Coquelet technology was well documented in the designers' house magazine, ACEC REVUE No 3-4, 1970, in the article "Les T�l�imprimeurs, T�l�chiffreurs et Transcodeurs ACEC - Syst�me Coquelet". This important paper includes an excellent analysis of why it is advantageous to limit the number of symbols per character to one or two.

Coquelet was conceived as an electromechanical system, using tuned reeds to generate the tones, and the same reeds as receiver filters. Because the tones were in two sets, low and high, one for each tone of the pair, sync could be recovered directly. Coquelet was designed to operate directly with existing ITA2 teleprinter equipment, so ran at the equivalent speed. The original system was asynchronous (i.e. stop-start; there could be gaps between tone pairs with hand sent text), and the equipment converted the signals directly from and to conventional teleprinter asynchronous signals. ACEC also manufactured ITA2 to Coquelet two-way converters.

The first tests were at Crowborough (a photograph is extant). Two way communications was first established with Delhi in 1965, and then with Singapore, using the 32 tone system, one tone per letter of the alphabet! The system proved extremely successful, and communications between Britain and Singapore often continued for an hour after standard teleprinter links had faded out.

Initial transmissions were on AM, soon to be followed by SSB. The 6 and 12 tone systems were introduced in the late 1970s to further save bandwidth. The 6 tone system is still in limited use today. Current equipment is made by Racal. Like Coquelet, Piccolo is a tone sequential-tone system, using six or twelve tones, however Piccolo was from the outset a completely electronic system, with the exception of the symbol clock, which was at first motor driven!

The early 32 tone version is well documented in the paper "Multi-tone signalling system employing quenched resonators for use on noisy radio- teleprinter circuits, published by J.D. Ralphs et al, Proc. IEE Vol 110 No. 9, September 1963.

This paper describes the use of quenched lossless LC filters for reception and tone generation, and has an interesting discussion of how to achieve orthogonal signalling in a non-coherent FSK environment. The discussion is in the time domain, and nicely parallels modern frequency domain arguments about tone spacing, which reach the same conclusions in different language.

The early Piccolo system related each of the ITA-2 characters to a single tone, so operating at 10 baud achieved a remarkable 100 WPM. Like Coquelet, the later Piccolo systems are two-tone techniques, i.e. transmitting sequential tone pairs. Synchronism is achieved in all systems by 10% AM modulating the transmitter with a character rate square wave.

Unlike Coquelet, Piccolo uses the minimum tone spacing for orthogonal detection (spacing = baud rate) and has always used electronic tone generation. The integrate and dump detection system allowed this level of performance, and the receiver filters were also used as triggered oscillators to generate moderately phase synchronous tones. Many versions of Piccolo exist - the most well known versions can be summarised as follows: