通过AVAssetReader读取audio样本

如何通过AVAssetReader读取audio样本？ 我find了使用AVAssetReader进行复制或混合的例子，但这些循环总是由AVAssetWriter循环控制。 是否有可能只是创build一个AVAssetReader并通读它，获取每个示例，并将每个audio样本的int32投入数组？

谢谢。

Answer 1

要扩展@ amrox的答案，你可以从CMBlockBufferRef获得一个AudioBufferList，例如

CMItemCount numSamplesInBuffer = CMSampleBufferGetNumSamples(buffer); AudioBufferList audioBufferList; CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer( buffer, 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 } } //Release the buffer when done with the samples //(retained by CMSampleBufferGetAudioBufferListWithRetainedblockBuffer) CFRelease(buffer); 

Answer 2

 AVAssetReader *reader = [[AVAssetReader alloc] initWithAsset:asset error:&error]; AVAssetTrack *track = [[asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0]; NSDictionary *settings = @{ AVFormatIDKey : [NSNumber numberWithInt:kAudioFormatLinearPCM] }; AVAssetReaderTrackOutput *readerOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:track outputSettings:settings]; [reader addOutput:readerOutput]; [reader startReading]; CMSampleBufferRef sample = [readerOutput copyNextSampleBuffer]; while ( sample ) { sample = [readerOutput copyNextSampleBuffer]; if ( ! sample ) { continue; } CMBlockBufferRef buffer = CMSampleBufferGetDataBuffer(sample); size_t lengthAtOffset; size_t totalLength; char *data; if ( CMBlockBufferGetDataPointer( buffer, 0, &lengthAtOffset, &totalLength, &data ) != noErr ) { NSLog(@"error!"); break; } // do something with data... CFRelease(sample); } 

Answer 3

这里的答案不是通用的。 当AudioBufferList需要不同的大小时，调用CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer可能会失败。 举例来说，当非Intererleaved样本。

正确的方法是调用CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer两次。 第一个调用查询AudioBufferList所需的大小，第二个调用实际填充AudioBufferList 。

 size_t bufferSize = 0; CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer( sampleBuffer, &bufferSize, NULL, 0, NULL, NULL, 0, NULL ); AudioBufferList *bufferList = malloc(bufferSize); CMBlockBufferRef blockBuffer = NULL; CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer( sampleBuffer, NULL, bufferList, bufferSize, NULL, NULL, kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment, &blockBuffer ); // handle audio here free(bufferList); CFRelease(blockBuffer); 

在现实世界的例子中，你必须执行error handling，也不应该malloc每一帧，而是cachingAudioBufferList 。