Digital Terrestrial Television (DTT) uses a transmission system based in the DVB-T norm. The most important part of this norm covers the use of a modulation system, COFDM, designed to offer a solution to the digital television signal broadcast with special focus in the multipath reflections.
There are cases where the same frequency is used from several different radio stations to cover a given geographical area. We call them single frequency networks or SFN. COFDM modulation also incorporates mechanisms to preserve the quality of the signal in this type of SFN networks, taking into account that apart from the typical reflections of terrestrial transmission we will also be receiving the signal from more than one transmitter simultaneously.
Those networks that do not reuse the frequencies in the same geographical area are known as multi-frequency networks or MFN.
The guard interval
COFDM modulation is based on the principle of sending small quantities of information in a pulsed way, alternating on activity times and pause times. The duration of the total cycle, known as symbol time, is one millisecond (1 ms). The pause times are known as guard intervals (GI).
The basic idea is that, knowing reflections (echoes) will be produced during the transmission, the pause times or guard intervals will allow these echoes to fade away and do not affect the signal reception. Comparing with what happens with the sound, it would be like playing notes in a piano allowing for enough silence time between them so that their sounds do not mix.
DTT is commonly using a guard interval of 224 us., GI = 1 / 4. In general we can say that the echoes arriving within the guard interval do not affect the signal reception as opposite to those arriving outside.
In order to increase the transmission capacity and somehow compensate for the slow symbols and pause cycles, the COFDM sends data in "parallel" using thousands of small sub-carriers within the channel bandwidth. Continuing on with the example above it would be something like playing accords instead of single notes.
The echoes in DTT
Any receiver situated in the main transmitter coverage area will receive the signal on the direct path together with the set of reflections and echoes that may have been created on the way.
In the Dynamic Echoes diagram we represent, in a time scale, the exact time in which the echoes arrive. The main signal is represented as a vertical line, with 0 dB level, placed at the precise moment in which the silence begins, in other words the guard interval.
The echoes will be represented in the same way by vertical lines drawn at a certain distance of the main echo depending on the relative delay and attenuation they are received with.
Since the echoes are due to the different paths traveled by signals in space and knowing that their speed is 300.000 km/s, we could also define a new scale equivalent to “time” in "distance”. As a rule of thumb 30 km are equivalent to 100us.
Echoes in MFN
In this type of networks there is only one transmitter using a frequency in a given area. As a consequence any echo received will be the result of a multi-path reflection of the main signal. Therefore, in general, the strongest echoe will be that of the direct path which will also have traveled the shortest distance. All other echoes will be weaker and will arrive later so they will be considered POST-ECHOES. So the situation will be like in the figure below:
The echoes falling within the guard interval will not affect the correct reception of the signal unless they are particularly strong, up from -5 dBc, which is not common.
Echoes in SFN
In general these networks will concentrate most of the problems with echoes. A receiver located in the coverage area of several transmitters operating in SFN will receive their signals simultaneously and will take them as a main signal with multiple echoes.
COFDM receivers embedded in iDTVs and DTT set top boxes, been aware of the presence of echoes in the received signal and the correction mechanisms available, will locate the strongest echo and identify it as “main signal”. The remaining echoes will therefore be weaker and may arrive either later or before compared to the main signal. Delayed echoes are known as POST-ECHOES while the other ones are known as PRE-ECHOES.
Once the primary and all the secondary echoes have been identified, the receiver calculates the ideal position for the guard interval so as to embrace the maximum number of echoes possible and thus minimize their impact on signal reception. This process is repeated continuousl.
There are differences among receivers, models and brands, as to the procedure used to recalculate the optimal position of the guard interval. In a critical situation that can lead to completely different behaviours.
Again, the echoes that fall within the guard interval will not affect the correct signal reception unless they are particularly strong or are located near its limits.
Although they are more common in SFN networks they could also be found in MFN. These echoes are very short, so close to each other that the receiver is not able to determine which one should be considered the main signal and which one the echo.
In the case of SFN these are typically found when the receiver is located in an area at the same distance from more than one transmitter. If those echoes are sufficiently close to each other and have similar power levels they may even make the reception impossible. This is an effect that is very difficult to detect and whose consequences may vary greatly from one receiver to another.
There are many situations in which the presence of echoes can degrade or severely affect reception of DTT. The installer exposed to them can only play with the location and orientation of the antennas in order to minimize the negative impact that these echoes can have on signal reception.
The antennas commonly used in TV reception installations are YAGI type and have a radiation diagram like the one in the figure.
The diagram shows the gain with which the antenna receives a signal depending on the angle it is coming from. In the front side the antenna has maximum gain while this decreases in the sides and the rear.
Therefore the dynamic analysis of echoes, made in the way the PROMAX field strength meters do, happens to be nowadays an essential function.
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