Communication Networks/Cable Television Network

From Wikibooks, open books for an open world
Jump to: navigation, search

The cable television network is something that is very near and dear to the hearts of many people, but few people understand how cable TV works. The chapters in this section will attempt to explain how cable TV works, and later chapters on advanced television networks will discuss topics such as cable internet, and HDTV.

coax cable has a bandwidth in the hundreds of megahertz, which is more than enough to transmit multiple streams of video and audio simultaneously. Some people mistakenly think that the television (or the cable box) sends a signal to the TV station to tell what channel it wants, and then the TV station sends only that channel back to your home. This is not the case. The cable wire transmits every single channel, simultaneously. It does this by using frequency division multiplexing.

TV Channels[edit]

Each TV channel consists of a frequency range of 6MHz. Of this, most of it is video data, some of it is audio data, some of it is control data, and the rest of it is unused buffer space, that helps to prevent cross-talk between adjacent channels.

Scrambled channels, or "locked channels" are channels that are still sent to your house on the cable wire, but without the control signal that helps to sync up the video signal. If you watch a scrambled channel, you can still often make out some images, but they just don't seem to line up correctly. When you call up to order pay-per-view, or when you buy another channel, the cable company reinserts the control signal into the line, and you can see the descrambled channel.

A descrambler, or "cable black box" is a machine that artificially recreates the synchronization signal, and realigns the image on the TV. descrambler boxes are illegal in most places.

NTSC[edit]

NTSC, named for the National Television System Committee, is the analog television system used in most of North America, most countries in South America, Burma, South Korea, Taiwan, Japan, Philippines, and some Pacific island nations and territories (see map). NTSC is also the name of the U.S. standardization body that developed the broadcast standard.[1] The first NTSC standard was developed in 1941 and had no provision for color TV.

In 1953 a second modified version of the NTSC standard was adopted, which allowed color broadcasting compatible with the existing stock of black-and-white receivers. NTSC was the first widely adopted broadcast color system. After over a half-century of use, the vast majority of over-the-air NTSC transmissions in the United States were replaced with ATSC on June 12, 2009, and will be, by August 31, 2011, in Canada.

PAL[edit]

SECAM[edit]

HDTV[edit]