User Guide: Don't cut the cable - coax is still going strong
Cabling for TV
"Cabling, widgets and glue" for routing AV signals have never been more important to the broadcast industry: and that includes good old coax cable, as Adrian Pennington discovered when he spoke to Canford's Iain Elliott.
Celebrating its fortieth birthday in 2016, Canford has certainly seen change throughout its existence: the difference now being the speed of change. Its broadcast customers range from equipment end-users, studio technicians, studio system integrators and OB truck builders.
“In essence, the routing of signals around a building or on location boils down to the loss of performance in the equipment at the receiving end,” says founder and chief designer Iain Elliott.
Attenuation is the power loss caused by a coax cable. The longer the coax, the greater the loss, but the loss is also frequency dependent, broadly rising with frequency (or waveform degradation).
Until recently different vendors used different tests to measure their equipment’s performance and the industry lacked a direct comparison between them.
Canford began in the analogue domain and encountered the first digital explosion with the AES digital audio specification for 110 ohms. On introduction in 1985 it caused some concern about how far a signal could travel without debilitating attenuation, an issue long since alleviated.
“Exactly the same concerns occurred around SDI and now the hot topic is how far a SDI signal will go over 6G coax,” observes Elliott.
Until very recently there was no standard definition of what signal loss was acceptable to receiving kit for 6G. That meant different vendors used different tests to measure their equipment’s performance and the industry lacked a direct comparison between them.
Now that SMPTE ST-2081 has been published these assumptions can be laid to rest. Accurate “what’s the maximum run length” figures can now be calculated for each design of coax cable, although manufacturers may or may not include safety margins which makes direct comparisons less straightforward.
“When 6G equipment first emerged I did some calculations based on the SMPTE 3G standard and basically found that you can’t go much more than 50 metres without having to use very large copper cable,” says Elliott. “That makes it very expensive and difficult to handle and terminate.”
With the increasing pace of the move from specialist broadcast cables and connectors to universal standard Category cables (Cat6 and Cat6A) and fibre, the product ranges Canford carries to support this technology are changing fast. But these changes always have knock-on effects. Finding an RJ45 connector large enough to fit onto Cat6A is not trivial, but exactly the sort of problem solving that is behind much of the firm’s product sourcing.
While the bulk of cable is installed, deployable Category cables are now required to support location production. “Category cables were not designed to be flexible,” says Canford. “They don’t lie flat on the ground because of the necessity to fix the geometry of the paired wires in relationship to one another.”
Canford was the first to introduce a truly deployable Cat5E cable that lies obligingly flat on the ground like a mic cable. “That cable proved extremely successful, but inevitably some users then requested a deployable Cat6, so we did it,” he adds.
“But the principal method of attaining successful transmission of higher data rates is the accurate retention of the cable geometry, which is a direct conflict with achieving flexibility. A deployable Cat6A looks very challenging to produce economically. But we’ve already started the trial development work.”
Japanese group Senko also have DIY fibre termination kits suitable for location work, “another evolutionary product”. Canford sales of Cat6A are predicted to be significantly higher next year, based on a notable hike in interest now, but this does not signify a collapse in the market for coax.
“Coax is still there because there’s a broader market beyond broadcast in industrial, signage and education that wants BNC or Micro-BNC connectors and SDI coax. IP is coming, but for the vast majority of people there is a long life in SDI and coax. It’s interesting to note that the classic BBC PSF1/3M coax cable has refused to die, even though it’s now a pretty inefficient design.”
For the majority of co-axial connectors, it is critical to have a connector that is specific to the cable to be used. Most connector manufacturers use a cable group coding system, but they are all different, as might be expected.
Since a substantial number of Canford’s original range of co-axial cables were BBC designs, it is logical therefore to use the BBC’s own classification, extended to cover newer designs from Canford’s own range.
When ordering connectors, it is essential to check that the correct crimp die is being used when making crimp terminations. Because there are small detail variations in similar connectors from each manufacturer, different crimp dies may be needed.
A similar looking connector from another manufacturer, on the same size cable, does not guarantee that the crimp die regularly used will still work.
While Elliott highlights products from Draka (manufacturers of the Canford SDV series cables) and Belden as the dominant quality brands “you should not find any difference in performance between them. It’s understood [among manufacturers] that you don’t design a cable that needs a new connector but stick to a standard family of sizes of which 0.6/2.8 and 1.0/4.8 are the two most common.”
The laws of physics
“The laws of physics command that there is only so much you can achieve with current cable technology,” says Elliott.
“You could gain an advantage by having a slightly larger centre wire but when manufacturers need to get higher precision in BNC performance, they tighten the tolerances and so constrain the centre wire closer to the primary specification.”
One technique that has improved cable performance is the use of gas injection for producing the physical foam dielectric of coaxial cables.
For the highest level of performance you have to get a very consistent formation in the dielectric.
The ideal dielectric would consist of a vacuum, but the practical need to insulate and accurately space both the inner and outer conductors demands a dielectric material with tough physical and electrical characteristics.
Traditionally, that was made with a chemical mix injected into the cable, but newer methods made of nitrogen gas deliver a more consistent performance. This is particularly beneficial where a coaxial cable is exposed to crushing, squashing, and several bending manoeuvres during installation. The gas-injected dielectric also ensures a better life span and stable attenuation values.
“However, it requires a very, very expensive gas injector in the manufacturing process. For the highest level of performance you have to get a very consistent formation in the dielectric. Beyond that, there’s not much
more you can do.”
The 24G SDI roadmap
The seemingly hasty arrival of IP networking could see portions of the industry leap like lemmings into investment, but broadcasters could also choose to upgrade their SDI; for which the roadmap includes a 24G standard capable of 4K at 120 frames a second.
Meanwhile, the IT industry is upping bandwidth at such velocity that, already, technologies of 25GbE, 40GbE and even 100GbE are emerging with the cost reducing every day. Imagine Communications, which is outfitting Disney/ABC with IP, has ‘proof of concepts’ in its labs with 40GbE and even 100GbE backbones.
“IP networking offers a lot of different possibilities to the discrete video channels of coax but much of that is down to the capital kit that an operator decides they need for type of work they are going to do,” says Elliott.