Re: ISB Receivers
Thanks very much for the detailed and lucid summary.
That certainly would explain the apparent paucity of solid state “three-channel” ISB receivers. In that light, the pattern can now be seen quite early on with the Marconi group receivers. The Hydrus ISB and Apollo marine SSB receivers, both dedicated to their respective functions, appeared circa 1969-69, at the beginning of the serious solid state era, at least if one discounts the early false starts with germanium transistors. The Eddystone EC958 general coverage receiver appeared at about the same time. Quite soon the EC958/5 was being rebadged as the Marconi Nebula for marine applications, and in 1974 the EC958/12, with ISB capability, arrived. So the GC receiver, probably the least costly of the three, was taking over the specialist receiver duties. And the EC958 was something of stepping-stone model, largely following valve receiver topology and delivering proximate performance, except perhaps for higher stability, but well short of the RA1772 level. (Although at the time Marconi might have argued otherwise.)
I wonder if SSB and ISB broadcast relays still retain the pilot carrier, so that the receiver can track Doppler shifts that might be noticeable on music. The VOA used – might still use – ISB relays and I can recall finding these back in the 1990s. The Liniplex F2 receiver, with its tracking PLL, would lock on to them – it was quite tenacious in that regard – and then one could simply switch between the two sidebands. But the reduced carrier upset the agc system a bit, given that it was designed around conventional AM transmissions with full carrier, and the result was a much higher audio level.
Re the Marconi Hydrus, it would be interesting to ascertain whether it largely followed the preceding valve topology, using filtering and limiting for carrier recovery, or whether it had a PLL, like the EC958/12.
Cheers,
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