无法正确反转 AVAsset 音频。唯一的结果是白噪声
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【中文标题】无法正确反转 AVAsset 音频。唯一的结果是白噪声【英文标题】：Can't reverse AVAsset audio properly. The only result is white noise 【发布时间】：2016-02-23 15:28:55 【问题描述】：我正在尝试反转 AVAsset 音频并将其保存到文件中。为了清楚起见，我用问题https://github.com/ksenia-lyagusha/AudioReverse.git做了简单的应用程序
应用程序从包中获取mp4 视频文件，将其作为单个m4a 文件导出到沙箱中的临时文件夹，然后尝试从那里读取它，反转并保存结果文件。
临时的m4a 文件没问题。
我反向部分的唯一结果是Sandbox 中带有白噪声的音频文件。
下面有一段代码，负责反转AVAsset。它基于相关问题
但是，它对我不起作用。
OSStatus theErr = noErr;
UInt64 fileDataSize = 0;
AudioFileID inputAudioFile;
AudioStreamBasicDescription theFileFormat;
UInt32 thePropertySize = sizeof(theFileFormat);
theErr = AudioFileOpenURL((__bridge CFURLRef)[NSURL URLWithString:inputPath], kAudioFileReadPermission, 0, &inputAudioFile);
thePropertySize = sizeof(fileDataSize);
theErr = AudioFileGetProperty(inputAudioFile, kAudioFilePropertyAudioDataByteCount, &thePropertySize, &fileDataSize);
UInt32 ps = sizeof(AudioStreamBasicDescription) ;
AudioFileGetProperty(inputAudioFile, kAudioFilePropertyDataFormat, &ps, &theFileFormat);
UInt64 dataSize = fileDataSize;
void *theData = malloc(dataSize);
// set up output file
AudioFileID outputAudioFile;
AudioStreamBasicDescription myPCMFormat;
myPCMFormat.mSampleRate = 44100;
myPCMFormat.mFormatID = kAudioFormatLinearPCM;
// kAudioFormatFlagsCanonical is deprecated
myPCMFormat.mFormatFlags = kAudioFormatFlagIsFloat | kAudioFormatFlagIsNonInterleaved;
myPCMFormat.mChannelsPerFrame = 1;
myPCMFormat.mFramesPerPacket = 1;
myPCMFormat.mBitsPerChannel = 32;
myPCMFormat.mBytesPerPacket = (myPCMFormat.mBitsPerChannel / 8) * myPCMFormat.mChannelsPerFrame;
myPCMFormat.mBytesPerFrame = myPCMFormat.mBytesPerPacket;
NSString *exportPath = [NSTemporaryDirectory() stringByAppendingPathComponent:@"ReverseAudio.caf"];
NSURL *outputURL = [NSURL fileURLWithPath:exportPath];
theErr = AudioFileCreateWithURL((__bridge CFURLRef)outputURL,
kAudioFileCAFType,
&myPCMFormat,
kAudioFileFlags_EraseFile,
&outputAudioFile);
//Read data into buffer
//if readPoint = dataSize, then bytesToRead = 0 in while loop and
//it is endless
SInt64 readPoint = dataSize-1;
UInt64 writePoint = 0;
while(readPoint > 0)
UInt32 bytesToRead = 2;
AudioFileReadBytes(inputAudioFile, false, readPoint, &bytesToRead, theData);
// bytesToRead is now the amount of data actually read
UInt32 bytesToWrite = bytesToRead;
AudioFileWriteBytes(outputAudioFile, false, writePoint, &bytesToWrite, theData);
// bytesToWrite is now the amount of data actually written
writePoint += bytesToWrite;
readPoint -= bytesToRead;
free(theData);
AudioFileClose(inputAudioFile);
AudioFileClose(outputAudioFile);
如果我将AudioFileCreateWithURL 中的文件类型从kAudioFileCAFType 更改为另一个，则根本不会在Sandbox 中创建结果文件。
感谢您的帮助。
【问题讨论】：
奇怪的噪音是什么样的？白噪声？ @yun.cloud 问题已编辑，现在可以更清楚了。 YES，最终输出文件中有白噪声。 只是想确认一下，但采样/比特率与原始文件是否相同？从我看来它看起来是正确的 - 但是当你只写文件而不反转它时会发生什么（本质上是复制数据） 【参考方案1】：您会收到白噪音，因为您的输入和输出文件格式不兼容。您有不同的采样率和通道，可能还有其他差异。要完成这项工作，您需要在读取和写入之间使用一种通用 (PCM) 格式。对于新的（ish）AVAudio 框架来说，这是一项合理的工作。我们从文件读取到 PCM，洗牌缓冲区，然后从 PCM 写入文件。这种方法并未针对大文件进行优化，因为所有数据都一次性读入缓冲区，但足以让您开始使用。
您可以从您的 getAudioFromVideo 完成块中调用此方法。为了清楚起见，忽略了错误处理。
- (void)readAudioFromURL:(NSURL*)inURL reverseToURL:(NSURL*)outURL
//prepare the in and outfiles
AVAudioFile* inFile =
[[AVAudioFile alloc] initForReading:inURL error:nil];
AVAudioFormat* format = inFile.processingFormat;
AVAudioFrameCount frameCount =(UInt32)inFile.length;
NSDictionary* outSettings = @
AVNumberOfChannelsKey:@(format.channelCount)
,AVSampleRateKey:@(format.sampleRate);
AVAudioFile* outFile =
[[AVAudioFile alloc] initForWriting:outURL
settings:outSettings
error:nil];
//prepare the forward and reverse buffers
self.forwaredBuffer =
[[AVAudioPCMBuffer alloc] initWithPCMFormat:format
frameCapacity:frameCount];
self.reverseBuffer =
[[AVAudioPCMBuffer alloc] initWithPCMFormat:format
frameCapacity:frameCount];
//read file into forwardBuffer
[inFile readIntoBuffer:self.forwaredBuffer error:&error];
//set frameLength of reverseBuffer to forwardBuffer framelength
AVAudioFrameCount frameLength = self.forwaredBuffer.frameLength;
self.reverseBuffer.frameLength = frameLength;
//iterate over channels
//stride is 1 or 2 depending on interleave format
NSInteger stride = self.forwaredBuffer.stride;
for (AVAudioChannelCount channelIdx = 0;
channelIdx < self.forwaredBuffer.format.channelCount;
channelIdx++)
float* forwaredChannelData =
self.forwaredBuffer.floatChannelData[channelIdx];
float* reverseChannelData =
self.reverseBuffer.floatChannelData[channelIdx];
int32_t reverseIdx = 0;
//iterate over samples, allocate to reverseBuffer in reverse order
for (AVAudioFrameCount frameIdx = frameLength;
frameIdx >0;
frameIdx--)
float sample = forwaredChannelData[frameIdx*stride];
reverseChannelData[reverseIdx*stride] = sample;
reverseIdx++;
//write reverseBuffer to outFile
[outFile writeFromBuffer:self.reverseBuffer error:nil];
【讨论】：
谢谢，太棒了。我花了一大笔钱做这件事。顺便问一下，您能推荐任何类似的倒车视频方法吗？ @Kseny，看看github上的this project【参考方案2】：我无法在您的代码中找到问题，但是我建议您使用 AVAssetWriter 反转 AVAsset。以下代码基于iOS reverse audio through AVAssetWritet。我在那里添加了其他方法以使其正常工作。最后我得到了反向文件。
static NSMutableArray *samples;
static OSStatus sampler(CMSampleBufferRef sampleBuffer, CMItemCount index, void *refcon)
[samples addObject:(__bridge id _Nonnull)(sampleBuffer)];
return noErr;
- (void)reversePlayAudio:(NSURL *)inputURL
AVAsset *asset = [AVAsset assetWithURL:inputURL];
AVAssetReader* reader = [[AVAssetReader alloc] initWithAsset:asset error:nil];
AVAssetTrack* audioTrack = [[asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
NSMutableDictionary* audioReadSettings = [NSMutableDictionary dictionary];
[audioReadSettings setValue:[NSNumber numberWithInt:kAudioFormatLinearPCM]
forKey:AVFormatIDKey];
AVAssetReaderTrackOutput* readerOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:audioTrack outputSettings:audioReadSettings];
[reader addOutput:readerOutput];
[reader startReading];
NSDictionary *outputSettings = @AVFormatIDKey : @(kAudioFormatMPEG4AAC),
AVSampleRateKey : @(44100.0),
AVNumberOfChannelsKey : @(1),
AVEncoderBitRateKey : @(128000),
AVChannelLayoutKey : [NSData data];
AVAssetWriterInput *writerInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeAudio
outputSettings:outputSettings];
NSString *exportPath = [NSTemporaryDirectory() stringByAppendingPathComponent:@"reverseAudio.m4a"];
NSURL *exportURL = [NSURL fileURLWithPath:exportPath];
NSError *writerError = nil;
AVAssetWriter *writer = [[AVAssetWriter alloc] initWithURL:exportURL
fileType:AVFileTypeAppleM4A
error:&writerError];
[writerInput setExpectsMediaDataInRealTime:NO];
writer.shouldOptimizeForNetworkUse = NO;
[writer addInput:writerInput];
[writer startWriting];
[writer startSessionAtSourceTime:kCMTimeZero];
CMSampleBufferRef sample;// = [readerOutput copyNextSampleBuffer];
samples = [[NSMutableArray alloc] init];
while (sample != NULL)
sample = [readerOutput copyNextSampleBuffer];
if (sample == NULL)
continue;
CMSampleBufferCallForEachSample(sample, &sampler, NULL);
CFRelease(sample);
NSArray* reversedSamples = [[samples reverseObjectEnumerator] allObjects];
for (id reversedSample in reversedSamples)
if (writerInput.readyForMoreMediaData)
[writerInput appendSampleBuffer:(__bridge CMSampleBufferRef)(reversedSample)];
else
[NSThread sleepForTimeInterval:0.05];
[samples removeAllObjects];
[writerInput markAsFinished];
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
dispatch_async(queue, ^
[writer finishWritingWithCompletionHandler:^
// writing is finished
// reversed audio file in TemporaryDirectory in the Sandbox
];
);
代码的已知问题。
-
如果音频过长，内存可能会出现问题。
音频文件的持续时间比原始文件长。 （作为一种快速修复，您可以像往常一样将其削减为 AVAsset）。
【讨论】：
它总是增加输出时间。最后没有音频。 @AsifHabib 最终在已知问题中指出该方法的缺点以上是关于无法正确反转 AVAsset 音频。唯一的结果是白噪声的主要内容，如果未能解决你的问题，请参考以下文章