Experienced aerial and satellite installer Mark Bartlett has been running the Solutions Group trade counter in Poole for the past four years so knows only too well the problems installers can face. With Digital Terrestrial Television now entering a new era, Mark tackles some frequently asked questions about DTT and offers some useful tips.

Is Analogue being switched off?
In my opinion the question should not be is it being switched off but when is it being switched off. An educated guess would be somewhere between four and eight years from now.

What does this mean to the rigger in the street?
With the new free-to-air digital terrestrial receivers now available, the whole DTT market is set to explode. There are approximately 16 million households across the country that don’t want to pay a monthly subscription or have a dish on the wall, but would be more than willing to view clear digital pictures through a terrestrial aerial on their existing television set at the right price.

How extensive is DTT coverage now across the country?
In some areas of the country DTT signals are received with absolutely no problems at all. In other areas though, picture break-up and loss occurs. This still occurs in a lot of the installations already completed. The complicated nature of detecting these problems means that traditional rules do not tend to apply i.e one set top box or integrated television may work better than another. However some tips will enable you to reduce and in most cases alleviate installation problems completely.

What sort of test equipment should I be using?
To be able to recognise the problems that can occur on DTT transmissions, your meter should be able to read these three parameters.

1. C/N (CARRIER TO NOISE RATIO)
2. BER (BIT ERROR RATIO)
3. DCP (DIGITAL CARRIER POWER)

Some meters in the market place simply give you a pass or fail display. These can and do work in a number of installations. However, I am aware of quite a few occasions where these test meters indicate good reception (pass) but when the set top box is installed it breaks up on a number of channels. This can also happen vice versa when the meter says fail the set top box goes and works (normally when another company has actually fitted a box and reaped the benefits). It is essential you understand the limitations of your meter.

Can’t I use my old Analogue meter to read DTT transmissions?
NO! NO! NO! Using your existing analogue meter will result in inaccurate measurements being taken.

What does a digital terrestrial set top box need to work?
Readings can be improved if you follow some simple steps.

Remember that DCP for a set top box can be as low as 55 dBµV which means extra high gain amplifiers may cause you problems. A good quality matched aerial should be used for reliability and performance.

Try to avoid masthead amplifiers if you can. If a masthead is required then try and keep its gain to a minimum (13 -15 dB). The use of screened mastheads when used with other high performance components has been found to significantly improve quality. ‘F’ type amplifiers, such as the V20-100 from Vision which has a gain of 13dB, have also been found to improve pictures and reduce interference.

Good quality double screened cable is also necessary (CT100/H109F). Advise the customer that the electrician’s low cost single hair braid cable down the cavity is really not good enough!

Avoid joins totally, even if this means drilling the outlet plate and bringing the cable directly in from the outside of the house. By improving measurement on DCP and C/N you will directly affect the BER reading. Experimentation is the key here.

I've had those readings and better and it has still failed. Why?
Noise spikes (impulses) are a big factor affecting signal reception and can be caused by a number of things, including a moving car on the street below. These are often not detected by the average test meter because they happen so fast.

To reduce the problems caused by environmental noise, it is imperative you use screened equipment along with good quality cable. F type aerials and screened accessories should be part of every installer’s stock list. Another way to reduce the effect of these spikes is to use something to shield the aerial, such as the roofline of a house, which can protect the signal against the noise being generated from passing cars etc.

Several other forms of noise have also been detected in and around the household. Fluorescent lighting and central heating thermostats are just two of many already being reported by installers at the trade counter. Bad earthing both in domestic and in communal dwellings has also caused DTT transmissions to break up.