07. Cable TV Systems
7.1 Introduction
Cable Television (CATV) was initially introduced for the benefit of communities living in shadow zones and in areas that were beyond the range of broadcast transmitters. However, with the advent of satellite television, reception of signals form a large number of TV stations located farther away have become possible. Cable TV has now expanded into a multi channel system with the availability of a large frequency spectrum. Modern cable networks have provisions for two way interactive communication with computer base data to provide services like interactive TV, pay TV etc.
7.2 Cable Signal Sources
It is essential for cable TV to collect desired signals from available sources. Then after necessary amplification and conversion, these corresponding signals are delivered to signal processing units located in the cable head end room. The different signal sources for cable TV are:
1. Satellite signals: The main signal source is from various satellites. The signals from satellites are weak and at high frequency. Therefore a high gain antenna with parabolic dish antenna of large diameter amplifies the weak signal. When the antenna is correctly aligned and oriented towards the desired satellite, the feed horn receives signals reflected towards the desired satellite, the feed horn receives signals reflected towards it by the dish surface. It delivers those to the Low noise block converter (LNBC). LNBC, also called as the front end converter, amplifies the signal further and down converts the higher band frequency into the intermediate frequency. It is often necessary to install more number of dish antenna units with associated feed horn and LNBC to collect signals from different satellites.
2. Terrestrial signals: The signal reception from VHF and UHF terrestrial broadcast is done by installing conventional TV antennas mounted usually on a tall building. If these transmissions are also available via satellites, it is better to use signals picked up from dish antenna to obtain noise free quality reception.
3. Outside broadcast Signals: Events like local sports and cultural programs are normally received through contribution links. As these programs are not directly aired by TV stations, they can be obtained from TV studios in order to distribute to cable subscribers.
4. Local studio signals: Local news and current affairs programs can also be gathered with camcorders by trained staff. It may be distributed at regular intervals.
5. Recorded Signal: Programs like movies, plays, songs and religious speeches earlier recorded on either video tapes/discs are also distributed on one or two channels of the cable network. For this, one or more DVD/CD players or video tape recorders or VCRs are made available in the control room.
6. Additional Services: Other services like pay TV, commercial data retrieval, on-screen data display for breaking news can also be provided on cable network. But this may demand for two way interactive cable systems with high quality distribution cables. Digital cable systems have such additional benefits.
7.3 Cable Signal Processing
The signal processing units are also called the Head End equipment. They consist of power dividers, satellite receivers, channel modulators, signal processors, amplifiers, VCRs, DVD players, monitors and a combining network. Figure 7.1 shows the block diagram of the cable Head End system. The sequence of processing different types of signal received from various sources is described here:
1. Satellite Signals: The signals on reception from satellite are amplified and down converted o IF signals. In order to divide the signal into different paths, the IF signal from LNBC is delivered to a signal splitter. It is commonly called a power divider because it enables equal division of signal power to its output ports. The satellite receivers accepts inputs from these ports on the power divider. The receiver tuner may be set to select any desired channel out of many available from multiplexed satellite signals. Then it demodulates the signal of the selected channel to provide base band signal. The two separate outputs, video and audio (V and A) signals, are fed into one of the channel modulators.
2. Terrestrial TV Signals.
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