Fibre is playing an increasingly important role in broadcast infrastructures, says Josh Simons The idea of sending video and audio signals down a fibre optic cable, rather than copper, is not new. Despite its many advantages, however, there has been considerable reluctance to its widespread use for internal infrastructures, the interconnectivity inside a broadcast facility. […]
Fibre is playing an increasingly important role in broadcast infrastructures, says Josh Simons
The idea of sending video and audio signals down a fibre optic cable, rather than copper, is not new. Despite its many advantages, however, there has been considerable reluctance to its widespread use for internal infrastructures, the interconnectivity inside a broadcast facility.
That situation is rapidly changing, and fibre is playing an increasingly important role. In some parts of the world Scandinavia, for example fibre has almost completely replaced copper in new installations. Here in the Middle East, there is surprisingly strong use of fibre. I would estimate it represents about 10% of installations today, and that proportion continues to grow.
The fundamental advantage of fibre is that it can carry the high bandwidth signals digital HD calls for upto 3Gb/s over long distances with ease. That is not the case for co-axial cables, which have to be carefully routed to avoid exceeding bend radius not clamped tight to trays and tie bars and carefully terminated with a true 75 ohm tuned BNCs and they will still only be reliable upto around 100m.
Fibre is the better medium for carrying digital video and audio. The question has to move from whether it should be included in an installation towards thinking about how such a system can be managed over its lifetime. This is because just as fibre has big advantages in carrying the signals, it has several disadvantages too, particularly in physical resilience.
A single mode fibre optic cable is a strand of glass 9 microns in diameter an order of magnitude narrower than a human hair. Trapping it, crushing it or forcing it to bend too tightly will break that tiny strand of glass, rendering the cable useless. Effecting a proper repair to a fibre cable calls for a skilled engineer and specialist equipment, so it is often seen as easier to simply replace the cable. Better solutions are required.
What I advocate is that you adopt a systems approach to cabling. Rather than running cables from point to point, which is always a management challenge, design systems that consolidate all signals at an optical distribution frame and run to termination or a patch panels with a multi-way cable.
Patch panels are now a practical proposition. Passive self-normalling fibre panels are now available, as are soft and highly flexible patch cords, so fibre can be treated exactly the way that co-ax has been used up to now.
24-way multi-core single mode fibre cable bundles are readily available for the trunk run from termination point to termination point. They look rather like a black co-ax cable, and this has caused problems in some installations. Engineers in a hurry, particularly from non-broadcast departments like building services, have a tendency to cut cables to re-route them, on the basis that it is a five-minute job with a couple of BNCs to repair them.
Even ignoring the fact that repairing a high-definition co-ax is not recommended, this is of course disastrous if the cable is not a co-ax but a bundle of mission-critical fibre circuits. So up to now, we have been recommending that fibres and multi-core fibres are routed through dedicated containment or at least flexible conduit within a common duct or cable tray. That provides protection from being accidentally cut, and stops the fibres being crushed.
Coming on to the market now are new cables which are fibre bundles inside a flexible micro-duct, providing all the protection required without the need to first lay conduit then thread the fibres through. The jointed rings around the cable give it the required degree of flexibility while being highly resistant to crushing and reducing the cables OD.
Most important, it provides more than enough crush resistance to protect the circuits against damage from other cables being thrown on top of the tray or forced through the duct. One of the models we recommend can withstand a crushing force of 1400 Newtons per metre.
The construction also adds another layer of protection, in that it allows the cable to bend but only to a certain degree, ensuring that the fibres within do not reach their minimum radius so will not break.
This minimum bend radius may be tighter than you expect. Traditionally, the recommendation was to loosely coil fibre in the bottom of the rack rather than form it around rails because tight bends were likely to cause damage. That is no longer the case.
Some years ago, we worked with one of our cable suppliers, Draka, to develop the first bend insensitive fibre. This was designed to be extremely resistant to bend damage, by using a special material for the cable jacket which not only provides extra resilience, it incorporates a special core technology which reflects light back into the fibre, greatly reducing losses.
Such cables are ideal for challenging applications like patch cords, but the same bend insensitivity technology has now spread out into other fibre cables. It means that single mode fibres and fibre bundles can now be formed into tight curves, with simplex patch cables having a minimum bend resistance as low as 5mm.
In summary, then, all the elements are in place to allow fibre to form a significant part of a modern broadcast infrastructure.
Multicore cables can now be formed into neat paths, and provide their own protective ducting to ensure signals are not lost because the fibre is crushed or it is inadvertently cut. These multicore circuits can easily be broken out into individual fibres through optical distribution frames or in the sort of passive, self-normalling patch panels that broadcast engineers find very familiar. The patch cords themselves are extremely resilient and will stand up to the everyday abuse that is typical in a broadcast installation.
To this must be added the key advantage of fibre, which is that it allows 3Gb/s HD video and any future formats like 3D or Super Hi-Vision to be carried with minimal attenuation or interference, and over distances which are effectively unrestricted. Certainly there are no limitations on equipment placement as there must be when co-ax is used.
Together, they allow the system designer to take a fresh approach to connectivity, laying out equipment and operational areas logically and efficiently, then connecting them through fibre cables which are both physically resilient and provide a trouble-free signal path.
