Early System Layouts

In Figure 17.1 we showed an early CATV distribution system (ca. 1968). Taps and couplers (power splits) are not shown.1 These systems provided from 5 to 12 TV channels. An LOS microwave system might bring in channels from distant cities. We had direct experience with an Atlantic City, NJ, system where channels were brought in by microwave from Philadelphia and New York City. A 12-channel system was derived and occupied the entire assigned VHF band (i.e., channels 2-13).

As UHF TV stations began to appear, a new problem arose for the CATV operator. It was incumbent on that operator to keep the bandwidth as narrow as possible. One approach was to convert UHF channels to vacant VHF channel allocations at the headend.

'Definitions:

Tap. A device for extracting a portion of the CATV signal from the cable. Coupler. A device used to combine signals or divide signals.

Power Split or Power Splitter. A device used to divide a signal between or among paths and not necessarily equally.

Satellite reception at the headend doubled or tripled the number of channels that could be available to the CATV subscriber. Each satellite has the potential of adding 24 channels to the system. Note how the usable cable bandwidth is "broadened" as channels are added. We assume contiguous channels across the band, starting at 55 MHz. For 30 channels, we have 55-270 MHz; for 35 channels, 55-300 MHz; for 40 channels, 55-330 MHz; for 62 channels, 55-450 MHz; and for 78 channels, 55-550 MHz. These numbers of channels were beyond the capability of many TV sets of the day. Set-top converters were provided that converted all channels to a common channel, an unoccupied channel, usually channel 2, 3, or 4, to which the home TV set is tuned. This approach is still very prevalent today.

In the next section we discuss CATV transmission impairments and measures of system performance. In Section 17.4, hybrid-coaxial cable/fiber-optic systems are addressed. The fiber replaced coaxial cable trunks, which made a major stride toward better performance, greater system extension, and improved reliability/availability.

Was this article helpful?

0 0

Post a comment