The Glass Cable

In 1976, a new sort of cable system debuted. This system used
fiber-optic cable for the trunk cables that carry signals
from the CATV head-end to neighborhoods. The head-end is
where the cable system receives programming from various
sources, assigns the programming to channels and retransmits
it onto cables. By the late 1970s, fiber optics had
progressed considerably and so were a cost-effective means of
carrying CATV signals over long distances. The great
advantage of fiber-optic cable is that it doesn't suffer the
same signal losses as coaxial cable, which eliminated the
need for so many amplifiers. In the early fiber-optic cable
systems, the number of amplifiers between head-end and
customer was reduced from 30 or 40 down to around six. In
systems implemented since 1988, the number of amplifiers has
been further reduced, to the point that only one or two
amplifiers are required for most customers. Decreasing the
number of amplifiers made dramatic improvements in signal
quality and system reliability.

Another benefit that came from the move to fiber-optic cable
was greater customization. Since a single fiber-optic cable
might serve 500 households, it became possible to target
individual neighborhoods for messages and services. In the
1990s, cable providers found this same neighborhood grouping
to be ideal for creating a local-area network and providing
Internet access through cable modems.

In 1989, General Instruments demonstrated that it was
possible to convert an analog cable signal to digital and
transmit it in a standard 6-MHz television channel. Using
MPEG compression, CATV systems installed today can transmit
up to 10 channels of video in the 6-MHz bandwidth of a single
analog channel. When combined with a 550-MHz overall
bandwidth, this allows the possibility of nearly 1,000
channels of video on a system. In addition, digital
technology allows for error correction to ensure the quality
of the received signal.

The move to digital technology also changed the quality of
one of cable television's most visible features: the
scrambled channel.

The first system to "scramble" a channel on a cable system
was demonstrated in 1971. In the first scrambling system, one
of the signals used to synchronize the television picture was
removed when the signal was transmitted, then reinserted by
a small device at the customer's home. Later scrambling
systems inserted a signal slightly offset from the channel's
frequency to interfere with the picture, then filtered the
interfering signal out of the mix at the customer's
television. In both cases, the scrambled channel could
generally be seen as a jagged, jumbled set of video images.

In a digital system, the signal isn't scrambled, but
encrypted. The encrypted signal must be decoded with the
proper key. Without the key, the digital-to-analog converter
can't turn the stream of bits into anything usable by the
television's tuner. When a "non-signal" is received, the
cable system substitutes an advertisement or the familiar
blue screen.