ios - 如何在iOS中将2个单声道文件转换为单个立体声文件？

Question

我正在尝试将2个CAF文件本地转换为单个文件。这两个CAF文件是单声道流，理想情况下，我希望它们是立体声文件，这样我就可以从一个声道获取麦克风，从另一个声道获取扬声器。

我最初从使用AVAssetTrack和AVMutableCompositionTracks开始，但是我无法解决混合问题。我合并的文件是一个单声道流，将两个文件交织在一起。所以我选择了AVAudioEngine路线。

根据我的理解，我可以传入两个文件作为输入节点，将它们附加到混音器，并具有一个能够获取立体声混音的输出节点。输出文件具有立体声布局，但是似乎没有音频数据写入其中，因为我可以在Audacity中打开它并查看立体声布局。在installTapOnBus调用周围放置一个dipatch sephamore信号也无济于事。任何见识将不胜感激，因为CoreAudio一直是理解的挑战。

// obtain path of microphone and speaker files
NSString *micPath = [[NSBundle mainBundle] pathForResource:@"microphone" ofType:@"caf"];
NSString *spkPath = [[NSBundle mainBundle] pathForResource:@"speaker" ofType:@"caf"];
NSURL *micURL = [NSURL fileURLWithPath:micPath];
NSURL *spkURL = [NSURL fileURLWithPath:spkPath];

// create engine
AVAudioEngine *engine = [[AVAudioEngine alloc] init];

AVAudioFormat *stereoFormat = [[AVAudioFormat alloc] initStandardFormatWithSampleRate:16000 channels:2];

AVAudioMixerNode *mainMixer = engine.mainMixerNode;

// create audio files
AVAudioFile *audioFile1 = [[AVAudioFile alloc] initForReading:micURL error:nil];
AVAudioFile *audioFile2 = [[AVAudioFile alloc] initForReading:spkURL error:nil];

// create player input nodes
AVAudioPlayerNode *apNode1 = [[AVAudioPlayerNode alloc] init];
AVAudioPlayerNode *apNode2 = [[AVAudioPlayerNode alloc] init];

// attach nodes to the engine
[engine attachNode:apNode1];
[engine attachNode:apNode2];

// connect player nodes to engine's main mixer
stereoFormat = [mainMixer outputFormatForBus:0];
[engine connect:apNode1 to:mainMixer fromBus:0 toBus:0 format:audioFile1.processingFormat];
[engine connect:apNode2 to:mainMixer fromBus:0 toBus:1 format:audioFile2.processingFormat];
[engine connect:mainMixer to:engine.outputNode format:stereoFormat];

// start the engine
NSError *error = nil;
if(![engine startAndReturnError:&error]){
    NSLog(@"Engine failed to start.");
}

// create output file
NSString *mergedAudioFile = [[micPath stringByDeletingLastPathComponent] stringByAppendingPathComponent:@"merged.caf"];
[[NSFileManager defaultManager] removeItemAtPath:mergedAudioFile error:&error];
NSURL *mergedURL = [NSURL fileURLWithPath:mergedAudioFile];
AVAudioFile *outputFile = [[AVAudioFile alloc] initForWriting:mergedURL settings:[engine.inputNode inputFormatForBus:0].settings error:&error];

// write from buffer to output file
[mainMixer installTapOnBus:0 bufferSize:4096 format:[mainMixer outputFormatForBus:0] block:^(AVAudioPCMBuffer *buffer, AVAudioTime *when){
    NSError *error;
    BOOL success;
    NSLog(@"Writing");
    if((outputFile.length < audioFile1.length) || (outputFile.length < audioFile2.length)){
        success = [outputFile writeFromBuffer:buffer error:&error];
        NSCAssert(success, @"error writing buffer data to file, %@", [error localizedDescription]);
        if(error){
            NSLog(@"Error: %@", error);
        }
    }
    else{
        [mainMixer removeTapOnBus:0];
        NSLog(@"Done writing");
    }
}];

}

Answer 1

使用ExtAudioFile进行此操作涉及三个文件和三个缓冲区。两个单声道用于阅读，一个立体声用于写作。在一个循环中，每个单声道文件都会将一小段音频读取到其单声道输出缓冲区，然后复制到立体声缓冲区的正确“一半”。然后，在立体声缓冲区充满数据的情况下，将该缓冲区写入输出文件，重复进行直到两个单声道文件读完为止(如果一个单声道文件长于另一个单声道文件，则写入零)。

对我而言，最有问题的领域是正确设置文件格式，core-audio要求非常特定的格式。幸运的是，存在AVAudioFormat可以简化某些常见格式的创建。

每个音频文件读取器/写入器都有两种格式，一种表示数据存储的格式(file_format)，另一种指示读取器/写入器的输出格式(client_format)。如果格式不同，读取器/写入器中会内置格式转换器。

这是一个例子:

-(void)soTest{


    //This is what format the readers will output
    AVAudioFormat *monoClienFormat = [[AVAudioFormat alloc]initWithCommonFormat:AVAudioPCMFormatInt16 sampleRate:44100.0 channels:1 interleaved:0];

    //This is the format the writer will take as input
    AVAudioFormat *stereoClientFormat = [[AVAudioFormat alloc]initWithCommonFormat:AVAudioPCMFormatInt16 sampleRate:44100 channels:2 interleaved:0];

    //This is the format that will be written to storage.  It must be interleaved.
    AVAudioFormat *stereoFileFormat = [[AVAudioFormat alloc]initWithCommonFormat:AVAudioPCMFormatInt16 sampleRate:44100 channels:2 interleaved:1];




    NSURL *leftURL = [NSBundle.mainBundle URLForResource:@"left" withExtension:@"wav"];
    NSURL *rightURL = [NSBundle.mainBundle URLForResource:@"right" withExtension:@"wav"];

    NSString *stereoPath = [documentsDir() stringByAppendingPathComponent:@"stereo.wav"];
    NSURL *stereoURL = [NSURL URLWithString:stereoPath];

    ExtAudioFileRef leftReader;
    ExtAudioFileRef rightReader;
    ExtAudioFileRef stereoWriter;


    OSStatus status = 0;

    //Create readers and writer
    status = ExtAudioFileOpenURL((__bridge CFURLRef)leftURL, &leftReader);
    if(status)printf("error %i",status);//All the ExtAudioFile functins return a non-zero status if there's an error, I'm only checking one to demonstrate, but you should be checking all the ExtAudioFile function returns.
    ExtAudioFileOpenURL((__bridge CFURLRef)rightURL, &rightReader);
    //Here the file format is set to stereo interleaved.
    ExtAudioFileCreateWithURL((__bridge CFURLRef)stereoURL, kAudioFileCAFType, stereoFileFormat.streamDescription, nil, kAudioFileFlags_EraseFile, &stereoWriter);


    //Set client format for readers and writer
    ExtAudioFileSetProperty(leftReader, kExtAudioFileProperty_ClientDataFormat, sizeof(AudioStreamBasicDescription), monoClienFormat.streamDescription);
    ExtAudioFileSetProperty(rightReader, kExtAudioFileProperty_ClientDataFormat, sizeof(AudioStreamBasicDescription), monoClienFormat.streamDescription);
    ExtAudioFileSetProperty(stereoWriter, kExtAudioFileProperty_ClientDataFormat, sizeof(AudioStreamBasicDescription), stereoClientFormat.streamDescription);


    int framesPerRead = 4096;
    int bufferSize = framesPerRead * sizeof(SInt16);

    //Allocate memory for the buffers
    AudioBufferList *leftBuffer = createBufferList(bufferSize,1);
    AudioBufferList *rightBuffer = createBufferList(bufferSize,1);
    AudioBufferList *stereoBuffer = createBufferList(bufferSize,2);

    //ExtAudioFileRead takes an ioNumberFrames argument.  On input the number of frames you want, on otput it's the number of frames you got.  0 means your done.
    UInt32 leftFramesIO = framesPerRead;
    UInt32 rightFramesIO = framesPerRead;



    while (leftFramesIO || rightFramesIO) {
        if (leftFramesIO){
            //If frames to read is less than a full buffer, zero out the remainder of the buffer
            int framesRemaining = framesPerRead - leftFramesIO;
            if (framesRemaining){
                memset(((SInt16 *)leftBuffer->mBuffers[0].mData) + framesRemaining, 0, sizeof(SInt16) * framesRemaining);
            }
            //Read into left buffer
            leftBuffer->mBuffers[0].mDataByteSize = leftFramesIO * sizeof(SInt16);
            ExtAudioFileRead(leftReader, &leftFramesIO, leftBuffer);
        }
        else{
            //set to zero if no more frames to read
            memset(leftBuffer->mBuffers[0].mData, 0, sizeof(SInt16) * framesPerRead);
        }

        if (rightFramesIO){
            int framesRemaining = framesPerRead - rightFramesIO;
            if (framesRemaining){
                memset(((SInt16 *)rightBuffer->mBuffers[0].mData) + framesRemaining, 0, sizeof(SInt16) * framesRemaining);
            }
            rightBuffer->mBuffers[0].mDataByteSize = rightFramesIO * sizeof(SInt16);
            ExtAudioFileRead(rightReader, &rightFramesIO, rightBuffer);
        }
        else{
            memset(rightBuffer->mBuffers[0].mData, 0, sizeof(SInt16) * framesPerRead);
        }


        UInt32 stereoFrames = MAX(leftFramesIO, rightFramesIO);

        //copy left to stereoLeft and right to stereoRight
        memcpy(stereoBuffer->mBuffers[0].mData, leftBuffer->mBuffers[0].mData, sizeof(SInt16) * stereoFrames);
        memcpy(stereoBuffer->mBuffers[1].mData, rightBuffer->mBuffers[0].mData, sizeof(SInt16) * stereoFrames);

        //write to file
        stereoBuffer->mBuffers[0].mDataByteSize = stereoFrames * sizeof(SInt16);
        stereoBuffer->mBuffers[1].mDataByteSize = stereoFrames * sizeof(SInt16);
        ExtAudioFileWrite(stereoWriter, stereoFrames, stereoBuffer);

    }

    ExtAudioFileDispose(leftReader);
    ExtAudioFileDispose(rightReader);
    ExtAudioFileDispose(stereoWriter);

    freeBufferList(leftBuffer);
    freeBufferList(rightBuffer);
    freeBufferList(stereoBuffer);

}

AudioBufferList *createBufferList(int bufferSize, int numberBuffers){
    assert(bufferSize > 0 && numberBuffers > 0);
    int bufferlistByteSize = sizeof(AudioBufferList);
    bufferlistByteSize += sizeof(AudioBuffer) * (numberBuffers - 1);
    AudioBufferList *bufferList = malloc(bufferlistByteSize);
    bufferList->mNumberBuffers = numberBuffers;
    for (int i = 0; i < numberBuffers; i++) {
        bufferList->mBuffers[i].mNumberChannels = 1;
        bufferList->mBuffers[i].mData = malloc(bufferSize);
    }
    return bufferList;
};
void freeBufferList(AudioBufferList *bufferList){
    for (int i = 0; i < bufferList->mNumberBuffers; i++) {
        free(bufferList->mBuffers[i].mData);
    }
    free(bufferList);
}
NSString *documentsDir(){
    static NSString *path = NULL;
    if(!path){
        path = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, 1).firstObject;
    }
    return path;
}