830,848. Electric selective signalling systems. STANDARD TELEPHONES & CABLES Ltd. Nov. 1, 1957 [Nov. 5, 1956], No. 34174/57. Class 40 (1). [Also in Group XL (b)] In a signal transmission or storage system, information in a binary, i.e. two condition, code form is converted into a three-condition code of which two of the conditions correspond to the two conditions employed in the binary form and the third condition indicates the prolongation of the previously signalled condition, which must be one of the first mentioned two conditions, until the other of the first mentioned two conditions is signalled and a receiver or reconstituting means is included for reconverting the three condition code into the binary form. In the conversion of the teleprinter start-stop code to the three-condition code, marks and spaces are represented by equal pulses of positive and negative polarity respectively, and the extension of the marking or spacing condition is represented by a neutral or zero pulse persisting until the next change to spacing or marking respectively occurs. In the three-condition code two pulses of the same polarity must be separated by a pulse of the opposite polarity. In the simplified transmitting arrangement, Fig. 2, the code pulses at the teleprinter.transmitter contacts 2 produce in the secondary winding of a transformer 3 short positive and negative pulses which on application to a gÞnerator 4 comprising two monostable generators V1, V2 generate respective positive and negative pulses of 20 msecs. duration. The three condition signal may be sent by frequency modulation, the neutral condition being represented by frequency fo and the other two conditions by fo Œ fm -. In the receiving arrangement, Fig. 4, for marks and spaces transmitted as fo + 425 c.p.s. and fo - 425 c.p.s. respectively, which also incorporates error-detecting means, the signals received at 5 are passed by filters 6, 7 to operate a trigger V3 or V4 to provide a mark or space signal respectively to the receiver coil 8. The output from the filter 6 or 7 is also passed to the trigger V5 so that it is in the triggered or reset condition respectively. If a further pulse is passed from filter 6 when V5 is triggered, the tube V6 will be fired to operate an alarm 9. Similarly if a further pulse is received from filter 7 when the trigger V5 is in the reset condition a pulse will be provided to trigger V6 and the alarm 9 will be operated. The circuit of Fig. 4 fails to detect the error in the revised code if an " extra " space is received immediately after a mark so that an extended space occurs between two marks, as indicated by the signal for the letter I in Fig. 5 (not shown). The operation of an alternative error correcting device depends on the fact that in the three-condition code correctly received, the positive and negative pulses are of unit length. The circuit of the triggers V5, V6 may be modified so that the alarm 9 is also operated if the pulse applied to V5 exceeds the normal length of 20 msecs. Alternatively, the receiver may include a sensing circuit triggered to examine a received pulse at predetermined intervals for example, twice during its normal duration. The sensing circuit would be disabled by the neutral condition and if the number of pulses counted was one or more than two, an alarm would be operated. In an alternative errordetection method, since in all correct signals, the counter of changes from the neutral condition is always an even number, a number operated by such changes could be arranged to operate an alarm if it indicated an odd count at the end of a signal. The incoming signals can be regenerated by a sensing circuit which provides two pulses for examining each element of the incoming signal, and the regenerator circuit is locked by the first examining pulse of the period of neutral condition to its previous condition, and is not unlocked so as to be free to change its condition until the arrival of two mark or two space pulses. As indicated by Fig. 6 (not shown), the frequency representing the neutral condition is not necessarily intermediate those representing marks and space, and these may be sent as narrow pulses of the higher frequencies. Specification 683,843 is referred to.