从线性PCM中提取音频通道

Question

我想提取一个声道音频从一个LPCM原始文件，即提取左声道和右声道的立体声LPCM文件。LPCM是16位深度，交错，2通道，小端。从我收集的字节顺序是{LeftChannel，RightChannel，LeftChannel，RightChannel...}，因为它是16位深度，所以每个通道将有2个字节的样本，对吗？

所以我的问题是，如果我想提取左声道，那么我会取0,2,4,6...n*2地址中的字节？而右通道将是1,3,4，...(n*2+1)。

另外，在提取音频通道后，我是否应该将提取通道的格式设置为16位深度,1通道？

提前感谢

这是我目前用来从AssetReader中提取PCM音频的代码。这段代码适用于在不提取音乐文件通道的情况下编写音乐文件，所以它可能是由格式或其他原因引起的。

    NSURL *assetURL = [song valueForProperty:MPMediaItemPropertyAssetURL];
AVURLAsset *songAsset = [AVURLAsset URLAssetWithURL:assetURL options:nil];
NSDictionary *outputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                [NSNumber numberWithInt:kAudioFormatLinearPCM], AVFormatIDKey, 
                                [NSNumber numberWithFloat:44100.0], AVSampleRateKey,
                                [NSNumber numberWithInt:2], AVNumberOfChannelsKey,
                            //  [NSData dataWithBytes:&channelLayout length:sizeof(AudioChannelLayout)], AVChannelLayoutKey,
                                [NSNumber numberWithInt:16], AVLinearPCMBitDepthKey,
                                [NSNumber numberWithBool:NO], AVLinearPCMIsNonInterleaved,
                                [NSNumber numberWithBool:NO],AVLinearPCMIsFloatKey,
                                [NSNumber numberWithBool:NO], AVLinearPCMIsBigEndianKey,
                                nil];
NSError *assetError = nil;
AVAssetReader *assetReader = [[AVAssetReader assetReaderWithAsset:songAsset
                                                            error:&assetError]
                              retain];
if (assetError) {
    NSLog (@"error: %@", assetError);
    return;
}

AVAssetReaderOutput *assetReaderOutput = [[AVAssetReaderAudioMixOutput 
                                           assetReaderAudioMixOutputWithAudioTracks:songAsset.tracks
                                           audioSettings: outputSettings]
                                          retain];
if (! [assetReader canAddOutput: assetReaderOutput]) {
    NSLog (@"can't add reader output... die!");
    return;
}
[assetReader addOutput: assetReaderOutput];


NSArray *dirs = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectoryPath = [dirs objectAtIndex:0];

//CODE TO SPLIT STEREO
[self setupAudioWithFormatMono:kAudioFormatLinearPCM];
NSString *splitExportPath = [[documentsDirectoryPath stringByAppendingPathComponent:@"monoleft.caf"] retain];
if ([[NSFileManager defaultManager] fileExistsAtPath:splitExportPath]) {
    [[NSFileManager defaultManager] removeItemAtPath:splitExportPath error:nil];
}

AudioFileID mRecordFile;
NSURL *splitExportURL = [NSURL fileURLWithPath:splitExportPath];


OSStatus status =  AudioFileCreateWithURL(splitExportURL, kAudioFileCAFType, &_streamFormat, kAudioFileFlags_EraseFile,
                                          &mRecordFile);

NSLog(@"status os %d",status);

[assetReader startReading];

CMSampleBufferRef sampBuffer = [assetReaderOutput copyNextSampleBuffer];
UInt32 countsamp= CMSampleBufferGetNumSamples(sampBuffer);
NSLog(@"number of samples %d",countsamp);

SInt64 countByteBuf = 0;
SInt64 countPacketBuf = 0;
UInt32 numBytesIO = 0;
UInt32 numPacketsIO = 0;
NSMutableData * bufferMono = [NSMutableData new];
while (sampBuffer) {


    AudioBufferList  audioBufferList;
    CMBlockBufferRef blockBuffer;
    CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(sampBuffer, NULL, &audioBufferList, sizeof(audioBufferList), NULL, NULL, 0, &blockBuffer);
    for (int y=0; y<audioBufferList.mNumberBuffers; y++) {
        AudioBuffer audioBuffer = audioBufferList.mBuffers[y];
        //frames = audioBuffer.mData;
        NSLog(@"the number of channel for buffer number %d is %d",y,audioBuffer.mNumberChannels);
        NSLog(@"The buffer size is %d",audioBuffer.mDataByteSize);






        //Append mono left to buffer data
        for (int i=0; i<audioBuffer.mDataByteSize; i= i+4) {
            [bufferMono appendBytes:(audioBuffer.mData+i) length:2];
        }

        //the number of bytes in the mutable data containing mono audio file
        numBytesIO = [bufferMono length];
        numPacketsIO = numBytesIO/2;
        NSLog(@"numpacketsIO %d",numPacketsIO);
        status = AudioFileWritePackets(mRecordFile, NO, numBytesIO, &_packetFormat, countPacketBuf, &numPacketsIO, audioBuffer.mData);
        NSLog(@"status for writebyte %d, packets written %d",status,numPacketsIO);
        if(numPacketsIO != (numBytesIO/2)){
            NSLog(@"Something wrong");
            assert(0);
        }


        countPacketBuf = countPacketBuf + numPacketsIO;
        [bufferMono setLength:0];


    }

    sampBuffer = [assetReaderOutput copyNextSampleBuffer];
    countsamp= CMSampleBufferGetNumSamples(sampBuffer);
    NSLog(@"number of samples %d",countsamp);
}
AudioFileClose(mRecordFile);
[assetReader cancelReading];
[self performSelectorOnMainThread:@selector(updateCompletedSizeLabel:)
                       withObject:0
                    waitUntilDone:NO];

音频文件服务的输出格式如下：

        _streamFormat.mFormatFlags = kLinearPCMFormatFlagIsSignedInteger | kLinearPCMFormatFlagIsPacked;
    _streamFormat.mBitsPerChannel = 16;
    _streamFormat.mChannelsPerFrame = 1;
    _streamFormat.mBytesPerPacket = 2;
    _streamFormat.mBytesPerFrame = 2;// (_streamFormat.mBitsPerChannel / 8) * _streamFormat.mChannelsPerFrame;
    _streamFormat.mFramesPerPacket = 1;
    _streamFormat.mSampleRate = 44100.0;

    _packetFormat.mStartOffset = 0;
    _packetFormat.mVariableFramesInPacket = 0;
    _packetFormat.mDataByteSize = 2;

Answer 1

听起来几乎是对的-你有一个16位的深度，所以这意味着每个样本将需要2个字节。这意味着左声道数据将以字节{0,1}，{4,5}，{8,9}等为单位。交错表示样本是交错的，而不是字节。除此之外，我会试一试，看看你的代码是否有任何问题。

在提取音频通道后，我是否应该将提取通道的格式设置为16位深度,1通道？

提取后，两个通道中只剩下一个通道，所以这是正确的。

Answer 2

我有一个类似的错误，音频听起来‘慢’，原因是你指定的mChannelsPerFrame为1，而你有一个双声道声音。将其设置为2，将加快播放速度。如果你这样做了，输出“听起来”是否正确……:)

Answer 3

我正在尝试将立体声音频分割成两个单声道文件(split stereo audio to mono streams on iOS)。我一直在使用你的代码，但似乎不能让它工作。您的setupAudioWithFormatMono方法的内容是什么？