Filling Up the TV Dial
Much of the discussion about cable TV these days revolves
around various digital issues. Even where true DTV is not available, the cable companies
have been able to use digital techniques to maximize the number of channels they deliver.
But the truth is that most signals on cable are still analog, and that's what most of us
watch.
But even with analog, it's hard not to be impressed at the
cable companies' ability to deliver such a large number of signals using what is
essentially primitive technology. That's not to suggest that the signals are always as
pristine as they might be, but it can't be denied that the cable outfits succeeded in
providing far more variety than the North American television system was designed for.
For reasons both historical and technical, the signals
delivered to your TV set's input terminals by an antenna and by cable are very different,
even though they may carry the same programming and connect to the same jack on the back
of your TV or VCR.
A television signal occupies an enormous amount of the
radio spectrum. When the North American TV system was set up, it was impossible to find a
chunk of bandwidth big enough to hold 12 channels on adjacent frequencies. As a result,
the VHF band occupies three fairly widely spaced chunks of the radio band: channels 2-4,
then a gap, channels 5 and 6, another, bigger gap, then channels 7-13. It's because of
this discontinuity that channels are identified by arbitrary numbers rather than their
frequencies, and the click-stop action of the original tuners simply jumped over the gaps.
Originally, cable was designed simply to deliver these channels, so the frequencies of
2-13 are the same over cable or over the air to this day.
It didn't take long for those channels to fill up, however.
For broadcasters, the UHF band was established at a much higher frequency, but cable
systems soon realized that UHF signals didn't travel well over their wires. But although
the gaps in the VHF band were filled with various communications services over the air, on
the cables they were blank.
Extra channels could be inserted in these spaces, and in
the frequencies just above and below the VHF band proper, and be carried by cable with no
problem. Some companies simply numbered them from 14 upward, others gave them letters, but
they had nothing whatever to do with the original frequencies of the material they
contained.
Originally, to receive these extra channels, you needed an
external converter (which the cable companies would happily rent you for a monthly fee).
The first of these were unwieldy affairs that consisted of a control box that looked like
a berserk concertina, connected by a wire to a box that you tucked behind the television.
The maximum number of channels this rig could handle was 30, but in those days it seemed
inconceivable that there would ever be enough channels to use up that capacity.
Converters became more sophisticated over time, but they
remained a necessity for a number of years. It was only when cable started to take off in
the United States -- long after Canada was heavily wired -- that the manufacturers began
to build tuners into their TV sets that could access the cable channels directly. They
varied, however, as to how many of the extra channels could be tuned, which is why some
older sets can't pull in all the signals being offered today.
By my count, there are 80-odd analog channels available at
the moment on my local cable system, which is modest by some standards. My receiving
equipment is probably typical: a monitor and VCR, each about five years old. Both can tune
up to 125 channels, but presumably some spacing has to happen, so I don't imagine it would
be long before these were effectively used up.
The answer is digital. Satellite systems pioneered the use
of data reduction to cram an enormous number of channels into a relatively narrow bit of
bandwidth, and terrestrial cable -- both wired and so-called "wireless" -- has
recently followed suit. There are some 50 digital channels available locally, and to get
them, we're back to the old set-top box.
Eventually, all that will be irrelevant. Digital cable is
designed to be viewed on analog televisions. Real digital television (DTV) uses a
completely different system, and that is undoubtedly what we'll all be watching.
Eventually.
...Ian G. Masters
ian@mastersonaudio.com
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