MPEG Audio Formats Jason Leung Wednesday, February 5, 2014

Introduction – MPEG Moving Picture Experts Group Officially: ISO/IEC JTC1/SC29 WG11 Established 1988 Sets standards for audio/video transmission 2 Hiroshi Yasuda (Nippon Telegraph and Telephone) Leonardo Chiariglione, Chair Official logo

MPEG Standards 3 MPEG-1 Coding of moving pictures and associated audio at up to 1.5 Mb/s MPEG-1 Coding of moving pictures and associated audio at up to 1.5 Mb/s MPEG-2 Generic coding of moving pictures and associated audio MPEG-2 Generic coding of moving pictures and associated audio MPEG-4 Coding of audio- visual objects MPEG-4 Coding of audio- visual objects MPEG-7 Multimedia content description interface MPEG-7 Multimedia content description interface MPEG-21 Multimedia framework MPEG-21 Multimedia framework MPEG-A Application formats MPEG-B MPEG system technologies MPEG-C MPEG video technologies MPEG-D MPEG audio technologies MPEG-E MPEG multimedia middleware MPEG-V Media context and control MPEG-M Multimedia service platform technologies MPEG-U MPEG rich media user interface MPEG-H High efficiency coding and media delivery in heterogeneous environments MPEG-DASH Dynamic Adaptive Streaming over HTTP

MPEG-1 Coding of moving pictures and associated audio at up to about 1.5 Mbit/s Standard for lossy compression ◦ VHS-quality video  26:1 compression ratio ◦ CD-quality audio  6:1 4 Part 1 System Part 2 Video Part 3 Audio Part 4 Conformance testing Part 5 Reference software

MPEG-1, Part 3: Audio Lossy compression Psychoacoustic model (perceptual coding) ◦ Reduces/discards parts we can’t hear  Outside audible range  Masking Compression ratios 5 CD kb/s 128 kb/s 160 kb/s 192 kb/s 9:1 7:1 11:1 MPEG-1

MPEG-1, Part 3: Audio Very strictly defines output/decoder ◦ Does not define encoding ◦ Different encoder = Different output 6 Digital audio signal Analysis Filterbank Quantization and Coding Encoding of bitstream Perceptual Model Encoded Bitstream Decoding of bitstream Inverse Quantization Synthesis Filterbank Audio out (Brandenburg and Popp 2000)

MPEG-1, Part 3: Audio 3 hierarchical layers 7 Layer I (MP1)Layer II (MP2)Layer III (MP3) Sampling Frequency32, 44.1, 48 kHz Bit rates32–448 kb/s32–384 kb/s32–320 kb/s Minimum encoding/ decoding delay 19 ms35 ms59 ms Psychoacoustic modelTime-domain Freq-domain Bit allocationBlock, 12 sub- band samples Block, 36 sub- band samples Adaptive block size QuantisationUniform Non-uniform Entropy coding of quantised values No Yes (Huffman) Subjective test performance Excellent at 384 kb/s Excellent at 256 kb/s Excellent at 192 kb/s (Ambikairajah, Davis, Wong 1997)

MPEG-1, Part 3: Audio Does this mean MP3 is the best? MP2 preferred in Digital Audio Broadcasting 1 ◦ Lower delays = faster transmission ◦ MP3 only for Internet broadcasts  Bandwidth considerations MP2 scores higher in subjective testing 2 ◦ Comparison based on expected quality  [MP2] 224 kb/s vs. [MP3] 192 kb/s ◦ Better for complex, random, transient signals  Voice, orchestra, percussion, applause 8 1.European Broadcasting Union EBU Tech pp. 7–9. 2.Ibid. pp. 51.

What’s next? MPEG-1 is 26 years old ◦ Designed 1988–1992 (published 1993) ◦ Designed for studio use  MP3 not meant to be standalone ◦ Designed for equipment of the early 90s MPEG-2 ◦ Designed 1990–1995 (published 1996) ◦ Part 3: Audio – updated MP2, MP3 slightly 9

Advanced Audio Coding (AAC) MPEG-2, Part 7 ◦ First published 1997 Successor to MP3 ◦ More sampling frequencies: 8–96 kHz ◦ Up to 48 channels  MP3 has 2 (MPEG-1) or 5.1 (MPEG-2) ◦ Higher efficiency for stationary signals ◦ Higher accuracy for transient signals 10

Advanced Audio Coding (AAC) Revised as MPEG-4, Part 3: Audio (1999) New tools ◦ Perceptual Noise Shaping  Noise  pseudo-random data New formats ◦ AAC-LD (low delay), 2000 ◦ MPEG-4 SLS (Scalable to Lossless), 2006 ◦ MPEG-4 ALS (Audio Lossless Coding),

Conclusions MP3 here to stay (for a while) ◦ Omnipresent, default format ◦ Playback guaranteed ◦ Berger 2009: iPod generation prefers MP3 fidelity over CDs AAC is default format for: ◦ YouTube, iTunes, Nintendo 3DS/DSi, PS3 ◦ HD content (MP4 = MPEG-4 format) ◦ Not finalized (as of Jan 2014) 12

References Ambikairajah, E., A.G. Davis, and W.T.K.Wong, Auditory masking and MPEG-1 audio compression. Electronics and Communication Engineering Journal 9(4): Brandenburg, K MP3 and AAC Explained. In Proceedings of AES 17 th International Conference on High Quality Audio Coding. Florence, Italy. Brandenburg, K., and H. Popp, An introduction to MPEG layer- 3. EBU Technical Review. June: European Broadcasting Union, EBU Evaluations of Multichannel Audio Codecs. EBU Tech Geneva, Switzerland. Liebchen, T An Introduction to MPEG-4 Audio Lossless Coding. Proceedings of the ICASSP III: Pan, D A Tutorial on MPEG/Audio Compression. IEEE Multimedia, IEEE Computer Society 2(2): Wikipedia! 13

MPEG Audio MPEG-1 ◦ Part 3: Audio (MP1, MP2, MP3) MPEG-2 ◦ Part 3: Audio ◦ Part 7: Advanced Audio Coding (AAC) MPEG-4 ◦ Part 3: Advanced Audio Coding MPEG-D ◦ Part 1: MPEG Surround ◦ Part 2: Spatial Audio Object Coding ◦ Part 3: Unified Speech and Audio Coding 14

Psychoacoustic Model Masking effect: loud sounds cover soft sounds 15 (Wikipedia. Psychoacoustics.

MPEG-1, Part 3: Audio 2 channels 3 hierarchical layers ◦ Increasing complexity, efficiency  [MP2] 192 kb/s = [MP3] 128 kb/s (quality) ◦ Backwards-compatible 16