ios - 无法正确反转 AVAsset 音频。唯一的结果是白噪声
<p><p>我正在尝试反转 <code>AVAsset</code> 音频并将其保存到文件中。为了清楚起见，我用问题 <a href="https://github.com/ksenia-lyagusha/AudioReverse.git" rel="noreferrer noopener nofollow">https://github.com/ksenia-lyagusha/AudioReverse.git</a> 做了一个简单的应用程序。 </p>
<p>应用程序从包中获取 <code>mp4</code> 视频文件，将其作为单个 <code>m4a</code> 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回来。
临时 <code>m4a</code> 文件没问题。</p>
<p>我反向部分的唯一结果是 <code>Sandbox</code> 中带有白噪声的音频文件。 </p>
<p>下面有一段代码，负责反转<code>AVAsset</code>。它基于相关问题</p>
<ul>
<li> <a href="https://stackoverflow.com/questions/6593118/how-to-reverse-an-audio-file" rel="noreferrer noopener nofollow">How to reverse an audio file?</a> </li>
<li> <a href="https://stackoverflow.com/questions/6707998/ios-audio-manipulation-play-local-caf-file-backwards" rel="noreferrer noopener nofollow">iOS audio manipulation - play local .caf file backwards</a> </li>
</ul>
<p>但是，它对我不起作用。</p>
<pre><code>OSStatus theErr = noErr;
UInt64 fileDataSize = 0;
AudioFileID inputAudioFile;
AudioStreamBasicDescription theFileFormat;
UInt32 thePropertySize = sizeof(theFileFormat);
theErr = AudioFileOpenURL((__bridge CFURLRef), 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 = ;
NSURL *outputURL = ;
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);
</code></pre>
<p>如果我将 <code>AudioFileCreateWithURL</code> 中的文件类型从 <code>kAudioFileCAFType</code> 更改为另一个，则根本不会在 <code>Sandbox</code> 中创建结果文件。</p >
<p>感谢您的帮助。</p></p>
<br><hr><h1><strong>Best Answer-推荐答案</ strong></h1><br>
<p><p>您会收到白噪声，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 channel ，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)<code>AVAudio</code> 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 写入到文件。这种方法没有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。</p>
<p>您可以从 <code>getAudioFromVideo</code> 完成 block 中调用此方法。为清楚起见，忽略了错误处理。</p>
<pre><code>- (void)readAudioFromURL:(NSURL*)inURL reverseToURL:(NSURL*)outURL {
//prepare the in and outfiles
AVAudioFile* inFile =
[ initForReading:inURL error:nil];
AVAudioFormat* format = inFile.processingFormat;
AVAudioFrameCount frameCount =(UInt32)inFile.length;
NSDictionary* outSettings = @{
AVNumberOfChannelsKey:@(format.channelCount)
,AVSampleRateKey:@(format.sampleRate)};
AVAudioFile* outFile =
[ initForWriting:outURL
settings:outSettings
error:nil];
//prepare the forward and reverse buffers
self.forwaredBuffer =
[ initWithPCMFormat:format
frameCapacity:frameCount];
self.reverseBuffer =
[ initWithPCMFormat:format
frameCapacity:frameCount];
//read file into forwardBuffer
;
//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;
float* reverseChannelData =
self.reverseBuffer.floatChannelData;
int32_t reverseIdx = 0;
//iterate over samples, allocate to reverseBuffer in reverse order
for (AVAudioFrameCount frameIdx = frameLength;
frameIdx >0;
frameIdx--) {
float sample = forwaredChannelData;
reverseChannelData = sample;
reverseIdx++;
}
}
//write reverseBuffer to outFile
;
}
</code></pre></p>
<p style="font-size: 20px;">关于ios - 无法正确反转 AVAsset 音频。唯一的结果是白噪声，我们在Stack Overflow上找到一个类似的问题：
<a href="https://stackoverflow.com/questions/35581531/" rel="noreferrer noopener nofollow" style="color: red;">
https://stackoverflow.com/questions/35581531/
</a>
</p>
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