HF broadcast transmitters operate for long hours with frequency changes of channel. The equipment must be reliable and protected against dangerous fault conditions as remote controlled, un-attended operation is normal for most operators. Rapid automatic frequency changing and output matching circuits are necessary, the power conversion efficiency must be high and in the event of faults diagnosis and repair must be straight forward and fast. The optimum cost of ownership results from the right combination of all these features. In 25 years time it becomes mandatory for HF broadcasting to use a single, upper sideband mode with a carrier wave which is 12 dBs below the peak envelope power. In order that suitable receivers can be introduced alongside conventional DSB equipment, broadcasters are encouraged to change to SSB transmission now. To give compatibility with DSB receivers in the interim period a carrier 6 dBs below the peak envelope power is to be used. Such a service although compatible will suffer increased distortion with DSB reception and high modulation levels.
展开▼
