4 METHODS FOR SATELLITE SIGNAL DISTRIBUTION IN CABLE NETWORKS
The matrix switching system
Pros: Standard DSTV decoders can be used, no special configuration is required.
Cons: Very labour intensive, originally designed for smaller systems. To avoid the many problems typical in the L-Band spectrum, the systems layout for larger systems will be very costly and complex.
This is due to the many active and passive components required in the system and the resulting mismatching and noise. In these high frequencies the transitional losses on all connections increase with time, making such systems unreliable and troublesome. Expansions of such systems (to include additional satellites or new devices such as PVR's) are very expensive and complicated.
One wire IF systems
Pros: Standard DSTV decoders can be used, single cable design and components are more cost effective than matrix system.
Cons: Very difficult to balance signal levels, high losses mean extra amps introducing noise. All amplifiers must include a slope adjustment. In these high frequencies the transitional losses on all connections increase with time. Such systems require a high amount of maintenance.
Digital Conversion Systems
Pros: Ideal for medium to larger systems. As all signals are within the cable spectrum (47 to 862 MHz) signal levels can be maintained without problem. Depending on the component quality (domestic, commercial, professional), the systems are stable and reliable.
These system are software driven (none volatile memory) highly flexible and can be reprogrammed to changing requirements and additional services.
Cons: Require re-converters on the subscriber side and special LO settings. Depending on the decoder software, opposite polarities on the transponders are not necessarily, a problem. As full transponders are converted (minus roll off), large frequency chunks are required out of the available spectrum.
This could be improved if the decoders use self-seeking software. Additional services require adding a converter to the head end for each new transponder.
64 and 128 QAM Transmodulation
Pros: This is the most economic way of transporting large numbers of digital services over cable TV networks.
The transmodulation technique converts the QPSK satellite signal to an 8MHz QAM modulated signal. The software driven, modular Head End System has a re-programmable NIT table and transponder split facility (enables the digital information of one heavily modulated transponder to be split over 2 8MHz channels).
There are no problems in adding additional services from various satellites to the system.
Cons: Requires QAM decoders. Additional services require adding a converter to the head end for each new transponder.
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