The earliest cable television systems were started out of necessity. The four available broadcast networks sent out their signals through a series of tower antennas, which meant that individual television sets had to be in a direct line of sight for best reception.
Those who lived in low-lying areas or mountainous regions would only receive weakened signals or no signal at all. Some communities began placing large antennas on a mountaintop and stringing cable lines to individual televisions. There was no organized cable company to speak of- residents were free to set up their own antennas or tap into an existing line.
The problem with this informal cable system is that the signal would weaken over distance and homes located too far from the antenna would not benefit at all. Signals would have to be boosted electronically every thousand feet or so. Since most homeowners were not capable of maintaining all of these boosters and other paraphenalia, private companies formed to maintain community cable lines.
The main signals were still coming from broadcast television antennas, but reception was improved by amplifying the signal coming through the cable itself. Since this service did involve some investment of time and equipment, individual users were eventually asked to become paid subscribers. Money generated from monthly fees would go towards technical improvements, maintenance, and the eventual addition of channels.
Modern cable companies still provide television signals through a coaxial cable, along with other services such as internet access and digital music
channels. Microwave receivers have replaced the antennas on the mountaintops and many of the lines have been replaced with fiber optics, but the same basic system is still utilized.
First of all, it helps to understand the principle of ‘bandwidth’. Imagine an older television set with a spinning UHF tuner. You may be aware that Channel 43 shows movies and Channel 61 shows cartoons and Channel 25 shows public television programs. Even if you couldn’t read the numbers on the dial, you could turn the dial until a station becomes tuned.
Within a few minutes, you’d probably realize which channel you had found based on the programming. Each television channel receives permission to broadcast its signal on a specific wavelength assigned by the FCC. Once your tuner reaches this frequency, the television takes those signals and converts them to a watchable picture with sound. Because no other station exists between Channel 25 and Channel 43, your tuner will only receive static.
Once it finds Channel 43, the picture returns. In theory, there could be a channel 14, 15, 16, and so on. Each channel would be assigned a specific frequency (bandwidth) but the tuner would only send one signal to the television set at a time. All of these channels would continue to broadcast a signal, but only the tuned channel would appear on the television screen.
This is the principle behind a cable television system. The FCC assigns every channel a specific section of bandwidth approximately 6 megahertz in size. All of these channels broadcast a constant signal at their assigned frequencies, much like the Channel 14, 15, 16 example above. Coaxial cable can transmit all of these signals simultaneously, much like a phone line can handle many conversations at once. The cable company sends the signals they receive from satellites or broadcast towers through the coaxial cable and eventually into individual homes.
The coaxial cable is plugged into a special box provided by the cable company or directly into a television considered ‘cable ready’. Essentially, the box contains a series of tuners and descramblers which can focus on individual channels. If the viewer wishes to see CNN, for example, the cable company assigns a specific channel number.
Once that number is dialed in, the tuner receives only the signal generated within the specific frequency reserved by the owners of CNN. These signals are converted by the television into recognizable images and sound. If the tuner were even slightly out of line, it might pick up signals from the channel to the left or right of CNN.
These bandwidth assignments were created at a time when most television signals were analog, not digital. Through the advances of fiber optics and a device which converts analog signals into digital signals, it is now possible to add more channels within the 6 Mhz bandwidth. Owners of the original cable channels can either create more programming of their own or allow others to use these new channel frequencies.
Theoretically, cable systems could support thousands of individual channels. The cable boxes would be able to tune into very specific frequencies, and the information can be sent through digital signals instead of analog. Analog signals work on a principal of controlling voltage on a wavelength. It takes a lot of energy to keep an analog system functioning efficiently.
Digital systems take less energy to receive and broadcast signals, so they take up less space on a traditional coaxial cable line. Viewers might not immediately notice a difference, but digital signals tend to be more stable once they have been processed by the cable box’s tuner.