History of Television (1 / 2)

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History of Television (1 / 2)

• During the 1870's Sir William Crookes constructed
  a vacuum tube , this produced a beam of
  electrons.
• In 1873 Joseph May, a telegraph operator
  discovered photoelectric effect.
• In 1875 George Carey proposed using a system
  where all points in an image were simultaneously
  scanned and transmitted.
• In 1881, Constantin Senlecq proposed converting
  the picture into a number of elements which could
  then be transmitted in series.
• A mechanical scanning system was found by Paul
  Nipkow in 1884. This involved a spinning disk at
  either end of transmission.
• By 1911 A A Cambell Swinton suggested making use
  of the cathode ray tube in a television system.
  
• In 1930 interlaced scanning was introduced in
  order to improve frame rate without otherwise
  altering equipment
• As early as 1928, John Logie Baird was able to
  demonstrate a working system for colour
  television which is the basis for the current
  system.
• In 1938, Georges Valensi suggested that it should
  be possible to achieve dual compatibility colour
  programmes should be transmitted such that
  viewing on a black and white television was
  possible.

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History of Television (2 / 2)

• In 1940, Peter Goldmark demonstrated a colour
  system involving the three primaries being
  transmitted sequentially.
• In America the NTSC (National Television System
  Committee) colour system was developed and
  launched in 1954.
• In 1961 the SECAM (Sequential Couleur á Memoire)
  system was developed in France, followed by its
  variant the PAL (phase alternation by line)
  system developed in Germany in 1963 which was
  launched in the UK in 1967.
• in 1980, Nippon Hoso Kyotai (NHK) published a
  standard for HDTV system
• In 1986 the USA joined Japan in pushing for the
  NHK system within a few months a group of
  companies and broadcasters from Europe known as
  'Eureka EU95' had begun to develop its own HDTV
  standard.
• In 1993 a new group was formed, initially called
  the European Launching Group and later the DVB
  (Digital Video Broadcasting) project.
• In 1994, the leaders of the Eureka EU95 project
  began research to convert their analogue HDTV
  system to using digital technologies.
• The American ATSC (Advanced Television Systems
  Comittee) system, based on the Grand Alliance
  work was also launched towards the end of 1998.

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Analog TV standards comparing

• The first three systems have the following
  similarities
• Despite the encoding differing, all
  nevertheless use a similar type of colour system,
  involving a luminance and two chrominance signals
  based on red, green and blue primaries.
• All use interlaced scanning.
• All use two separate carrier signals, one FM
  modulated for sound and one AM modulated for
  picture.

HDTV standards are not compatible with the old
standards.
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Advantages of digital television

• More digital channel in the same band of an
  analog channel thanks to MPEG-2 compression
  system
• CD quality stereo sound thanks to MPEG Layer II
  (Musicam)
• Service information, such as on-screen programme
  guide
• Near Video On-Demand (NVOD)
• Simulcasting (HDTV SDTV)
• Adaptivity to the channel
• QEF
• Wideband Internet Now

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Structure of the DVB project
The project was officially started in 1993, all
members pay an annual membership fee. Commercial
Modules have the task to formulate requirements
on the systems, these form the basis for the work
in the Technical Module, ad-hoc groups works on
special subjects. After the completion of
development work the commercial working groups
verify the specifications for the new systems and
forward them, if necessary, to the 'Steering
Board' for final decision. Promotion Module is a
group whose task it is to look after interested
persons in all parts of the world.
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DVB standards
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DVB wideband now
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DVB-SI (1/2)
DVB-SI adds information that enables DVB IRDs to
automatically tune to particular services and
allows services to be grouped into categories
with relevant schedule information, it is based
on four tables. Each table contains descriptors
outlining the characteristics of the
services/event being described

• NIT The Network Information Table groups together
  services belonging to a particular network
  provider (for example more satellites located at
  a single orbital position form a satellite
  network). It contains all the tuning information
  that might be used during the set-up of an IRD.
  It is also used to signal a change in the tuning
  information.
• SDT The Service Description Table lists the names
  and other parameters associated with each service
  in a particular MPEG multiplex. The receiver use
  SDT to compile and display a list of available
  services.
• EIT The Event Information Table provide
  information ( event name, start time,
  duration,...) in chronological order regarding
  the events contained within each service.
• TDT The Time and Date Table carries time and data
  information in UTC format.

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DVB-SI (2/2)
In addition there are three optional SI tables

• BAT The Bouquet Association Table provides a
  means of grouping services that might be used as
  one way an IRD presents the available services to
  the viewer, for example childrens channels ,
  sports channels , etc.
• RST The Running Status Table its used to rapidly
  update the running status of one or more events,
  this may be necessary when an event starts early
  or late due to scheduling change. The Running
  Status sections are sent out only once, at the
  time the status of an event changes, unlike the
  other SI tables which are normally repetitively
  transmitted.
• ST The Stuffing Tables may be used to replace or
  invalidate either sub-tables or complete SI
  tables.

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DVB-S
In satellites channels there are power
limitation so the main design objective is for a
sistem ruggedness against noise and interference,
this is achieved by means of QPSK and
concatenation of convolutional and RS codes that
provide a BER of 10-11 this means an error every
hour.
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Framing structure
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Randomization for energy dispersal
Randomizing is necessary to decorrelate the
transmitted spectrum from the data content, this
is achieved by means of a Pseudo Random Binary
Sequence generator

• Polinomial for PRBS shall be 1X14X15
• Sequence 100101010000000 loaded at start of
  every 8 packet
• Sync byte not randomized
• Also operating if there is no input stream
• SYNC1 bit-wise inverted to initialize the
  descrambler

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Outer Coding (1/3)
RS codes are a block coding technique, the data
stream is broken up into blocks and redundant
data is then added to each block, the data is
further subdivided into a number of symbols. DVB
uses a RS (204,188) code, utilizing sixteen check
symbols per 188 information symbols for a total
codeword length of 204 symbols. RS encoding then
consists of the generation of these check symbols
from the original data. The process is based upon
finite field arithmetic so named because the
result of any operation is still an element of
the field. The field elements are all values from
0 to 2 m -1, where m is the number of bits per
symbol. The field polynomial is used to determine
the order of the elements in the finite field,
DVB uses 8 bit symbols m 8 and a field
polynomial of 285 p(x) x8x4x3x21 . The last
item that needs to be known to generate a
particular RS implementation is the generator
polynomial starting root, DVB uses a generator
polynomial starting at root zero g(x) (x?0)
(x?1) (x?15) con ? 02HEX .
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Outer Coding (2/3)

• DVB RS 204,188 code can correct (204-188)/2 or
  8 errors per 204-symbol codeword. A burst error
  of 57 to 64 consecutive bits (dependent upon
  whether the error starts on a symbol boundary)
  can be corrected by that RS code. If these same
  errors are more evenly spaced within the
  codeword, however, it will require many more
  check symbols to correct all of the errors. For
  this reason, RS codes are generally combined with
  other coding methods such as Viterbi, which is
  more suited to correcting evenly distributed
  errors.
• The shortened RS code may be implemented by
  adding 51 bytes, all set to 0, before the
  information bytes at the input of a (255,239)
  encoder. After the RS coding procedure these null
  bytes shall be disharged.

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Outer Coding (3/3)
Input data stream is clocked back out of the
function while being fed back into the check
symbol generation circuitry. A series of finite
field adds and multiplies results in each
register containing one check symbol after the
entire input data stream has been entered. Check
symbols are shifted out at the end of the
original message.
On receiver side the incoming symbols are divided
into the generator polynomial in the Syndrome
calculation block.
The check symbols, which form the remainder in
the encoder section, will cause the syndrome
calculation to be zero in the case of no errors.
If there are errors, the resulting polynomial is
passed to the Euclid algorithm, where the factors
of the remainder are found. The result is then
evaluated for each of the incoming symbols over
many iterations, and any errors are found and
corrected.
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Convolutional interleaving
It is not economic to cover every code word
against burst because they do not occur often
enough. The solution is to use a technique known
as interleaving.
Based on the Forney approach, it is composed of
I12 branches , each one of these contain a shift
register whose dept depend on the branch. For
syncronization purpose, the sync bytes shall
always be routed in the branch 0 corrisponding
to a null delay.
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Inner coding
In DVB Viterbi convolutional coding may be used
to prevent random errors from reducing the power
of the interleave scheme. Following interleave,
the data are fed to a shift register. The
contents of the shift register produce 2 outputs
that represent different parity checks on the
input data so that bit errors can be corrected.
There will be 2 output bits for every input bit
therefore the coder is described as a ½ coder but
any rate between 1 and ½ may be realized by means
of puncturing , in this way it is possible to
adjust the correcting power as a function of the
link quality.
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Baseband shaping Modulation
Prior to modulation, the I and Q signals shall be
square root raised cosine filter with a roll-off
factor a that shall be 0,35 .
The system shall employ conventional Gray-coded
QPSK modulation with assolute mapping (no
differential coding).
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DVB-C
The cable network system has the same core as the
satellite system, but the modulation system is
based on quadrature amplitude modulation (QAM)
rather than QPSK, and no inner-code forward
error-correction is needed. The system is centred
on 64-QAM, but lower-level systems, such as
I6-QAM and 32-QAM can also be used. In each case,
the data capacity of the system is traded against
robustness of the data. Higher-level systems,
such as 128-QAM and 256-QAM may also become
possible, but their use will depend on the
capacity of the cable network to cope with the
reduced decoding margin. In terms of capacity,
an 8MHz channel can accommodate a payload
capacity of 38.5 Mbit/s if 64-QAM is used,
without spill-over into adjacent channels.
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Byte to symbol mapping
After convolutional interleaving, an exact map of
bytes into symbols shall be performed, the
process is illustrated for the case of a 64-QAM.
The two MSB of each symbol shall then be
differentially coded in order to obtain a p/2
rotation-invariant QAM constellation.
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QAM constellations
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DVB-T
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Hierarchical modulation
The splitter produces two streams, HP LP

• The LowPriority stream is of higher bitrate, but
  lower robustness than the HP one.
• For example in a 64QAM the 2 MSB could be used to
  map HP in a rugged QPSK while the other bits are
  used to map the LP with higher bit rate but lower
  robustness
• On LP HP are applied different code rates.

Scope

• The HP stream is received also with really bad
  channel
• It is possible to receive the same signal with
  different quality receivers.

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Inner interleaving

• Hierarchical and non hierarchical inner
  interleaving provide mapping of input bit onto
  output modulation symbols in this case is shown
  64-QAM.
• For hierarchical mode, the two MSB are mapped in
  a QPSK .

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OFDM why ?
Differently from satellite communication where we
have one single direct path from transmitter to
receiver in the classical terrestrial
broadcasting scenario we have to deal with a
multipath- channel The transmitted signal
arrives at the receiver in various paths of
different length. Since multiple versions of the
signal interfere with each other (inter symbol
interference (ISI)) it becomes very hard to
extract the original information.
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OFDM the concept
The common representation of the multipath
channel is the channel impulse response (cir) of
the channel which is the signal at the receiver
if a single pulse is transmitted
Let's assume a system transmitting discrete
information in time intervals T. The critical
measure concerning the multipath-channel is the
delay tmax of the longest path with respect to
the earliest path. A received symbol can
theoretically be influenced by (tmax / T)
previous symbols.
The scenario we are dealing with in DVB-T is
characterized by the following conditions
Transmission Rate R 1/T
7.4Msym/s Maximum channel delay tmax
224µs For the single carrier system this results
in an ISI of tmax / T 1600 Instead in the case
that the original data stream of rate R is
multiplexed into N parallel data streams of rate
Rrnc 1/Trnc R/N the ISI will be tmax / Trnc
tmax / (N T) 0.2 if N 8192
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OFDM the orthogonality principle (1/2)
In OFDM the subcarrier pulse used for
transmission is chosen to be rectangular. This
has the advantage that the task of pulse forming
and modulation can be performed by a simple
Inverse Discrete Fourier Transform (IDFT) which
can be implemented very efficiently as a I Fast
Fourier Transform (IFFT). Accordingly in the
receiver we only need a FFT to reverse this
operation. According to the theorems of the
Fourier Transform the rectangular pulse shape
will lead to a sin(x)/x type of spectrum of the
subcarriers
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OFDM the orthogonality principle (2/2)

• Obviously the spectrums of the subcarriers are
  not separated but overlap. The reason why the
  information transmitted over the carriers can
  still be separated is the so called orthogonality
  relation giving the method its name. By using an
  IFFT for modulation we implicitly chose the
  spacing of the subcarriers in such a way that at
  the frequency where we evaluate the received
  signal (indicated as arrows) all other signals
  are zero. In order for this orthogonality to be
  preserved the following must be true
• The receiver and the transmitter must be
  perfectly synchronized.
• The analog components, part of transmitter and
  receiver, must be of very high quality.
• There should be no multipath channel.
• In particular the last point is quite a pity,
  since we have chosen this approach to combat the
  multipath channel. Fortunately there's an easy
  solution for this problem The OFDM symbols are
  artificially prolonged by periodically repeating
  the 'tail' of the symbol and precede the symbol
  with it.

At the receiver this so called guard interval is
removed again. As long as the length of this
interval ? is longer than the maximum channel
delay tmax all reflections of previous symbols
are removed and the orthogonality is preserved.
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OFDM frame structure (1/2)

• Each symbol is constituited by a set of 6817
  carriers in the 8K mode or 1705 carriers in the
  2K mode, it is transmitted with a duration TS
  TU? .
• Each trame has a duration og TF and consist of 68
  symbol.
• Four frame constitute one super-frame.

In addition to the transmitted data an OFDM frame
contain also

• Scattered pilot cells
• Continual pilot carriers
• TPS carriers

The number of useful data carriers is 6048 for 8K
mode and 1512 for 2K mode.
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OFDM frame structure (2/2)
Pilots can be used for syncronization and channel
estimation , are modulated according to a PRBS
corrisponding to their respective carrier index

• The scattered pilots use some carriers inside the
  symbol and this information is transmitted at
  boosted power level
• There are 177 continual pilot for the 8K mode and
  45 continual pilot for the 2K mode.

TPS sygnal is referred to an OFDM frame
constituited of 68 symbols, each one of this
convey one TPS bit so we have 68 bits so used
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DVB-CA (1/2)
A conditional access system makes use of both
scrambling and encryption techniques in order to
prevent reception by unauthorised persons. An
encrypted message, known as an Entitlement
Control Message (ECM) or multi-session key is
used to send coded keys, known as control words
to the receiver. This gives it the information
necessary to descramble the signal. However, this
will only occur if authority is given by an
Entitlement Management Message (EMM). The control
word is frequently changed (several times a
minute) while the ECM is changed far less often,
about once a month.
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DVB-CA (2/2)
After early attempts to create a single standard
encryption system failed to provide the level of
security required and different systems began to
emerge, actually on the market are used
Simulcrypt With the use of MPEG-2 multiplexing of
multiple signals into a single transmission, it
is possible to have many separate ECM's which are
sent together with the program information. This
allows several different CA systems to operate
together within a single scrambled broadcast.
This requires a Common Scrambling Algorithm
throughout the CA systems involved.
Multicrypt In a Common Interface environment,
it is possible for a user to receive a service
from a different CA system provider (which does
not make use of Simulcrypt with other CA systems)
by the addition of a module to the interface.
This would allow the receiving of multiple
signals from different sources, all with
different CA systems, providing all of the
different modules were available. This does
however require the modules to be made for each
of the CA systems in question. This is unlikely
to occur given the cheaper alternative of
Simulcrypt.
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DVB against all