A test of MPEG using time-inverted spoken audio

We excerpted a 20 second sample from a DAT-mastered talking book segment and encoded it at 32 and 48 kbit/sec using MPEG I, layer 3. We also encoded the same segment at 80 kbit/sec. using MPEG I, layer 2. We then encoded a time-inverted version of the material in the same way. After decoding, we put the inverted segments back into normal sequence and compared them with the corresponding segments encoded in normal temporal order. We did the comparison by means of an ABX test with volunteer listeners. Naive listeners were unable to reliably distinguish between material coded in normal temporal order and the same material coded in inverted order. Trained listeners could reliably make the distinction in layer 3 at 32 and 48 kbit/sec. but not in layer 2 at 80 kbit/sec.