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Is it at the
end of the
line? by Robin Palmer
T he familiar BNC
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