Re: ISB Receivers
I also checked the Racal RA1772 manual, which confirmed that pairs of this receiver could be used for space diversity reception, in the master/slave configuration, including for SSB and ISB. Audio combining looks to have been a bit more sophisticated than in the Eddystone case, but then the RA1772 probably cost a lot more than the EC958/12.
“The audio diversity combiner circuit consists basically of an electronic switch which selects either channel A or channel B dependent on which channel has the highest level of AGC voltage. An audio signal zero-crossover detector circuit ensures that switching between channels occurs only at the zero-crossover point, to avoid the introduction of switching distortion and clicks.”
So in the 1970s, if you wanted space diversity, you installed a pair of multi-purpose HF receivers and fitted the appropriate accessories and interconnections. Or perhaps you stepped up to the Marconi Hydrus or something similar, which was available in diversity versions but still a lot more compact than the older Marconi HR21 and like monsters.
Cheers,
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