Transcript 4.2 Audio compress

Chapter 4 Audio and video compression
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4.1 Introduction
4.2 audio compression
4.3 Video compression
4.1 introduction
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Both audio and most video signals are
continuously varying analog signals
The compression algorithms associated with
digitized audio and video are different from
close
4.2 Audio compress
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Pulse code modulation(PCM)
Bandlimited signal
The bandwidth of the communication
channels that are available dictate rates that
are less than these.This can be achieved in
one of two ways:
 Audio signal is sampled at a lower rate
 A compression algorithm is used
4.2.1 Differential pulse code modulation
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DPCM is a derivative of standard PCM and
exploits the fact that,for most audio signals,
the range of the differences in amplitude
between successive samples of the audio
waveform is less than the range of the actual
sample amplitudes.
Figure4.1
4.2.1 Differential pulse code modulation –
cont (figure 4.1)
4.2.2 Adaptive differential PCM
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Additional savings in bandwidth –or improved
quality –can be obtained by varying the
number of bits used for the difference signal
depending on its amplitude
A second ADPCM standard ,which is G.722.It
added subband coding.
A third standard based on ADPCM is also
available.this is defined in G.726.This also
uses subband coding but with a speech
bandwidth of 3.4kHz
4.2.3 Adaptive Predictive Coding(APC)
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Even higher levels of compression-but at
higher levvels of complexity-can be obtained
by also making the predictor coefficients
adaptive.This is the principle of adaptive of
adaptive predictive coding
4.2.4 Linear predictive coding
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There are then quantizized and sent and the
destination uses them,together with a sound
synthesizer,to regenerate a sound that is
perceptually comparable with the source
audio signal.this is LPC technique.
Three feature which determine the perception
of a signal by the ear are its:
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Pitch
Period
Loudness
Basic feature of an LPC encoder/decoder:
figure 4.4
4.2.4 Linear predictive coding -cont
(figure 4.4)
4.2.5 Code-excited LPC
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Code-excited LPC
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The synthesizers used in most LPC decoders are
based on a very basic model of the vocal tract
In the CELP model,instead of treating each
digitized segment independently for encoding
purpose
All coders of this type have a delay associated
with them which is incurred while each block
of digitized samples is analyzed by the
encoder and the speech is reconstructed at
the decoder
4.2.6 Perceptual coding
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Perceptual encoders have been designed for
the compression of general audio
Perceptual coding since its role is to exploit a
number of the limitation of the human ear.
Sensitivity of the ear
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A strong signal may reduce the level of sensitivity
of the ear to other signals which are near to it in
frequency
4.2.6 Perceptual coding -cont
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The Sensitivity of the ear varies with the
frequency of the signal,the perception threshold of
the ear – that is, its minimum level of sensitivityas a function of frequency is show in figure 4.5(a)
Most sensitive to signals in the range 2-5kHz
Shown 4.5(b) shows how the the sensitivity
of the ear changes in the vicinity of a loud
signal
4.2.6 Perceptual coding -cont (figure4.5)
4.2.6 Perceptual coding -cont
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The masking effect also varies with frequency as
show in figure 4.6
Critical bandwidth
Temporal masking:
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When the ear hears a loud sound,it takes a short
but finite time before it can hear a quieter sound
SHOW 4.7
4.2.6 Perceptual coding-cont (figure4.6)
4.2.6 Perceptual coding-cont (figure4.7)
4.2.7 MPEG AUDIO CODERS
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ENCODING
 Input signal is first sampled and quantized
using PCM
 The bandwidth that is available for
transmission is divided into a number of
frequency subbands using a bank of
analysis filters
 Scaling factor:
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THE analysis filter band also determines the
maximum amplitude of the 12 subband
samples in each subband
4.2.7 MPEG AUDIO CODERS -cont
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Discrete Fourier transform(DFT)
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The 12 set of 32 PCM samples are first
transformed into an equivalent set of frequency
components using a mathematical technique
Signal-to-mask ratios(SMRs)
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Using the known hearing thresholds and masking
properties of each subband,the model determines
the various masking effects of this set of signals
4.2.7 MPEG AUDIO CODERS -cont
(figure4.8)
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Frame format,show figure 4.8(b)
4.2.7 MPEG AUDIO CODERS -cont
table 4.2
Table 4.2 Summary of MPEG layer1,2 and 3 perceptual encoders
Layer
1
2
3
Apllication
Digital audio
cassette
Digital audio and
digital video
broadcasting
CD-quality
Compressed
bit rate
32-448kbps
32192kbps
64kbps
Quality
Example
input-to-output
delay
Hi-fi quality at 192
kbps per channel
20ms
Near CD-quality
at 128 kbps per
channel
40ms
CD-quality of
64kbps per
channel
60ms
4.2.8 Dolby audio coders
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MPEG V.S Dolby AC-1 ,show figure 4.9
 MPEG:
 Advantage: psychoacoustic model is required
only in the encoder
 Disadvantage:a significant portion of each
encoded frame contains bit allocation
information
 Dolby AC-1:
 Use a fixed bit allocation strategy for each
subband which is then used by both the
encoder and decoder
4.2.8 Dolby audio coders -cont (figure4.9)
4.2.8 Dolby audio coders -cont
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Dolby AC-2 standard which is utilized in many
applications including the compression
associated with the audio of a number of PC
sound cards
The hybrid approach is used in the Dolby AC3 standard which has been defined for use in
a similar range of applications as the MPEG
audio standards including the audio
associated with advanced television(ATV)
4.3 Video compression
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The digitization format defines the sampling
rate that is used for the luminance ,Y ,and
two chrominance,Cb and Cr
4.3.1 video compress principles
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Frame type
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I-frame:
 I-frames are encoded without reference to any
other frames
 GOP:The number of frame between I-frames
P-frame:
 encoding of a p-frame is relative to the
contents of either a preceding I-frame or a
preceding P-frame
4.3.1 video compress principles -cont
The number of P-frames between I-frame is
limited since any errors present in the first Pframe will be propagated to the next
B-frame:their contents are predicted using search
regions in both past and future frames
PB-frame:this does not refer to a new frame type
as such but rather the way two neighboring P- and
B-frame are encoded as if they were a single
frame
D-frame:only used in a specific type of application.
It has been defined for use in movie/video-ondemand application
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4.3.1 video compress principles –cont
(figure4.11)
4.3.1 video compress principles -cont
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Motion estimation and compensation
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P-frame Macroblock structure ,show figure 4.12(a)
P-frame Encoding procedure,show figure 4.12(b)
 Best match macroblock
 Motion vector
 DCT+ Quantization +run-length & V
 Huffman
B-frame encoding procedure,show figure 4.13
4.3.1 video compress principles –cont
(figure4.12)
4.3.1 video compress principles –cont
(figure4.13)
4.3.1 video compress principles –cont
(figure4.14)
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Implementation issues ,show figure4.14
4.3.1 video compress principles –cont
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Performance - Compression ratio
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I-frame:10:1 – 20:1
P-frame:20:1-30:1
B-frame:30:1-50:1
4.3.2 H.261
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For the provision of video telephony and
videoconferencing services over an ISDN
Transmission channels multiples of 64kbps
Digitization format used is either the common
intermediate format(CIF) or the quarter
CIF(QCIF)
CIF:Y=352X288, Cb=Cr=176X144
 QCIF:Y=176X144, Cb=Cr=88X72
H.261 encoding format show figure 4.15
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4.3.2 H.261 -cont
4.3.2 H.261 -cont
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H.261 video encoder principles figure 4.16(a)
4.3.2 H.261 -cont
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Two threshold
 Low
 high
4.3.3 H.263
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Over wireless and public switched telephone
networks(PSTN)
Include video telephony videoconferencing ,
security surveillance ,interactive game
Low bit rates
Digitization formats
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QCIF:Y=176X144 , Cb=Cr=88X72
S-QCIF:Y=128X96, Cb=Cr=64X68
4.3.3 H.263 -cont
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Frame types:
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I-frame
P-frame
B-frame
PB-frame:because of the much reduced encoding
overhead
Unrestricted motion vectors
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To overcome this limitation ,for those pixels of a
potential close-match macroblock that fall outsize
of the frame boundary
4.3.3 H.263 -cont
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Error resilience
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Cause error propagation,show figure4.17(a)
Error tracking and resilience,show figure4.17(b)
 When an error is detected , decoder send NAK
to encoder
Independent segment decoding
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Prevent these errors from affecting neighboring
GOBs in succeeding frames
Show figure 4.18
4.3.3 H.263 -cont (figure 4.17)
4.3.3 H.263 -cont (figure 4.18)
4.3.3 H.263 -cont (figure 4.19)
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Reference picture selection(figure 4.19 )
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NAK mode ,show figure 4.19(a)
ACK mode,show figure 4.19(b)
4.3.4 MPEG
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MPEG-1
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Source intermediate digitization format(SIF)
Resolution:352X288
VHS-quality audio
Video on CD-ROM at bit rates up to 1.5Mbps
MPEG-2
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Four level
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LOW
MAIN
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High 1440
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high
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4.3.4 MPEG -cont
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MPEG-4
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Similar h.163
Low bit rate range from 4.8 to 64kbps
Interactive multimedia application
4.3.5 MPEG-1
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Support two type spatial resolutions
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Frame type:I,P,B-frame,(figure 4.20)
Based on the h.261,there are two main
differences:
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NTSC
PAL
Temporal
B-frame was increased
Video bitstream structure (figure 4.21)
4.3.5 MPEG-1 -cont (figure 4.20)
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Figure 4.20
4.3.5 MPEG-1 -cont (figure 4.21)
4.3.6 MPEG-2
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Support four levels and five profiles
MP@ML
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For digital television broadcasting
Resolution of either 720X480 pixels at 30Hz or
720X576 pixels at 25Hz
Bit rate from 4Mbps – 15Mbps
Use interlaced scanning,show 4.22(a)
Field mode(figure 4.22(b))
Frame mode(figure 4.22(c))
4.3.6 MPEG-2 -cont (figure4.22)
4.3.6 MPEG-2 -cont
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HDTV(Grand Alliance)
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ITU-R HDTV
16/9 ASPECT RATIO
 MP@HL
 Audio: Dolby AC-3
DVB HDTV
 4/3 ASPECT RATIO
 SSP@H1440-SPATIALLY-SCALEABLE PROFILE
AT HIGH 1440
 MPEG audio layer 2
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4.3.7 MPEG-4
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Scene composition
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Content-based functionalities
Audio-visual object(AVOs)
Object descriptor
Binary format for scenes
Scene descriptor
Video object planes(VOPs)(figure 4.23)
Audio and video compression(figure 4.24)
4.3.7 MPEG-4
-cont (figure4.23)
4.3.7 MPEG-4
-cont (figure4.24)
4.3.7 MPEG-4
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-cont
Transmission format(figure 4.25)
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Transport stream
Packetized elementary
Elementary stream(ES)
FlexMux layer
Synchronization layer
Elementary stream descriptor(ESD)
Composition and rendering block
4.3.7 MPEG-4
-cont (figure4.25)
4.3.7 MPEG-4
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-cont
Error resilience techniques (figure 4.26)
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Use of fixed-length
Based on reversible VLCs
Error occur
 macroblock
 header
4.3.7 MPEG-4
-cont (figure4.26)
4.3.7 MPEG-4
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-cont
Reversible VLCs (figure 4.27)
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The associated set of RVLCs is then produced by
adding a fixed—length prefix and suffix to each of
the corresponding VLCs
Forward direction scan
Reverse direction scan
The error at difference points in the bitstream
resulting in an overlap region
4.3.7 MPEG-4
-cont (figure4.27)