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Digit is on a horizon Probably, not a one question causes so many disputes amongst professionals of broadcasting market, as prospect of the transition to digital television. The spectrum of the judgments and estimations, which herewith are expressed, lies within the range from possibility of the fast transition to digital broadcasting in nearest three-five years to refusal of foreseeable future from “digitalization” of air in general and localization of technological innovations in cable and satellite networks only. That is for the last, that process there, as it is spoken, is already going, however this particularly scarcely worries the suppressing majority of Russians, whose television sets are connected to united information space by means of air antennas with VHF and UHF ranges solely. But TV-transmitters are hereunder equipment of the “last mile”, which allows the lucky owners of almost 90 millions TV sets not to feel itself as lonely on immense open spaces of our country. Today these transmitters are analog at whole (the exceptions are not in count), but not later than in 2015 all of these must become digital, anyway; this target date is specified as deadline in “The concepts of the development of the communication services in the field of broadcasting in Russian Federation for a period to 2010 and in perspective to 2015”, which was designed on target of Russian Ministry of communications. So, we already know — when, it is remained to understand — how. It is sorrowful, that Russia is not in a range of pioneers in broadcasting equipment production sphere. However, every cloud has a silver lining — there is a possibility to study the experience of more deft neighbors. For present-day day in the world three standards of the digital terrestrial television are existed: American ATSC, European DVB-T and, finally, Japanese ISDB-T. If Japanese standard is only determined by modification DVB-T, that American differs from the last very vastly. Since the main advantage of digital TV is a possibility to send in band of the analog signal of 6, 7 or 8 MHz width not one, but several programs simultaneously, that the target, confronted before developers of the new standard, was a technical realization of this remarkable principle. From theory of information it is known that reception capacity of the channel (the transmission rate of the data) is the product of the width of the band in frequency area on the dynamic range. Obviously that the most simplest way of the digital binary signal transmission is two-level amplitude modulation, however maximum transmission rate in this case is 2F bit/s, what, certainly, absolutely it is not enough. Transition to multi-layered modulation allows to raise proportionally the reception capacity of the channel (since the maximum rate is nF bit/s, where n is a number of amplitude levels of the signal, and F is a band width). These simple considerations lie on base of the ATSC standard with 8-VSB modulation system (8-level system with vestigial side band). The system, accepted in USA, uses one carrier and ensures the delivery 19.29 Mbit/s digital stream within terrestrial broadcasting channel with 6MHz band. Unlike to simple main target of the American standard, the European DVB-T standard differs by elegance and theoretical refinement. The last standard is based on method of coded orthogonal frequency division multiplexing (COFDM). COFDM is the multi-frequency broadcasting system, in which frequency spectrum divides among great number of carriers, each of they is modulated by digital flow with very low transmission rate. The DVB-T provides two working modes: 2k and 8k. The main parameters, characterizing the data communication in DVB-T system, are given in the following table.
The first look at parameters, given in table, can provide in despondency even experienced engineer, so a digital modulator, forming several thousands of carrying oscillations, is introduced as too much complex and high-priced unit. This circumstance, however, has not upset the Russian specialists and after several years of debates, which, to word, did not die down, already nearly nobody has no doubt that standard of digital terrestrial broadcasting in Russia will become DVB-T exactly. In practice for realization of the COFDM methods it is not required the use of thousands of generators and modulators. Actually, the formula, describing COFDM signal, presents itself real part of the Furrier inverse transform (FIT) and, a propos, is an important part of DVB-T standard since exactly its is defines the algorithm to practical realization of the proposed modulation mode. The modern digital processors, realizing quick FIT, provide wholly acceptable hardware embodiment of the COFDM. And though cost of the DVB-T modulator at present it is enough high (not less 6-8 thousands dollars USA), year from year she steadily falls, inspire an optimism in supporters of the most quick transition to digital television. To a considerable extent the possibilities of microcircuit engineering is explained the presence two modes COFDM — 2k and 8k. Obviously that advantage of the DVB-T is the higher the greater number of carriers are used for forming the signal on the digital transmitter output. In particular, the maximum separation between transmitter in single frequency network, equal to 67.2 km, is possible to provide only under 6817 carriers (the 8k mode), as this is can see from the table. However reached to 1995 (when the DVB standard was taken) the electronic technology level did not allow to handle of such number of carriers. At that time many specialists considered that for the acceleration of standard reception it is possible to down the requirements to single frequency network and to limit the amount of carriers by range from 1500 to 2000 that has served as a reference for appearance of the 2k mode. In reality, this is the recent history today, and modern digital TV-transmitters allow working in any of the named modes. The structured circuit of the DVB-T modulator is shown on figure. The modulators delivered on the market, as a rule, in standard configuration have two input interfaces — serial ASI and parallel SPI. Formation of the COFDM signal occurs as follows. For each carrier is defined, what data it must to transfer. In accordance with this on base of accepted modulation circuit (usually QPSK, 16-QAM or 64-QAM) necessary amplitude and phase of each carrier are computed, whereupon by means of fast FIT operation the form of the signal in time area is computed. In produced serial DVB-T modulator the output COFDM signal is formed on intermediate frequency (IF), equal to 36.15 MHz (rarely equal to 35.5 MHz). Exactly such IF value is chosen to provide the attachment with existing exciters of analog transmitters; for conversion of the last in digital it is enough simply to connect the DVB-T modulator output to input of the converter IF-HF block.
Regrettably, not any analog exciter is easy possible to transform in digital. For this he must, as minimum, have an IF input, as well as a control block, allowing to work not only with analog, but also with digital signal. One more feature of digital TV-transmitter from analog is a method of the amplifying line building. As is well known, two versions of realization of the television analog transmitter are existed: with separate and joint tract of the forming and reinforcement of picture and audio channels. In the last case the requirements to the linearity of the power amplifier are greatly higher since interaction of the carriers of picture and audio signal fluctuations on nonlinear amplifying tract elements brings to the combinational frequencies forming, getting in TV-channel band. Obviously that difficulty of the amplifying tract forming in joint reinforcement systems increases with level of output power growing. As a rule, the modern analog TV-transmitters with power levels in sync pulse peak to 5 kW are formed only on circuit with joint reinforcement. At levels, exceeding 5 kW, the producers of transmitting equipment, alongside with joint reinforcement, quite often offer the separate circuits of the transmitter forming too. However the last are little suitable for the following transformation in units, functioning in digital broadcasting format, so all DVB-T transmitters are already on principle of the signal forming built solely on circuit with joint reinforcement. But if realization of lines with joint reinforcement meets with difficulties already under transmission of the analog signal with two carriers that what can we speak about a need to transfer without distortions the digital stream, formed from several thousands oscillations with strictly determined amplitude-phase correlations! However, almost any power amplifier of analog TV-transmitter, built on circuit with joint reinforcement, can provide the transmission of the digital signal with given distortion level. The question is only in that, on what power level this will be reach. In countries, which are proceeding to the building of digital TV-broadcasting networks at the end of past century, the term “Digital ready transmitter” that is to say literally, “transmitter ready to digit”, is introduced. Such unit, originally intended for analog TV-signal transmission, is converted in digital transmitter by change of a modulator (or exciter at whole) and if required, an output filter. As to power amplifier, it has high values of linearity features needing for transmission of the digital signal with the greatest possible power level. The leading producers are declared the correlation of the power values in sync pulse to average power of the digital signal of the Digital ready transmitter, equal to 3:1 or, at the worst, 4:1. Such correlations practically unattainable when use the traditional analog transmitters, in which the measures on increasing of the amplifying line linearity are not accepted. Instead of term “digital ready” in domestic literature the term “hybrid” or “analog-to-digital” transmitter is used, that introduces not wholly exact unit feature since here the semantic accent is transferred to transformation readiness (in the future) on already existing possibility, being absent objectively. As was it already noted above, an analog transmitter, prepared to the following transfer in digital broadcasting format, must provided by the following features:
The use of such transmitters in broadcasting network will allow to the operators and/or broadcasters to optimize the expenses on equipment buying with the provision of transition perspective to digital format. But since this perspective, as overseas, so and in Russia, lies within planned lifetime of newly gained transmitters, such in obligatory order must be hybrid (or digital ready — as to whom more likes). However a simple presence of the set of the mentioned features in the unit it is not enough absolutely! It is necessary that transmitter guarantee will be provide the beforehand given correlation of the values of powers in sync pulse peak to average power of the digital signal. Otherwise it is impossible to define the service zone under the following transition to “digit”. Follows to remember that digital signal with reference to the picture on television set screen, unlike analog signal, falls into category of thresholds — either he is, or he is not.
The structured circuit of transfer systems of the digital television is shown on figure. So (or approximately so) will look the customer equipment set, successfully overcame all difficulties of the intermediate period and went in air in full-fledged digital format. The differences from given circuit can be concluded, in particular, either in program numbers, simultaneously transfer in standard TV-channel 8 MHz band (up to 6 under high quality of the received signal), or in presence of the data transfer channel (for instance, Internet-traffic with asymmetric access). If this will be used the DVB-T standard, that, unlike American ATSC system, an operator will get the additional following advantages:
The last point we shall consider in details. The ability to hierarchical modulation is an important particularity of the broadcasting systems with COFDM. Data on the multiplexer output of the transport stream are sliced to two independent MPEG-2 transport streams, to which the miscellaneous priority levels are assigned. The stream with high priority is coded for the reason of high noise reduction; the stream with lowest priority is coded for the reason of the transferring data high velocity ensuring. Then both coded streams are integrated and are sent together. Thereby, it is appear the possibility of the transmission of two different programs or one TV-program in two versions on one channel. The first version is characterized by high noise reduction, but has limited definition, the second is characterized by high definition, but limited by factor of the noise reduction. This opens the new possibilities. So, on stationary antenna by means of high-end receiver a version with high definition can be received, but this program will is received by a simple and cheap receiver i with limited definition. The noise protected version will also be received in heavy reception condition, for instance, for signal receiving during movement or on room antenna. Under change of the receiving conditions it is possible to switch the receiver from one version on another. The leading broadcasting equipment producers offer the broad spectrum of digital TV-transmitters for functioning with all known standards. Already at present are available some serial produced units with digital signal power output up to 20 kW that equivalent to the power of the analog signal, equal to 60 kW. Practically all DVB-T transmitters on the market support 2k and 8k modes, as well as QPSK, 16-QAM and 64-QAM modulation. The possibility of the functioning inside single frequency networks with synchronizing with the help of GPS-receiver and ensuring of hierarchical modulation is mostly offered optionally. Thereby, the industry of the broadcasting equipment has already prepared all types of units required for transition to “digit”. It is remained only to answer on the sacramental question: where we can take the money for this transition? |
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