With the recent introduction of Digital Terrestrial Television, many different opinions are being given about the best way to handle DTT installations, writes Patrik Lagerstedt, Technical Director at Horizon Global Electronics.

I have experience in the practical approaches of a number different countries, including Sweden, Italy, the UK, Australia and Germany. Each country has taken their proven experience from the analogue TV industry and have tried to apply this in different ways to the new digital age. Of course, the old school of signal level/carrier noise does to some extent apply, but with DTT, a more sophisticated approach is needed. This article seeks to summarise the new challenges and needs that face a rigger on a DTT installation.

Analogue signals

Traditionally, CCIR bandwidth is 8MHz, with PAL-I consuming 7.61MHz within that channel. It contained a vision carrier, possibly a sub-carrier and sound carrier. The carriers were narrow, and with analogue information were sensitive to noise which would affect the signal adding interference like snow, fading or ghosting. A multi-path of signals (the same channel allocation) or adjacent channels caused the same problems. However, a strong RF signal level would normally make the carrier noise less noticeable resulting in better reception. Figure 1 demonstrates these inherent features of analogue signals.
DTT signals

The DTT signal is very different from the standard analogue PAL signal. The DVB-T standard was designed to use a COFDM (Coded Orthogonal Frequency Division Multiplex) modulation together with FEC (Forward Error Correction). This provides a signal that is more or less immune to ghosting, noise and multi-path interference. By applying FEC, interference effects are reduced. So, whereas with analogue
the signal had a vision carrier, sub-carrier and sound carrier, a DTT signal appears to be the whole bandwidth. Figure 2 shows this. The DTT signal is actually a multiplex carrier, with the data (zeroes and ones) combined together with the error correction code. For 2K mode, there are 1705 carriers, and for 8K mode there are 6817 carriers. Figure 3 demonstrates the difference.

Practical differences

These differences mean a very different approach is needed during installations, similar to the changes when mobile phones changed from analogue to digital. Analogue phones pretty much always worked, even with sparks, noise or faintness, but you could generally talk over them. With new digital GSM phones, reception is perfect, but in poor signal areas, sudden dropout is not unusual. The same problems apply to DTT signals. A signal or install needs to be over a certain threshold to receive a perfect picture in differing conditions, but the nature of the DTT signal means it's very robust and immune to interference.

To some degree, the old way of looking at high signals is applicable, but measuring Carrier/Noise as proof of quality is not valid any more. “Right,” I hear you say, “which of the carriers is the one to measure? It would be impossible to measure them all in real time.” Well, it's not the same as analogue signals where carrier noise effects adjacent channels. This means it also becomes necessary to measure the Flatness of the carrier. However, comparing this simple series of RF measurements at various points doesn't say anything about the actual data reception.

Measuring the difference

What is needed is a TV Meter that checks all aspects of the signal quality, and enables real time adjustment of the aerial. The practical way is to read the pre-BER (uncorrected Bit Error Rate) register of the COFDM decoder. The BER is very often used in digital equipment and is a measure of the amount of received error the FEC part of the COFDM decoder receives. TV Meters should have an in-built COFDM decoder, just like the receiver or TV set, so that it's pre-BER that is being read. The intricacies of the COFDM and FEC system mean that once the FEC is applied for a verti-BER reading, no fine tuning can be made. Reading the verti-BER is a great way of confirming that a good install has been made.
As DTT signals sometimes have multiplexes (more than 1 channel) in the same channel space, all channels that are to be received need to be checked. Practically, one of the channels will probably be weaker than the others. It's the installer's job to make sure all have a sufficient pre-BER reading, I recommend a reading of 1E-3
to avoid problems during adverse weather conditions. This reading simply means 1 error in 1000 pieces of data received.

Cabling and connectors can make a great difference to the install, with good quality double screening necessary, reducing the impact of mains interference and speeding up the job. If mains interference continues to be a problem an additional mains filter may be needed for the source or reception equipment. Any amplifiers used to redistribute the signal should also be of good quality.

To summarise, I would recommend the following checklist for an easy and successful DTT installation.