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The Mpeg Handbook Chapter 2' Fundamentals

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Actual sounds are converted variations in air pressure and air velocity to electrical signals ... Attenuate by 6.02dB by dividing samples by two ... – PowerPoint PPT presentation

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Title: The Mpeg Handbook Chapter 2' Fundamentals


1
The Mpeg HandbookChapter 2. Fundamentals
  • ? ? ?

2
Audio and Video
  • What is an audio signal?
  • Actual sounds are converted variations in air
    pressure and air velocity to electrical signals
  • What is an video signal?
  • Television is to allow a moving picture, which is
    a two-dimensional image, which changes as a
    function of time
  • This is a three-dimensional information source
  • Distance across the screen, distance down the
    screen, and time
  • Solution
  • Convert the three-dimensional moving image into a
    series of still pictures, taken at the frame rate
  • Two-dimensional images are scanned as a series of
    lines

3
Type of video
RGB video
-Single channel television broadcast -PC cable
Component
-Transform-based compression -DVDP
Composite
Chroma
-Differential coding system
4
What is digital signal ? (1)
  • Digital equipment is smaller extent than analog
    and at lower cost or same performance at much
    lower cost

5
What is digital signal ? (2)
  • Digital signal is used as the input of
    compression system and output of decoding stage
  • Quality is determined by ADC and performance of
    the coder

6
Sampling (1)
Hz
Cycle-per-millimeter
Cycle-per-degree
Cycle-per-picture-height Cycle-per-picture-width
7
Sampling (2)
8
Sampling (3)
-Beat-frequency ???? ?? ?? ? ???? ???? ???
?????? ??? ?3? ???
9
Sampling (4)
Fb
Fb
10
Sampling (5)
  • Sampling system consist of a pair of filters
  • Anti-aliasing filter before the sampling process
  • Reconstruction filter after the sampling process
  • Low-pass filter

11
Reconstruction (1)
  • Input must be band limit by an ideal linear phase
    low-pass filter
  • Ideal low-pass filter is a sin(x)/x
  • Reconstruction filter has the same frequency
    response

12
Reconstruction (2)
  • Optics dont have negative light
  • The restriction to positive-only impulse response
    limits the sharpness of optical filters
  • Filter must be symmetrical
  • non-causal
  • Delay of one-half the window period
  • (in chapter 3)

13
Aperture effect
  • Real sample impulse cannot be infinitely small in
    time
  • Zero-Order Hold
  • Pulse width equal to the sample period
  • About 4dB down at the Nyquist frequency

14
Choice of audio sampling rate (1)
  • Nyquist criterion is only the beginning of the
    processing
  • For professional products, there is a need to
    operate at variable speed for pitch correction

15
Choice of audio sampling rate (2)
  • In the early days of digital audio, video
    recorders were adapted to store audio samples by
    creating a pseudo-video waveform
  • Sampling rate is constrained to relate simply to
    the field rate and field structure of TV standard
  • 525/60
  • 35 blanked lines, 490 lines per frame, 245 lines
    per field for sample
  • 60245344.1kHz
  • For practical and economic reasons digital audio
    now has essentially three rates to support
  • 32 kHz for broadcast, 44.1 kHz for CD, 48 kHz for
    professional use

16
Video sampling structures
  • Luminance samples appear at half the spacing of
    colour difference samples
  • Other luminance sample is co-sited with a pair of
    colour difference samples
  • Figure 2.22 shows 420 subsampling
  • Colour data are vertically low-pass filtered

17
The phase-locked loop (1)
  • Voltage Controlled Oscillator
  • Run at a range of frequencies according to the
    voltage applied to a control terminal
  • Driven by a phase error measure between the
    output and some reference

18
The phase-locked loop (2)
19
Quantizing (1)
  • Process of expressing some infinitely variable
    quantity by discrete or stepped values

20
Quantizing (2)
  • Divides the voltage range up into quantizing
    intervals Q
  • Mid-tread quantizer is universally used in audio
    and video
  • Audio muting or video blanking is half-way up a
    quantizing interval
  • -1/2ltQuantizing errorlt1/2

21
Quantizing error (1)
  • Quantizing error waveform can be thought of as an
    unwanted signal
  • The quantizing process adds to the perfect
    original
  • Additive broadband noise uncorrelated with the
    signal
  • Quantizing noise
  • At low levels, quantizing error becomes a
    function of the input waveform and the quantizing
    structure
  • Unwanted signal becomes a deterministic function
    of the wanted signal
  • Distortion rather than noise
  • With a large signal , there are so many steps
    involved
  • Staircase with many steps appears to be a slope

22
Quantizing error (2)
  • The non-linearity of the transfer function
    results in distortion, which produces harmonics
  • These harmonics are generated after the
    anti-aliasing filter, and so any which exceed
    half the sampling rate will alias

23
Dither (1)
  • Job of decorrelation by making the action of the
    quantizer unpredictable
  • Dither causes a slight reduction in the SNR, but
    small price to pay for the elimination of
    non-linearities
  • Linearity is an essential requirement for digital
    audio and video
  • Ideal quantizer can be dithered by linearly
    adding a controlled level of noise either to the
    input signal or to the reference voltage

24
Dither (2)
  • View of system position
  • The addition of dither means that successive
    samples effectively find the quantizing intervals
  • Quantizing error becomes a function of the dither
  • Unacceptable distortion is converted into
    broadband noise
  • The dither has resulted in a form of duty cycle
    modulation
  • System resolution has been extended indefinitely
  • View of transfer function of quantizer position
  • Transfer function which is a perfect staircase
    becomes straight

25
Introduction to digital processing
  • Only two basic types of element in use
  • Combined in some way and supplied with a clock to
    implement
  • Strength of binary logic
  • Considerable noise and distortion can be
    tolerated
  • Signal is compared with a threshold
  • Can pass through any number of stages without
    degraded
  • Reclocking
  • Use of a storage element

26
Logic element (1)
  • Exact levels are not of much consequence
  • Interfacing between different logic families
  • Driving external devices
  • There is two states
  • High and low
  • Positive logic
  • High voltage represents a true logic condition
  • Low voltage represents a false logic condition
  • In binary, the column position specifies the
    power of two
  • Parallel system is most convenient inside
    equipment or short distances
  • It is inexpensive
  • Single signal path is convenient for cables
    between pieces of equipment
  • The connectors require fewer pins
  • It can be called digital system

27
Logic element (2)
  • Important simple gates and their derivatives
  • Normally, a high voltage level is a binary 1 and
    a low voltage level is a binary 0

28
Storage elements (1)
  • Base memory element in logic circuits is the
    latch
  • Edge-triggered device
  • Change state at transition
  • Level-triggered device
  • Change state at level
  • Shift register can be made from a series of
    latches
  • Connecting the Q output of one latch to the D
    input of the next
  • All the clock inputs in parallel
  • Useful for converting between serial and parallel
    data formats

29
Storage elements (2)
  • When large numbers of bits are to be stored,
    cross-coupled latches are less suitable
  • More complicated to fabricate inside integrated
    circuit than dynamic memory
  • Consume more current
  • In large RAMs, the data bits ard stored as the
    presence or absence of charge in a tiny capacitor
    (MOS)
  • Charge will suffer leakage after few milliseconds
  • Delay needed is less than this (read out before
    decay)
  • Where longer delays are necessary, memories must
    be refreshed periodically
  • Desired bit has to be addressed before it can be
    read or written
  • Size of the chip package restricts the number of
    pins available
  • Large memories use the same address pin
  • Bits are arranged internally as rows and columns
  • The low address and the column address are
    specified sequentially on the same pins

30
Storage elements (3)
  • Basic volatile RAMs will lose data if power is
    interrupted
  • Non-volatile RAMs or NVRAMs retain the data in
    the absence of power
  • ROM
  • A type of memory which is written once is called
    a read-only-memory
  • Data are fixed
  • UVROM
  • Can be written electrically
  • Need to be erased by exposure to ultraviolet
    light
  • EAROM (electric alterable)
  • Can be rewritten electrically a limited number of
    times

31
Binary coding (1)
  • Practical digital hardware places a limit on the
    wordlength
  • Occur overflow and underflow
  • The pulses to be counted are fed to the clock
    input of a D-type latch
  • Divide-by-two counter
  • As a result of the fixed wordlength, the infinite
    range of real numbers is mapped onto the limited
    range of a binary code of finite wordlength

32
Binary coding (2)
  • Mathematically the pure binary mapping from an
    infinite scale to a finite scale is known as
    modulo arithmetic
  • For a broadcast standard luminance signal
  • Only handle active line
  • Sync pulses go off the bottom of the scale
  • Small offset in order to handle slightly
    misadjusted inputs
  • The extremes of the range are reserved for
    synchronizing
  • Colour difference video signals are bipolar
  • Blanking is in the centre of the signal range

33
Binary coding (3)
  • Digital audio mixing
  • Same quantizing interval size, no offset
  • achieved by adding sample values
  • Others (non-uniform or offset quantizing)
  • Cannot be processed
  • Binary numbers are not proportional to the signal
    voltage
  • If two offset binary streams are added
  • May lead to an overflow
  • Attenuate by 6.02dB by dividing samples by two
  • This approach is not suitable for audio or colour
    difference signals

34
Binary coding (4)
  • In twos complement system
  • All numbers clockwise from zero are positive and
    have the MSB reset
  • All numbers anticlockwise from zero are negative
    and have the MSB set
  • MSB is sign bit
  • ( 1minus )

35
Binary coding (5)
  • Real ADC configure
  • MSB inversion may be selectable by an external
    logic level

36
Binary coding (6)
  • Linear sum of the two waveforms obtained by
    adding pairs of sample values

37
Binary coding (7)
  • Twos complement adding process
  • Effectively both twos complement numbers to be
    added contain an offset of half full scale
  • Code consists of moving one full rotation round
    the circle of numbers
  • Offset has no effect and is effectively
    eliminated
  • Sometimes necessary to phase reverse or invert a
    digital signal
  • Process of inversion in twos complement
  • Invert to form the ones complement, and one is
    added
  • Performing a second inversion gives the original
    sample values
  • Subtraction can be performed using adding logic
  • Radix point
  • Numbers to the right of it are added
  • 1100.1 is not -4.5, it is -40.5-3.5

38
Binary coding (8)
  • When mixing by adding sample values, care has to
    be taken to ensure overflow
  • If the MSB of both input is zero, the numbers are
    both positive, thus the sum has the MSB set, the
    output replaced with the maximum positive code

39
Binary coding (9)
  • Storage element can be combined with an adder to
    obtain a number of useful functional blocks
  • Latch is connected in a feedback loop around an
    adder
  • Accumulator
  • Discrete time integrator in filtering
  • Addition of an inverter allows the difference
    between successive inputs to be obtained
  • Digital differentiation

40
Gain control
  • When processing digital audio or image data the
    gain of the system will need to be variable
  • In digital domain by multiplying each sample
    value by a coefficient
  • Multiplication in binary circuits can be
    performed by bit shifting
  • The samples to be multiplied must have been
    uniformly quantized

41
Floating-point coding
  • Floating-point coding allows a much greater range
    of numbers
  • Floating-point is the computers equivalent of
    lossy compression
  • Without the inaccuracy of floating-point coding
    by using techniques such as double precision

42
Multiplexing principles
  • Multiplexing is used where several signals are to
    be transmitted down the same channel
  • In time-division multiplexing the timebase of the
    transmission is broken into equal slots, one for
    each signal

43
Packets
  • In multiplexing system, much easier to organize
    if each signal is in the form of data packets of
    constant size
  • Each packet consists of two components
  • In more complex system, check that packets are
    not lost or repeated
  • Packet continuity count in the header

44
Statistical multiplexing
  • The multiplexer has to ensure that the total bit
    rate does not exceed the rate of the channel
  • With variable-rate inputs, creating null packets
    which are generally called stuffing or packing
  • In MPEG environment, statistical multiplexing can
    be extremely useful
  • Allow for the varying difficulty of real program
    material

45
Timebase correction (1)
  • Accurate control of delay is the essence of
    timebase correction
  • Compression result in a variable amount of data
    and effectively the picture period varies
  • Buffering will be needed at the encoder and
    decoder

46
Timebase correction (2)
  • Shift register approach and the RAM approach to
    delay are very similar
  • Addressing of the RAM is given by ring-like
    structure
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