MP3 Overview John Ehrhardt Elena Silenok CSE228 – Spring 03

Where did MP3 come from? In 1987 the Fraunhofer IIS (Institut Integreierte Schaltungen) started work on audio encoding In 1991 with Prof. Deiter Seitzler from the University of Erlangen they devised a very powerful algorithm that was standardized as ISO-MPEG Audio Layer-3

Why “MP3” and what is MP3? Windows introduced the.mp3 file extension for MPEG-1 Layer III encoder and decoder software Files encoded with the MPEG-2 lower sampling rate extension of Layer III are also known as mp3s

MPEG Audio In MPEG audio, one may achieve a typical data reduction of 1:4 by Layer 1 (corresponds to 384 kbps for a stereo signal), 1:6...1:8 by Layer 2 (corresponds to kbps for a stereo signal), 1:10...1:12 by Layer 3 (corresponds to kbps for a stereo signal), While maintaining CD quality sound.

MP3 Encoding Much more than just reducing the sampling rate and the resolution of the samples Consists of 3 major stages: – Hybrid Filter Bank Analysis – Perceptual Modeling – Quantization & Coding

Polyphase filter bank –Divides the audio signal into 32 equal-width frequency subbands –Correlates subbands according to human perception of sound frequencies Modified Discrete Cosine Transform –Since it’s not 64 values, the DCT has been modified to be used for 32. –Increases the frequency resolution 18 times higher than that of layer 2 Hybrid was chosen for compatibility with layers 1 & 2 that do not use the MDCT. Hybrid Filter Bank Analysis

Perceptual Modeling Provides a masking threshold that allows the quantization and coding step to know if the results are perceptually indistinguishable from the original signal. –A strong tonal signal in one subband will mask weak noise in close frequencies Most important aspect when determining the quality of an encoder.

Quantization & Coding Quantization –Power-law: larger values have less accuracy –Huffman coding Coding – Attempts to quantize the resulting MDCT from the Filter Bank at level that meets both the bitrate and the masking requirements – Huffman Coding and Quantization level provide feedback for bitrate – Scale factors for each subband are adjusted until they meet the masking threshold.

Bitstream Layout Divided into 1152 samples per block One block is encoded within one MPEG-1 audio frame (header and data.) Header (First 4 bytes of a frame) – No file header – Contains: Frame Sync, MPEG Layer, Sampling Frequency, Number of Channels, CRC, etc. – Variable bit rate mp3’s switch bitrate between frames

Audio Tag: ID3v1 Contains information about the artist, title, published year, genre, etc. The last 128 bytes of the MPEG audio file. ID3v2 much more complicated – See for more detailswww.id3.org

Watermarking Set of secondary digital data embedded in the primary digital media Provides ownership protection, copy control, fingerprinting, authentication, and control over information Robust vs. fragile, invisible vs. visible, public vs. private (detection w/ or w/o the original unmarked image)

Current State of MP3 3 million of MP3 tracks downloaded every day (International Federation of Phonographic Industries) – mostly pirated Forrester Research says MP3/other online music sales have reached 7% by $1.1bn a year MP3 alternative launched in December 1998: the Secure Digital Music Initiative

Secure Digital Music Initiative Started in 1998, currently over 200 members Spearheaded by RIAA, IFPI, RIAJ and major recording companies SDMI intended to secure music in all forms, across all delivery channels 2 phases, to finally incorporate dual watermarking or other protection scheme

References x.html hannover.de/project/mpeg/audio/faq/mpeg1.html “MP3 And AAC Explained,” Karlheinz Brandenburg, Fraunhofer Institute for Integrated Circuits FhG-IIS A, Erlangen, Germany

Thank you for your attention!