I am a bit of a communications history buff. I've been doing a little research about telecom in the days before transoceanic phone service before cables and satellites. The only way to bridge the oceans was hf radio. It's interesting to note that anyone with a shortwave radio could listen to all the international point-to-point phone traffic. I am going to assume that a form of independent sideband was used with a maximum of two or four circuits going to a specific country. Ssb is very easy to receive even with a standard shortwave radio of the day as long as it had a bfo to demodulate and recover the signals. I would like to know if any form of primitive encryption was used to make the circuits a bit more secure. It must have been very easy to literally monitor all the international traffic to and from a given nation. Anyone who can shed light on this subject would be appreciated to help scratch my historical itch.
Dave Marthouse email@example.com