Audio Coding
Introduction
Lecture
WS 2013/2014
Prof. Dr.-Ing. Karlheinz Brandenburg [email protected]
Prof. Dr.-Ing. Gerald Schuller [email protected]
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Organisatorial Details - Overview
• Lectures:
– 14 lectures read by Prof. Brandenburg and Prof. Schuller
• Practice lessons:
– 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)
• Exam:
– Written exam at the end of the semester, 90 minutes
– Agree to this method by signing the document that is passed around
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 2026 01:00-02:30pm
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Organisatorial Details - Timeline
Lecture: Date: Read by:
1. Introduction 14.10.2013 Prof. Brandenburg
2. Psychoacoustics 21.10.2013 Dipl.-Ing. Werner
3. Basics of Multirate Signal Processing 28.10.2013 Prof. Schuller
4. Filter Banks 1 04.11.2013 Prof. Schuller
5. Filter Banks 2 11.11.2013 Prof. Schuller
6. Quantization & Coding 18.11.2013 Prof. Brandenburg
7. MPEG 1 / MPEG 2 BC Audio 25.11.2013 Prof. Brandenburg
8. MPEG 2 / 4 AAC 02.11.2013 Prof. Brandenburg
9. Prediction and Lossless Audio Coding 09.12.2013 Prof. Schuller 10. Audio Coding for Communication (ULD) 16.12.2013 Prof. Schuller
11. Coding of Stereophonic Signals 06.01.2014 Prof. Brandenburg
12. Parametric Coding of High-Quality Audio 13.01.2014 Prof. Brandenburg
13. Dolby AC3, DTS 20.01.2014 Prof. Schuller
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
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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)
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
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Psychoacoustics (Masked Threshold)
L 0 20 40 60 dB 80 0,02 0,05 0,1 0,2 0,5 1 2 5 10 20 kHz T f m f =0,25 T m f =1kHz f m =4kHz
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)
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The Basic Paradigm of T/F Domain Audio Coding
Digital Audio Input Filter Bank Psychoacoustic Model Bitstream Formatting Signal to Mask Ratio Quantized Samples Encoded Bitstream Bit or Noise Allocation
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
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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)
History of Audio Coding
• 1979 - the „Critical Band Coder“
• 1982 - „classic ATC“ for Music
• 1985 - MSC • 1987 - OCF • 1987 - MASCAM • 1987 - PXFM • 1990 - ASPEC, MUSICAM • 1992 - MPEG 1 • 1996 - ePAC • 1997 - MPEG 2 AAC • 1999 - MPEG 4 AAC • 2002 - HE AAC • 2012 - USAC
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The time line for near-CD-quality
• 1990 256 kbit/s ASPEC, MUSICAM would fail today’s listening tests
• 1992 192 kbit/s MPEG-1 Layer-3
• 1994 128 kbit/s MPEG-1 Layer-3 (".mp3") including combined joint stereo coding
bad quality for some signals
• 1997 96 kbit/s MPEG-2 Advanced Audio Coding better than MP3 at 128, not fully transparent
• 2000 64 kbit/s AAC-based MPEG-4
• 2003 48 kbit/s MPEG-4 HeAAC (AAC+ in 2000) e.g. used for XM Radio
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
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Demo: Can you hear it (Version 4, 2000) ?
Each “?” corresponds to eitherO (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 ? ? ?
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) _ _ _
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Extra Material
Organisatorial Details – Overview (Repetition)
• Lectures:
– 14 lectures read by Prof. Brandenburg and Prof. Schuller
• Practice lessons:
– 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)
• Exam:
– Written exam at the end of the semester, 90 minutes
Page ‹Nr.›
Organisatorial Details – Timeline (Repetition)
Lecture: Date: Read by:
1. Introduction 14.10.2013 Prof. Brandenburg
2. Psychoacoustics 21.10.2013 Dipl.-Ing. Werner
3. Basics of Multirate Signal Processing 28.10.2013 Prof. Schuller
4. Filter Banks 1 04.11.2013 Prof. Schuller
5. Filter Banks 2 11.11.2013 Prof. Schuller
6. Quantization & Coding 18.11.2013 Prof. Brandenburg
7. MPEG 1 / MPEG 2 BC Audio 25.11.2013 Prof. Brandenburg
8. MPEG 2 / 4 AAC 02.11.2013 Prof. Brandenburg
9. Prediction and Lossless Audio Coding 09.12.2013 Prof. Schuller 10. Audio Coding for Communication (ULD) 16.12.2013 Prof. Schuller
11. Coding of Stereophonic Signals 06.01.2014 Prof. Brandenburg
12. Parametric Coding of High-Quality Audio 13.01.2014 Prof. Brandenburg
13. Dolby AC3, DTS 20.01.2014 Prof. Schuller
History of Audio Coding
• 1979 - the „Critical Band Coder“
• 1982 - „classic ATC“ for Music
• 1985 - MSC • 1987 - OCF • 1990 - MUSICAM • 1990 - ASPEC • 1992 - MPEG 1 • 1996 - PAC • 1997 - MPEG 2 AAC • 1999 - MPEG 4 AAC • 2002 - HE AAC • 2012 - USAC
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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
„classic ATC“ for Music
•
Universität Erlangen-Nürnberg, 1982
•
First real-time music coder
•
Sampling rate between 30-32 kHz
•
Does not use a psycho-acoustic model →
bad quality for some music pieces
•
Block length of 128 samples (about 4 ms)
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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
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)
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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)
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
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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)
MPEG-1 (2)
• Layer III
Window length: 1152 samples (24 ms)
Frequency resolution: 576/192 subbands
Quantization: non-uniform with Huffman coding
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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
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)
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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)
MPEG 2 AAC (1)
• first named MPEG-2 NBC (non backwards compatible), later named AAC (advanced audio coding)
• MPEG-2 AAC (ISO/IEC 13818-7)
• 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
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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.
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
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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
