Question

我正在尝试从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（例如滤镜）。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。

知道实现这一目标的最佳方法是什么？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，因为这个过程非常耗时。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，因为它非常耗时。

这种方法的一个缺点是整首歌被加载到内存中，这当然是有限的。

Answer 2

除汤姆欧文的答案外，我建议更换

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

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

与

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

避免双重处理样本，并减少内存使用开销。

或者，你可以将自动发布池中的[NSMutableData dataWithLength：length]包装为一个不相关但相似的问题{/ 3}}。

Answer 3

我认为你想在那里确保它的PCM ......

NSDictionary* outputSettingsDict = [[NSDictionary alloc] initWithObjectsAndKeys:

                        [NSNumber numberWithInt:kAudioFormatLinearPCM],AVFormatIDKey,
                   //     [NSNumber numberWithInt:44100.0],AVSampleRateKey, /*Not Supported*/
                   //     [NSNumber numberWithInt: 2],AVNumberOfChannelsKey,    /*Not Supported*/

                        [NSNumber numberWithInt:16],AVLinearPCMBitDepthKey,
                        [NSNumber numberWithBool:NO],AVLinearPCMIsBigEndianKey,
                        [NSNumber numberWithBool:NO],AVLinearPCMIsFloatKey,
                        [NSNumber numberWithBool:NO],AVLinearPCMIsNonInterleaved,

                        nil];

AVAssetReaderTrackOutput* output = [[AVAssetReaderTrackOutput alloc] initWithTrack:songTrack outputSettings:outputSettingsDict];

Answer 4

对于歌曲的持续时间，我相信你可以简单地查询歌曲资产：

float songTimeInSeconds = CMTimeGetSeconds(songAsset.duration);
    int songMinutes = (int)(songTimeInSeconds/60.);
    int songSeconds = (int)(songTimeInSeconds - 60.0*songMinutes);

提取iPod库原始PCM样本并播放声音效果

4 个答案: