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The Mpeg Handbook Chapter 2' Fundamentals


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

The Mpeg HandbookChapter 2. Fundamentals
  • ? ? ?

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

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

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

Sampling (1)
Cycle-per-picture-height Cycle-per-picture-width
Sampling (2)
Sampling (3)
-Beat-frequency ???? ?? ?? ? ???? ???? ???
?????? ??? ?3? ???
Sampling (4)
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

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

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)

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

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

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

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

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

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

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

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

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

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
  • 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

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
  • System resolution has been extended indefinitely
  • View of transfer function of quantizer position
  • Transfer function which is a perfect staircase
    becomes straight

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

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

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

Storage elements (1)
  • Base memory element in logic circuits is the
  • Edge-triggered device
  • Change state at transition
  • Level-triggered device
  • Change state at level
  • Shift register can be made from a series of
  • 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

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
  • Charge will suffer leakage after few milliseconds
  • Delay needed is less than this (read out before
  • 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

Storage elements (3)
  • Basic volatile RAMs will lose data if power is
  • 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
  • Can be written electrically
  • Need to be erased by exposure to ultraviolet
  • EAROM (electric alterable)
  • Can be rewritten electrically a limited number of

Binary coding (1)
  • Practical digital hardware places a limit on the
  • 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

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
  • Colour difference video signals are bipolar
  • Blanking is in the centre of the signal range

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
  • 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

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 )

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

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

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
  • 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
  • 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

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

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
  • Accumulator
  • Discrete time integrator in filtering
  • Addition of an inverter allows the difference
    between successive inputs to be obtained
  • Digital differentiation

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

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

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

  • 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

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

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

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