ExtAudioFileWrite 生成不可用的 m4a aac 文件

Question

我正在尝试将单声道线性 pcm 文件转换为 aac 文件。经过一番努力，我终于使用以下设置输出了它，但现在该文件无法播放。我真的不知道该去哪里寻找 - 我找到的所有示例都与我已有的类似。

我见过的所有示例都有 destFormat.mBytesPerPacket = 0 但我无法让它写入（我在写入时得到 -66567，它解析为 kExtAudioFileError_MaxPacketSizeUnknown）。

ExtAudioFileRef sourceFile = 0;
ExtAudioFileRef destinationFile = 0;
OSStatus        error = noErr;

AudioStreamBasicDescription srcFormat, destFormat;
UInt32 size = sizeof(srcFormat);
error = ExtAudioFileOpenURL((CFURLRef)self.track.location, &sourceFile);
if(error != noErr)
    NSLog(@"conversion error: %i", error);
error = noErr;

ExtAudioFileGetProperty(sourceFile, kExtAudioFileProperty_FileDataFormat, &size, &srcFormat);

destFormat.mFormatID = kAudioFormatMPEG4AAC;
destFormat.mSampleRate = 22000;
destFormat.mFormatFlags = 0;
destFormat.mBytesPerPacket = 2; // must have a value or won't write apparently
destFormat.mFramesPerPacket = 0;
destFormat.mBytesPerFrame = 0;
destFormat.mChannelsPerFrame = 1;
destFormat.mBitsPerChannel = 0;
destFormat.mReserved = 0;

//create the output file

NSString *destURL = [self.track.location absoluteString];
NSLog(@"source url: %@", destURL);
destURL = [destURL substringToIndex:([destURL length] - 3)]; //remove caf extension
NSLog(@"source url with no extension: %@", destURL);
destURL = [NSString stringWithFormat:@"%@m4a",destURL]; //add acc extension
NSLog(@"dest url with correct extension: %@", destURL);
NSURL *destinationURL = [NSURL URLWithString:destURL];

size = sizeof(destFormat);
AudioFormatGetProperty(kAudioFormatProperty_FormatInfo, 0, nil, &size, &destFormat);

error = ExtAudioFileCreateWithURL((CFURLRef)destinationURL, kAudioFileM4AType, &destFormat, NULL, kAudioFileFlags_EraseFile, &destinationFile);
if(error != noErr)
    NSLog(@"conversion error: %i", error);
error = noErr;

//canonical format
AudioStreamBasicDescription clientFormat;
clientFormat.mFormatID = kAudioFormatLinearPCM;
clientFormat.mSampleRate = 22000;
int sampleSize = sizeof(AudioSampleType);
clientFormat.mFormatFlags = kAudioFormatFlagsCanonical;
clientFormat.mBitsPerChannel = 8 * sampleSize;
clientFormat.mChannelsPerFrame = 1;
clientFormat.mFramesPerPacket = 1;
clientFormat.mBytesPerPacket = sampleSize;
clientFormat.mBytesPerFrame = sampleSize;
clientFormat.mFormatFlags |= kAudioFormatFlagIsNonInterleaved;

//set the intermediate format to canonical on the source file for conversion (?)
ExtAudioFileSetProperty(sourceFile, kExtAudioFileProperty_ClientDataFormat, size, &clientFormat);

//get the converter
AudioConverterRef audioConverter;
size = sizeof(audioConverter);
error = ExtAudioFileGetProperty(destinationFile, kExtAudioFileProperty_AudioConverter, &size, &audioConverter);
if(error != noErr)
    NSLog(@"error getting converter: %i", error);
error = noErr;

/*UInt32 bitRate = 64000;   
error = AudioConverterSetProperty(audioConverter, kAudioConverterEncodeBitRate, sizeof(bitRate), &bitRate);
if(error != noErr)
    NSLog(@"error setting bit rate: %i", error);
error = noErr;*/

// set up buffers
UInt32 bufferByteSize = 32768;
char srcBuffer[bufferByteSize];

NSLog(@"converting...");

int i=0;
while (true) {
    i++;
    AudioBufferList fillBufList;
    fillBufList.mNumberBuffers = 1;
    fillBufList.mBuffers[0].mNumberChannels = 1;
    fillBufList.mBuffers[0].mDataByteSize = bufferByteSize;
    fillBufList.mBuffers[0].mData = srcBuffer;

    // client format is always linear PCM - so here we determine how many frames of lpcm
    // we can read/write given our buffer size
    UInt32 numFrames = bufferByteSize / clientFormat.mBytesPerFrame;

    error = ExtAudioFileRead(sourceFile, &numFrames, &fillBufList); 
    if(error != noErr)
        NSLog(@"read error: %i run: %i", error, i);

    if (!numFrames) {
        // this is our termination condition
        error = noErr;
        break;
    }

    //this is the actual conversion
    error = ExtAudioFileWrite(destinationFile, numFrames, &fillBufList);

    if(error != noErr)
        NSLog(@"conversion error: %i run: %i", error, i);
}

if (destinationFile) ExtAudioFileDispose(destinationFile);
if (sourceFile) ExtAudioFileDispose(sourceFile);

Answer 1

您必须设置kExtAudioFileProperty_ClientDataFormat才能编码为非pcm格式（根据文档）。客户端格式是您要在应用程序中处理音频数据的格式（通常是 pcm）。

Answer 2

此处发布的好示例： 使用 AVMutableAudioMix 调整资产内曲目的音量

这并不完全是你想要的，但它会在 m4a 中读取 ->转换为 pcm ->改变音量->然后保存回m4a。

Answer 3

您需要在目标文件上设置客户端数据格式以匹配源文件上的客户端格式。您现在设置的方式是，目标文件希望您将 AAC 传递给它，但事实并非如此。

Answer 4

Core Audio 确实很神秘，错误结果通常与真正的错误没有什么关系。我看到的最明显的问题是 22000 的采样率对于 AAC 来说不是有效的。一些有效的采样率为 32000、44100 或 48000。我不确定是否支持其他采样率，但通常它们是这些数字的偶数倍或整除。