Question

我想在iOS上反转现有的音频文件（例如WAV，CAF，...）。有关如何实现这一目标的任何建议？开源库？

Answer 1

我参与了一个示例应用，它会记录用户说的内容并向后播放。我使用CoreAudio来实现这一目标。链接到应用代码。

每个样本的大小为16位（2字节）（单声道）。您可以通过从记录结束开始并向后阅读将每个样本复制到不同的缓冲区中来一次加载每个样本。当你到达数据的开头时，你已经反转了数据并且播放将被颠倒。

    // set up output file
AudioFileID outputAudioFile;

AudioStreamBasicDescription myPCMFormat;
myPCMFormat.mSampleRate = 16000.00;
myPCMFormat.mFormatID = kAudioFormatLinearPCM ;
myPCMFormat.mFormatFlags =  kAudioFormatFlagsCanonical;
myPCMFormat.mChannelsPerFrame = 1;
myPCMFormat.mFramesPerPacket = 1;
myPCMFormat.mBitsPerChannel = 16;
myPCMFormat.mBytesPerPacket = 2;
myPCMFormat.mBytesPerFrame = 2;


AudioFileCreateWithURL((__bridge CFURLRef)self.flippedAudioUrl,
                       kAudioFileCAFType,
                       &myPCMFormat,
                       kAudioFileFlags_EraseFile,
                       &outputAudioFile);
// set up input file
AudioFileID inputAudioFile;
OSStatus theErr = noErr;
UInt64 fileDataSize = 0;

AudioStreamBasicDescription theFileFormat;
UInt32 thePropertySize = sizeof(theFileFormat);

theErr = AudioFileOpenURL((__bridge CFURLRef)self.recordedAudioUrl, kAudioFileReadPermission, 0, &inputAudioFile);

thePropertySize = sizeof(fileDataSize);
theErr = AudioFileGetProperty(inputAudioFile, kAudioFilePropertyAudioDataByteCount, &thePropertySize, &fileDataSize);

UInt32 dataSize = fileDataSize;
void* theData = malloc(dataSize);

//Read data into buffer
UInt32 readPoint  = dataSize;
UInt32 writePoint = 0;
while( readPoint > 0 )
{
    UInt32 bytesToRead = 2;

    AudioFileReadBytes( inputAudioFile, false, readPoint, &bytesToRead, theData );
    AudioFileWriteBytes( outputAudioFile, false, writePoint, &bytesToRead, theData );

    writePoint += 2;
    readPoint -= 2;
}

free(theData);
AudioFileClose(inputAudioFile);
AudioFileClose(outputAudioFile);

Answer 2

WAV文件只包含原始PCM样本。如果以相反的顺序播放它们，您将获得反向声音。 See this question for details. （以及this search）

Answer 3

Kiran在Swift 2.2中的回答：

    let forwardAudioURL: NSURL = ... // wherever your original audio is
    let reversedAudioURL: NSURL = ... // wherever you want the reversed file to go

    // Load forward audio into originalAudioFile
    var originalAudioFile: AudioFileID = nil
    let possibleError1 = AudioFileOpenURL(forwardAudioURL,
                                          AudioFilePermissions.ReadPermission,
                                          0,
                                          &originalAudioFile)

    // Load the size in bytes of the original audio into originalAudioSize variable
    var originalAudioSize: Int64 = 0
    var propertySize: UInt32 = 8
    let possibleError2 = AudioFileGetProperty(originalAudioFile,
                                              kAudioFilePropertyAudioDataByteCount,
                                              &propertySize,
                                              &originalAudioSize)

    if possibleError1 != 0 || possibleError2 != 0 {
        // Handle errors if you want
    }

    // Set up file that the reversed audio will be loaded into
    var reversedAudioFile: AudioFileID = nil
    var format = AudioStreamBasicDescription()
    format.mSampleRate = 16000
    format.mFormatID = kAudioFormatLinearPCM
    format.mChannelsPerFrame = 1
    format.mFramesPerPacket = 1
    format.mBitsPerChannel = 16
    format.mBytesPerPacket = 2
    format.mBytesPerFrame = 2
    AudioFileCreateWithURL(reversedAudioURL,
                           kAudioFileCAFType,
                           &format,
                           AudioFileFlags.EraseFile,
                           &reversedAudioFile)

    // Read data into the reversedAudioFile
    var readPoint: Int64 = originalAudioSize
    var writePoint: Int64 = 0
    var buffer: Int16 = 0
    while readPoint > 0 {
        var bytesToRead: UInt32 = 2;
        AudioFileReadBytes(originalAudioFile,
                           false,
                           readPoint,
                           &bytesToRead,
                           &buffer)
        AudioFileWriteBytes(reversedAudioFile,
                            false,
                            writePoint,
                            &bytesToRead,
                            &buffer)
        writePoint += 2
        readPoint -= 2
    }
    AudioFileClose(originalAudioFile)
    AudioFileClose(reversedAudioFile)

Answer 4

使用libsndfile，我能够读取录制的音频样本。下面的代码片段显示了如何打开输入文件，并以相反的顺序写入输出。您应该注意，所有数据都在内存中读取！

SF_INFO info_read;
info_read.format = 0;
SNDFILE* sndfilein = sf_open("my_input_file.caf", SFM_READ, &info_read);

SF_INFO info_write;
info_write.format = info.format;
info_write.channels = info.channels;
info_write.frames = info.frames;
info_write.samplerate = info.samplerate;
info_write.sections = info.sections;
info_write.seekable = info.seekable;

SNDFILE* sndfileout = sf_open("my_output_file.caf", SFM_RDWR, &info_write);

int* buf = new int[8000 * 30];
int framesRead = sf_readf_int(sndfilein, buf, info.frames);
for (int i = 0; i < info.frames; i++)
    sf_writef_int(sndfileout, &buf[info.frames - i], 1);
delete[] buf;

sf_close(sndfilein);
sf_close(sndfileout);

如何反转音频文件？

4 个答案: