Measurements and amplification of DTT (DVB-T) signals

The benefits and possibilities offered by digital broadcasting have increased the interest in receiving the programming by the users of analog TV.

In all countries changing the way television is broadcast from analog to digital there is a transition period when both systems operate simultaneously. It creates considerable challenges for the designers and installers of antenna systems. When designing and implementing the systems it is necessary to take into account both kinds of signals, even if some of the systems are designated exclusively for reception of digital terrestrial television channels. The quality of the signals can only be verified by measurements, which in the case of digital signals requires increased competence of the installer.

The minimum signal level required for proper reception of digital terrestrial television is lower than in the case of analog TV (e.g. by 6-12 dB in the case of 64-QAM modulation, depending on FEC). It is due to the much greater efficiency of digital modulation, which is a combination of amplitude and phase modulation of the RF signal. Digital modulation schemes complicate the implementations of the transmitters and receivers, but the transmitters of the same power as the older analog ones can cover larger areas.

When designing modern installations distributing DVB-T signals, the antenna installer should take into account the recommended signal levels shown in the table below.

As shown in the table, DVB-T technology imposes on the RF signals quite different limits than those of analog TV. In Poland (64-QAM, FEC 3/4) the limits are 48 dBuV and 74 dBuV.

The parameters of the signal with a guard interval of 1/4 allow for total capacity of a multiplex of 22 Mb/s. It limits the bitrates of SD channels to 3 Mb/s, and of HD channels to 7 Mb/s.
Given that satellite channel bitrates range from 12-18 Mb/s, the above parameters of DVB-T signal do not guarantee the highest quality - this modulation scheme imposes some limitations.

The method of measuring the level of DVB-T signals is different than in the case of analog TV. It is the result of completely different frequency spectrum of these signals. The vast majority of analog TV meters measures in fact the level of video carrier, so a very limited channel segment. Due to the fact that COFDM digital signal is actually containing several thousand QAM-modulated orthogonal subcarriers, to properly measure its level it is necessary to take into account all of them, in a whole 8 MHz channel. This basic difference is crucial when someone tries to measure the level of a DVB-T channel with an meter dedicated for measuring analog signals. Such a device, testing only a fragment of the channel (about the video carrier frequency of the corresponding analog channel) will give unreliable, biased result. At best, the result will be lower by several decibels.

The spectrum of analog and digital signals. In this example, the first channel on the left (22) is a DVB-T channel, having all spectrum components at the same level throughout its 8 MHz bandwidth, whereas the adjacent channel (23) is an analog channel (two distinct peaks visible - audio and video carriers). It is clear that the level of analog video carrier is greater than the balanced levels of DVB-T subcarriers. The digital signals are not detected by analog receivers (they are treated as noise).
Source: BBC R&D www.bbc.co.uk

View of DigiAir Pro R10510 meter. Possibility of measuring analog signals (using Constant Wave)
and digital signals (using Channel Power) methods

Consequently, the measuring device should distinguish between analog and digital signals. Anyone being interested in buying a DVB-T meter should also consider the possibility of measuring analog channels. It is worth to make such measurements, even when antenna systems are dedicated solely for digital channels, especially in the case of using broadband amplifiers. The problem of amplifying TV signals in transitional period will be presented in the further part of the article.

The signal level is not the only parameter which must be taken into account in measuring DTT signals. At least equally important are MER Modulation Error Ratio (MER) and Bit Error Rate (BER).

From the perspective of an installer or a viewer of DVB-T, an encyclopedic definition of the MER parameter is not very interesting. However, it should be borne in mind that MER carries information on the level and type of noise that interferes with the digital signal (phase noise, amplitude noise etc.). To truly understand the importance of measuring the MER parameter, there is the need to explain the phenomenon of so-called digital cliff.
In analog TV, the decline of the carrier to noise ratio (C/N) causes proportional decrease of image quality, but some large elements of the picture can still be recognized even in the case of the lowest C/N value allowing any reception. For example, the signal with C/N of 44 dB generally provides a very good picture quality. A decrease in the C/N by about 10-15 dB will increase fine "grain" in the image, but the majority of the viewers will consider it acceptable. Only with further drop of C/N by next 10 to 15 dB will the image become unacceptable (although visible) due to high noise level.

The cliff effect in digital DVB-T. The quality of analog TV picture decreases gradually.
(source: en.wikipedia.org/wiki/Digital_cliff)

In the case of a digital signal, the boundary between an ideal image quality and loss of reception is very sharp. In DVB-T, similarly to other digital broadcasting, we have to cope with "digital cliff" effect. This means a sudden jump from a normal reception to a complete lack of content. Operation on the boundary is the reason that the picture appears to freeze up or show up like a mosaic.

MER can be treated as a measure of the distance of the received signal to the digital cliff. Without knowing this parameter, the antenna installer cannot guarantee stable reception in adverse weather conditions or other situations when the signal can fluctuate (e.g. interfered by a passing truck). The recommended and minimum values of MER required for proper reception depend on the modulation technique. In the case of 64-QAM modulation, they are 31 dB and 26 dB respectively.

The third important parameter that should be taken into account when measuring DVB-T signals is BER. This parameter is well known to all installers of satellite dishes and describes the quality of the received signal. It informs about the proportion of erroneous bits (caused by interferences occurring in the transmission channel) in relation to all transmitted bits. There are several types BER. Although from the perspective of the installer only one of them is really interesting, we should be conscious of the kinds of measurements meters can take (most measuring instruments can measure two types of BER).

Digital signals transmitted via RF channels (DVB-S/S2 and DVB-T) are broadcast using 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. It means that in addition to the necessary information the transmitter also sends a number of correction bits. The proportion of the information bits to all bits is shown by FEC (Forward Error Correction). It usually takes 3/4 or 5/6, which means 25% or 17% of redundant data (respectively) in relation to to the total transmitted stream. Of course, better correction leads to limitation of the capacity of the channel.

The redundantly encoded digital signal is received and decoded at the receiver using correction algorithms. Due to a relatively simple hardware implementation, the preferred algorithm is Viterbi. The principle of the algorithm is not important here, but the fact that the signals before and after the decoding/correction are two different signals in terms of the number of errors. At this point we can define two types of BER parameter:

A good quality signal features BER of 1E-4 or less. Such signal is recognized as Quasi Error Free (QEF).

The question that we face in the era of implementation of digital terrestrial TV is whether there is a category of amplifiers specially dedicated for DVB-T. The most common answer is that there are no such amplifiers and the RF equipment so far employed in analog TV systems can be successfully used for receiving DVB-T broadcasts. Generally, this is true - only very old, low-performance amplifiers with high noise levels may have problems with proper operation with digital signals.

However, the amplification of television signals in the transitional period requires additional attention. Awareness of the existence of certain phenomena, and the experience of countries where DVB-T has been broadcasting for years, will allow for quicker diagnosis of problems that may occur with the use of broadband TV amplifiers.

Due to the previously mentioned differences in the levels of analog and digital signals, we should be particularly careful when they are simultaneously amplified. Intermodulation products, resulting from the use of active components and interference of two or more channels, can interfere or disrupt the reception of some of them. In particular, the interferences generated when amplifying strong analog channels can significantly affect the reception of "weaker" DVB-T channels. Consequently, the amplifiers should be carefully selected and adjusted, without excessive gain levels.

The above figure shows the spectral characteristics of the signals at the output of a broadband amplifier. The level of the intermodulation products can, in extreme cases, exceed the levels of the DVB-T signals. The greater the difference in the level of the received analog and digital signals, the higher the likelihood of problems after their amplification. Therefore, it is extremely important to measure input levels of all signals - even in situations where the installation is dedicated only for DVB-T. In this situation, a good practice is to apply attenuators of the strongest analog channels at the input of the amplifier.

Based on the previous information, the conclusion is that in the transitional period the most effective devices are low-noise amplifiers with small and medium values of gain, possibly with adjustable gain level. A special series of such amplifiers has been designed by Terra. The ABxxx masthead amplifiers can be powered with 5 VDC or 12 VDC power source, so including the power option offered by many DVB-T receivers. The parameters of the preamplifiers are shown in the table below:

Another good practice is application of pre/amplifiers with gain control performed by switch/es and not with potentiometers. The former method usually ensures better precision and stability of the adjusted gain.

In the case of MATV/SMATV systems, with dozens or more outlets, the application of broadband amplifiers at inputs of the systems is not a proper solution. Only channel amplifiers allow for equalization of all channels and high output levels without a risk of interference.

Alcad ZG channel amplifiers are used in Europe for several years. The professional equipment
provides optimum results both in the case of analog TV channels and DVB-T multiplexes.

Analyzing the performance of an RF amplifier, one of the basic parameters is its internal noise level, described by the noise figure. This parameter is particularly important in the case of DVB-T antenna systems, in which the signal levels are considerably lower lower than those of analog TV. The majority of amplifiers dedicated for small building systems have noise figure within 3-5 dB. The representatives of a group with noise figure at the lower limit are the amplifiers from the Terra HS series.

An important factor influencing the noise level is the location of gain control (in the structure of the amplifier), especially taking into account amplifiers with two or more amplification stages. Much better results (less noise added to the signal) are provided by amplifiers with interstage gain control (instead of reducing input signal).

Block diagram of the HS-016 R82030 amplifier. Separate VHF and UHF paths and interstage gain control ensure very good noise parameters of the device.

The amplifier is dedicated for small/medium antenna systems, with a few to a dozen outlets. It is recommended for use in DVB-T systems, also in the transitional period (with continued analog broadcasts)