Patent GB-697744-A (GB)
Title: Improvements in or relating to digital information transmission systems
IPC: H04L1/16
Filed: 1951-01-10
Abstract: 697,744. Electric selective signalling. WESTERN ELECTRIC CO., Inc. Jan. 10, 1951 [Jan. 11, 1950], No. 717/51. Class 40 (i) [Also in Group XIX] In binary permutation code transmission systems, an error in a received code group is automatically detected and corrected. In the simplest embodiment, only one error per code group can be detected and corrected, but by additional apparatus the system may be arranged for single error correction with double error detection, or by further complexity for double error correction, &c. Assuming each code group to consist of m information elements, a number k of check elements are added to the code group at the transmitter. The number of check elements added varies with the number of information elements as shown in table I in the Specification, and the combination of check elements required for each information code group is determined according to odd or even " parity checks " among arbitrarily selected sub-groups of the group code elements as shown by means of tables II, III in the Specification. It is shown in the Specification that the therein defined " geometrical spacing " of the code groups must be at least "three units'' to achieve single error correction, higher spacings of the code group elements being necessary to achieve double error detection with single error correction, double error correction, &c. If, however, the necessary spacing already exists between all the information code group elements for the desired degree of detection-correction, then no check elements need be added. At the receiver, register relays are actuated in accordance with the code received, contacts of these relays being arranged in a "parity check" circuit. If the system is for single error correction, then if the code is incorrectly received, a number of check relays are energized over the check circuit in a combination indicative of the transmission line on which the error has arisen, and serve to compensate the error and operate an alarm. The two conditions of the binary code elements may be signalled by mark or space, +ve or -ve current, dot or dash, or be represented by perforations in cards. Transmission of the elements of a code group may, as described, be simultaneous, but may also be effected sequentially by distributors as in start-stop telegraphy. Vacuum tubes may be used in lieu of relays. The code groups may be transmitted through delay networks while the parity checks of the sub-groups are being made, the delay being such that a code group does not appear at the delay network output until a possible error has been located, the output then being corrected accordingly. As shown, Fig. 1, a four element binary information code to be transmitted is set up at INP by closing a combination of the switches 1, 2, 3, 4, representing the number or other information to be sent. Corresponding relays M1, M2, M3, M4 are thereby operated and over make contacts m1-m4 extend earth over the corresponding transmission line cm1-cm4 to actuate corresponding register relays MR1, -MR4, at the receiver. According to the combination' of code information, relays M1-M4 operated, check element impulses determined by sending parity circuits K1, K2, K3 involving contacts of relays M1-M4, are simultaneously sent over the transmission check lines ck1, ck2, ck3 to operate corresponding register check relays KR1, KR2, KR3. Contacts of the register relays MR1 - MR4, KRI-KR3 are connected in a receiving " parity check " circuit E1 analogous to the sending parity circuits, whereby if the seven (four information and three check) code elements have been transmitted correctly all the check relays Cl, C2, C3 are operated. If one of the received code elements is erroneous, however, a different combination of the check relays is energized indicative of the error. Contacts of the check relays are connected in an error locating or tree circuit EL to energize one of the error relays E1-E7 corresponding to the transmission line on which the error has occurred, or, if there is no error, to ground a check lead extending direct to the corrector ER. Here earth is extended over the check lead and contacts of the error locating relays El, E7 and contacts of the receiving register relays to the information code output relays OM1-OM4 such that if there is no error those relays OM1-OM4 are energized from the earth on the check lead corresponding to the operated register relays MR1-MR4. If there is an error, however, on one of the information lines cm1- cm4, and hence one of the register relays MR1- MR4 is in the wrong condition, the signal (mark or space) on the corresponding information output relay OM1-OM4 is reversed by the operated error locating relay E4-E7 so that the output code relays OM1-OM4 are actuated in correspondence with. the input relays M1-M4. Errors arising on the check lines are, of course, also detected, but these have no effect on the output. Additional contacts on the error relays may actuate an alarm. In a modification, Fig. 3 (not shown), the operating speed of the system is increased by energizing the check lead directly from the receiving parity check circuit contacts when no error is detected. A further system, Fig. 4 (not shown), is described where an additional check element is added to the system of Fig. 1 to permit also of double-error-detection.