Patent GB-1235302-A (GB)
Title: Monitoring arrangement for telephone systems
Inventor: FRITSCHI WALTER WILLIAM
IPC: H04Q3/00
Filed: 1968-08-27
Abstract: 1,235,302. Automatic exchange systems. WESTERN ELECTRIC CO. Inc. 27 Aug., 1968 [28 Aug., 1967], No. 40923/68. Heading H4K. In an arrangement for tracing malicious calls whereby a called subscriber such as CSB operates a key to connect a tone TS on to the line, that interexchange trunk circuit which carries the tone, such as TA or TB from a group of 128 such circuits, is identified by a pair of detectors DET1, DET2, each of which is normally connected to monitor half of the trunk circuits over a tree of switches TG0-TG62, the arrangement being such that if one detector indicates that a trunk in its half of the group carries tone a control circuit operates switch TGO to connect both detectors to that half of the group so that each now monitors a quarter of the group, that detector which now indicates that a trunk in its quarter of the group carries tone causing the switches in the next level of the tree to be operated to switch both detectors to that quarter of the group so that each detector minotors an eighth of the group, and so on until at the last level one pair of trunk circuits is connected one to each detector so that the detector then identifying tone and the setting of the switches in the tree identifies the trunk and records it in equipment such as an automatic message accounting device. Each of the tree switches TG has two pairs of inputs and one pair of outputs, the left-hand input pair being connected to the left-hand output and the righthand input pair being connected to the righthand output in the normal reset condition of the tree. Depending on which detector DET is active the switches of each level of the tree are operated to disconnect one input pair and connect the left- and right-hand inputs of the remaining pair to the corresponding outputs. In one embodiment, Figs. 2 to 6 (not shown), the switches TG are each comprised of eight gated amplifiers the states of which are controlled for each level of the tree by two bi-stable circuits. A third bi-stable circuit associated with each level determines that the switches of each level shall be switched in one direction when, because tone is present on more than one trunk, both detectors are active at the same time. The third bi-stable is switched at each identification cycle to vary the direction of preference. A counter in the control circuit steps from level to level of the tree and holds for a number of steps at the last level to provide a continuity test on the received tone before recycling and resetting the switch tree. The detectors comprise tuned receivers with guard circuits, the sensitivity of the circuits being increased on switching of the last level as an additional guard against noise. Should tone disappear, or a spurious signal cease, before the identification cycle is completed the counter is reset with the switch tree to stand-by. In a second embodiment, Figs. 8 to 10 (not shown), each trunk has a tuned receiver to detect the trace tone and operate a relay to mark an input to a switch GT31-GT62. Each switch GT comprises a pair of magnetically latching relays contacts of which determine the connection of input pairs to output pair. Each latching relay has a lamp to indicate when it is operated. The detectors are relays responding to the marks presented by the receivers over the switch tree, one or other of a pair of control relays for each level of the tree being operated according to which detector relay responds. The energization of a control relay at one level switches control upwards to the next level. For each identification cycle a bi-stable relay pair is shifted from one state to the other, contacts of one of the relays being associated with the pair of control relays for each level so that in alternate cycles either one or the other control relay has preference to operate when both detector relays respond to tone detected in each of their trunk groups. If at any level neither detector relay responds the control and switch relays are released thereby setting the tree to stand-by.