Freesat Freeview Euro Tv Sky The best kit, The best programmes
ABC guide to... Step-by-step guides to understanding digital TV
Freeview amplification Want to see more?
Visit Wotsat.com for daily news, reviews and updates from the world of digital TV, or join our forums.
ABC guide to
Freeview amplification It’s possible to squeeze more signal from your Freeview setup using boosting or amplifying kit, provided you have the right tools for the job If you don’t have the perfect aerial in the perfect reception area, connected by perfect cable to a perfect TV set, the chances are that your Freeview system may need a helping hand for the signal in the form of a ‘booster’ or aerial amplifier. Why use an amplifier?
01 Not surprisingly, an aerial amplifier is used when the signal at the TV or receiver is too low. However, an amplifier will not make a poor aerial better and it’s rarely much use amplifying the signal if the output from the aerial itself is too low. You can test this by connecting up your TV or receiver as close to the aerial as possible, using a very short length of cable. If the signal is then still too weak you need to fit a better aerial – one with a higher gain or better interference rejection (or both). An amplifier is used to compensate for the reduction of the (good) signal as it passes along the cables to the TV or receiver. You may be able to avoid using an amplifier by fitting better cable (to lose less signal along the way). Where should the amplifier go? 02 The amplifier should always go as close to the aerial as possible. It will amplify whatever is at the input, and that includes any noise already in the signal. So if you put the amplifier at the TV end of a long cable it will amplify all the noise in the signal at that point. It’s like trying to hear the news from a badly tuned (analogue) radio. Turning up the volume doesn’t help to distinguish
the speech from the crackle and hiss. Instead, the amplifier is connected at the aerial end of the cable, so the signal is amplified before the cable reduces the signal and introduced noise. There will be less noise in the signal close to the source and because the signal starts down the cable at a much higher level the noise added along the way is less overwhelming at the receiver. A ‘masthead’ amp is usually the best choice. This fits either to the aerial mast (as the name suggests) or to the wall close by. So, either way, it is right next to the aerial. Sometimes it’s not practical to fit the amplifier this close to the aerial and
(a) No amplification is needed with a good signal and short cable (b) A longer cable will require boosting the signal with an amplifier that can be mains-powered (c) An external amplifier needs a remote power supply (d) Feeding two or more TVs needs a distribution amplifier with an attenuator on the shorter cable(s)
a second choice is to fit it in the loft near to where the aerial cable enters the building. How to power a masthead
03 amplifier
Aerial amplifiers require electrical power to operate and many indoor models are simply plugged into a mains outlet. However, fitted in the loft, there is often no suitable mains outlet available and amplifiers fitted outside are never mains powered. So masthead amps are usually powered remotely. Power is sent up the signal cable in much the same way as the power for a
DISTRIBUTION AMPLIFIER
AMP AMP
ATTENUATOR REMOTE POWER SUPPLY (a)
2 What Satellite & Digital TV
(b)
(c)
(d)
abc guide to... satellite LNB is sent up the LNB cable. A power supply unit is used indoors to introduce the power to the signal cable, and this has loopthrough co-ax connectors for the signal cable and is powered from the mains. The power supply unit can be fitted anywhere along the signal cable from the masthead amplifier where there is mains available, and is often positioned near the TV, at the end of the signal cable. Some TVs produce a 5V power on the aerial input. However, relatively few masthead amplifiers can operate on anything but the 9V-12V that is normally supplied by the power supply units.
THRESHOLD
THRESHOLD
How much amplification is
04 needed?
Every bit of cable and every connection reduces the signal level. Different cables (and connectors) have a different effect (the thicker and more expensive the cable, the less the reduction). It is possible to calculate the exact reduction that your system will introduce but a rule of thumb (not suitable for poor quality cables – but these should be replaced anyway) is 0.2dB loss for every metre of cable. So if you have 20m of cable from the aerial to the TV, the signal level is reduced by 4dB and fitting an amplifier with a gain of 4dB will restore the level at the TV to the same as it is at the aerial. In practice, amplifiers are only available with certain gains and the estimate may be out anyway. So it’s best to buy an amplifier with a variable gain of up to a value greater than your estimate suggests. How to use an amplifier for 05 multiple TVs Using a single aerial to feed more than one TV will also reduce the signal available at each TV or receiver. Each time the signal is split, using a passive ‘Y-splitter’, the two signals produced each suffer a 3-4dB loss. So you can insert a 4dB amplifier before the splitter to bring the two signals back up to the right level. Just like cable-compensating amplifiers, this amplifier for the splitter should ideally be positioned as close to the aerial as possible. If the split signals are going to be travelling along long cables, then the amplifier should be specified to compensate for the cable loss as well, adding the cable loss gain to the splitting gain to arrive at the overall amplifier gain required. A more convenient system is to use an amplified splitter or distribution amplifier. This combines the electronics into one box, and two-way, four-way and larger versions are available, typically with a slight overall gain of about 6-8dB (often
AMP
THRESHOLD
AMP
variable) in addition to the compensation for the signal division to provide for cable loss. If one cable from the splitter is substantially longer than the other, then the gain should be selected to cope with the long cable and an attenuator (of a value equal to the loss from the difference between the cable lengths) used on the shorter cable and positioned close to the TV or receiver. How to use a set-back amplifier
06 For the most part, don’t! Set back amplifiers are probably the type most often bought as they are powered by the mains (often built into a mains plug) or batteries and simply sit behind your TV connected to the aerial downlead and the TV’s aerial input. A set back amplifier will give a boost to the signal before it gets to your TV but as the TV will usually have a better amplifier built in all the set back amp will do is to amplify the noise as well and make the reception worse, or even increase the problem of interference from nearby radio signals such as taxi radios. The occasion when a set back amp is helpful is when the UHF output from, say, a Freeview receiver kept under the main TV (and connected to it by Scart or HDMI) is to be fed to a secondary (analogue) TV over a very long run or divided between
several TVs. Then the set back booster is in the right place to give the signal enough oomph to survive the trip. How to use filtering
07 Freeview systems can be subject to many forms of interference – from taxi radios, amateur radio and the TETRA Police radio system, as well as passing vehicles, electric trains, house wiring and thermostats. Powerful analogue TV transmissions and other radio broadcasts (even outside the Freeview frequency range) may overload the receiver’s input and destroy reception. The effect is heightened by amplification of the signal so you should fit either a separate passive filter (requiring no power) before the amplifier to limit the signal to the Freeview frequency range only, or – even better – use an amplifier with such filtering built in. If interference is within the Freeview frequency range you may need a filter to block a part of the range. But you cannot, of course, remove unwanted signals at a frequency without removing wanted reception at that frequency too. So for some interference the solution is to use a more directional aerial, accurately aligned on the transmitter, or to shield the aerial from a local source of interference n Geoff Bains
Over a long cable run the signal level is reduced and noise is added (top). Fitting an amplifier at the TV will raise the level of the signal but also amplify the noise (middle). Instead, the amplifier should be fitted at the aerial so the signal is amplified to compensate for the losses in the cable but the noise is not (bottom).
What Satellite & Digital TV 3