Extract iPod Library raw PCM samples and play with sound effects

Question

I am trying to extract raw PCM samples from an MP3 in the iPod Library so that I can play the song and manipulate the pitch, tempo, and apply sound effects (such as filters)

Answer 1

Additionally to Tom Irving's Answer, I suggest replacing

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

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

with

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

which avoids double handling the samples, and reduces memory usage overhead.

alternatively, you can wrap [NSMutableData dataWithLength:length] in an auto release pool as demonstrated in this answer to an unrelated but similar question.