On British submarine telegraph cable systems, repeater stations and receiving sites employed tuning forks. Repeater sites were at cable traffic junctions or islands; e.g., Ascension and St Helena Islands in the Atlantic, Cocos Keeling in the Indian Ocean, Norfolk Island (junction) and Fanning (repeater) in the Pacific.
The purpose of the tuning fork was to govern transmitted symbol rates for outgoing traffic. “Cable code” is a bipolar form of international telegraph code (almost exclusively what one hears on the radio today), where both the “dot” and “dash” elements have equal duration. Equal-duration dots and dashes saved time, meaning more revenue-generating traffic could be sent. This was especially important when cables had no compensation, and the transmission rates were extremely slow due to large cable capacitance. Dots and dashes in cable code are instead distinguished by their polarity. Alternating dot-dash sequences, when they occurred, created polarity reversals on the cable used to recover the best point for pulse sampling. When no telegrams were being sent, an idling polarity-reversal sequence was periodically injected into the cable (every 15-20 seconds or so) to maintain pulse detection synchronization with the distant receiver. The tuning fork rate was governed by a Synchronome master clock (and its backup). An implementation of electro-mechanical frequency control (EMFC?) employed a stepper relay to move the weights on the fork by small amounts to maintain frequency synchronization with the Synchronome. Here’s one surviving system, the master clock and tuning fork for PK <http://telegraphmuseum.org/object/synchronome-clocks/> (Porth Curno), the landing point in Cornwall England used for most of the Empire’s submarine telegraph cables networks (and for many optical fiber cables today). The Submarine Telegraph Museum (originally established by Cable & Wireless) on this site is a fascinating visit. Frequencies around 15 Hz were common on early 20th century cables, depending on the degree of success in compensating for the inherent capacitance on a cable thousands of miles long surrounded by conductive sea water. Cable compensation is an entirely separate subject outside the scope of a time-nuts forum. Basically, every function we see today in fiber optic or electrical synchronous transmission systems (timing, encoding, transmission, pulse regeneration, reception, decoding, printing) was invented in electro-mechanical form for submarine telegraphy — and realized in beautiful brass & mahogany machinery.
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