Wednesday, December 8, 2010

What would cable TV be without the cable?

Look behind your TV set and you’ll probably find one.  It all seems so simple now.  Take a wire, surround it by a dielectric, insulating shield, surround that by a conducting shield and surround the whole thing by a plastic insulator.  It was not so simple on December 8, 1931 when Messrs. Lloyd Espenschied of Kew Gardens New York and Herman A. Affel of Ridgewood New Jersey were awarded U.S. Patent No. 1,835,031 for a “Concentric Conducting System.”  Per their employment agreement, they assigned their patent to the American Telephone and Telegraph Company. 

Telephone calls were sent down a “twisted pair” cable.  Transmitting one call was reasonably easy.  All you needed was a long enough pair of wires and a signal that was amplified enough to overcome the electrical resistance of the long wire.  One wire in the twisted pair carried the signal while the other wire was grounded.

Sending 10 calls or 100 calls on a twisted pair was only slightly more difficult.  The signals for each of the calls were divided into short slices of time on one end.  Each slice was given a turn on the wire and all you needed was a “carrier” signal of sufficiently high frequency to time them so that they could be reassembled on the other end.  Here is the problem:  more calls require higher frequencies – and the longer the cable, the more the multiplexed signal would bleed off to ground.

What was needed was a low capacitance cable, in which the interaction between the carrier signal and ground could be minimized.  If you have that, you can send not only telephone calls but many TV signals – and of course, many internet signals at the same time.  That's where the coaxial cable comes in; without which, even this blog would not be possible.

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