Cable TV
Cable TV sometimes called CATV is a system
of delivering the TV signal to home receivers
by way of a coaxial cable rather than over the
air by radio wave propagation. A cable TV
company collects all the available signals arid
programs and frequency multiplexes them on
a single coaxial cable that is fed to the homes
of subscribers. A special cable decoder box is
used to receive the cable signals, select the de
sired channel, and feed a signal to the TV set.
CATV Background
Many companies were established to offer TV
signals by cable. They put up very tall high
gain TV antennas. The resulting signals were
amplified and fed to the subscribers by cable.
Similar systems were developed for apartments
and condos. A single master antenna system
was installed at a building, and the signals were
amplified and distributed to each apartment or
unit by cable.
Modern Cable TV Systems
Today, cable TV companies collect signals and
programs from many sources, multiplex them,
and distribute them to subscribers (see Fig.
1). The main building or facility is called
the headend. The antennas receive local TV
stations and other nearby stations plus the spe
cial cable channel signals distributed by satel
lite. The cable companies use parabolic dishes
to pick up the so called premium cable chan
nels. A cable TV company uses many TV an
termas and receivers to pick up the stations
whose programming it will redistribute. These
signals are then processed and then combined
or frequency multiplexed onto a single cable.
The main output cable is called the trunk
cable. In older systems it is a large, low loss
coaxial cable, although newer cable TV sys
tems use fiber optic cable. The trunk cable is
usually buried and extended to surrounding areas. A junction box containing amplifiers takes the signal and redistributes it to smaller cables called feeders, which go to specific areas and neighborhoods. From there the signals are again rejuvenated with amplifiers and then sent to individual homes by coaxial cables called drops.
The coaxial cable (usually 75 ohm RG- 59/U) comes into a home and is connected to a cable decoder box, which is essentially a special TV tuner that picks up the cable channels and provides a frequency synthesizer and mixer to select the desired channel. The mixer output is heterodyned to TV channel 3 or 4 and then fed to the TV set antenna terminals. The desired signal is frequency translated by the cable box to channel 3 or 4 that the TV set can receive.
Cable TV is a popular and widely used service in the United States. More than 70 percent of U.S. homes have cable TV service. This service eliminates the need for antennas. And because of the direct connection of amplified signals, there is no such thing as poor, weak, noisy, or snowy signals. In addition, many TV programs are available only via cable, for example, the premium movie channels. The only downside to cable TV is that it is more expensive than connecting a TV to a standard antenna.
Signal Processing
The TV signals to be redistributed by the cable
provider usually undergo some kind of pro
cessing before they are put on the cable to the
TV set. Amplification and impedance matching
are the main processes involved in sending
the signal to remote locations over what is
sometimes many miles of coaxial cable. However
, at the headend, other types of processes
are involved.
straight Through Processors
In early cable ystems, the TV signals from local stations vere picked up with antennas and the signal was amplified before being multiplexed onto the main cable. This is called straight through processing. Amplifiers called strip amplifiers that are tuned to the received channels pass the desired TV signal to the combiner. Most of these amplifiers include some kind of gain control or attenuators that can reduce the signal level to prevent distortion of strong local signals. This process can still be used with local VHF TV stations, but today heterodyne processing is used instead.
Heterodyne Processors
Heterodyne processing translates the incomning TV signal to a different frequency. This is necessary when satellite signals are involved. Microwave carriers cannot be put on the cable, so they are
down converted to some available 6 MHz TV
channel. In addition, heterodyne processing
gives the cable companies the flexibility of
putting the, signals on any channel they want to
use.
The cable company uses modules called
heterodyne processors to translate the received
signals to the desired channel (see Fig. 2).
The processor is a small TV receiver. It has a
tuner set to pick up the desired over- the- air
channel. The output of the mixer is the normal
TV 1F's of 45.75 and 41.25 MHz. These pictures and sound IF signals are usually separated by filters,
rated by filters, and they incorporate AGC and processing
provide for individual gain control to make
fine tuning adjustments. These signals are then
sent to a mixer where they are combined with a local oscillator signal to up convert them to the final output frequency. A switch is usually provided to connect the input local oscillator to the output mixer. This puts the received signal back on the same frequency. In some cases that is done. However, setting the switch to the other position selects a different local oscillator frequency that will up convert the signal to another desired channel frequency.
Some heterodyne processors completely demodulate the received signal into its individual audio and video components. This gives the cable company full control over signal quality by making it adjustable. In this way, the cable company could also employ scrambling methods
if desired. The signals are then sent to a modulator unit that puts the signals on carrier frequencies. The resulting signal is then upconverted to the desired output channel frequency.
All the signals on their final channel assignments are sent to a combiner which is a large special purpose linear mixer. Normally, directional couplers are used for the combining operation.
Figure 2 shows how multiple directional couplers are connected to form the combiner or multiplexer. The result is that all the signals are frequency multiplexed into a composite signal that is put on the trunk cable.
Cable TV Converter
The receiving end of the cable TV system at the customer's home is a box of electronics that selects the desired channel signal from those on the cable and translates it to channel 3 or 4, where it is connected to the host TV receiver through the antenna input terminals. The cable 717 box is thus a tuner that can select the special cable TV channels and convert them to a frequency that any TV set can pick up.
Figure 3 shows a basic block diagram of a cable TV converter The 75 ohm RG 59/U cable connects to a tuner made up of a mixer and a frequency synthesizer local oscillator capable of selecting any of the desired channels. The synthesizer is phase locked and microprocessor controlled. Most control processors provide for remote control with a digital infrared remote control like that used on virtually every modem TV set.
The output of the mixer is sent to a modulator that puts the signal on either channel 3 or 4. The output of the modulator connects to the TV set antenna input. The TV set is then set to the selected channel and left there. All channel changing is done with the cable converter remote control.
Today, the cable converters have many advanced features, among them automatic identification and remote control by the cable
company. Each processor contains a unique ID code that the cable company uses to identify the customer. This digital code is transmitted back to the cable company over a special reverse channel. There are several 6 MHz channels below channel 2 that can be used to transmit special signals to or from the cable converter. The digital ID modulates one of these special reverse channels. These low channels can also be used by the cable company to turn on or disable a cable converter box remotely. A digital signal is modulated onto a special channel and sent to the cable converter. It is picked off by a special tuner or with a bandpass filter as shown in Fig. 1. The signal is demodulated, and the recovered signal is sent to the microprocessor for control purposes. It can be used to lock out access to any special channels for which the customer has not subscribed. The reverse channels can also be used for simple troubleshooting.
A cable modem is an
extension of the cable TV converter. This device encodes and
decodes digital data over one of the 6 MHz channels on the cable.
The cable modem permits users to connect their personal computer
to the Internet by way of the cable system. Because of the wide
bandwidth of a cable TV channel, a very high data rate from 10 to
30 Mbits/s can be achieved. This is significantly higher than that
obtainable on conventional telephone lines .Some cable TV
companies offer data services to users, and more are expected to
offer this service in the future.
