TV-BAY Magazine

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BNC Is it at the end of the line? by Robin Palmer T he familiar BNC connector has been around a long time. It was invented in the late 1940s, primarily for military radio frequency and microwave interconnections. The initials derive from the style plus the names of the inventors (Paul Neill and Carl Concelman): Bayonet Neill– Concelman. The original development work was done by Octavio M. Salati so maybe a more appropriate name for the connector would have been BOMS. The BNC was probably widely accepted into the broadcast television industry because of its locking feature made possible by the bayonet lugs on the female connector. Also the connector’s (at the time) relatively small size. The RF boys prefer 50 ohms but BNC (having a 75 ohm variant) better suited baseband video and the power requirements in distribution amplifiers in those early days. For the next four decades until the1990s, the whole world sent its studio video signals via 75 ohm cable connected up with BNCs, save for a few weird MUSA patch panels left over from the second world war. If you mixed up 75 with 50 ohm connectors because they look the same, it did not matter at baseband video frequencies. It all just worked. Then came digits: digital television at standard definition, initially on horrible 25 pin parallel connectors. Advances in chip technology allowed serialisation and transmission over 75 ohm cable. What could be more convenient than to use the old BNCs again? At standard definition the 270 MHz SDI clock speed is well within the BNC’s acceptable parameters. However, to handle this the coax cable had to a lot better than the usual baseband video cable. Engineers now struggled when many BNCs were packed together on the back plates of some equipment bays. The BNC did not seem miniature any more but it was the standard. Once digital TV advanced to high definition in the mid 2000s, further developments in chip technology took digital TV signal serialisation to 1.5 GHz via HD-SDI. This allowed a 10 bit 4:2:2 in any HD format to be carried over a single 75 ohm coaxial cable (but much higher spec), still with the same old trusted BNC at the end. It you wanted to connect 4:4:4 RGB, there was no bandwidth left for the extra bits. So the initial solution was dual-link with two cables and connectors for each signal path. More recently the chip wizards have managed to serialise at 3 GHz. This standard is confusingly referred to as 3G, not to be confused with third- generation mobile phone technology. Now 3G serial video has all the bandwidth for 4:4:4 RGB and higher frame rates too in 4:2:2. With this format the cable and connector are much more critical and sometimes does not even work. Cable length is now becoming restricted at 3G compared with SD and baseband. The old BNC is really not fit for purpose above 3 GHz and a 3G serial signal does have some harmonic content above this which can leak out of the BNC slots. You certainly can’t get away with naughty tricks like T pieces, double or open terminations. Even BNC barrels used to extend a cable run can spoil the serial digits if they date from the pre-HD-SDI era. A big problem is that all BNCs look the same but can have very different signal characteristics at the very frequencies in use today. The emerging products for 4K sport four BNCs per input or output because that huge payload of serial digital TV needs a quad–link HD-SDI. So what future for the humble BNC? Could it ever handle 4K? Well maybe with mild data compression, but why bother and have more confusion. Is there even a future for copper here? With optical connections there is enormous bandwidth for future television standards and all those of the past: all on one link. There was a time when the BNC (the 50 ohm one) was also used for 10BaseT data networking. However the future of studio television signal surely has to be on 10 GHz Ethernet which is already available and low cost. All it needs is a new metal lockable connector a bit like a BNC but tiny and without the venerable legacy. Robin Palmer is Managing Director of Cel-Soft and is fascinated by solutions for 3D & TV quality control and measurement technology. 40 | TV-BAY MAGAZINE TV-BAY076APR13.indd 40 26/03/2013 16:47