As more broadcasters gear up for HD 1080p or 3D transmissions, cables that can support 3Gb/s will be in high demand. Josh Simons of Argosy offers some suggestions for those considering 3G solutions.
Over the last couple of years there has been a great deal of talk about 3Gb/s infrastructures. The original thinking was to support 1080p HD signals, but the recent rise in interest in stereoscopic 3D television may also need 3Gb/s transmission, to include both “eyes” as conventional HD signals in a single stream.
But despite all the talk of 3Gb/s, there is still relatively little equipment out there which supports it. And those pieces of equipment that do already exist vary considerably in quality. The challenge for the 3Gb/s infrastructure is that this is a very high data rate indeed, and if a single part of the chain is not performing at the highest level — or even is not perfectly matched to the rest of the system, even if its individual performance is fine — then there will be problems. Structures that should work will lose data, which in a broadcast application means the feed will stop.
Important points for consideration are the equivalent cable length reproduced by the PCB track between the input interface and the EQ point, essentially, the link between two pieces of equipment is not just the cable.
At the sending device, it is the line driving chip and how robustly it delivers a sharply differentiated digital signal that matters. At the receiving device, it is the ability to detect the digital signal and re-clock to it. Our tests suggest that some combinations of equipment perform better than others.
The solution for the systems engineer is to do a lot of research, not least to try to determine which chip vendor is providing the send and receive on each device, as matched chips seem to perform better. Other than that, there is no alternative but to test equipment in context, not just on the bench but in a real installation in combination with the other equipment likely to be used.
That is fine when you are building a complete system today, but in reality most people are simply preparing for the 3Gb/s future. By installing any new cabling to the right standard, you should be prepared.
My first recommendation would be to set a house standard. Choose a good quality cable and matched connectors, and decide how far your maximum connectivity will be. You may not want to risk as much as 90m on IMAGE 360: you might set a lower limit at, say, 60m and then step up to larger cables IMAGE 1000 and 2000. Any runs beyond this and you will go to fibre.
Second, install the cable to the highest standards, adhere to the cable bend radius sharp bends have to be avoided when looming. Crushing the cable will dramatically alter its capacitance at that point, which will have a significant effect on its ability to transmit what are in effect RF signals. So clipping cables ties up tight is, again, is bad practice. A good alternative to nylon is to use Velcro which is a softer material and has a wider foot print. Make sure the BNC connector is a good match to the cable.
Your supplier should be able to recommend which plug goes with which cable type. Draka and Belden tune their cables to certain brands of connector. And make sure that your tools are in perfect condition: dull cutters or worn crimp tools will, again, distort the insulation layers and affect the performance. New tools are very inexpensive, so replace them regularly if you are working to 3Gb/s standards.
And the final recommendation is to keep testing. Test each cable link as it goes in, then when the new equipment arrives test it carefully, on the bench and in the rack. It may seem like a lot of work, but putting the effort in should ensure that 3Gb/s installations remain trouble-free.
Draka’s range of HDTV coaxial cables certified 3G compatible
A recent test by measurement specialist Tektronix found that a few key cable suppliers in the market place complied fully with SMPTE 424M requirements for 3G compatibility.
For broadcast engineers, HD SDI 1.5 Gb/s operation is now common practice so many broadcasting units are presently looking at 3 Gb/s as the next step. Cable manufacturer Draka Communications emerged a clear winner in this category.
“With an increasing demand for HD transmissions broadcasters are demanding guarantees of 3G compatibility right down to cable and connector behaviour,“ says Oli Hentschel, Draka product manager, Studio Broadcast. ”We recently concluded a series of thorough tests for our entire range with the leading measurement expert, Tektronix , who certified that our range of HDTV cables are fully in line with SMPTE 424M requirements.”
The practical tests performed by Tektronix offer both a guide and tool for broadcast engineers to rely upon claimed maximum transmission distances for 3Gb/s operations. Extensive tests were performed in the U.S. offices of Tektronix using a signal transmitted at 1080P HD maximum resolution, revealing that the Draka cables meet 1080p requirements with ease end even outperformed expectations.
Following the 1080P benchmarks, Draka HD Pro 1.0/4.8AF cable, with a calculated transmission length of 80 metres, in practice tested with a maximum application length of 150 metres. Draka HD Pro 0.8/3.7AF cable has a tested transmission length of 120 metres (instead of a 50 metres calculated application length). The full table of calculated application length compared with 3Gb/s HD 1080P actual maximum application for all Draka HDTV cables is available at the end of this release.
Argosy builds image
Argosy, a UK-based international supplier of HD broadcast cables and studio infrastructure products has a collection of video, audio and power cables in its portfolio, including those made by Belden and Draka, that are suitable for 3Gb/s applications. Its range of IMAGE HD video cables is manufactured by Draka to Argosy’s own specification. Surpassing the requirements of SMPTE 424M (1080p/60) — and suitable for 3Gb/s installations — the IMAGE range is widely used in worldwide HD installations and has one of the most enviable track records in the industry. IMAGE cables are designed to maximise operational bandwidth while offering the lowest possible return and attenuation loss.