Diplex

The communications companies are often faced with the problem of increasing their facilities and occupying more efficiently existing channels, usually by the installation of new transmitters. To take advantage of existing equipment, use has been made of diplex (two messages in one direction), employing one transmitter whose output was commutated from one frequency to another at a rhythm superior to the keying rhythm. The various radio-frequency components of modulated signals create a frequency-diversity effect at the receiver which attenuates somewhat the random effect offading. A particular way of switching is described, taking place when aerial current is zero, thus avoiding excessive sidebands due to abrupt wave front. In the "synchronous" version, the keying is delayed, and stored for a fraction of a cycle of the modulating frequency, then released at the beginning of the next cycle. The rise and fall of the keying is then substantially sinusoidal and sidebands reduced. The original diplex system used by Transradio since 1937 was later improved to decrease the bandwidth; several units of the improved diplex are now in common use. It must be emphasized that no synchronism is required at both ends, neither keying speed bears any relation to the diplexing frequency.