Reading samples via AVAssetReader

It’s unclear from your question whether you are talking about video samples or audio samples. To read video samples, you need to do the following:

1. Construct an AVAssetReader:

   asset_reader = [[AVAssetReader alloc] initWithAsset:asset error:&error];
(error checking goes here)

2. Get the video track(s) from your asset:

   NSArray* video_tracks = [asset tracksWithMediaType:AVMediaTypeVideo];
AVAssetTrack* video_track = [video_tracks objectAtIndex:0];

3. Set the desired video frame format:

   NSMutableDictionary* dictionary = [[NSMutableDictionary alloc] init];
   [dictionary setObject:[NSNumber numberWithInt:<format code from CVPixelBuffer.h>] forKey:(NSString*)kCVPixelBufferPixelFormatTypeKey];

   Note that certain video formats just will not work, and if you’re doing something real-time, certain video formats perform better than others (BGRA is faster than ARGB, for instance).

4. Construct the actual track output and add it to the asset reader:

   AVAssetReaderTrackOutput* asset_reader_output = [[AVAssetReaderTrackOutput alloc] initWithTrack:video_track outputSettings:dictionary];
   [asset_reader addOutput:asset_reader_output];

5. Kick off the asset reader:

   [asset_reader startReading];

6. Read off the samples:

   CMSampleBufferRef buffer;
   while ( [asset_reader status]==AVAssetReaderStatusReading )
         buffer = [asset_reader_output copyNextSampleBuffer];