Digital Audio Compression - Berkeley Design Technology, Inc

16 Pages·2000·0.38 MB·English

Digital Audio Compression: Why, What, and How
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1
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(cid:1) Why Compress?
(cid:1) What is Audio Compression?
(cid:1) How Does it Work?
(cid:1) Conclusions
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© 2000 Berkeley Design Technology, Inc.
ICSPAT 2000 Page 1 October 18, 2000
Digital Audio Compression: Why, What, and How
WWhhyy:: TToooo MMuucchh DDaattaa!!
z “CD quality”
(cid:132) Rate: 2 audio channels × 44,100 samples/sec × 16
bits = 1.4 Mbit/second audio data
(cid:132) Audio CD capacity: 0.8 gigabytes audio data
z “Cinema quality”
(cid:132) ~6 audio channels ×
48,000 samples/sec × 16 bits =
4.6 Mbit/second, ~1 Gbyte/hour
z Typical compression today: few hundred
kbit/sec for even 6 channels
3
CCrroossssiinngg TThhrreesshhoollddss
Memory Capacity - Mbytes/chip
1000
Audio CD 512
100 DVD Sound Track 128
32 MP3 Album
i
h 10 8
c

/ 2
s 1 MP3 Song
e 0.5
t
y
b 0.1 0.125
M
0.032
0.01 0.008
0.001
1975 1980 1985 1990 1995 2000 2005 2010
Year
4
© 2000 Berkeley Design Technology, Inc.
ICSPAT 2000 Page 2 October 18, 2000
Digital Audio Compression: Why, What, and How
CCoommmmeerrcciiaall AApppplliiccaattiioonnss
(cid:1) Cinema (digital film sound)
(cid:1) Consumer devices
(cid:132) Mini-disc (Sony)
(cid:132) Handheld players
(cid:132) Games
(cid:132) DVD
(cid:1) Distribution
(cid:132) Internet distribution
(cid:132) Audio over cable (set-top box)
(cid:132) Satellite/terrestrial audio broadcast
(cid:132) Digital television
5
WWhhaatt:: CCoommpprreessssiioonn GGooaallss
(cid:1) Reduced bandwidth and/or storage
(cid:1) Make decoded signal as close as
possible to original signal
(cid:1) Lowest implementation complexity
(cid:1) Reasonable arithmetic requirements
(cid:1) Applicable to as many signal types as
possible
(cid:1) Robust
(cid:1) Scalable
(cid:1) Extensible
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© 2000 Berkeley Design Technology, Inc.
ICSPAT 2000 Page 3 October 18, 2000
Digital Audio Compression: Why, What, and How
PPssyycchhooaaccoouussttiiccss
(cid:1) What does it cover?
(cid:132) Relationship between what arrives at the
ear and what we hear
(cid:1) Why is it important for compression?
(cid:132) Don’t transmit what the ear can’t hear
(cid:1) How to figure out what ear can’t hear?
(cid:132) Range of human hearing
(cid:132) Masking
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RRaannggee ooff HHuummaann HHeeaarriinngg
ZZwwiicckkeerr//FFaassttll pp.. 1177
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© 2000 Berkeley Design Technology, Inc.
ICSPAT 2000 Page 4 October 18, 2000
Digital Audio Compression: Why, What, and How
AAuuddiittoorryy MMaasskkiinngg
(cid:1) One signal
pp MMaasskkeerr SShhiifftteedd
can make mm
AA.. hheeaarriinngg
another
tthhrreesshhoolldd
inaudible
FFrreeqq..
SSiiggnnaall nnoott
HHeeaarriinngg
mmaasskkeedd
tthhrreesshhoolldd
MMaasskkeedd
ssiiggnnaallss
9
AAuuddiittoorryy MMaasskkiinngg ((ccoonntt’’dd))
(cid:1) Temporal Masking
AAfftteerr ZZwwiicckkeerr//FFaassttll pp.. 7788,,
BBuusseerr//IImmbbeerrtt pp.. 4477
→→ Pre-
el el
vv masking
ee
LL
ure ure (“Back- Shifted Post-masking
ss ward”)
eses Threshold (“Forward”)
PrPr
d d
nn
uu Masker
oo
SS
Duration
TTiimmee →→
10-30 100-200 msec
msec
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© 2000 Berkeley Design Technology, Inc.
ICSPAT 2000 Page 5 October 18, 2000
Digital Audio Compression: Why, What, and How
PPeerrcceeppttuuaall CCoommpprreessssiioonn
Calculate
Thresholds
Remove
Window Spectral
Inaudible
(Snapshot) Analysis
Components
Pack
Coding
Quantize Data into
Scheme
Frames
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SSppeeccttrraall AAnnaallyyssiiss//SSyynntthheessiiss
(cid:1) What is it?
(cid:132) Break a signal into spectrum
(cid:132) Recover the signal from its spectrum
Forward (analysis)
Reverse/inverse
(synthesis)
Time domain Frequency domain 12
© 2000 Berkeley Design Technology, Inc.
ICSPAT 2000 Page 6 October 18, 2000
Digital Audio Compression: Why, What, and How
SSppeeccttrraall AAnnaallyyssiiss ((ccoonntt’’dd))
(cid:1) How is it done?
1. Filter bank
2. Transform
++
. .
. .
. .
Time Frequency Time
domain domain domain
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SSppeeccttrraall AAnnaallyyssiiss ((ccoonntt’’dd))
(cid:1) Does reconstructed output = input?
Yes, if:
(cid:132) Don’t change transform data
(no filtering)
(cid:132) Design window correctly
(cid:132) Window input often enough
(cid:132) Overlap windows in analysis/resynthesis
properly
(cid:1) If yes: Identity System
(cid:132) Solid basis for further changes
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© 2000 Berkeley Design Technology, Inc.
ICSPAT 2000 Page 7 October 18, 2000
Digital Audio Compression: Why, What, and How
HHooww:: AAnnaallyyssiiss
(cid:1) Spectral analysis
(cid:132) DCT
(cid:132) Wavelet
(cid:132) ...
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Davidson et al., 1994
HHooww:: AAnnaallyyssiiss
(cid:1) Calculate masking thresholds
(cid:132) “Perceptual model”
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Davidson et al., 1994
© 2000 Berkeley Design Technology, Inc.
ICSPAT 2000 Page 8 October 18, 2000
Digital Audio Compression: Why, What, and How
HHooww:: NNooiissee AAllllooccaattiioonn
(cid:1) Remove inaudible components
(cid:1) Quantize remaining components
(cid:132) Use minimum # bits
(cid:1) Adds noise
(cid:132) Keep below
masking
threshold
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Davidson et al., 1994
HHooww:: MMoorree TTrriicckkss
(cid:1) Filter
(cid:132) Bandlimit input signal
(cid:139) LFE bandlimited to <120 Hz
(cid:1) Differential coding of spectral values
(cid:1) Coupling
(cid:132) Across time
(cid:132) Across channels
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© 2000 Berkeley Design Technology, Inc.
ICSPAT 2000 Page 9 October 18, 2000
Digital Audio Compression: Why, What, and How
HHooww:: CCooddiinngg SScchheemmee
(cid:1) Direct coding
(cid:1) Entropy (e.g., Huffman) coding
(cid:132) MPEG: “noiseless coding”
(cid:132) PAC: “information-theoretic coding”
(cid:1) Quantization table
(cid:1) Run-length
(cid:1) Vector quantization
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AArrttiiffaaccttss:: ““PPrree--eecchhoo””
Original Resynthesized
(5.3 msec)
(cid:1) Quantization noise spread
≥
(cid:1) Noise components 1-2 msec before
impulsive signal not masked
(cid:1) Fix: Shorter window; wavelets (ePAC)
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© 2000 Berkeley Design Technology, Inc.
ICSPAT 2000 Page 10 October 18, 2000