[Synchrodyne]

Moving back a couple of decades, the attached items from Wireless World 1949 cover what was claimed to be the first shipboard SSB/ISB installation, on the vessel Caronia, using STC equipment. This was used for passenger telephone communications; the vessel being fitted with conventional equipment for regular marine R/T and W/T MF/HF communications (which did not change to SSB until the 1970s).

I imagine that at the time, point-to-point communications was changing over to SSB/ISB, and that would have included those shore stations that handled international telephone calls, including those to/from ships.

From the STC advertisement:

“The R.X.9 Independent-sideband receiver. Capable of receiving either or both channels of a double-channel circuit. Whilst one channel is busy with ‘subscriber’ calls, the other can be used as an ‘order wire’. Suitable for reception of high-fidelity double-sideband telephony. Automatic frequency control.”

From that one could deduce that the R.X.9 was typical of ISB receivers of its time, and might even have been a point-to-point model adapted for the marine application. In respect of its mention of double-sideband telephony, STC might have been saying that its ISB receiver did a better job of receiving incoming DSB telephone calls – by minimizing selective fading distortion – but it could also have recognized that it was a better way of receiving HF broadcasts for shipboard redistribution.

Cheers,