Patent GB-1460468-A (GB)
Title: Detecting and correcting transmission errors in fixed-weighting  codes
IPC: H04L1/00
Filed: 1974-02-12
Abstract: 1460468 Data transmission; error correction SIEMENS AG 12 Feb 1974 [21 Feb 1973] 06415/74 Heading H4P [Also in Division B7] In a method of detecting and correcting errors in a data transmission system employing 3/2 ratio five bit code groups, e.g. 3-0's and 2-1's, and additional 4 bit test group composed of elements designated according to a Hamming code plus an even parity bit, also complying with the 3/2 rule is transmitted for each data group, in order that single errors may be detected and corrected. On reception each data signal group and test group are tested to detect for a determined relation between data groups and test groups; hamming error correction is effected when the test reveals a single error. Information to be transmitted may be represented by a plurality of signal groups, at least one indicating the nature of information. If no error is detected, received groups are applied to an operational device. On occurrence of a single error groups are applied to a disturbance indicator which compares data groups with associated test signal groups to ascertain whether they have a determined relation and comply with the 3/2 rule or to a disturbance indicator if there is a multiple error. The method may be applied to a train signalling system where certain indications, i.e. a STOP signal must be conveyed unambiguously and if necessary result in retardation of a train. Other signals, i.e. indicating train position are considered not so vital. A transmitter on a train (1) interrogates a series of fixed transceivers (3-6, Fig. 7, not shown) in turn during passage along a track and reply is stored at 8 divided into data and test groups which are passed respectively into stores 9, 10. Not shown is an optional device preceding 8 which passes signals only on reception of a determined number of like groups. If received data groups have an incorrect ratio, e.g. 1/4, 0/5, further interrogation is normally made but if time precludes this, emergency braking is applied if operation inhibition has not been imposed i.e. in the event of positional signal transmission. However a false 2/3 signal may be received which is not identified by 10 hence 9, 10 pass their signals to allocator 12 which performs a test in accordance with a table (Fig. 4, not shown). If faultless, signals are forwarded. If faulty a disturbance indication is emitted by 11 which also receives error signals via gate 14, from 9, 10. If however only a single error occurs, from 9 through an inhibiting input on device 16 data is placed in certain positions and from 10 through decoder 19 certain sequences are passed unchanged through gate 20 into other determined positions. Through intermediate store 22, output from 17 is passed to parity check 23 which determines the position of error by four checks and succeeding device 24 corrects it then passes the resultant back to 23 for a further check before issue to operating device 13; this motivates indication/alarm/retardation of train, &c. With switch 25 in reverse position, output at 24 effects correction of data and tests the groups in storage devices (not shown) before passing to 9, 10 and output from 12 is direct to 13 when the test is positive i.e. no error. The radio transmission may be by a.m., length, polarity, pulse number or frequency. It is envisaged in the Specification that the method may be applied to other applications but no examples are given.