Audio Coding Introduction

Transcription

1 Audio Coding Introduction Lecture WS 2013/2014 Prof. Dr.-Ing. Karlheinz Brandenburg Prof. Dr.-Ing. Gerald Schuller Page Nr. 1

2 Organisatorial Details - Overview Lectures: 14 lectures read by Prof. Brandenburg and Prof. Schuller Practice lessons: Exam: Instructors: Dr. Andreas Franck M. Sc. Javier Frutos-Bonilla Periodic homework assignments, which will count 30% towards the final grade. Small groups (2-3 people) to solve the homework and deliver a single solution for the whole group. Homework presentation during the lessons (laptop with Octave or Matlab running) Written exam at the end of the semester, 90 minutes Agree to this method by signing the document that is passed around Page Nr. 2

3 Organisatorial Details Time and Place Lectures: Monday, 03:00-04:30pm, Room Sr K 2026 Practice lessons: Monday, 7:15-8:45am, Room Sr K 2003B, odd weeks (bi-weekly) Suggestion: Shift to other time, for instance Thursday K :00-02:30pm Page Nr. 3

4 Organisatorial Details - Timeline Lecture: Date: Read by: 1. Introduction Prof. Brandenburg 2. Psychoacoustics Dipl.-Ing. Werner 3. Basics of Multirate Signal Processing Prof. Schuller 4. Filter Banks Prof. Schuller 5. Filter Banks Prof. Schuller 6. Quantization & Coding Prof. Brandenburg 7. MPEG 1 / MPEG 2 BC Audio Prof. Brandenburg 8. MPEG 2 / 4 AAC Prof. Brandenburg 9. Prediction and Lossless Audio Coding Prof. Schuller 10. Audio Coding for Communication (ULD) Prof. Schuller 11. Coding of Stereophonic Signals Prof. Brandenburg 12. Parametric Coding of High-Quality Audio Prof. Brandenburg 13. Dolby AC3, DTS Prof. Schuller 14. SAOC and USAC Dr. Franck Page Nr. 4

5 Current Applications (1) Digital audio broadcasting - EU 147 (Layer 2) - WorldSpace (Layer 3) - XM Radio (HeAAC) ISDN Transmission of Audio Digital TV - MPEG-1/2 Layer 2 - Dolby AC-3 multichannel coding - MPEG- 2 AAC Storage of large music volumes (archives) DVD - Dolby Digital - DTS Page Nr. 5

6 Current Applications (2) Internet and Network Audio - MPEG-1/2 Layer 3 (.mp3, all software player) - AAC (Apple ITunes Music Store) - AAC-LD (real-time video conference systems) - others (WMA) Audio on portable phones -.mp3 - HeAAC (recommended by 3GPP) Solid state portable music player (mp3, AAC, WMA) Page Nr. 6

7 Basics of High Quality Audio Coding The goal: transparent coding of music signals The source is not known in advance Use information about the sink, not the source The solution: Modeling of the masking threshold of the ear The quantization noise has to be kept below the masked threshold Page Nr. 7

8 Psychoacoustics (Masked Threshold) 80 db 60 f =0,25 m f =1kHz f m m =4kHz L T ,02 0,05 0,1 0,2 0, khz f T Page Nr. 8

9 Demo: The "13 db-miracle" Original signal Original + white noise, SNR = 13,6 db Original + noise at threshold, S/N = 13,6 db Difference (modulated white noise) Difference (noise at threshold) Page Nr. 9

10 The Basic Paradigm of T/F Domain Audio Coding Digital Audio Input Filter Bank Bit or Noise Allocation Quantized Samples Bitstream Formatting Encoded Bitstream Signal to Mask Ratio Psychoacoustic Model Page Nr. 10

11 Differences between Audio and Speech Coding (1) Generic audio coding is similar to speech coding except: Larger bandwidth speech coders usually use up to 7 khz bandwidth Fewer audible artifacts Use of psycho-acoustic model for irrelevancy removal Page Nr. 11

12 Differences between Audio and Speech Coding (2) Different requirements for bitrate speech aims for as small as possible (e.g. GSM: <=13kbps) audio demands more for quality (>=64 kbps, decreasing) Not specialized to speech model Page Nr. 12

13 History of Audio Coding the Critical Band Coder classic ATC for Music MSC OCF MASCAM PXFM ASPEC, MUSICAM MPEG epac MPEG 2 AAC MPEG 4 AAC HE AAC USAC MPEG-H: Coding for 3D audio Page Nr. 13

14 The time line for near-cd-quality kbit/s ASPEC, MUSICAM would fail today s listening tests kbit/s MPEG-1 Layer kbit/s MPEG-1 Layer-3 (".mp3") including combined joint stereo coding bad quality for some signals kbit/s MPEG-2 Advanced Audio Coding better than MP3 at 128, not fully transparent kbit/s AAC-based MPEG kbit/s MPEG-4 HeAAC (AAC+ in 2000) e.g. used for XM Radio Page Nr. 14

15 What quality can be reached today? Define the quality to reach for first: High end: don t call it transparent (hard to prove) best listening conditions listeners need years to be trained large number of samples for statistics near CD - quality: defined as good enough, no formal definition much more important for practical purposes example: mp3 at 128 kbit/s for stereo Page Nr. 15

16 Demo: Can you hear it (Version 4, 2000)? Each? corresponds to either O (Original, 1536 kbit/s for two channels) or C (Coded, 48 kbp/s for two channels) (HeAAC, demo provided by Coding Technologies) Trumpet solo O??? Speech O??? Abba O??? Page Nr. 16

17 Did you hear it? O (Original, 1536 kbit/s for two channels) or C (Coded, 48 kbp/s for two channels) (HeAAC, demo provided by Coding Technologies) Trumpet solo (O) _ Speech (O) _ Abba (O) _ Page Nr. 17

18 Extra Material Page Nr. 18

19 Organisatorial Details Overview (Repetition) Lectures: 14 lectures read by Prof. Brandenburg and Prof. Schuller Practice lessons: Exam: Instructors: Dr. Andreas Franck M. Sc. Javier Frutos-Bonilla Periodic homework assignments, which will count 30% towards the final grade. Small groups (2-3 people) to solve the homework and deliver a single solution for the whole group. Homework presentation during the lessons (laptop with Octave or Matlab running) Written exam at the end of the semester, 90 minutes Agree to this method by signing the document that is passed around Page Nr. 19

20 Organisatorial Details Timeline (Repetition) Lecture: Date: Read by: 1. Introduction Prof. Brandenburg 2. Psychoacoustics Dipl.-Ing. Werner 3. Basics of Multirate Signal Processing Prof. Schuller 4. Filter Banks Prof. Schuller 5. Filter Banks Prof. Schuller 6. Quantization & Coding Prof. Brandenburg 7. MPEG 1 / MPEG 2 BC Audio Prof. Brandenburg 8. MPEG 2 / 4 AAC Prof. Brandenburg 9. Prediction and Lossless Audio Coding Prof. Schuller 10. Audio Coding for Communication (ULD) Prof. Schuller 11. Coding of Stereophonic Signals Prof. Brandenburg 12. Parametric Coding of High-Quality Audio Prof. Brandenburg 13. Dolby AC3, DTS Prof. Schuller 14. SAOC and USAC Dr. Franck Page Nr. 20

21 History of Audio Coding the Critical Band Coder classic ATC for Music MSC OCF MUSICAM ASPEC MPEG PAC MPEG 2 AAC MPEG 4 AAC HE AAC USAC MPEG-H: Coding for 3D audio Page Nr. 21

22 The Critical Band Coder M.A. Krasner, MIT Lincoln Laboratories, 1979 First coder to use a psycho-acoustic model Sampling rate of 30kHz Analysis/Synthesis Filter QMF Filter Tree of depth 2 to 7 Filter bandwidths ranging from 117 Hz to 3.75 khz No calculation of the Threshold in Quiet, just looked at worst case scenarios Quantization with Block-companding, fixed bit distribution from psycho-acoustic criteria Bitrate of kbps Page Nr. 22

23 classic ATC for Music Universität Erlangen-Nürnberg, 1982 First real-time music coder Sampling rate between khz Does not use a psycho-acoustic model bad quality for some music pieces Block length of 128 samples (about 4 ms) Bitrate: 3bits/sample (about 100 kbps) Page Nr. 23

24 MSC (Multiple Adaptive Spectral Audio Coding) Krahe and others, Universität Duisburg, 1985 First Coder to use both psycho-acoustic model and transformation-coding Analysis/Synthesis: FFT with conversion of Amplitude & Phase window length of 1024 samples window ends sine-tapered with an overlap of 64 samples Threshold estimation is only using in-band masking Quantization uses block-companding with 2 bits per sample Page Nr. 24

25 OCF (Optimum Coding in Frequency Domain) Brandenburg, Universität Erlangen, 1987, 1988 MDCT-Filter bank with window length of 1024 or 512 Explicit calculation of the masking threshold with a simple model Calculation per critical band No tonality criteria used Maximum calculation instead of convolution Non-uniform quantization (quantization noise dependant on amplitude) Huffman coding from pairs of spectral values Page Nr. 25

26 ASPEC- Adaptive Spectral Perceptual Entropy Coding (1) Uni Erlangen, FhG, AT&T Bell Labs, Deutsche Thomson-Brandt, CNET, 1990 Analysis/Synthesis: MDCT with switchable block lengths Use of 2 models for psycho-acoustic Simple: like OCF Better: like PXFM + 1/3 Frequency grouping resolution + local tonality criteria (like Hybrid) Quantization/Coding: like OCF, Choice of Huffman-code-books Further division of the spectrum Control of window length (switching the number of bands) Page Nr. 26

27 MUSICAM - Masking-pattern Universal Subband Integrated Coding and Multiplexing (1) IRT, CCETT, Philips, Matsushita 1990 Subband-coding, that is good time resolution, bad frequency resolution First version used QMF-tree as filter bank Newest version uses 32 channel polyphase-filter bank Parallel FFT for fine calculation of masking Tonality criteria by local comparison of the spectral values Block-companding of the subband signal Page Nr. 27

28 MPEG-1 (1) Layer I Window length: 384 samples (8 ms) Frequency resolution: 32 subbands Quantization: Block-companding (12 samples) Layer II Window length: 1152 samples (24 ms) Frequency resolution: 32 subbands Quantization: Block companding (12 samples) Use of Scalefactor select information (SFSI) Page Nr. 28

29 MPEG-1 (2) Layer III Window length: 1152 samples (24 ms) Frequency resolution: 576/192 subbands Quantization: non-uniform with Huffman coding Use of Scalefactor Select Information Page Nr. 29

30 MPEG (1) December 1988 First meeting of Audio Expert Group July 1989 Call for Proposals (14 proposals received) Fall 1989 Clustering of similar proposals July 1990 Listening tests of Coders December 1990 Adoption the Committee Draft Page Nr. 30

31 MPEG (2) The results of the Stockholm-Tests showed 2 proposals were best, ASPEC and MUSICAM Listening tests show that ASPEC is better especially at low bitrates In comparison of complexity parameters MUSICAM is better RESULT: collaboration between ASPEC & MUSICAM in a Layered solution (hence Layer 1, Layer 2, & Layer 3) Page Nr. 31

32 PAC Resulted from split of AT&T and Lucent Technologies Branched off from MPEG-AAC, proprietary instead of standardized technology Used in American Satellite Broadcast System (XM, Sirius) Page Nr. 32

33 MPEG 2 AAC (1) first named MPEG-2 NBC (non backwards compatible), later named AAC (advanced audio coding) MPEG-2 AAC (ISO/IEC ) offers very high quality compressed audio Allows 1 to 48 channels, Sampling rates from 8 to 96 khz, with multi-channel, multi-lingual, and multi-program possibilities. AAC works at bit-rates from 8 kbit/s for mono Speech signals and up to 160 kbit/s/channel for very high quality, allows tandem coding Page Nr. 33

34 MPEG 2 AAC (2) 3 Profiles from AAC with varying levels of complexity and scalability. Joint Stereo -Mode is more flexible compared to MP3 in that it is switchable for individual scale factor bands whereas MP3 was only switchable for the whole spectrum. Page Nr. 34

35 MPEG 2 AAC Basic Features High frequency resolution filter bank-based coder (1024 subbands MDCT with 50% overlap) 1: 8 block switching (1024/128 subbands MDCT) Non- uniform quantizer Noise shaping in half critical bands (scalefactor bands) Huffman coding of scalefactors and spectral coefficients Page Nr. 35

36 HE AAC Combination of the MPEG-4 AAC Low Complexity (LC) Object and the MPEG-4 Spectral Band Replication (SBR) Object SBR: parametric coding of high frequency envelope with small amount of control data Parametric stereo and multi-channel coding Backwards compatible to AAC 5.1 surround sound at 128 kbps Good quality stereo at 32 kbps or above Page Nr. 36