AAC, like MP3, introduces a padding of silence at the beginning of the stream. Because modern QT container files do not compensate for this, all audio and video streams within this type of QT file will be off sync by default. QT playback software waits for the audio stream to begin (waits for silence padding to end) before video playback begins, even though the streams themselves line up 1:1 in the container file. This is lazy engineering, not an advanced feature.