Report 1991-02 Download
In February and March of 1990, the BBC was offered equipment for transmission and reception of Digital Audio Broadcasting signals using COFDM (Coded Orthogonal Frequency Division Multiplex) and MASCAM (Masking-pattern Adapted Sub-band Coding and Multiplexing).
This report gives details of the trials that were carried out in the London area using a UHF frequency. The conclusions are that the signal is robust in the presence of echoes. As a consequence, COFDM was found to be a suitable modulation system for broadcasting high quality digital sound to the home, to mobile receivers and to portable receivers. The quality of the MASCAM sound signals was very good in the mobile environment.
The signal fails when the signal strength is too low. Thus conventional planning techniques (suitably modified for the different height of the receiving antenna) can be used to identify the service area. Because buildings and hills cast a fairly sharp shadow at UHF, the failure point was usually found to be the point at which line-of-sight propagation to the transmitter was no longer maintained.
Additionally it may be concluded that a lower frequency than UHF would be preferable. Higher powers and/or more transmitters, would be needed if UHF is adopted for a service.
The implementation of a co-channel active deflector shows that a single frequency network of transmitters is feasible. However, the problem of ensuring the isolation between the antennas is high enough to prevent distortion or oscillation is not trivial, and may severely restrict the maximum gain of the repeater. In practice the useable gain will depend on the nature of the transmitter site.