Communication Networks/Analog and Digital TV

Coaxial Cable

Coaxial cable has an incredibly high bandwidth (compared to twisted pair), and it distorts very little over long distances. For this reason, coax is able to carry a large number of analog television channels to a very large audience.

Bi-Directional Cable

The original implementation of the television network only needed to move data in one direction: from the station to the homes. For this reason, a number of amplifiers were installed in the network that take the signal from the base station, and amplify that signal towards the homes. However, a problem arises when cable internet users want to transmit data back to the base station (upload).

The original cable TV network had a very large amount of available bandwidth, but it wasn't designed to transmit data from the user back to the network. Instead, the entire network was set up with directional amplifiers, that would amplify data going to the user, but wouldnt affect data coming back from the user.

HDTV

HDTV is the next generation of television, and actually allows better resolution, larger frame size, and lower bandwidth than the traditional analog signals. Also, digital signals are less prone to cross-talk between channels, so channels don't need to be spaced as far apart in the frequency domain as analog signals are.

This chapter will discuss the next generations of the cable TV network.

Channels

The TV channels in an analog TV scheme carry channels spaced every 6 MHz from about 150 MHz to 500 MHz. Below 150 MHz was considered originally to be too susceptible to noise, and there was simply no need to expand above 500 MHz. However, with the advent of cable internet, the system needed to be revamped.

Bandwidth

A new band was set aside, from 55 MHz to 75 MHz, to allow traffic to be uploaded from the user. Also, another band was set aside, from 550 to 750 MHz to allow for cable internet downloads. A cable modem would be able to demodulate these two bands of data, without interfering with the TV signal.

Problems

200 MHz of download bandwidth seems like a lot, but every household on a given line (and there could be 100 or more) all need to share this bandwidth, which can slow down the system, especially in heavily populated areas.