Patent GB-609120-A (GB)
Title: Improvements in or relating to electrical signalling systems
IPC: H04B3/04
Filed: 1946-03-06
Abstract: 609,120. Transmission systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd., TERRONI, T. B. D., and PARKER, J. E. T. March 6, 1946, No. 6961. [Class 40 (iv)] In a modulated carrier current signalling system having automatic gain control which operates in response to changes in attenuation in the transmission circuit, the gain control makes a correction up to a predetermined maximum which is less than the maximum possible correction and at which point an alarm may be given. The pilot frequency is selected in tuned circuits L1, C1, L2, C2, amplified in valve VA and rectified by MRA. The circuit is so set that if the level is normal no effect is produced, but, if it is at least ¢ db. up or down, relay SL operates in the appropriate direction. If it is incorrect by 4 db. or more, relay SP is operated. Thus, if the level is ¢ db. or more too low, SL1(L) closes and operates IG in series with PCDM. IG3 operates delay relay DS whose contacts DS1, DS2 reverse the charge on condenser C7 which charges through VR2 until gas-filled valve NA fires and operates PA. Thereupon, in sequence, SC operates and locks and releases DS, PA releases and operates A, A operates B and energizes stepping magnet PCIM, B short-circuits A which in turn shortcircuits B. The switch PC cuts in predetermined pads at each step and stepping continues with repeated operation of A and B until the level is restored to normal, whereupon SL releases IG followed by SC. If the change in level is in the opposite sense, SL1(H) closes and operates DS directly. The operations otherwise are the same as the foregoing except that with IG normal the stepping circuit includes the " decrease " magnet PCDM instead of PCIM. If the correction required is 4 db. or more, relay SP operates in the appropriate direction, but the resulting functions depend on the setting of a manual switch AS. In position 1, any large change in level operating SP immediately operates alarm relay LA. This energizes AD followed by DS to restart the delay circuit and at the end of the predetermined time PA operates and gives an alarm on lead 15. In position 2, SC is operated by PA after the initial delay period and AD then restarts the timing circuit, but with VR3 in the charging circuit in place of VR2. PA cannot thus reoperate to give an alarm until after the PC switch has had time to make eight steps, i.e. a change of 4 db., unless the level has already been restored to normal. In position 3 of AS, both relays SP and SL operate on a change of Œ 4db. and at the end of the initial delay period PA operates SC to release DS and reset the delay circuit. The appropriate PC switch steps under control of VR3 which may allow six steps, i.e. a 3 db. correction. If relay SP is still operated, indicating that the level is still more than 4 db. from normal, AD operates and an alarm is given, the stepping circuit being interrupted by PAI. If the level is, however, now less than 4 db. from normal, the contacts of SP1 are open, AD and DS are released and the PC switch steps under control of the ¢ db. relay SL until the necessary correction is made. Position 4 of AS allows PC to step under control of VR3, which may allow up to eight steps. If the necessary correction is reached within this limit, stepping ceases, but otherwise an alarm is given when the eighth step is reached. In each of the four settings of the switch AS the removal of the link DAL renders the special facilities given above effective only in the " decrease gain " direction and stepping progresses normally in the opposite direction until the permissible maximum position is reached, when an alarm is given. The automatic operation of the equipment can be put out of use by cut-off key KAGCO which interrupts the circuits to relay contacts SP1, SL1 and lights indicator lamp AGCOL. In this condition the switch PC can be operated step-by-step by manual keys KSU, KSD. Valve failure is indicated by the release of AA which lights lamp AAL and cuts off positive battery from the control relays. A test key KAGCT inserts a predetermined pad in the line to test the response of the equipment.