Transmitters for digital terrestrial TV broadcasting

By Alexander Artamonov, Konstantin Kopiev, Yury Pogoreltsev, Lev Protopopov

Review contents:

Digit is on a horizon The standardization of the digital TV broadcasting transmitter features Digital and digital-analog (hybrid) TV-transmitters Transmitters for digital terrestrial TV broadcasting Low-power DVB-repeaters from ELTI Company Spot: filling gaps in DVB-T networks with digital repeaters NEC transmitters for digital TV Transmitters of Rohde & Schwarz for broadcasting Radio broadcasting transmitters of Technosystem and Screen Service Italia Transmitters of Thales Broadcast & Multimedia for digital television broadcasting Measurement of path parameter of digital TV radio transmitters

In this paper the results of the designing and exploitation in experienced areas of radio transmitters for digital terrestrial TV broadcasting are given. On base of the requirements, given in explanatory note to the Project 1 of the standard draft, the main correlations between required parameters and the amplified oscillation features are determined. The got data were used at designing and adjusting of the transmitter pilot models.

The distinctive particularity of American (ATSC) and European (DVB-T) system of air digital TV broadcasting (DTV) is an use of relatively complex signal system, the correct processing of which requires the maintenance of the special conditions on all stages of the forming and emitting of the signal. In accordance with standards, specifying the algorithms of the transformation of the data and signal stream on all stages from transmitting camera to receiving monitor, should be designed and all component parts of the system.

The structured scheme of transmitting DTV complex is given on the figure. On the left are shown the signal sources and equipment, which is needed for working with these signal sources. The programs transmitted through a satellite, enter in DVB-S receiver. The analog programs are subjected to coding by means of MPEG-2 coder. A server can be used in the case, when operator forms the own broadcasting program.

It is necessary to note that unlike analog, the digital broadcasting provides the transmission of several programs in single television channel; moreover these programs can be others than television. This can be audio programs, a special purpose TV-program (i. e. confravision), any other data. For instance, the operator from Saint Petersburg (ZAO “Telemedium”) has comprised in broadcasting content the commercials, which is displayed on electronic city panels. The main requirement to transmitted information is that its was packed in format of the MPEG-2 stream.

Program forming is provided by multiplexor, uniting the input streams in single transport stream.

Formed transport stream is entered to modulator, providing the processing of input discrete sequence in accordance with broadcasting standard and radio signal forming. Than this signal is carried in determined radio link by means of exciter, is increased, filtered and emitted.

The wording of the requirements to the radiated signal features of the DVB-T system preceded the high volume of the experimental scientific studies. Emphases in the course of experiment, carried out Nizhny Novgorod, Moscow and Saint Petersburg from 2000 to 2002, was spared to questions to electromagnetic compatibility with analog TV. So in Nizhny Novgorod the digital 50th TV channel transmitter was supplied with analog 49th channel and 51st channel transmitters (see Figure). Here by means of output filter the limiting values of out-of-band emissions were established in near frequency area and the influence of the digital signal on receiving of analog channels was determined.

The oscillation spectrum at the output of 50th channel transmitter installed in Nizhny Novgorod

For digital broadcasting In Saint Petersburg 34 channel is chosen. The nearest analog transmitters are 33 channel and 36 channel, with 25 kW and 5 kW power accordingly.

The oscillation spectrum at the output of 50th channel transmitter installed in Saint Petersburg

Estimation of the influence of the signal from digital transmitter on receiving of analog channels was carried out under receiving on the individual antenna, antenna of the collective receiving and in cable network. The results show the possibility of the accommodation of analog and digital transmitter, working in DVB-T standard, in nearby channels when performing the requirements to out-of-band emissions that creates the premise for revising of frequency plans at transition period to digital TV-broadcasting.

The block diagram of the “ONEGA-1C” transmitter

Simultaneously with functioning in experienced areas the MART Company has carried out the development of some digital transmitters, has made the pilot models and has carried out their certification. At April 2002 the first domestic transmitter “ONEGA-1C” for digital air broadcasting in DVB-T standard was produced, which was installed on Broadcast Center (Saint Petersburg), and at present its provides the commercial broadcasting on 34 channel.

“ONEGA-1C” transmitter

Test results have allowed formulating the main requirements to digital transmitter parameters. From our point of view the most important is the parameter named “Equivalent energy degradation” which gives the integral estimation of transmitter quality as a whole.

For the purpose of the determination of the requirements to different elements of the transmitter path by means of mathematical modeling the relationships of END from nature and factor values, influencing upon this factor were studied. The main reason, causing raise of END are nonlinear (amplitude and phase) distortions in power amplifier, the phase fluctuations at exciter oscillator and irregularity of frequency-phase features in antenna-feeder path. With provision of modeling results a few transmitters were designed, which main features are given in table.

Main parameters
Output power, Peff, W	100, 200, 500, 1000
Frequency range, MHz	470…790
Bandwidth, MHz	8
TV-standard	DVB-T TS 300744
Modulation	2K, 8K COFDM
Load, Ohm	50/75
Level of the harmonicas, dB, not more than	-60
Input parameters
Input signal	Multiplexed transport MPEG-2 stream with rates from 4.98 to 31.67 Mbit/s
Interfaces	DVB — SPI, 2×DVB — ASI
Main features
Accuracy of the frequency setting, Hz	±100
Frequency stability	1×10-7
Equivalent energy losses, dB, not more than	0,5 in 64-QAM mode and R=7/8
Relative level of out-of-band spectrum components*, dB, not more than	-35
Working conditions
Temperature,°C	From + 5 to + 45
Humidity	not more than 80% at 20°C
Attitude under sea level, m	not more than 2500
* It is determined by working conditions; the necessary value of the additional suppression of these components is provided by filter, which features are agreed with customer

The power amplifier of these transmitters was carried out with BLF861 transistors from Philips. The original system of the air cooling has allowed to provide from each amplifying unit the efficient output power equal to 23 W (in similar products from R&S company this factor is equal to 30 W). The power density is 1.67 W in 1 dm³, power requirement is 7.7 kW.

To a considerable extent the power factors of the power amplifier are defined by peak-factor value and working mode of amplifying units. On the base of conducted studies the recommendations to the mode challenge and to the peak-factor value were created. As a result of optimization of the total effect, provided by influence of different mechanisms, the requirements to peak-factor value, correcting pre-corrector actions, and suppression factor of the filter of out-of-band spectral components (OSC) were determined. To the best power features the following features are corresponded: the peak-factor value — 2.3; the OSC level at power amplifier output — 30 dB; the correcting effect of pre-corrector — 5 dB; the suppression factor of the specter off-band components level in filter — 25 dB.

In table the set of the measuring equipment, needing for digital transmitter parameters measurement, is given. Here, there are general-purpose units (e. g. the power meter, analyzer of the spectrum, frequency meter), so the specialized units. To specialized units are pertained:

• Test signal generator MPEG-2 G-420, is designed by NIIT, Saint Petersburg;
• Measuring receiver.

Name	Type
HF power meter	MK3-68, M3-56
Test signal generator	G-420 (NIIT)
Spectrum analyzer	C4-82
Frequency meter	C3-64
Measuring receiver	EFA 40/43 (R&S)
DTVM2000 (T) Bt

The last is a most complex position since domestic measuring receivers for present day are absent, and these are the most expensive units.

Analyzing the requirement to analog and digital transmitters and considering inevitability of enough long transition period, we have designed and introduced the concept of the double use of the modern transmitting means, named as the hybrid transmitters. This concept provides the unity of the structure and maximum unification of the components that will vastly simplify the transition from one broadcasting type to the other and will reduce the expenses. The structure of such transmitter at figure is given.

Broadcasting signals are formed by analog or digital (DVB-T) modulators. Hereinafter follows the pre-corrector, provided for compensation of nonlinear and frequency distortions of the subsequent path. A converter provides the signal transferring in working channel, and amplifying path provides a necessary output power. Basically, the transition from one broadcasting type to another in this structure is provided by modulator change.

The scheme of the parameter control of digital television radio transmitter

As previously, the main requirement to amplifier is a linearity in dynamic range of the signal, which defines the level of out-of-band spectrum components. For transmitter with joint amplifying, as well as for digital transmitters, this else the level of combinational components inside the channel band. As always, here is carried out the search of the compromise between signal quality and energy efficiency of the amplifying components supply.

For oscillator, using in digital transmitter, the increased requirements to phase noise are presented. Besides it is necessary to take into account the nominal values of the intermediate frequencies. So for supply of foreign digital modulator the oscillator realignment can be needed.

Material is provided by MART Company