Crystal Clear Satellite and Television installation services

Digital Television Using Optical Technology

Community Installations

Now available for community systems is the latest technology to be able to run cables for miles with no interference to the signal using fibre optic cable that will not rot or rust inside the protective robust and flexible sheath.

Technology

Just as digital electrical connections use pulses of electricity down a metal cable, fibre optics relies on light pulses down a single fine fibre of glass inside a protective sheath. Electrical signals are turned into light by a laser and focused into the fibre at one end. At the other end, a detector converts the light pulses back to an electrical signal.

Although light normally travels in a straight line only, the light can follow bends in the fibre because of ‘total internal reflection’. For the light, the fibre is like a hollow pipe and it operates just like a speaking tube, or, indeed, the waveguide sections of a satellite antenna.

To work effectively the glass used must be very clear and very thin. FibreMDU uses the thinnest ‘single mode’ fibre, only about 8um in diameter, which allows through only one light path, unlike cheaper, thicker multimode fibres that allow the light to travel in many paths through the pipe resulting in some mutual interference along the way and some distortion of the signal.

Fibre cable

A fibre optic cable uses a single ‘conductor’ and is completely immune to electrical interference, you can run these alongside a mains cable for miles and there will be no interference to the signal. The fibre will not rot or rust and inside the protective sheath is both robust and flexible.

The fibreMDU system uses fibre cable in six layers. Stripping it apart, the smallest part that you can easily see is the central core, which itself contains the actual fibre embedded in a plastic coating. Around this is a soft plastic buffer layer, surrounded by wound steel armour. Next out is a layer of Kevlar strands for strength with a PVC jacket protecting the lot from the elements. The whole cable is less than 3mm diameter, very easy to carry around, bend around corners and thread through inaccessible parts of a building.

Most impressive of all is the almost complete lack of ‘losses’. Only a tiny amount of signal is lost along the cable, in the order of 0.3db a kilometre, compared with about 0.3db a meter for normal satellite co-ax. So you can lay cable runs of hundreds of metres without even thinking about amplifications.

Optical LNB

The optical Universal LNB looks much like any Astra-type LNB (albeit a pretty large one), but this is where the fibre technology starts, as close as possible to where the satellite signal is first received.

A bit like a Quattro LNB, the optical Universal receives all four sub-bands, vertical low band, horizontal low band, vertical high band and horizontal high band, at the same time. It does not output these through four separate outputs each across the frequencies 950-2150MHz but stacks them one above the other in one band of 950-5450MHz and outputs the combined signal from the laser through the optical connection.

Of course, the LNB needs electrical power for all this and it cannot get that from the receiver (because the fibre optic cable does not carry electrical power) so the LNB has two connections, the FC connector for the fibre optic cable and a standard F-connector for a 12V input. That 12V (at about 400mA) is provided by a separate (and tiny) mains power supply that connects to the other end of the power cable.

So if you are converting an existing electrical system to fibre optics you can use the co-ax cable already in place to carry the power to the LNB.

Converter boxes

To be any use for the receiver by the TV, the optical signal from the LNB has to be turned back to an electrical one like those from ‘normal’ LNBs. This is done by special converter boxes that act like a virtual LNB.

The converter boxes have a maximum input level of 14dBm in the standard mode while the LNB outputs + 7dBm. This means that if you are not optically splitting the signal many ways or using several kilometres of optical cable, you need to insert an optical attenuator at the converter box input to avoid dazzling the converter with an over-powered optical signal. It needs sunglasses to see properly in the sunshine of the optical LNB.

Performance

Putting together an optical system is, if anything, simpler than an electrical one, having no cable losses to worry about and access to the entire received spectrum at all points.

Once it is all together it works staggeringly well. Signals of the right level and with no added noise just fall out of the converter box pretty much regardless of what lies between it and the LNB. This makes it perfect for multi-home systems, especially those stretched over large buildings or many buildings. This is where it starts to pay for itself, saving on expensive copper cable, multi-switches, amplifiers, equalisation stages, the higher installation costs, earthing costs and a more pricey design stage.