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MP3 Overview

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

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MP3 Overview

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  1. MP3 Overview John Ehrhardt Elena Silenok CSE228 – Spring 03

  2. 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

  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

  4. 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 256..192 kbps for a stereo signal), 1:10...1:12 by Layer 3 (corresponds to 128..112 kbps for a stereo signal), While maintaining CD quality sound.

  5. MP3Encoding • 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

  6. Hybrid Filter Bank Analysis • 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.

  7. 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.

  8. 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.

  9. 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

  10. 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 www.id3.org for more details

  11. 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)

  12. 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 2003 - $1.1bn a year • MP3 alternative launched in December 1998: the Secure Digital Music Initiative

  13. 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

  14. References • http://www.stanford.edu/~udara/SOCO/lossy/mp3 • http://www.iis.fraunhofer.de/amm/techinf/layer3/index.html • http://www.tnt.uni-hannover.de/project/mpeg/audio/faq/mpeg1.html • http://www.dv.co.yu/mpgscript/mpeghdr.htm • “MP3 And AAC Explained,” Karlheinz Brandenburg, Fraunhofer Institute for Integrated Circuits FhG-IIS A, Erlangen, Germany • http://www.sdmi.org

  15. Thank you for your attention!

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