核心音频指定要解码的音轨

Core Audio specify which audio track to decode

本文关键字：解码 音轨 音频 核心      更新时间：2023-10-16

我能够使用Core Audio API成功获取音频文件的解码PCM数据。以下是还原代码，该代码显示了我如何做：

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）？

一种方法是将所有轨道解码，然后将所需的曲目数据（复制）数据（用于交错的立体声等）中提取到另一个缓冲区，数组或文件中。与解码时间相比，额外的复制时间微不足道。

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