提取iPod库原始PCM样本并播放声音效果

Question

我试图从iPod库中的MP3中提取原始PCM样本,以便我可以播放歌曲并操纵音调,速度和应用声音效果(如滤镜).我已经走了AVPlayer和AVAudioPlayer的路线,它们都不能完全控制播放.

以下代码就我所知.我现在处于一个不知道如何处理我的while循环中的CMSampleBufferRef的地步,因为我不知道使用哪个框架来播放音频并应用这些效果.

知道什么是实现这一目标的最佳方法？我已经查看了使用AVAssetWriter转换文件的情况,但这不会为我删除它,因为该过程太耗时.当然我可以将PCM样本读入内存进行播放,而不必先将它们写入磁盘？

注意:我知道下面的代码引用了项目中的一个mp3,但是我知道这种方法与我从MPMediaPropertyAssetURL中提取NSURL的方法相同

-(IBAction)loadTrack:(id)sender {

 NSString *songPath = [[NSBundle mainBundle] pathForResource:@"Smooth_Sub Focus_192" ofType:@"mp3"];
 NSURL *assetURL = [[NSURL alloc] initFileURLWithPath:songPath];

 AVURLAsset *songAsset = [AVURLAsset URLAssetWithURL:assetURL options:nil];

 NSError *assetError = nil;
 AVAssetReader *assetReader = [[AVAssetReader assetReaderWithAsset:songAsset
                error:&assetError] retain];
 if (assetError) {
  NSLog (@"Error: %@", assetError);
  return;
 }

 AVAssetReaderOutput *assetReaderOutput = [[AVAssetReaderAudioMixOutput assetReaderAudioMixOutputWithAudioTracks:songAsset.tracks
                           audioSettings: nil] retain];
 if (![assetReader canAddOutput:assetReaderOutput]) {
  NSLog (@"Incompatible Asser Reader Output");
  return;
 }

 [assetReader addOutput: assetReaderOutput];
 [assetReader startReading];

 CMSampleBufferRef nextBuffer;
 while (nextBuffer = [assetReaderOutput copyNextSampleBuffer]) {
  /* What Do I Do Here? */
 }

 [assetReader release];
 [assetReaderOutput release];

}

Answer 1

我在自己的代码中做了类似的事情.以下方法为AVURLAsset返回一些NSData:

- (NSData *)extractDataForAsset:(AVURLAsset *)songAsset {

    NSError * error = nil;
    AVAssetReader * reader = [[AVAssetReader alloc] initWithAsset:songAsset error:&error];

    AVAssetTrack * songTrack = [songAsset.tracks objectAtIndex:0];
    AVAssetReaderTrackOutput * output = [[AVAssetReaderTrackOutput alloc] initWithTrack:songTrack outputSettings:nil];
    [reader addOutput:output];
    [output release];

    NSMutableData * fullSongData = [[NSMutableData alloc] init];
    [reader startReading];

    while (reader.status == AVAssetReaderStatusReading){

        AVAssetReaderTrackOutput * trackOutput = (AVAssetReaderTrackOutput *)[reader.outputs objectAtIndex:0];
        CMSampleBufferRef sampleBufferRef = [trackOutput copyNextSampleBuffer];

        if (sampleBufferRef){
            CMBlockBufferRef blockBufferRef = CMSampleBufferGetDataBuffer(sampleBufferRef);

            size_t length = CMBlockBufferGetDataLength(blockBufferRef);
            UInt8 buffer[length];
            CMBlockBufferCopyDataBytes(blockBufferRef, 0, length, buffer);

            NSData * data = [[NSData alloc] initWithBytes:buffer length:length];
            [fullSongData appendData:data];
            [data release];

            CMSampleBufferInvalidate(sampleBufferRef);
            CFRelease(sampleBufferRef);
        }
    }

    if (reader.status == AVAssetReaderStatusFailed || reader.status == AVAssetReaderStatusUnknown){
        // Something went wrong. Handle it.
    }

    if (reader.status == AVAssetReaderStatusCompleted){
        // You're done. It worked.
    }

    [reader release];

    return [fullSongData autorelease];
}

我建议在后台线程上执行此操作,因为它很耗时.

该方法的缺点是整首歌曲被加载到存储器中,这当然是有限的.

Answer 2

除了汤姆欧文的答案,我建议更换

       UInt8 buffer[length];
       CMBlockBufferCopyDataBytes(blockBufferRef, 0, length, buffer);

        NSData * data = [[NSData alloc] initWithBytes:buffer length:length];

同

        NSMutableData * data = [[NSMutableData alloc] initWithLength:length];
        CMBlockBufferCopyDataBytes(blockBufferRef, 0, length, data.mutableBytes);

这避免了双重处理样本,并减少了内存使用开销.

或者,您可以将[NSMutableData dataWithLength:length]包装在自动发布池中,如此答案中所示,这是一个不相关但相似的问题.