For a broadcast engineer, choosing the big equipment is always exciting. Even comparing specifications and functionality on core products like routers or DAs is interesting.
Then you have to connect it all together – and the temptation is just to reach for the nearest drum of co-ax and bag of BNCs. There surely cannot be much fascination in comparing cables and plugs, can there?
Today, as it has been for decades, the co-axial cable connects the majority of broadcast equipment. In our experience, a typical studio gallery or MCR contains around 10km of co-ax, along with around 5000 BNCs. That adds up to close to half a tonne, by the way.
More to the point, perhaps, the direct cost of the cable and connectors, if you use the high quality HD products we would recommend, would be somewhere around £7500. Compared with the equipment around it – the vision mixer, the graphics generator, the central router – this is a very small sum of money indeed.
Yet some would try to shave a few percentage points off the cost of the cable, by choosing what we would regard as an inferior product. In the context of any size of installation, the amount you save is tiny. But the potential risk is enormous.
Think about what a co-ax cable is for a moment. It has a solid copper conductor which carries our video signal, at up to 3Gb/s which is radio frequency. Wrapped around that is the dielectric, which today is commonly a foamed polymer. Wrapped around that is the earth conductor, which protects the core from interference as well as providing the return half of the circuit. And finally the whole is wrapped in a protective jacket.
The only way to bring down the price of a cable is to change one of those four elements.
Copper is a globally traded commodity, so every manufacturer pays the same price and uses the same metal in every range. To reduce the cost, your only option is to reduce the volume of copper you use per metre of cable: make the conductor thinner. Not only does that increase the end to end resistance, it makes any inconsistencies in the diameter more significant, resulting in impedance changes. It also makes it more fragile.
The dielectric is a critical part of the cable’s performance. Sending digital signals, and especially HD, down co-ax cables is a challenge, and one that works because the capacitance of the cable is absolutely consistent. It should be well known by now that you cannot put sharp bends into digital HD cables, or pinch them with tight cable ties, because that changes the capacitance of the cable at that point, which reduces its ability to deliver digital signals transparently.
The best dielectric is air, so when we talk about foamed dielectrics we mean a plastic compound which contains air bubbles trapped in a rigid plastic. The air bubbles have to be distributed evenly throughout the material for consistent capacitance, and they must not break down over time so the plastic must be completely stable at all normal temperatures and environments. Use inferior materials and the cable becomes unpredictable at best: you might get away with a 100m run, or you may find that the digital signal is lost after 50m.
In the best cables the earth screen is a conductive foil around the dielectric, surrounded by a braid of tinned copper. You can save money by not including the foil. You can use less copper in the braid by not making it so tight, so that there are gaps in it rather than wrapping the cable completely. The quality of the braid wrap is traditionally measured in the percentage of overlap. You need to ensure it is 90% or more. Our IMAGE cable for digital video has a 95% braid wrap.
The braid is also the primary source of mechanical strength in the cable, both in the run of the cable and to attach it to the BNC connector. If a cable gets accidentally tugged, the last thing you want is for it to simply pull out of the plug.
Finally, the outer jacket protects the cable. You need it to resist the accidental damage that can easily happen in a typical installation. If a cable gets trapped when you are pushing a piece of equipment back into a rack, you do not want to have to pull it out and replace it because the outer insulating jacket has got damaged.
The same considerations apply to connectors: there really are technical reasons why some connectors are more expensive than others. The BNC – the Bayonet Neill-Concelman connector, named after its two inventors – has been around for 60 years, being patented in 1951. It has survived the move from analogue to digital and from SD to HD, and even now replacements are only being considered for high density applications.
We talk about the broadcast BNC has being a 75Ω connector, but in the past we have got away with a very approximate impedance. Digital HD, and certainly 3Gb/s infrastructures, is much less tolerant, and so you need to choose a connector which has a true 75Ω performance.
You also need to consider how the connector fits onto the cable. There is no point in carrying the precious digital payload carefully down the length of co-ax if you lose it all because the BNC does not match the dimensions of each element of the cable so cannot give a solid termination without insertion loss.
As well as picking the right cable and the right connector, you need the right tool to fit one on the other. Grabbing a pair of pliers to crimp the connector is not a good idea: you will damage the conductor, adding an impedance pinch point. Similarly stripping the insulation back with a knife means you may create an unscreened section of the cable, again introducing unnecessary risks to the signal.
My recommendation is simple: talk to a good specialist cable supplier, and listen to their recommendations. At Argosy we carry out extensive testing, with our own facilities as well as those at our manufacturers. We know what works and what does not, and can give you solid advice.
We stock a range of materials from different manufacturers, to meet your needs. As the digital HD revolution started we realised there was a need for a new type of cable, which would provide the high specifications needed, and we commissioned our own from Draka, our IMAGE range. We can also supply cables from Belden and other vendors if you prefer.
We will tell you what cable types we believe you need for your application, and we will ensure that you have connectors and tools to match. Cable and connectors are our mainstays, and we take them very seriously indeed. Trust your supplier and you should get a trouble-free installation: the risks are too great to compromise.