反转音频文件 Swift/Objective-C

有没有办法可以反转并导出 .m4a 音频文件？我找到了反转音轨的解决方案here，但它似乎只适用于 .caf 文件格式。如果唯一的方法是使用 .caf，是否有办法先将 .m4a 文件转换为 .caf？

Update: In 另一个帖子我发现 AVAssetReader 可用于从音频文件中读取音频样本，但我不知道如何以相反的顺序写回样本。下面的代码片段是直接来自帖子的答案。任何帮助，将不胜感激。谢谢

+ (void) reverseAudioTrack: (AVAsset *)audioAsset outputURL: (NSURL *)outputURL {
NSError *error;

AVAssetReader* reader = [[AVAssetReader alloc] initWithAsset:audioAsset error:&error];
if (error) {NSLog(@"%@", error.localizedDescription);}

AVAssetTrack* track = [[audioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];

NSMutableDictionary* audioReadSettings = [NSMutableDictionary dictionary];
[audioReadSettings setValue:[NSNumber numberWithInt:kAudioFormatLinearPCM]
                     forKey:AVFormatIDKey];

AVAssetReaderTrackOutput* readerOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:track outputSettings:audioReadSettings];
[reader addOutput:readerOutput];
[reader startReading];

CMSampleBufferRef sample; //= [readerOutput copyNextSampleBuffer];
NSMutableArray *samples = [[NSMutableArray alloc] init];

// Get all samples
while((sample = [readerOutput copyNextSampleBuffer])) {
    [samples addObject:(__bridge id)sample];
    CFRelease(sample);
}

// Process samples in reverse
AudioChannelLayout acl;
bzero(&acl, sizeof(acl));
acl.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo;

AVAssetWriter *writer = [[AVAssetWriter alloc] initWithURL:outputURL
                                                   fileType:AVFileTypeAppleM4A
                                                      error:&error];
if (error) {NSLog(@"%@", error.localizedDescription);}
NSDictionary *writerOutputSettings = [ NSDictionary dictionaryWithObjectsAndKeys:
                                      [ NSNumber numberWithInt: kAudioFormatAppleLossless ], AVFormatIDKey,
                                      [ NSNumber numberWithInt: 16 ], AVEncoderBitDepthHintKey,
                                      [ NSNumber numberWithFloat: 44100.0 ], AVSampleRateKey,
                                      [ NSNumber numberWithInt: 1 ], AVNumberOfChannelsKey,
                                      [ NSData dataWithBytes: &acl length: sizeof( acl ) ], AVChannelLayoutKey, nil ];

AVAssetWriterInput *audioWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeAudio outputSettings:writerOutputSettings];

[writer addInput:audioWriterInput];
[writer startWriting];
[writer startSessionAtSourceTime:CMSampleBufferGetPresentationTimeStamp((__bridge CMSampleBufferRef)samples[0]) ];

// (1) Would it work if I loop in reverse here?
for (NSInteger i = 0; i < samples.count; i++) {
    CMBlockBufferRef buffer = CMSampleBufferGetDataBuffer((__bridge CMSampleBufferRef)samples[i]);

    CMItemCount numSamplesInBuffer = CMSampleBufferGetNumSamples((__bridge CMSampleBufferRef)samples[i]);
    AudioBufferList audioBufferList;
    CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer((__bridge CMSampleBufferRef)samples[i],
                                                            NULL,
                                                            &audioBufferList,
                                                            sizeof(audioBufferList),
                                                            NULL,
                                                            NULL,
                                                            kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment,
                                                            &buffer
                                                            );

    for (int bufferCount = 0; bufferCount < audioBufferList.mNumberBuffers; bufferCount++) {
        SInt16* samples = (SInt16 *)audioBufferList.mBuffers[bufferCount].mData;
        for (int i=0; i < numSamplesInBuffer; i++) {
            // amplitude for the sample is samples[i], assuming you have linear pcm to start with

            // (2) What should I be doing to write the samples into an audio file?
        }
    }
    CFRelease(buffer);
}

就在这里a您可以通过这种方式处理并导出 iOS 支持的任何音频文件。

然而，大多数这些格式（mp3仅举一例）是有损和压缩的。您必须首先解压缩数据，应用转换，然后重新压缩。您将应用于音频信息的大多数转换可能应该在原始 PCM 级别完成。

结合这两个语句，您可以通过几遍来完成此操作：

1. 将原始文件转换为kAudioFormatLinearPCM兼容的音频文件，例如AIFF
2. 处理该临时文件（反转其内容）
3. 将临时文件转换回原始格式

就像您将转换应用于压缩的文件一样jpeg图像，在此过程中会出现退化。最终的音频至多会再经历一个压缩周期。

因此，这种方法的真正数学答案实际上是否定的。

仅供参考，这里是 swift 3 中的一些起始代码。它需要进一步细化以跳过文件头。

var outAudioFile:AudioFileID?
var pcm = AudioStreamBasicDescription(mSampleRate: 44100.0,
                                      mFormatID: kAudioFormatLinearPCM,
                                      mFormatFlags: kAudioFormatFlagIsBigEndian | kAudioFormatFlagIsSignedInteger,
                                      mBytesPerPacket: 2,
                                      mFramesPerPacket: 1,
                                      mBytesPerFrame: 2,
                                      mChannelsPerFrame: 1,
                                      mBitsPerChannel: 16,
                                      mReserved: 0)

var theErr = AudioFileCreateWithURL(destUrl as CFURL!,
                                    kAudioFileAIFFType,
                                    &pcm,
                                    .eraseFile,
                                    &outAudioFile)
if noErr == theErr, let outAudioFile = outAudioFile {
    var inAudioFile:AudioFileID?
    theErr = AudioFileOpenURL(sourceUrl as! CFURL, .readPermission, 0, &inAudioFile)

    if noErr == theErr, let inAudioFile = inAudioFile {

        var fileDataSize:UInt64 = 0
        var thePropertySize:UInt32 = UInt32(MemoryLayout<UInt64>.stride)
        theErr = AudioFileGetProperty(inAudioFile,
                                      kAudioFilePropertyAudioDataByteCount,
                                      &thePropertySize,
                                      &fileDataSize)

        if( noErr == theErr) {
            let dataSize:Int64 = Int64(fileDataSize)
            let theData = UnsafeMutableRawPointer.allocate(bytes: Int(dataSize),
                                                           alignedTo: MemoryLayout<UInt8>.alignment)

            var readPoint:Int64 = Int64(dataSize)
            var writePoint:Int64 = 0

            while( readPoint > 0 )
            {
                var bytesToRead = UInt32(2)

                AudioFileReadBytes( inAudioFile, false, readPoint, &bytesToRead, theData)
                AudioFileWriteBytes( outAudioFile, false, writePoint, &bytesToRead, theData)

                writePoint += 2
                readPoint -= 2
            }

            theData.deallocate(bytes: Int(dataSize), alignedTo: MemoryLayout<UInt8>.alignment)

            AudioFileClose(inAudioFile);
            AudioFileClose(outAudioFile);
        }
    }
}