使用 iOS 的 ExtAudioFileWrite 将音频样本缓冲区写入 aac 文件

更新：我已经弄清楚了这一点并发布了我的解决方案作为我自己的问题的答案（如下）

我正在尝试使用 AAC 格式的 ExtAudioFileWrite 将简单的音频样本缓冲区写入文件。

我已经通过下面的代码实现了这一点，将单声道缓冲区写入 .wav 文件 - 但是，我无法对立体声或 AAC 文件执行此操作，而这正是我想要做的。

这是我到目前为止所拥有的......

CFStringRef fPath;
fPath = CFStringCreateWithCString(kCFAllocatorDefault,
                                  "/path/to/my/audiofile/audiofile.wav",
                                  kCFStringEncodingMacRoman);


OSStatus err;

int mChannels = 1;
UInt32 totalFramesInFile = 100000;

Float32 *outputBuffer = (Float32 *)malloc(sizeof(Float32) * (totalFramesInFile*mChannels)); 


////////////// Set up Audio Buffer List ////////////

AudioBufferList outputData; 
outputData.mNumberBuffers = 1;
outputData.mBuffers[0].mNumberChannels = mChannels; 
outputData.mBuffers[0].mDataByteSize = 4 * totalFramesInFile * mChannels;
outputData.mBuffers[0].mData = outputBuffer;

Float32 audioFile[totalFramesInFile*mChannels];


for (int i = 0;i < totalFramesInFile*mChannels;i++)
{
    audioFile[i] = ((Float32)(rand() % 100))/100.0;
    audioFile[i] = audioFile[i]*0.2;
}

outputData.mBuffers[0].mData = &audioFile;


CFURLRef fileURL = CFURLCreateWithFileSystemPath(kCFAllocatorDefault,fPath,kCFURLPOSIXPathStyle,false);

ExtAudioFileRef audiofileRef;

// WAVE FILES

AudioFileTypeID fileType = kAudioFileWAVEType;
AudioStreamBasicDescription clientFormat;
clientFormat.mSampleRate = 44100.0;
clientFormat.mFormatID = kAudioFormatLinearPCM;
clientFormat.mFormatFlags = 12;
clientFormat.mBitsPerChannel = 16;
clientFormat.mChannelsPerFrame = mChannels;
clientFormat.mBytesPerFrame = 2*clientFormat.mChannelsPerFrame;
clientFormat.mFramesPerPacket = 1;
clientFormat.mBytesPerPacket = 2*clientFormat.mChannelsPerFrame;


// open the file for writing
err = ExtAudioFileCreateWithURL((CFURLRef)fileURL, fileType, &clientFormat, NULL, kAudioFileFlags_EraseFile, &audiofileRef);

if (err != noErr)
{
    cout << "Problem when creating audio file: " << err << "\n";
}

// tell the ExtAudioFile API what format we'll be sending samples in
err = ExtAudioFileSetProperty(audiofileRef, kExtAudioFileProperty_ClientDataFormat, sizeof(clientFormat), &clientFormat);

if (err != noErr)
{
    cout << "Problem setting audio format: " << err << "\n";
}

UInt32 rFrames = (UInt32)totalFramesInFile;
// write the data
err = ExtAudioFileWrite(audiofileRef, rFrames, &outputData);

if (err != noErr)
{
    cout << "Problem writing audio file: " << err << "\n";
}

// close the file
ExtAudioFileDispose(audiofileRef);



NSLog(@"Done!");

我的具体问题是：

• 如何设置 AAC 的 AudioStreamBasicDescription？
• 为什么我无法让立体声在这里正常工作？如果我将通道数 ('mChannels') 设置为 2，那么我会正确获得左通道，而右通道会出现失真。

我非常感谢任何帮助 - 我想我已经阅读了几乎所有我能找到的页面，但我一无所知，因为虽然存在类似的问题，但他们通常从某些输入音频文件中派生 AudioStreamBasicDescription 参数，我无法看到结果。苹果文档也没有帮助。

提前谢谢了，

Adam

好吧，经过一番探索我已经弄清楚了。我将其包装为一个将随机噪声写入文件的函数。具体来说，它可以：

• 写入 .wav 或 .m4a 文件
• 以任一格式写入单声道或立体声
• 将文件写入指定路径

函数参数是：

• 要创建的音频文件的路径
• 通道数（最多 2 个）
• 布尔值：使用 m4a 压缩（如果为 false，则使用 pcm）

对于立体声 M4A 文件，该函数应调用为：

writeNoiseToAudioFile("/path/to/my/audiofile.m4a",2,true);

该函数的来源如下。我已尝试尽可能多地评论它 - 我希望它是正确的，它当然对我有用，但如果我错过了某些内容，请说“亚当，你做得有点错误”。祝你好运！这是代码：

void writeNoiseToAudioFile(char *fName,int mChannels,bool compress_with_m4a)
{
OSStatus err; // to record errors from ExtAudioFile API functions

// create file path as CStringRef
CFStringRef fPath;
fPath = CFStringCreateWithCString(kCFAllocatorDefault,
                                  fName,
                                  kCFStringEncodingMacRoman);


// specify total number of samples per channel
UInt32 totalFramesInFile = 100000;      

/////////////////////////////////////////////////////////////////////////////
////////////// Set up Audio Buffer List For Interleaved Audio ///////////////
/////////////////////////////////////////////////////////////////////////////

AudioBufferList outputData; 
outputData.mNumberBuffers = 1;
outputData.mBuffers[0].mNumberChannels = mChannels;    
outputData.mBuffers[0].mDataByteSize = sizeof(AudioUnitSampleType)*totalFramesInFile*mChannels;



/////////////////////////////////////////////////////////////////////////////
//////// Synthesise Noise and Put It In The AudioBufferList /////////////////
/////////////////////////////////////////////////////////////////////////////

// create an array to hold our audio
AudioUnitSampleType audioFile[totalFramesInFile*mChannels];

// fill the array with random numbers (white noise)
for (int i = 0;i < totalFramesInFile*mChannels;i++)
{
    audioFile[i] = ((AudioUnitSampleType)(rand() % 100))/100.0;
    audioFile[i] = audioFile[i]*0.2;
    // (yes, I know this noise has a DC offset, bad)
}

// set the AudioBuffer to point to the array containing the noise
outputData.mBuffers[0].mData = &audioFile;


/////////////////////////////////////////////////////////////////////////////
////////////////// Specify The Output Audio File Format /////////////////////
/////////////////////////////////////////////////////////////////////////////


// the client format will describe the output audio file
AudioStreamBasicDescription clientFormat;

// the file type identifier tells the ExtAudioFile API what kind of file we want created
AudioFileTypeID fileType;

// if compress_with_m4a is tru then set up for m4a file format
if (compress_with_m4a)
{
    // the file type identifier tells the ExtAudioFile API what kind of file we want created
    // this creates a m4a file type
    fileType = kAudioFileM4AType;

    // Here we specify the M4A format
    clientFormat.mSampleRate         = 44100.0;
    clientFormat.mFormatID           = kAudioFormatMPEG4AAC;
    clientFormat.mFormatFlags        = kMPEG4Object_AAC_Main;
    clientFormat.mChannelsPerFrame   = mChannels;
    clientFormat.mBytesPerPacket     = 0;
    clientFormat.mBytesPerFrame      = 0;
    clientFormat.mFramesPerPacket    = 1024;
    clientFormat.mBitsPerChannel     = 0;
    clientFormat.mReserved           = 0;
}
else // else encode as PCM
{
    // this creates a wav file type
    fileType = kAudioFileWAVEType;

    // This function audiomatically generates the audio format according to certain arguments
    FillOutASBDForLPCM(clientFormat,44100.0,mChannels,32,32,true,false,false);
}



/////////////////////////////////////////////////////////////////////////////
///////////////// Specify The Format of Our Audio Samples ///////////////////
/////////////////////////////////////////////////////////////////////////////

// the local format describes the format the samples we will give to the ExtAudioFile API
AudioStreamBasicDescription localFormat;
FillOutASBDForLPCM (localFormat,44100.0,mChannels,32,32,true,false,false);



/////////////////////////////////////////////////////////////////////////////
///////////////// Create the Audio File and Open It /////////////////////////
/////////////////////////////////////////////////////////////////////////////

// create the audio file reference
ExtAudioFileRef audiofileRef;

// create a fileURL from our path
CFURLRef fileURL = CFURLCreateWithFileSystemPath(kCFAllocatorDefault,fPath,kCFURLPOSIXPathStyle,false);

// open the file for writing
err = ExtAudioFileCreateWithURL((CFURLRef)fileURL, fileType, &clientFormat, NULL, kAudioFileFlags_EraseFile, &audiofileRef);

if (err != noErr)
{
    cout << "Problem when creating audio file: " << err << "\n";
}


/////////////////////////////////////////////////////////////////////////////
///// Tell the ExtAudioFile API what format we'll be sending samples in /////
/////////////////////////////////////////////////////////////////////////////

// Tell the ExtAudioFile API what format we'll be sending samples in 
err = ExtAudioFileSetProperty(audiofileRef, kExtAudioFileProperty_ClientDataFormat, sizeof(localFormat), &localFormat);

if (err != noErr)
{
    cout << "Problem setting audio format: " << err << "\n";
}

/////////////////////////////////////////////////////////////////////////////
///////// Write the Contents of the AudioBufferList to the AudioFile ////////
/////////////////////////////////////////////////////////////////////////////

UInt32 rFrames = (UInt32)totalFramesInFile;
// write the data
err = ExtAudioFileWrite(audiofileRef, rFrames, &outputData);

if (err != noErr)
{
    cout << "Problem writing audio file: " << err << "\n";
}


/////////////////////////////////////////////////////////////////////////////
////////////// Close the Audio File and Get Rid Of The Reference ////////////
/////////////////////////////////////////////////////////////////////////////

// close the file
ExtAudioFileDispose(audiofileRef);


NSLog(@"Done!");
}

不要忘记导入 AudioToolbox 框架并包含头文件：

#import <AudioToolbox/AudioToolbox.h>