Contents:

• A few words about DVB-T
• What are the benefits of digital TV?
• What antenna for DVB-T?
• Measurement of digital terrestrial TV signals

The digital switchover will be carried out in stages and individual regions of Poland will experience it at different times.

Similarly to many countries, for each area there will be two important dates:

Digital Video Broadcasting Projekt (DVB) je asociace sdružující přes 250 vysílačů, výrobců, síťových operátorů, poskytovatelů softwaru a regulátorů ve více než 35 zemích, založená k vývoji počátečních standardů pro dodávání digitální televize, multimédií a datových služeb. Služby podle DVB standardů jsou dostupné na všech kontinentech, přístupné více než bilionu přijímačů.

Asociace je autorem mnoha standardů. Jedním z nich je DVB-T standard pro digitální terestriálních TV (NTC), potvrzené v prosinci 1995, popsané v EN 300744. První přenos, který použil tento standard se uskutečnil v UK 15.září 1998. Od té doby se mnoho změnilo. Současný stav standardů a jejich implementace v různých zemích je možné procházet na web stránkách dvb.org .

Digitální terestriální televizní systémy přenáší komprimované dogitální audio/video přenosové signály (MPEG-TS). Výhodou tohoto řešení je možnost vysílání několika programů a poskytování služeb jako je EPG (Electronic Program Guide) v jednotlivém signálu. V závislosti na standardu systém používá MPEG-2 nebo více pokročilou MPEG-4/H.264 kompresi.

To broadcast a digital signal, DVB-T uses COFDM (Coded Orthogonal Frequency Division Multiplexing) modulation scheme. This scheme has been developed for systems that are susceptible to the effect of multipath propagation (typical for wireless signal transmission in urban areas). It consists in the fact that, due to various reflections, the radio signal reaches the receiver in several copies, which are shifted in time. The greater the throughput of the system, the more noticeable it becomes. COFDM modulation is a way to eliminate this problem. Instead of a single high-bandwidth signal the stream is divided into many sub-streams with much slower data rates, modulating a number of orthogonal subcarriers. The number of the subcarriers amounts to 1705 (2k mode) or 6817 (8k mode). Depending on the standard/version, each subcarrier is modulated using QPSK, 16QAM or 64QAM modulation scheme. The throughput of the entire DVB-T stream is within 4 - 32 Mb/s, depending on the version and method of modulation. This means that one multiplex (the band equivalent of a single analog TV channel) can carry up to eight digital television channels in standard definition (SD) or 3-4 channels in high definition (HD).

The main benefits are connected with the above-mentioned digitalization, data compression and transmission modes. First of all - digital TV saves bandwidth - a digital package of programs (multiplex) carries up to eight digital SD programs, taking the same band as one analog channel. It means that the released channels, previously occupied by analog television, can be used by other services.

Digital terrestrial television can be received in locations that were practically without possibility of receiving analog TV signals

An example from the UK - one of the countries leading in implementation of digital terrestrial television. Comparison of image quality of digital and analog reception in difficult conditions (source www.stevelarkins.freeuk.com)

Given the characteristics of the digital signal and so-called "guard interval", it is resistant to reflections that may completely spoil analog TV reception ("ghosts" - see the picture below).

Digital terrestrial TV eliminates the problem of reflections, often visible in the case of analog TV
(source: wikipedia.org)

However, DVB-T signals are also susceptible to interference from sources such as other radio and television transmitters, devices that emit electromagnetic energy, or even passing cars, and may require additional filters adapted to the situation prevailing in a given location.

New DVB-T multiplexes will offer HDTV channels.

Digital television provides many new opportunities, such as electronic program guides (EPG), video on demand (VOD), a selection of several audio tracks (e.g. the choice between different language versions), a number of selectable subtitles.

COFDM modulation allows to redistribute the same multiplex in neighboring areas, using the same band. The "cells" make up a Single Frequency Network (SFN). Due to implementation of the mentioned guard interval, even the receivers within the range of more than one transmitter will not have problems with interference. However, the key issue is the precise synchronization of all transmitters of the SFN, usually using GPS service. Such networks have already been successfully deployed in Germany and other countries.

The architecture of a Single Frequency Network

The question is often asked by our customers. Generally speaking, digital TV does not require to use new types of antennas. Yagi-Uda arrays or even typical broadband antennas can receive digitally modulated signals. Of course, everything depends on the place of reception and the local signal propagation conditions.
However, DIPOL has marked a certain group of antennas as "DVB-T". What is the reason? Analog terrestrial signals are broadcast at frequencies up to 782-790 MHz (channel 60). Therefore, the most installed UHF antennas have been designed to work within the range of 21 to 60 ch. DVB-T broadcasting can use channels up to 69, so the need for antennas working in the entire UHF band - from 21 to 69 channel.

The second point is that DVB-T receivers provide 5 VDC supplying voltage for antenna preamplifiers, instead of typical 12 VDC used in "normal" installations. Of course, the antenna with the amplifier powered from the dedicated 12 VDC power supply will work correctly, but in the case of using a new antenna for a DVB-T receiver it is very convenient to utilize the 5 VDC voltage provided at the antenna input.
DIPOL offers two high quality compact DVB antennas equipped with low-noise amplifiers powered with 5 VDC (directly from the DVB-T receiver or with the included AC/DC adapter and DC injector).

To amplify signals in large antenna systems one requires channel amplifiers - this also applies to the DVB-T installations.

The spectrum of DVB-T signal differs from the spectrum of a single analog channel, which is presented in the following figures. The challenge for the amplifier to increase the level of the DVB-T signal is well visible there. Due to steep slopes and use of subcarriers in the whole width of the channel (usually carrying a whole multiplex), the key parameters of the amplifier become flat gain across the channel and selectivity.

DIPOL recommends to use renown Alcad channel amplifiers. The ZG-421 R905039 amplifiers are tuned to ordered channel and feature perfect selectivity (17 dB - one channel gap, 56 dB - two-channel gap), high gain - 53 dB and a very large output level - 120 dB which allows to feed up to 200 subscriber outlets. The R905039 amplifiers ensure proper amplification of all subcarriers of digital signals.

Alcad ZG-421
R905039

Comparison of frequency characteristics of ZG-401 and ZG-421 amplifiers (the latter - in red - steeper slopes)

The ZG-421 R905039 amplifiers allow to equalize levels of signals in different channels, and due to perfect selectivity, can be used in systems that operate adjacent channels.

For analog signals the most important parameters that should be measured are the levels at the input of the distribution network and the C/N (carrier to noise) ratios.
Due to the nature of digitally modulated signals it is impossible to distinguish a single carrier - the signals are similar to noise and C/N measurements are not relevant. In addition, in the case of a digital signal the most important thing is not the signal level, but its quality which is determined by a number of parameters described below.

The basic parameter providing information about digital signal quality is BER (Bit Error Rate). It indicates the possibility of an error in the bit stream. Digital signals transmitted via RF channels (DVB-S/S2 and DVB-T) are broadcast using double redundancy coding. This increases the bandwidth requirements for signal transmission, but allows the receiver to correct errors arising in the transmission channel by means of appropriate corrective algorithms (Viterbi and Reed-Solomon corrections).
Most meters available on the market have the ability to measure two types of bit error rate - BER (Channel BER - CBER, also marked bBER or PreBER) and aBER (bit error rate after the first step of signal correction - Viterbi - also marked as PosBER). It is very important to distinguish these two parameters. From the perspective of the installer, the measurement of aBER, whose value is approximately one million times smaller than the BER, in most cases is pointless.
It is assumed that a digital signal with BER equal to 1E-4 (1 erroneous bit per 10000 bits) has a good quality - is Quasi Error Free (QEF). For HDTV signals the BER should be not higher than 1E-6.

Professional DVB-S/S2, DVB-T, DVB-C, and analog signals analyzer Rover Scout ST2+ R10830

Most installers, having meters with only basic functions, limit measurements to signal level and bit error rate. Meanwhile, the equally important parameter is Modulation Error Ratio (MER). In fact, it is an equivalent to S/N or C/N for analog signals, with the difference that aside from amplitude noise it also takes into account other noise components, such as phase noise - very important in the case of digitally modulated signals.
The difference (ratio) between the measured and minimum MER shows the margin possessed by an installation, protecting proper reception (margin to the point of cutting out altogether). A deterioration of this parameter practically does not affect the quality of reception, until it reaches this point - the minimum value (the border between proper reception and lack of reception can be as narrow as 1 dB).
Typical and minimum values of MER depend on modulation schemes (QPSK, 8PSK, QAM ,...) and FEC (Forward Error Correction) parameters. For example, in the case of digital terrestrial television (DVB-T with QPSK modulation) and FEC=2/3, the minimum and typical MER values are 14 dB and 17 dB respectively.

It is also important to analyze constellation diagrams. They carry information about the types and levels of noise influencing digital signals.

Constellation diagram is a presentation of the digitally modulated signal. It assigns to each symbol (the possible sequence of bits - for example, 16 symbols of 16QAM modulation, created by combinations of 4 bits) a constellation point in the complex plane.

A constellation diagram for rectangular 16-QAM, used in DVB-T
[wikipedia.org]

In the case of analog TV antenna systems the key parameter measured in addition to the levels of input and output signals (at the subscriber outlets) is C/N (carrier to noise) ratio. This parameter, however, carries no information about phase noise, equally important in typical digital modulations. A professional installer who wants to know and analyze the sources of noise and interference in the reception/distribution network, should choose for this purpose the constellation diagram.

Below there are examples of constellation diagrams of DVB-T signals and their interpretation.

Constellation diagrams of DVB-T signals - good signal on the left, poor signal on the right
[Sunrise Telecom Broadband]