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POST OFFICE TELEPHONY TRANSMITTER

Notes:

The first Intercontinental radio telephone service was, not surprisingly, between London and New York. Developed by AT&T and the GPO it opened in January 1927 and had been developed without full knowledge of Short Waves and hence operated in the LF band at 60 kHz using Single Sideband Modulation.

With the opening up of Short Waves, transmitters for International Telephony were developed by the Post Office using the expertise at Dollis Hill Research Centre and they were also purchased from STC.

The picture shows the Australian telephony transmitter at Rugby sometime after it commenced operation in 1930. (Look at the number of control knobs!)

Along with the radio frequency aspects of International Telephony, new technology had to be developed to deal with the base-band signals.

This included the conversion of two wire inland telephone circuits into four wire radio circuits, plus the invention of echo, or singing, suppressers and constant volume amplifiers to automatically maintain transmitter drive levels and line levels.