPerFrame = m_inputFormat.mChannelsPerFrame;
m_outputFormat.mBytesPerFrame = sizeof(short) * m_outputFormat.mChannelsPerFrame;
m_outputFormat.mBitsPerChannel = sizeof(short) * 8;
m_outputFormat.mFramesPerPacket = 1;
m_outputFormat.mBytesPerPacket = m_outputFormat.mBytesPerFrame * m_outputFormat.mFramesPerPacket;


ExtAudioFileSetProperty(m_audioFile, kExtAudioFileProperty_ClientDataFormat, sizeof(m_outputFormat), &m_outputFormat)

short* transformData = new short[sampleCount];    

AudioBufferList fillBufList;
fillBufList.mNumberBuffers = 1;
fillBufList.mBuffers[0].mNumberChannels = channels;
fillBufList.mBuffers[0].mDataByteSize = m_sampleCount * sizeof(short);
fillBufList.mBuffers[0].mData = (void*)(&transformData[0]);

ExtAudioFileRead(m_audioFile, &m_frameCount, &fillBufList);

我对感兴趣，如何指定我要解码的音轨(假设媒体文件包含多个)？

Answer 1

一种方法是解码所有轨道，然后将所需的轨道数据(隔行立体声的每个其他样本等)提取(复制)到另一个缓冲区，阵列或文件中。与解码时间相比，额外的复制时间微不足道。