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ABC guide to... Step-by-step guides to understanding digital TV
july 2010
Distributing TV signals around the home, pt 1 Want to see more?
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distribution
Be the boss of your AV system anywhere in your home by following our guide How to extend a Scart connection
When you have one central satellite TV receiver or PVR, one DTT receiver or recorder, or even a single DVD player in the household and you want to watch its output on one or more secondary TVs in other rooms, then you need to send, or distribute, the equipment’s signal around the home. Choice of method
01 The output from any video source is available in a number of different formats and what is available, what quality and facilities you expect at your secondary TV screen, and what you expect to pay will largely determine the means to carry the signal around the home. HDMI systems and Ethernet digital distribution provide the highest-quality pictures and sound, and these complex and expensive techniques will be explored at another time. A number of proprietary systems use CAT5 network cable to carry analogue AV signals with a quality approaching the very best, although these too can be expensive for simple domestic setups. The most common distribution method uses the UHF output from a receiver in the same way as an aerial signal can be sent around the home. However, the simplest technique is a direct connection, extending the Scart connection that’s provided on almost all types of AV source to another room.
The simplest way to add a second TV to your system is to directly connect it to the Scart socket – via a three-way phono lead for ease of routing through the building
How to go wireless
02 A Scart connection between a
03 When you physically can’t lay a
satellite or DTT receiver and a TV provides a very good (standard definition) picture and stereo sound, so if you have an unused Scart socket on the receiver it is relatively easy to extend this connection to another room and a second TV set. A Scart connection’s performance degrades over long distances but, depending on the quality of the cable used (get the best you can afford with individually shielded connectors inside), a 10m cable should operate OK and you may be able to get away with a 20m connection. This should cope with most normal home layouts, but you should remember that this is the actual length of the cable – laid along walls and so on – and not the distance ‘as the crow flies’. The cable will probably have to go through a wall or two and ready-made Scart cables will need a large hole to accommodate the plug. Instead, you can either solder on the plug after the cable is laid, or use a three-way phono lead (left audio, right audio, and video) with Scart converters at each end. This will probably also be cheaper for the cable but it restricts you to a composite video signal and precludes using RGB, which produces the best SD picture quality. Though Scart splitters and distribution systems are available, feeding more than one extra TV in this way is rarely practical or affordable.
cable to the second TV you may want to consider a wireless videosender. This two-unit setup has a transmitter to convert the video and stereo audio signals to a radio signal (usually at 2.4GHz) and send it to a receiver which converts it back to AV signals. Videosenders usually only cope with composite video signals but they do handle (analogue) stereo audio and usually include a return path for remote control commands. With some systems you can add further receiver units to pick up the signal for additional secondary TVs. But transmission power is minuscule and the signal will not pass through much in the way of walls and floors. Interference from other devices nearby using the same frequencies, including wireless networks, Bluetooth devices, some car alarms, toys, and microwave ovens may also occur. How to use UHF
04 The most popular technique for video distribution is by UHF. Receivers and recorders that include a UHF modulator loopthrough add their own output to the UHF signal from your aerial so you can tune to the receiver/recorder output as though it were another broadcasting channel. With a single extra TV the connection can be direct from the AV equipment to the TV with standard
02
Main TV Primary AV connection
Phono connection
Scart connection
Secondary TV
2 What Satellite & Digital TV
abc guide to... 04 Secondary TV Main TV Primary AV connection UHF distribution amplifier
Secondary TV UHF aerial feed
aerial coaxial cable (or, better, goodquality satellite LNB cable) laid in the most direct route the building allows. But this will preclude connecting the main TV to the aerial feed unless you have a Sky Digibox or other equipment with two UHF outputs for this purpose. It’s simple, but the downsides of using UHF are many. Picture quality is poor, sound is only mono, it requires an analogue tuner in the secondary TV, it can be tricky to find a vacant UHF frequency to use without interference, and many AV devices (especially DTT receivers and DVD players) do not include a UHF modulator. How to distribute UHF signals
05 If you want to feed more than one TV away from the living room AV equipment, then you will need to split the UHF signal or (more likely) use a distribution amplifier to compensate for signal level losses from splitting the signal and additional long cable runs. This is done in the same way as an aerial signal is amplified and distributed, but the amplifier is not placed close to the aerial as it is to use the combined signal from the AV equipment, and so it is often positioned behind the main TV and
equipment stack. Distribution amplifiers from two-way to 16-way are available, with different gains, variable gain, or even adjustable gain on each output. How to stay in control
06 When watching satellite TV in another room you will want to remain in control of the receiver to change channels and start or play recordings, therefore a remote control relay is essential alongside the video distribution system. Videosenders usually incorporate a relay system that converts and carries the infrared commands in the opposite direction to the AV signals. A UHF distribution system can use a ‘magic eye’ type relay (like the Sky TV-Link) that converts the IR commands to electrical pulses to be sent ‘back’ down the co-ax cable carrying the UHF signals to the living room, where they are converted back to infrared. The Sky Digibox’s second UHF output will accept the electrical commands with no need for a converter. The only proviso is that any amplifier used to distribute the UHF signals must be TV-Link-compatible to allow the electrical commands to pass through. If you are using a direct connection to
UHF distribution is the most commonly used technique to feed the picture and sound to secondary TV sets around the home
the secondary TV from the equipment’s Scart socket, then you need another sort of remote relay. The same type of radio relay built into videosenders is also available as a standalone system. The most popular is the Powermid, a pair of small pyramid-shaped units to pick up and beam the remote control commands from the second TV back to the living room so you can fully use the AV equipment as well as watch its output n Geoff Bains
Glossary Composite video
Analogue video connection with chrominance, luminance and sync picture information carried in one signal.
A videosender provides an alternative connection to one, or more, secondary TVs without the need for wires throughout the home
RGB
Analogue video connection with separate signals for red, green and blue picture information.
UHF
TV signal with picture and sound information modulated on a ultra high frequency radio carrier.
06 Videosender receiver
Secondary TV
Videosender receiver
Secondary TV
Main TV Primary AV connection
Videosender transmitter
What Satellite & Digital TV 3