有没有办法可以反转并导出.m4a音频文件？我找到了一个反转音轨here的解决方案，但它似乎只是在处理.caf文件格式。如果唯一的方法是使用.caf，有没有办法将.m4a文件首先转换为.caf？
更新 在another post中，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。以下代码片段直接来自帖子。任何帮助，将不胜感激。感谢
+ (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);
}
答案 0 :(得分：1)
是的，有 方式可以处理，然后导出任何有iOS支持的音频文件。
但是，大多数这些格式（名称为mp3）都是有损和压缩的。您必须首先解压缩数据，应用转换并重新压缩。您将应用于音频信息的大多数转换应该可以在原始PCM级别完成。
结合这两个陈述，你可以在几个过程中完成这个：
kAudioFormatLinearPCM的音频文件，例如AIFF 就像您正在对压缩的jpeg图像应用转换一样，过程中会出现降级。最后的音频最多只会遭受一次压缩周期。
因此，这种方法的真正数学答案实际上是没有。
仅供参考，以下是swift 3中的一些入门代码。需要进一步细化才能跳过文件头。
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);
}
}
}
答案 1 :(得分：0)
我从这个 repo 在 GitHub 上找到了一个解决方案：https://github.com/tomisacat/AudioReverse
下面的函数对我来说非常适合反转 m4a 文件
func reverse(fromUrl: URL) -> URL? {
do {
let inFile: AVAudioFile = try AVAudioFile(forReading: fromUrl)
let format: AVAudioFormat = inFile.processingFormat
let frameCount: AVAudioFrameCount = UInt32(inFile.length)
let outSettings = [AVNumberOfChannelsKey: format.channelCount,
AVSampleRateKey: format.sampleRate,
AVLinearPCMBitDepthKey: 16,
AVFormatIDKey: kAudioFormatMPEG4AAC] as [String : Any]
let outputPath = NSTemporaryDirectory() + "/" + "reverse.m4a"
let outputUrl = URL(fileURLWithPath: outputPath)
let outFile: AVAudioFile = try AVAudioFile(forWriting: outputUrl, settings: outSettings)
let forwardBuffer: AVAudioPCMBuffer = AVAudioPCMBuffer(pcmFormat: format, frameCapacity: frameCount)
let reverseBuffer: AVAudioPCMBuffer = AVAudioPCMBuffer(pcmFormat: format, frameCapacity: frameCount)
try inFile.read(into: forwardBuffer)
let frameLength = forwardBuffer.frameLength
reverseBuffer.frameLength = frameLength
let audioStride = forwardBuffer.stride
for channelIdx in 0..<forwardBuffer.format.channelCount {
let forwardChannelData = forwardBuffer.floatChannelData?.advanced(by: Int(channelIdx)).pointee
let reverseChannelData = reverseBuffer.floatChannelData?.advanced(by: Int(channelIdx)).pointee
var reverseIdx: Int = 0
for idx in stride(from: frameLength, to: 0, by: -1) {
memcpy(reverseChannelData?.advanced(by: reverseIdx * audioStride), forwardChannelData?.advanced(by: Int(idx) * audioStride), MemoryLayout<Float>.size)
reverseIdx += 1
}
}
try outFile.write(from: reverseBuffer)
return outputUrl
} catch let error {
print(error.localizedDescription)
return nil
}
}