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ABC guide to... Step-by-step guides to understanding digital TV
september 2010
Home video distribution over CAT5 Ethernet
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Distribution Part 3
Standard networking cable can also be used to set up a high-quality AV distribution system Although distribution of AV signals using UHF and video senders is simple to implement, the results are far from those that you get by direct connection on your main TV. To step up a level in picture and sound quality you need to invest in some special wiring about the home. The wiring most commonly used for this is ‘Category 5’ (or CAT5) cable that is designed for use in computer networks. Used instead of (several) conventional co-ax cables it serves well for video and audio signal distribution about the home.
a single wire and the surrounding shield connected to a common reference ground). This calls for some electronics at one end to convert the ‘normal’ signal to a balanced one and ‘transmit’ it down the cable, and a ‘receiver’ at the other end to convert it back again. There is no standard for the signals or the conversion, so this means that a system needs to be built from components from just one manufacturer.
What AV signals can be sent via
02 CAT5?
Because a CAT5 cable contains four twisted pairs it can carry four signals – usually left audio, right audio, video, and control signals and/or power. In a CAT5 distribution system it is usual to send composite video and stereo analogue audio to the secondary TV sets. For standard-definition TV this is adequate for most users and a big quality
How does CAT5 cable carry AV
01 signals?
CAT5 receiver
secondary TV
CAT5 (and CAT5e) cable consists of four ‘twisted pairs’ of wires in an outer sheath, the whole thing about 5mm diameter. Each pair can carry a specific signal and the two twisted conductors act to cancel out interference picked up along the way. They can only do this because the signals they carry are ‘balanced’, with a positive signal travelling down one wire while a mirror image negative signal travels down the other wire (as opposed to the ‘normal’ technique of a signal down
Glossary CAT5
Computer network data cable with four twisted pair connections originally designed for 100Mbit/s data transfer.
CAT5 cable
HDMI (High-Definition Multimedia Interface) Interface for transmitting uncompressed audio and video digital data, incorporating High-bandwidth Digital Content Protection (HDCP). Used to connect high-definition satellite receivers to TVs.
RJ45
Eight-way modular connector used with CAT5 cable for computer network and AV distribution systems. More correctly called an 8P8C (8 Position 8 Contact) modular connector.
2 What Satellite & Digital TV September 2010
CAT5 transmitter
Scart cable AV source
A simple Scart replacement CAT5 system will extend a Scart connection way beyond its normal reach, and some can be daisy-chained to take the composite video signal to several secondary TVs
abc guide to... improvement over the UHF video and mono audio obtained with UHF co-ax distribution. Special CAT5 systems are available to handle S-video and even component video (at the expense of losing audio transmission), but they have to be dedicated to these formats – you can’t, say, use the link to watch one source using component video and then remotely switch to composite video from another source.
CAT5 receiver secondary TV
Hub systems use a switching matrix to route any of the connected AV sources to any of the secondary TVs
How do I use CAT5 cable?
03 Some new-build homes are pre-wired between rooms with CAT5 cable (ready for a computer network) that can be used for AV distribution. Otherwise, wiring a home with CAT5 is as simple as wiring can be because the cable is cheap, flexible and robust – and easy to pull through inaccessible paths. What’s more, CAT5 can carry AV signals over huge distances without significant degradation (a mile in the case of the Blue Delta Milestone system) so you are not restricted by taking roundabout routes. Ready-made cables can be used or, because the connectors used are normally the same as those used for computer networks – usually called RJ45 (although that designation actually refers to a very similar phone connector). These are cheap and simple to fit with the correct crimping tool. Note that you cannot use the same CAT5 wiring for both AV distribution and a computer network.
CAT5 cable consists of four twisted wire pairs, with standard colour coding for local area network application
How is a CAT5 system put 04 together? CAT5 distribution systems come in two flavours – simple point-to-point arrangements and hub-based systems. Systems like Blue Delta Milestone and the Keene CAT5 Blaster use a CAT5 transmitter at one end and a receiver at the other. Each unit has a Scart socket, so the CAT5 cable can be treated as a very long Scart cable replacement. Several TVs can be fed using a splitter unit or daisy-chained with special tapping receivers.
CAT5 hub CAT5 cable
CAT5 transmitter
AV source
Scart cable
The hub-based systems such as Global’s AViLynx or AVNex’s Nexus use a switching matrix to route different AV sources to different rooms. These systems can cost thousands of pounds, although the AViLynx is aimed at the home market and more affordable. What about control?
05 Most CAT5 distribution systems use the fourth twisted pair to supply power to the transmitter or receiver at the far end of the cable and to carry infrared remote control commands from an ‘eye’ at the secondary TV back to the source equipment so you can change channel and so on. Other control features can be implemented too. The Milestone system extends the Scart’s pin 5 switching across the CAT5 link while the AViLynx has its own remote control handset to select between the sources connected to the hub, block control to certain TVs, and other complex functions.
How can I distribute HD signals?
06 The HDMI output from an HD
receiver can be routed to a secondary TV simply with a long HDMI cable. An HDMI output will reach about 15m at a pinch, although cables with built-in line amplifiers are available to reach up to 40m (but cost about £150). Because making your own cables is not practical, actually laying HDMI cables around the home can be both difficult and expensive. Instead, you can again use CAT5 cable, and there are now domestic HDMI-CAT5 converters available to replace an HDMI cable in much the same way as analogue Scart connections can be replaced by CAT5. However, these are more expensive than the composite video/analogue audio equivalents and a simple point-topoint HDMI-over-CAT5 kit costs more than £100. If you want to supply several different TVs then HDMI splitters for two, four, even eight TVs are available but the complex electronics involved to handle the high bandwidth signal and satisfy the content protection system means these are far more expensive than, say, Scart splitters and a four-way unit will typically cost more than £100. When you are going to the expense of HDMI wiring your home you might want to go the whole hog with an HDMI switch matrix that offers three or four separate HDMI inputs that can be fed to any or all of four outputs. Such a unit costs several hundred pounds on its own (without the many metres of HDMI wiring needed) but it provides a level of picture and sound quality to the whole house not found elsewhere n Geoff Bains
September 2010 What Satellite & Digital TV 3