The Purpose of a Satellite TV Multiplexer

Multiplexing is a term for several processes that take multiple information streams and combine them into a single flow of data. This allows a transmitter to send the information without using too much bandwidth.

When the messages reach their destination, a demultiplexer splits the combined stream into its original components, then sends each message to its appropriate destination. Computer networks, fiber-optic cable and communications satellites all use multiplexing.

Necessity

A single satellite has to receive multiple simultaneous transmissions from several different Earth stations. The satellite must then transmit these broadcasts back to Earth to the TV stations or subscribers who want to view them. To do this, the satellite multiplexes all the different broadcasts before sending them earthward. Each receiver that picks up the satellite transmission then demultiplexes the beam, selects the individual broadcast it's tuned to pick up, and disregards the rest.

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Methods

Communications satellites can multiplex signals based on frequency, time or code. Frequency-division multiplexing combines all the different broadcast frequencies into a single beam. Time-division multiplexing transmits segments of data in the order it receives them, so fast that the demultiplexed broadcast looks continuous. Code-division transmissions identify each segment of data with a code number in order to separate them properly for demultiplexing. Code division requires more bandwidth than the other approaches, which gives it the alternative name of spread-spectrum multiplexing.

Options

Time-division multiplexing isn't right for some forms of satellite communications because if the information stream is intermittent and a time slot goes unused, the slot is wasted. With continuous television transmissions, time-division multiplexing works well. Satellite companies can combine time with frequency divisions by dividing the available bandwidth into several channels, then assigning time slots within each channel. This gives the system greater flexibility, but it can be inefficient when dealing with a steady stream of transmission, such as broadcast television.

Multiplexing on the Ground

Sometimes a satellite television subscriber also may need to multiplex. For example, a hospital may want to distribute satellite transmissions to many televisions. A multiplexer can combine all the channels sent to the satellite dish and send them to the TVs in various rooms, where the broadcasts will be demultiplexed. Homeowners also can use a multiplexer to combine separate transmissions, for example from a satellite and a television antenna.

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