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MPEG- -1 1 MPEG Overview of MPEG- -1 Standard 1 Standard Overview of MPEG Introduction to perceptual and entropy codings codings Introduction to perceptual and entropy Contents Contents History History Psychoacoustics and

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MPEG MPEG-

  • 1

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Overview of MPEG Overview of MPEG-

  • 1 Standard

1 Standard

Introduction to perceptual and entropy Introduction to perceptual and entropy codings codings

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25 September 2009 25 September 2009 MPEG MPEG-

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Contents Contents

 

History History

 

Psychoacoustics and perceptual coding Psychoacoustics and perceptual coding

 

Entropy coding Entropy coding

 

MPEG MPEG-

  • 1

1

 

Layer I/II Layer I/II

 

Layer III (MP3) Layer III (MP3)

 

Comparison and Audio Quality Comparison and Audio Quality

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Introduction Introduction

 

Digitizing an analog signal is ( Digitizing an analog signal is (lossy lossy) compression ) compression

 

Digitizing introduces quantization noise Digitizing introduces quantization noise

 

Quantization noise imply loss of quality Quantization noise imply loss of quality

 

Linear quantization > 16 bit (98 dB) Linear quantization > 16 bit (98 dB)   inaudible noise (CD) inaudible noise (CD)

 

Linear quantization 4 bit (26 dB) Linear quantization 4 bit (26 dB)

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Contents Contents

 

History History

 

Psychoacoustics and perceptual coding Psychoacoustics and perceptual coding

 

Entropy coding Entropy coding

 

MPEG MPEG-

  • 1

1

 

Layer I/II Layer I/II

 

Layer III (MP3) Layer III (MP3)

 

Comparison and Audio Quality Comparison and Audio Quality

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History History

 

Moving Picture Expert Group (MPEG) Moving Picture Expert Group (MPEG)

 

Created in January 1988 Created in January 1988

 

Starts the development of MPEG Starts the development of MPEG-

  • 1 in May 1988

1 in May 1988

 

Publishes the MPEG Publishes the MPEG-

  • 1 standard in November 1992 (ISO/IEC 11172

1 standard in November 1992 (ISO/IEC 11172-

  • 3 for audio)

3 for audio)

 

MPEG MPEG-

  • 1 standard

1 standard

 

Defines bit Defines bit-

  • stream

stream

 

Defines decoding functions Defines decoding functions

 

DOES NOT define encoding techniques DOES NOT define encoding techniques

Inspired by MUSICAM (Masking pattern Universal Subband Integrated Coding And Multiplexing) )

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Contents Contents

 

History History

 

Psychoacoustics and perceptual coding Psychoacoustics and perceptual coding

 

Entropy coding Entropy coding

 

MPEG MPEG-

  • 1

1

 

Layer I/II Layer I/II

 

Layer III (MP3) Layer III (MP3)

 

Comparison and Audio Quality Comparison and Audio Quality

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Psychoacoustics Psychoacoustics

 

Masking effect Masking effect Critical bands Critical bands

z/Bark lower boundary higher boundary bandwidth central frequency 100 100 50 1 100 200 100 150 2 200 300 100 250 3 300 400 100 350 4 400 510 110 450 5 510 630 120 570 6 630 770 140 700 7 770 920 150 840 (Brandenburg)

Time domain masking (Pohlmann 2000) Frequency domain masking (Pohlmann 2000) Idealized critical bands (Painter & Spanias 2000)

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Perceptual Coding Perceptual Coding

 

Dividing the different Dividing the different subbands subbands

  • f a signal
  • f a signal

 

Ignoring masked audio Ignoring masked audio information information

 

Introducing inaudible Introducing inaudible quantization noise quantization noise

Bits association according to masking threshold (Pohlmann 2000) Quantization noise added according to masking threshold (Pohlmann 2000)

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Perceptual Coding Perceptual Coding

Perceptual Encoder/Decoder (Kahrs & Brandenburg 1998)

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Contents Contents

 

History History

 

Psychoacoustics and perceptual coding Psychoacoustics and perceptual coding

 

Entropy coding Entropy coding

 

MPEG MPEG-

  • 1

1

 

Layer I/II Layer I/II

 

Layer III (MP3) Layer III (MP3)

 

Comparison and Audio Quality Comparison and Audio Quality

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Entropic Coding Entropic Coding

 

Use information about the signal to code Use information about the signal to code efficiently efficiently

 

Entropy of a signal Entropy of a signal

 

Example 1: {0, 2, 2, 2, 0, 0, 0, 0, 0, 2, 0, 3, 2, 2, 0, 0, 0, 3 Example 1: {0, 2, 2, 2, 0, 0, 0, 0, 0, 2, 0, 3, 2, 2, 0, 0, 0, 3, 0, 0} , 0, 0} 20 symbols 20 symbols – – twelve 0 (0.6), zero 1 (0), six 2 (0.3), two 3 (0.1) twelve 0 (0.6), zero 1 (0), six 2 (0.3), two 3 (0.1) Entropy Entropy H H = 1.30 = 1.30

 

Example 2: {1, 2, 3, 0, 2, 1, 1, 2, 3, 0, 0, 1, 0, 3, 3, 3, 2, 0 Example 2: {1, 2, 3, 0, 2, 1, 1, 2, 3, 0, 0, 1, 0, 3, 3, 3, 2, 0, 1, 2} , 1, 2} 20 symbols 20 symbols – – five 0 (0.25), five 1 (0.25), five 2 (0.25), five 3 (0.25) five 0 (0.25), five 1 (0.25), five 2 (0.25), five 3 (0.25) Entropy Entropy H H = 2 = 2

 

Shannon theorem Shannon theorem

  It is impossible to code with less than It is impossible to code with less than H H bits/symbol bits/symbol   It is possible to code with less than It is possible to code with less than H+ H+1 1 bits/symbol bits/symbol

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Entropic Coding Entropic Coding

 

Huffman coding Huffman coding

 

Example 1: {0, 2, 2, 2, 0, 0, 0, 0, 0, 2, 0, 3, 2, 2, 0, 0, 0, 3 Example 1: {0, 2, 2, 2, 0, 0, 0, 0, 0, 2, 0, 3, 2, 2, 0, 0, 0, 3, 0, 0} , 0, 0} 20 symbols 20 symbols – – twelve 0 (0.6), zero 1 (0), six 2 (0.3), two 3 (0.1) twelve 0 (0.6), zero 1 (0), six 2 (0.3), two 3 (0.1) Entropy Entropy H H = 1.30 = 1.30

 

Immediate coding: Immediate coding: 0   “ “00 00” ” 1 1   “ “01 01” ” 2 2   “ “10 10” ” 3 3   “ “11 11” ” “ “0010101000000000001000111010000000110000 0010101000000000001000111010000000110000” ”

 

Huffman coding: Huffman coding: 0   “ “0 0” ” 1 1   “ “111 111” ” 2 2   “ “10 10” ” 3 3   “ “110 110” ” “ “010101000000100110101000011000 010101000000100110101000011000” ”

 

Efficiency: Efficiency:

 

Immediate coding: 2 bits/symbol Immediate coding: 2 bits/symbol

 

Huffman coding: 1.5 bits/symbol (statistically) Huffman coding: 1.5 bits/symbol (statistically)

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Contents Contents

 

History History

 

Psychoacoustics and perceptual coding Psychoacoustics and perceptual coding

 

Entropy coding Entropy coding

 

MPEG MPEG-

  • 1

1

 

Layer I/II Layer I/II

 

Layer III (MP3) Layer III (MP3)

 

Comparison and Audio Quality Comparison and Audio Quality

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MPEG MPEG-

  • 1

1

 

Sampling rate: 32, 44.1 and 48 kHz Sampling rate: 32, 44.1 and 48 kHz

 

Four modes: Four modes:

 

Mono: 1 channel Mono: 1 channel

 

Stereo: 2 channels Stereo: 2 channels

 

Dual: 2 channels independent (e.g. bilingual Dual: 2 channels independent (e.g. bilingual programmes programmes) )

 

Joint stereo: 2 channels coded together Joint stereo: 2 channels coded together

 

2 perceptual models 2 perceptual models

 

Floating point quantization (normalization) Floating point quantization (normalization)

 

Error checking: Error checking: Cyclic redundancy check (CRC)

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Contents Contents

 

History History

 

Psychoacoustics and perceptual coding Psychoacoustics and perceptual coding

 

Entropy coding Entropy coding

 

MPEG MPEG-

  • 1

1

 

Layer I/II Layer I/II

 

Layer III (MP3) Layer III (MP3)

 

Comparison and Audio Quality Comparison and Audio Quality

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MPEG MPEG-

  • 1 Layer I

1 Layer I

 

From 32 to 448 kbps From 32 to 448 kbps

 

32 32-

  • subband

subband polyphase polyphase filterbank filterbank

 

Bit allocation (0 Bit allocation (0-

  • 15)

15)

 

Max dynamic range > 120 dB Max dynamic range > 120 dB

 

Linear quantization Linear quantization

 

1 frame 1 frame   384 samples 384 samples Example: Philips Digital Example: Philips Digital Compact Cassette Compact Cassette

Example of Layer I encoder (Pohlmann 2000) Layer I frame format (Pohlmann 2000)

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MPEG MPEG-

  • 1 Layer II

1 Layer II

 

From 32 to 384 kbps From 32 to 384 kbps

 

Improvement of Layer I Improvement of Layer I

 

Improved FFT analysis Improved FFT analysis

 

Scale factor redundancy Scale factor redundancy

 

Finer quantization Finer quantization

 

1 frame 1 frame   1152 samples 1152 samples Example: Digital Audio Example: Digital Audio Broadcasting (DAB) Broadcasting (DAB)

Example of Layer II encoder (Pohlmann 2000) Layer II frame format (Pohlmann 2000)

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Contents Contents

 

History History

 

Psychoacoustics and perceptual coding Psychoacoustics and perceptual coding

 

Entropy coding Entropy coding

 

MPEG MPEG-

  • 1

1

 

Layer I/II Layer I/II

 

Layer III (MP3) Layer III (MP3)

 

Comparison and Audio Quality Comparison and Audio Quality

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MPEG MPEG-

  • 1 Layer III (MP3)

1 Layer III (MP3)

 

From 32 to 320 kbps From 32 to 320 kbps

 

Improvements: Improvements:

 

Finer psychoacoustics model Finer psychoacoustics model

 

Alias reduction (MDCT filters) Alias reduction (MDCT filters)

 

Nonuniform Nonuniform quantization quantization

 

Entropy coding Entropy coding

 

Adaptative Adaptative block size block size

 

Only Layer with patents Only Layer with patents

 

Inspired by: Inspired by:

ASPEC (audio spectral perceptual entropy coding)

OCF (optimal coding in the freq. domain)

Example of Layer III encoder (Pohlmann 2000) Layer III frame format (Pohlmann 2000)

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MPEG MPEG-

  • 1 Layer III (MP3)

1 Layer III (MP3)

 

Filtering: Hybrid Filtering: Hybrid polyphase polyphase filter/MDCT filter/MDCT

 

Steady Steady-

  • state signals: 18

state signals: 18-

  • point MDCT on every

point MDCT on every subband subband Frequency resolution: 41.67 Hz Frequency resolution: 41.67 Hz Time resolution: 24 ms Time resolution: 24 ms

 

Transient signals: 6 Transient signals: 6-

  • point MDCT

point MDCT Frequency resolution: 125 Hz Frequency resolution: 125 Hz Time resolution: 8 ms Time resolution: 8 ms

 

3 blocks modes 3 blocks modes

 

Pre Pre-

  • echo detection

echo detection

 

Quantization : power 3/4 Quantization : power 3/4

 

Entropy coding: Entropy coding:

 

Huffman tables Huffman tables

 

Run length coding Run length coding

Filtering stage in Layer III encoder (Pohlmann 2000) MDCT filterbank in Layer III encoder (Pohlmann 2000)

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Joint Stereo Coding Joint Stereo Coding

 

Intensity coding Intensity coding

 

Sum of left/right channels Sum of left/right channels

 

Coding of the sum and of left/right scale factors Coding of the sum and of left/right scale factors

 

Usually only for high Usually only for high-

  • frequency

frequency subbands subbands

 

Efficient for redundant audio channels Efficient for redundant audio channels

 

MS (mid/side) stereo coding MS (mid/side) stereo coding

 

Sum and difference of left/right channels Sum and difference of left/right channels

 

Coding of the two values Coding of the two values

 

Stereo masking Stereo masking

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Contents Contents

 

History History

 

Psychoacoustics and perceptual coding Psychoacoustics and perceptual coding

 

Entropy coding Entropy coding

 

MPEG MPEG-

  • 1

1

 

Layer I/II Layer I/II

 

Layer III (MP3) Layer III (MP3)

 

Comparison and Audio Quality Comparison and Audio Quality

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Audio Quality Audio Quality

Comparison with CD Comparison with CD-

  • quality at 48 kHz (16 bit

quality at 48 kHz (16 bit -

  • 1.412 Mbps)

1.412 Mbps)

 

Layer I: Layer I:

 

No perceptual difference for 384 kbps (stereo) No perceptual difference for 384 kbps (stereo) – – 2:1 compression 2:1 compression

 

Layer II: Layer II:

 

No perceptual difference for 256 kbps (stereo) No perceptual difference for 256 kbps (stereo) – – 4:1 compression 4:1 compression

 

Layer III Layer III

 

Increase of mean opinion score compared to Layer II at 256 kbps Increase of mean opinion score compared to Layer II at 256 kbps (stereo) for 128 kbps (stereo) (stereo) for 128 kbps (stereo) – – 8:1 compression 8:1 compression

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Comparison between Layers Comparison between Layers

 

Layers I and II are very similar Layers I and II are very similar

 

Each Layer has its defined decoder design Each Layer has its defined decoder design

 

Encoding/Decoding complexity: Layer I/II Encoding/Decoding complexity: Layer I/II   broadcasting broadcasting

 

Encoding/Decoding quality: Layer III Encoding/Decoding quality: Layer III   audio storage audio storage

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Conclusion Conclusion

Bibliography Bibliography: :

 

Ambikairajah Ambikairajah, E. & al. , E. & al. Auditory masking and MPEG Auditory masking and MPEG-

  • 1 audio compression

1 audio compression, , Electronics & Communication Engineering Journal, Electronics & Communication Engineering Journal, 1997 1997

 

Brandenburg, K. & Brandenburg, K. & Bosi Bosi, M. , M. Overview of MPEG Audio: Current and Future Overview of MPEG Audio: Current and Future Standards for Low Standards for Low-

  • Bit

Bit-

  • Rate Audio Coding

Rate Audio Coding, Journal of the Audio Engineering Society, , Journal of the Audio Engineering Society, 1997 1997, Vol. Vol. 45(No. 1/2) , Vol. Vol. 45(No. 1/2)

 

Painter, T. & Painter, T. & Spanias Spanias, A. , A. Perceptual Coding of Digital Audio Perceptual Coding of Digital Audio, Proceedings of IEEE , Proceedings of IEEE, , 2000 2000, Vol. Vol. 88(No. 4) , Vol. Vol. 88(No. 4)

 

Painter, T. & Painter, T. & Spanias Spanias, A. , A. A Review of Algorithms for Perceptual Coding of Digital A Review of Algorithms for Perceptual Coding of Digital Audio Signals Audio Signals, , Digital Signal Digital Signal Processing, Processing, 1997 1997

 

Pan, D. Pan, D. A Tutorial on MPEG/Audio Compression A Tutorial on MPEG/Audio Compression, IEEE , IEEE MultiMedia MultiMedia, IEEE , IEEE Computer Society, Computer Society, 1995 1995, Vol. 2(2), pp. 60 , Vol. 2(2), pp. 60-

  • 74

74

 

Pan, D.Y. Pan, D.Y. Digital Audio Compression Digital Audio Compression, Digital Technical Journal, , Digital Technical Journal, 1993 1993, Vol. 5 , Vol. 5

 

Kahrs Kahrs, M. and Brandenburg, K. , M. and Brandenburg, K. Applications of digital signal processing to audio Applications of digital signal processing to audio and acoustics and acoustics, , Kluwer Kluwer Academic Publishers, Academic Publishers, 1998 1998

 

Mallat Mallat, S. , S. Traitement Traitement du Signal du Signal, , Ecole Ecole Polytechnique, Polytechnique, 2000 2000

 

Pohlmann Pohlmann, K.C. , K.C. Principles of Digital Audio, Principles of Digital Audio, McGraw McGraw-

  • Hill Professional,

Hill Professional, 2000 2000