Equipment for Satellite Transmission of the TV Signal
By Sergey Poresh, Andrey Harlamov

The equipment for the digital compression of the TV signal was developed on the bases of the MPEG-2/DVB standards, the good description of which was given, for example, in the review of Constantine Glasman "MPEG, this is simple!" ("625" magazine 2000, #3) and in the monograph of Boris Loshkin "Digital broadcasting: from the studio to the audience". There are very few functionally different units that are used. This is coder, which allows transforming the analogous video and sound signals into the elementary digital flow, multiplexer which allows combining the elementary flows into the programs package, add the radio programs and other data to them, modulator, which transforms this flow into the phase-modulated signal which is appropriate for the relaying through the satellite transponder or the cable network, and, at last, receiver of the digital signal which transforms the signal into the image and sound. On the bases of this main set there exit a large number of modifications which were developed for the satisfaction of the customers' requirements and the uninterruptedly perfecting by the companies-producers.

Coder is the main element of the equipment, which defines the quality of the distributed digital signal. It is exactly where the new ideas of the developer of the algorithms of the pressing of the video signal are realized, for them the new element base is created. At present the leading companies-manufactures declared about the launching on the market the coders of fifth and sixth generation. From the facilities of the previous generations they vary in the significantly increased calculating opportunities of the processors, new more effective algorithms of the data processing and the higher pressing coefficients. In particular, the increased productivity of the modern processors allowed realizing the new methods of the full motion at which the calculation occurs on all the shot parallel to the few channels. That allows avoiding the resolution decrease, which is peculiar to the hierarchy method at the transfer of the fast moving objects.

The modern MPEG-2 coders, as a rule, have the preliminary signal processing blocks, which allow realizing such progressive methods of the video image processing as the adapting of the scene (noise elimination), the change in the spatial resolution, coder adapting to the scene. With the development of the systems of the television of high clearness there appeared the coders, capable of the signal of these systems processing in 4:3 and 16:9 formats.

The coders vary in the opportunities of the MPEG signal coding of the different profiles (High, Main, Low) and levels (Simple, Main, SNR, Spatial< High, 422). They can code the signal on the defined information speed not depending from the complexity of the information transferred in the signal (constant bit rate, CBR) or with the providing of the defined constant signal quality, but the variable exit speed (variable bit rate, VBR). The VBR coders are used in the cases when for the formation of the program package the method of the statistics multiplexing is used.

The coders usually have the module construction, which allows realizing the wide range of entering and exiting interfaces. The composite and component analogous video interfaces SDI (digitized, but uncompressed video signal). The amount of the digital and analogous sound entrances can vary in the wide limits and by adding the corresponding modules their amount can be increased up to six or even up to twenty stereo mates. Besides, usually there is and entrance for the high-speed data, which can be included into the general exiting flow for the distribution together with the TV signal. There are also existing the low-speed entrances (Ethernet, RS-232, RS-485) for the organization of the remote control and integration into the system with other functional modules.

As the exiting one or few anisochronous consecutive interfaces of the transport flows DVB-ASI are usually used. For the transfer of the data flow along the ATM or SDH networks some coders have the corresponding options of the exiting interfaces of different hierarchy levels for the electric and optical links. There are also can be found the Gigabit Ethernet interfaces as the reaction on the demand for the video distribution in the Internet networks.

At last, the coders can have the QPSK grid or the build-in multiplexer, which allows connecting them in a cascade for the broadcasting of two or more programs at the same time. These opportunities find themselves very important in the systems of the news collection when the mass-dimensional equipment parameters and the time spent on the bringing of the system from the transport condition into the operating one are highly important.

For the uniting of the elementary ASI flows, which were formed by the coders, into the single transport flow there are exist the multiplexers. The amount of the entering interfaces into the multiplexer can achieve few tens, but in that case the total speed of the elementary flows on the entrance is always limited by the defined value.

The majority of the multiplexers form one exiting transportation flow, but some companies produce the multiplexers, which are capable of organizing the few transport flows different in their content at the same time. The speed of the exiting transport flow (or the total speed in the case of few transport flows) is also limited. At present this limitations are in the 100...200 Mbit/sec limits.

Almost all of the contemporary multiplexers at the corresponding software maintain the statistic multiplexing function. This type of multiplexers can analyze the speed of the elementary flows, which are received by its entrances from the VBR coders and to fill in the exiting transportation flow according to the real value of these speeds. In that case there appears an opportunity of the more compact filling in of the exiting flow and organization of the services of different quality: with the guarantee quality, speed or with the providing of the channel depending on the opportunities of the real traffic. The multiplexers also can have the function, which allows checking of the entering information speed of the program and change it depending in the exiting transport flow and also to switch from VBR to CBR and back. Except for the coders other facilities like sound coders and IP encapsulater can be attached to the multiplexer. These facilities allow turning into the exiting transport flow the additional radio channels, data channels, satellite Internet, streaming broadcasting.

Such a sort of multiplexers as the remultiplexers, which are capable of receiving the program packages formed by other multiplexers and forming the exiting packages with the changed combinations of programs and overhead information are marked out. In spite of the fact that at present this function is realized also in some models of the multiplexers the remultiplexers are chipper. Very often the facilities called service filters (or the splitter of the transport flow), which can separate the part of the programs from the package and form the new transport flow out them according to the overhead information are often useful.

The multiplexers can also include the systems of the conditional access, which allow organizing the information security of the broadcasting programs. The modern multiplexers, as well as coders, have the module function. They can be equipped with the different entering interfaces, which make their connection with the SDH and ATM networks easier. Besides, the contemporary multiplexers have a series of opportunities of the input into transport flow of high- and low-speed data including IP and even some additional audio channels (Harmonic).

The compression systems where the common or statistical multiplexing is used usually operate under the control of the computer with the special program, which provides the check-up of the working capacity of the system and its reserving. This is one more type of the production, which the manufacture companies offer to the customers. The software allows controlling one or few compression systems at the same time, creating, saving and switching from one system configuration profile to another very fast, reserving not only the compression equipment, but also the controlling computers, taking stock of the fails of all the elements of the system, and also creating the archives of events that occurred in the system. In the case of the statistical multiplexing the controlling system also carries out the control of the elementary flows speed, which are formed by the coders.

All of the large compression apparatus complexes operate under the checking of the controlling system, which is usually build on the bases of Windows NT. But some companies for the building of the controlling system use the Unix platform. In any case the controlling system provides the checking and monitoring of all the facilities included into the compression system, their automated reservation, recordation of the found fails and breakdowns, generating of the overhead information, which is inputted into the exiting transport flow, routing of the exiting and entering signals (if the multiplexers maintain that function).

All that was said earlier about the types of the exiting interfaces for coders is absolutely equal for the exiting and entering interfaces of the multiplexers: the basic is DVB-ASI, but the options of connecting to the ATM or SDH networks is possible as well.

The modulators serve for the transformation of the digital information from the impulse form into one of the forms, which are appropriate for the relay through the satellite transponder, cable network or the broadcast transmitter. In the case of the satellite relay the next types of the modulation are used: square-phased (QPSK), eight-phased (8PSK) or amplitude-phased modulation (16QAM). The most well spread is the QPSK type of modulation, which is now used in the systems of the direct TV broadcasting and the distribution systems. Other types of the modulation are now used in the systems of the news collection. As these types have the advantages in the use of the satellite resources in the future they should get the wide spread occurrence. The main limiting reason is the relatively high cost of the receiving equipment.

Usually the modulators have one input ASI interface, sometimes two, what allows organizing the cross reserving of the grids. Besides some companies offer the modulators with the parallel SPI or LVDS interfaces. For the output reserving of the modulators the switches, which transfer the system on the reserve set or at the fallout of the carrier (or specter) from the main set or at the out-of-order call from the according output of the modulator are often used. The modulators can switch by the controlling system signals or have the build-in automatic reserving. In the second case it is necessary to join the according ports and the build-in Mute function provides the blocking of the output signal of the reserving modulator. The modulators that are used in the satellite transfer systems have the standard values of the output frequency 70 MHz, 140 MHz or L band.

One more type of the equipment offered by the manufacture companies is the digital satellite television receiver. The receiver consists of the demodulator, which provides the transformation of the phase-modulated signal into the impulse form and the decoder, which transforms the digital signal into the analogous form for the reproduction on the television or broadcast on the local television network. The receivers can significantly vary in prices: from the inexpensive domestic facilities assigned for the setting in the apartments to the professional used on the stations of broadcasting and cable television.

The contemporary professional receivers-decoders also have the module construction and can be completed with the wide range of the additional cards, which enlarge their opportunities. Here can be named the ASI cards of input/output, SDI digital cards with an opportunity of addressing of the each channel to the appointed port. The sound can be brought out in the analogous form as well as in the AES/EBU digital standard. Almost all of the receivers-decoders have the slots for the installing of the decoder cards of the conditional access systems. Lots of the models of the receivers have an opportunity of receiving of the last versions of the software through the order wire of the receiving program package. Besides some of the receivers have an opportunity of control of the external facilities (for example, turning on/off the studio videocassette recorder for the insertion of the regional advertising), using the signals of the temporary marks transferred in the intervals of the advance pulse shots slaking.

The most well spread in the satellite systems are the receivers that decode the signals with the QPSK modulation in the 4:2:0 and/or 4:2:2 formats, but there exist also the demodulators and the receivers that are capable of processing the signal with the 8PSK and 16QAM modulation.

The situation on the world market of digital television techniques makes the producers to stick to the DVB norms. This is clearly demonstrated by the regularly hold EBU tests on the compatibility of the coding and decoding equipment. The norms' observance allows to the TV companies completing the receiving network with the decoders not obligatory of the same producer, which established the compression equipment. The exception is the shutting down systems. The well known shutting down systems Power VU and Power Vu Plus of Scientific Atlanta doesn't allow using the receivers of any other producers.

During the delivery of the television signal to the consumer except for the channel creating equipment takes part the relatively long chain of equipment. At the direct broadcasting these are the increasing frequency transformers, transmitters, feeder sections, transmitting antennas, transponders of the geostationary satellites, receiving antennas with the decreasing frequency transformers. In the distributing television networks the broadcast transmitter with the help of which the signal gets to the consumer are added. The example of the direct broadcasting can be the distribution of the NTV+ programs; the example of the distribution network can be the distribution of ORT, RTR, "Culture" programs. In the second case the requirements to the energetic characteristics of the transponder are lower than at the direct broadcasting as the city air-broadcasting station can venture the setting of the receiving antenna of the bigger diameter than the individual user. The tendencies are next: the satellite transponders become more and more powerful and the differences slightly graded. For example, the space vehicles of the "Express-A" series have the transponders assigned for the delivery of the television signal to the distribution networks and at the same time allowing to receive this signal on the receiving antennas of the diameter less than a meter. After the starting of such a vehicle into the orbital position of 40° EL planned on the June of 2002 and the transfer of broadcasting of All-Russian programs on it by that the program package would be available not only to the broadcasting companies, but also to the separate citizens and house collectives that live on the European part of Russia and some of the CIS countries.

In the chain, which provides the satellite broadcasting after the grids there are situated the increasing frequency transformers. They realize the transformation of the signals of intermediate frequency into the frequency C, Ku DBS bands. The contemporary frequency transformers are the compact facilities of 1 or 2 RU height. They should provide the high stability of the exiting frequency, minimum level of the phase noises and of the cross-product distortion. There exist the transformers for the intermediate frequency of the 70 MHz, 140 MHz or L bands. On the exit the transformers provide the band of ± 40 MHz. For the compensation of losses in the cable the contemporary frequency transformers usually have the build-in equalizer, which allows planing the amplitude-frequency function form. Besides, some models of the converters can be additionally equipped with the equalizers of phase frequency function. As the elements of the schemes of the satellite broadcasting the frequency transformers foresee the reserving in the different variants. That can be the 1:1, N:1 or N:M reserving.

If the directional earth transmitter is remoted from the system on the big distance the intermediate frequency signal is formed, the optic intermediate frequency extenders, which transfer the intermediate frequency signals almost without losses on the distance of hundreds meters and even kilometers can be very useful.

From the output of the up-converters the inputs of the driver amplifier receive the signals of the radio frequency. The driver amplifier as the frequency transformers are assigned for the operation with the in the defined diapason. The amplifiers vary in power (small, medium and high power). By the type of the active element the amplifiers divide on the hard-bodied (transit) and vacuum (TWT, klystron). By the variant of the performance they divide on the internal, assigned for the setting in the enclosed space and external, that are usually set on the carrier constructions of the antenna,

The solid-state amplifiers have the good linearity, wide range of frequencies, high reliability and are produced in the power range from the units to the hundreds watt. Some companies offer the solid-state amplifiers of the power up to 1.0...1.2 kW. The main problem of the solid-state amplifiers is the high heat-evolution. If you look inside of the solid-state amplifier you will see that the body is represented as the solid very massive radiator, which can be added by the few fans. At the same power of the solid-state amplifiers are significantly more expensive than, for example, TWT amplifiers.

At present the TWT amplifiers are the main element of the modern earth stations of the satellite connection of middle and high power (up to hundreds watt) as they were the first developed wide band active elements. But they have the low linearity. Usually the linear part of the operating characteristics of TWT is limited with the spot of -10...-7 dB from the power of the lamp satiation. That means that the amplifier of the power 700 W, for example, in the multisignal condition can give out the antenna of not more than 70...140 W not to overcome the allowed level of the cross-product distortion. For the solution of this problem the TWT amplifiers are equipped by the, so-called, linearizers, which allow rising of the operating point up to the level of -4 dB. The live period of the TWT depending on its representation and the operation conditions can achieve 10...20 thousands hours.

In the cases when in is needed to radiate big and superb powers (over the kilowatt) the clyster power amplifiers are used. They have the high linearity but the very narrow operating frequency band. Usually the clyster transmitters cross one or two satellite stems, about 40...80 MHz on the band. The clyster power amplifiers were and are used in the systems of the analogous satellite broadcasting.

In the frameworks of the not big article we can only name the type of the equipment produced by the manufacture companies for the providing of such a roughly developing branch of techniques as the digital television broadcasting. The more detailed description will have to be provided to the more specialized reviews.

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