kAudioFormatLinearPCM 到 kAudioFormatMPEG4AAC

Question

我的代码使用 kAudioFormatLinearPCM formate 记录 wav 文件，使用此代码，

void AQRecorder::SetupAudioFormat(UInt32 inFormatID)
{
    memset(&mRecordFormat, 0, sizeof(mRecordFormat));

    UInt32 size = sizeof(mRecordFormat.mSampleRate);
    XThrowIfError(AudioSessionGetProperty(  kAudioSessionProperty_CurrentHardwareSampleRate,
                                        &size, 
                                        &mRecordFormat.mSampleRate), "couldn't get hardware sample rate");

    size = sizeof(mRecordFormat.mChannelsPerFrame);
    XThrowIfError(AudioSessionGetProperty(  kAudioSessionProperty_CurrentHardwareInputNumberChannels, 
                                        &size, 
                                        &mRecordFormat.mChannelsPerFrame), "couldn't get input channel count");

    mRecordFormat.mFormatID = inFormatID;
    if (inFormatID == kAudioFormatLinearPCM)
    {
        // if we want pcm, default to signed 16-bit little-endian
        mRecordFormat.mFormatFlags = kLinearPCMFormatFlagIsSignedInteger | kLinearPCMFormatFlagIsPacked;
        mRecordFormat.mBitsPerChannel = 16;
        mRecordFormat.mBytesPerPacket = mRecordFormat.mBytesPerFrame = (mRecordFormat.mBitsPerChannel / 8) * mRecordFormat.mChannelsPerFrame;
        mRecordFormat.mFramesPerPacket = 1;
    }
}

void AQRecorder::StartRecord(CFStringRef inRecordFile)
{
    int i, bufferByteSize;
    UInt32 size;
    CFURLRef url;

    try {       
        mFileName = CFStringCreateCopy(kCFAllocatorDefault, inRecordFile);

        // specify the recording format
        SetupAudioFormat(  kAudioFormatLinearPCM );

        // create the queue
        XThrowIfError(AudioQueueNewInput(
                                      &mRecordFormat,
                                      MyInputBufferHandler,
                                      this /* userData */,
                                      NULL /* run loop */, NULL /* run loop mode */,
                                      0 /* flags */, &mQueue), "AudioQueueNewInput failed");

        // get the record format back from the queue's audio converter --
        // the file may require a more specific stream description than was necessary to create the encoder.
        mRecordPacket = 0;

        size = sizeof(mRecordFormat);
        XThrowIfError(AudioQueueGetProperty(mQueue, kAudioQueueProperty_StreamDescription,  
                                         &mRecordFormat, &size), "couldn't get queue's format");

        NSString *recordFile = [NSTemporaryDirectory() stringByAppendingPathComponent: (NSString*)inRecordFile];    


        NSLog(recordFile);

        url = CFURLCreateWithString(kCFAllocatorDefault, (CFStringRef)recordFile, NULL);

        // create the audio file kAudioFileCAFType
        XThrowIfError(AudioFileCreateWithURL(url, kAudioFileWAVEType, &mRecordFormat, kAudioFileFlags_EraseFile,
                                          &mRecordFile), "AudioFileCreateWithURL failed");
        CFRelease(url);

        // copy the cookie first to give the file object as much info as we can about the data going in
        // not necessary for pcm, but required for some compressed audio
        CopyEncoderCookieToFile();

        // allocate and enqueue buffers
        bufferByteSize = ComputeRecordBufferSize(&mRecordFormat, kBufferDurationSeconds);   // enough bytes for half a second
        for (i = 0; i < kNumberRecordBuffers; ++i) {
            XThrowIfError(AudioQueueAllocateBuffer(mQueue, bufferByteSize, &mBuffers[i]),
                       "AudioQueueAllocateBuffer failed");
            XThrowIfError(AudioQueueEnqueueBuffer(mQueue, mBuffers[i], 0, NULL),
                       "AudioQueueEnqueueBuffer failed");
        }
        // start the queue
        mIsRunning = true;
        XThrowIfError(AudioQueueStart(mQueue, NULL), "AudioQueueStart failed");
    }
    catch (CAXException &e) {
        char buf[256];
        fprintf(stderr, "Error: %s (%s)\n", e.mOperation, e.FormatError(buf));
    }
    catch (...) {
        fprintf(stderr, "An unknown error occurred\n");
    }   

}

当尝试在 SetupAudioFormat( kAudioFormatLinearPCM ); 行中使用 kAudioFormatMPEG4AAC 而不是 kAudioFormatLinearPCM 格式时，代码可以正常工作，我的代码无法创建音频文件，am我错过了一些东西，知道如​​何解决吗