Question

我正在尝试将单声道线性pcm文件转换为aac文件。经过多次努力，我终于使用以下设置得到了输出，但现在文件将无法播放。我真的不知道在哪里看 - 我发现的所有例子都与我已有的相似。

我见过的所有样本都有destFormat.mBytesPerPacket = 0但是我无法写它（我在写入时得到-66567，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）。客户端格式是您希望处理应用中音频数据的格式（通常为pcm）。

Answer 2

这里贴出了很好的例子：Using AVMutableAudioMix to adjust volumes for tracks within asset

这不是你想要的，但它会在m4a中读取 - ＆gt;转换为pcm - ＆gt;改变音量 - ＆gt;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。您现在设置目标文件的方式是期望您将其交给AAC，而您不是。

Answer 4

Core Audio确实很神秘，错误结果通常与真正的错误无关。我看到的最明显的问题是22000的采样率不是AAC的有效率。一些有效的采样率是32000,44100或48000.我不确定是否支持其他人，但通常它们甚至是倍数或者可以被这些数字整除。

ExtAudioFileWrite生成不可用的m4a aac文件

4 个答案: