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Digital Television
The digitization has involved all the communication systems in a sort of Digital revolution.
The reason for this success lies in the ability of digital technology to allow treated data to be stored,
handled, packed and sent in a way more efficient and less expensive.
Fax transmission has become digital between '70s and '80s. The Digital Audio by Compact Disc has
replaced that of vinyl records and compact cassettes, relegating those means to niche applications.
The DVDs are increasingly popularity day by day. This digitization process could not save the television.
The seed for the birth of digital TV can be traced back to the early '90s when the Motion Picture
Expert Group (MPEG) has developed the first digital compression standard common for audio and video.
Without compression techniques, the spread of digital TV would be feasible for large data stream requires
that its transmission. But it is with the Definition of the Digital Video Broadcasting (DVB), which has
created the complex set of standards, what has provided the basis for digital TV. The DVB has adopted
the MPEG-2 as a system standard data compression, and different adaptations in different distribution
channels: DVB-Satellite; DVB-Cable; DVB-Terrestrial.
Broadcasting via satellite:
One of the practical applications of the space conquest is the artificial satellites orbiting around the earth.
They initially used only for military purposes and research, artificial satellites for civil applications have
had such a development that currently there is a myriad of satellites orbiting the earth in geostationary zone.
The geostationary orbit position is that particular area of the sky, about 35,800 km from the surface Earth,
that allows satellites to rotate the globe in solidarity so that they are always located above a particular point
on the equator, where there is no need of energy to maintain the position (gravitational balance). There are
satellites used for many purposes. There are weather satellites, for telephone communications, for data communication,
and since twenty years for television and radio broadcast. But, certainly the best known is the satellite system
GPS (Global Position System) without the which several transport systems would be blind. Theoretically, with a
satellite we can cover about 40% of the earth's surface, so with a system composed of only three satellites we
could cover the entire globe with the exception of a few bands around at the poles. The advantage of satellite
television transmission against that terrestrial is easy to understand. In fact, in addition to their coverage,
without shade, to almost throughout the "enlightened" land, a single satellite makes up the complex network of
terrestrial repeaters otherwise be necessary. In Europe there are two major groups of satellites owners for TV
transmissions, Eutelsat and
Astra. Each with its own fleet of satellites located
at different degrees of longitude. Those more interested to Italian viewers are called Hotbird satellites
(located at 13 degrees East), which spreads the Rai, Mediaset and Sky, and Astra 1b/c/e/f/g/h-2c (located at 19.2° East)
which broadcasts channels of French TV, German, etc.
Satellite TV channels initially broadcasted in analog mode were almost fully converted to the new technique of
digital broadcasting more convenient economically . In fact, in a frequency where there was only one analog channel
with various channels audio, using digital technology we can transmit many more. As we shall see later the number of
TV and radio channels transmitted in a single channel, which is called transponder, depending on certain parameters
of the flow of data transmitted as FEC, Symbol Rate, etc.
Differencies between analog and digital transmission, terrestrial and by satellite:
As we know, see on webpage operating principles TV,
the analog PAL color television transmission occurs via the diffusion of 25 frames (images) per second,
consisting of 625 lines. The lines are transmitted in an alternating way, first all the odds and then
all equals, with a frequency of 50 Hz to form full 25 frames per second. In digital world we think in pixels,
which is the unit of information displayed on the screen. In particular, for broadcast television, a 720 x 576
pixels format is used. That means that each picture is made up of 720 horizontally and 576 pixels vertically.
Another important factor is the bandwidth of a television channel. For terrestrial broadcasting using amplitude
modulation, that requires 7 MHz in VHF, and 8 MHz in UHF band. For satellite broadcasting we use the frequency
modulation that ensures a better quality, which is necessary given the distance the signal has to travel.
The bandwidth required to transmit an analogue TV channel frequency modulated can vary between 26 and 54 MHz,
but the most used ones are 27, 33, and 36 MHz.
Typical values of a digital channel are (for an explanation of various parameters, see next paragraph):
- Band Width: 33 MHz
- QPSK bit rate: 41.25 Mbps
- SymbolRate: 27.5 MSps
- FEC: 3/4
Considering that, by MPEG-2 compression, a digital TV channel of good level is obtained with only 8 Mbps,
it is easy to calculate how many channels can be transmitted instead of a single analog channel.
Modulation in digital transmission:
As we know the process of modulation is to vary the carrier (in Amplitude or Frequency) by a signal
containing the data to be transferred. In digital transmission, although the concept is still valid,
the modulation is completely different of analogue radio and television ones. The modulation is different
even for the three types of broadcasting, satellite, cable, terrestrial and is functional to the available
bandwidth. As a result, a receiver suitable for satellite reception is not for others types of diffusion
and vice versa. The Quadrature Phase Shift Keying (QPSK) modulation is used for satellite transmission.
That means that two carriers are radiated in phase quadrature (90° out of phase). Thus we have the
transmission of two bits, value 0 or value 1. The couple of bits is called Symbol Rate. Generally,
the symbol rate is shown in the size of Msps (Mega Symbol per second), so a SynbolRate of 27.5 Msps is equal
to 55 Mbps. Transmitting also the carrier 180° out of phase we double transmission capacity so we have 4 bits
(2 bits each Symbol). In the receiver the demodulator module extract data, filtering the carrier, to obtain
the simple binary values.
The figure below shows briefly the stages necessary for the transmission of a digital television channel.
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Briefly: a circuit for the video data and one for audio data, whose entrance a corresponding signal
is applyed, to compress data in MPEG-2. These two signals along with data services (EPG, etc..)
are injected into a module that is proposed to packaging and synchronize them, so that the sound
is on time with the images, and mixing all to a single data stream (Stream Transporter). The mixing
is implemented using the multiplexing technique. The data flow is used to modulate the high-frequency
signal (carrier) that will be irradiated by the radio station to the satellite, and from there to
the potential viewers. Note that in the modulator stage there is a protective system error called FEC
(Forward Error Correction). For an example, when we find in the tuning parameters of a television
channel FEC = 3/4 means that the flow of data is composed of 3 pieces of data useful to the vision
and 1/4 of data for error checking. In the receiver an inverse procedure is applyed with the same
number of stages and the addition of the circuit by which the user sets some personal parameters,
such as parental control, language, PPV, etc.
As shown, represents a typical FTA transmission channel (Free to Air), this means that any digital
receiver (also known as decoder) is able to receive and properly displayed on the screen of a TV
the signal received, with no decoding system. Commercial TVs radiate scrambled signal in order to
allow viewing programs broadcast only to its subscribers, who typically receive a smart card
when signing a contract of service delivery and paying the related fee.
Encryption systems are different, but all share a common coding system based of signal encryption
algorithm (scrambling) that can be decoded through a system of keys stored in smart cards.
Encryption Systems:
Encryption Systems more used in Europe are:
- BetaCrypt
- Conax
- Irdeto
- Mediaguard (also known as SECA)
- NDS
- TPScript
- Viaccess
over time some of these systems have evolved to combat Satellite piracy . This battle between Sat
operators and Sat hackers that takes place day by day. Currently there is version 2 of the SECA,
Viaccess and dell'Irdeto, but recently, the management of major Italian provider "SKY", considering
insecure the SECA2 system, has implemented a proprietary system called NDS. For each of these
systems (Conditional Access) is necessary to have a CAM (Conditional Access Module) where inserting
the smart card into, issued by the broadcasters, which will only work for that providers and for
that coding system. CAMs can be integrated in the decoder (Goldbox) which can then make it clear
only transmissions irradiated with that system, or inserted into receivers capable of receiving,
through special slots, diverse CAM and be authorized to decrypt multiple systems. These receivers
are the so called Common Interface. There are operators that in order to broaden the base of potential
subscribers, use double encoding transmissions. For example, an operator broadcast its transmission in
both SECA and Viaccess system, so that both owners of Goldbox and Common Interface with Viaccess CAM
receives, are able to decode the signal of that operator.
SECA MEDIAGUARD
This is a brief description of the Mediaguard (SECA) coding system structure.
The smart card plays a prominent role in the process of communication between the CAM and smartcard
itself in the sense that the smartcard directs the mode in which communication must take place.
After each reset (power on the receiver), the smartcard is recognized by the receiver/CAM.
Another way to reset the communications is to extract and reinsert the smart card. After a reset,
we said, the smart card communicates to the decoder as it can be updated. That routine is
standardized and strict, that must take place only in the manner specified by the ISO reference.
The type of information communicated, for example, the speed of transmission, if transmission is
in synchronous or asynchronous mode, the voltage and amperage necessary for updating, etc.
Mediaguard algorithm
SECA algorithm is based on a key, called "Crypted Controlword", of eight bytes. From this key,
the system gets the so-called "Decrypted Controlword". To do that two other keys called "Primary Key"
PK, and "Secondary Key" SK, are needed. These two keys are used together to form a key to 16 bytes
to decrypt the control key. There are two sets of keys other than those listed: The "Operational
Key" OP and "Management Key" MK. The operational key is the one that actually makes it clear the
channel tuned, while the administrative key is one that takes care of automatically changing the
operational key, according to the guidelines of the broadcaster. The operation of changing the
operating key by the administrative key is called Auto Update. Finally, but not least important,
are to mention the "Provider Keys" (key operator) and "SECA Keys". The first, are the keys that
allow the provider to activate, modify and disable the smartcard, while the second, are the keys
that authorize to make changes to the smartcard at all levels. Clearing that while there may be
Provider Keys for one operator, with the right SECA Keys one can add and remove multiple providers
and not only change it (channels that are part of the bouquet, but belong to another provider or
broadcaster).
As before said, after a reset the smartcard starts communicating with CAM. That first communication
takes the technical name of ATR (Answer To Reset). As the ATR is received by the CAM, it starts to
ask for information to the smartcard. The instructions CAM sends to the smartcard are called
Instruction Byte or abbreviated INS Byte. For a list of supported instructions
Click here.
Di seguito viene riportato un esempio di comunicazione tra CAM e smartcard per rendere l'idea a
chi non ha cognizioni di programmazione. L'esempio riguarda la richiesta delle informazioni sui
providers supportati dalla smartcard. La relativa istruzione è la INS 12. Siccome le cifre
numeriche sono in esadecimale (hex), la notazione prevede che queste siano precedute dai segni 0x
per distinguerle dalle cifre decimali. Quindi 0x12 indica la cifra 12 in esadecimale che corrisponde
al numero 19 decimale.
working on translation
The following is an example of communication between CAM and smartcard to make the idea to those
who have no knowledge of programming. The example concerns the request of information on
providers supported by smart card . The instruction is the INS 12. Since the numbers are in
hexadecimal (hex) format, the notation states that these numbers are preceded by 0x to distinguish
them from decimal ones. So 0x12 indicates the number 12 in hexadecimal that corrisponds to the decimal
number 19.
C1 12 00
00 19 where:
- C1 12 = INS 0x12 (Instruction Byte 0x12)
- 00 = Provider Number (provider number of which is asking info)
- 00 = SECA (ID number of SECA system)
- 19 = Answer Bytes (bytes number expecting in answered, 0x19 = 25)
The answer would be something like:
12 00 00
4C 49 54 49 FF 4C FF 4C 4C 49 54 49 FF 4C FF 4C
F5 49 99
FF FF 90 00 where:
- 12 = 0x12 Answer (answer to INS 0x12 request)
- 00 00 = Provider ID
- 4C 49 54 49 FF 4C FF 4C 4C 49 54 49 FF 4C FF 4C = Provider Name
- F5 49 = Smartcard Address
- 99 = Info still unknown
- FF FF = subscriber end Date
- 90 00 = standard SECA specification that means, all OK (to the end of communication)
Premise that the section concerning the description of the SECA is reporting from a page of a site
that deals with the subject, we do assume no responsibility for the accuracy of the content, nor
for any use that can be done of it. The data available on the subject are, for obvious reasons, very
little. Anyway I hope I've helped to give an idea on the communication between the smartcard and
decoder/CAM system and SECA to those who, like me, is curious to know and love the things of the world
where they live in. Recalling that the informations are given for educational purposes only, those who want
to deepen the argument can try to search in our "mare nostrum" INTERNET of which, I hope with dignity,
this site is part of.
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