752,503. Monitoring apparatus. BRITISH BROADCASTING CORPORATION. June 19, 1953 [June 19, 1952], No. 15480/52. Class 40 (4). [Also in Group XL (c)] Automatic monitoring apparatus for a transmission link comprises test equipment at the sending end of the link which controls the sending to the receiving end of one of three different monitoring signals according as the input to the link is below a first predetermined level, between the first and a second (higher) predetermined level, or above the second predetermined level respectively, and test equipment at the receiving end of the link which compares the monitoring signals received with the output of the link and gives an indication whenever the third monitoring signal is received and the output is below a third predetermined level intermediate the first and second levels, or the first monitoring signal is received and the output is above the third level. The indication may control switching over to an alternative link. As described, the link is a line 10 (Fig. 1) and the three monitoring signals are transmitted over the earth-phantom of line 10. Test equipment at sending end. This comprises two identical detectors 16, 17 (Fig. 1) which control the monitoring signal send unit 18. The circuit of a suitable detector 16 is shown in Fig. 3. Intelligence signals incoming to the link are applied to an amplifier comprising pentodes 31 and 32, the provision of a negative feedback path including the rectifiers 34, 35, which functions as described in Specification 653,099, [Group XL (c)], ensuring that the gain of the amplifier 31, 32 is substantially reduced at and above the second predetermined level of input. Thus the output of this amplifier is linearly related to input below this level but changes very little above it. The voltage appearing across the first secondary winding 48 of output transformer 47 is rectified by rectifier 50 and applied as a negative voltage to the grid of triode 53 which is normally biased to operate at the upper bend of its anode current/ grid voltage characteristic. The voltage appearing across the second secondary 49 is similarly rectified and applied to triode 63. The anode relays 56, 66 (Fig. 3) are normally operated in the absence of input signal to the link. Operation of send unit. Fig. 4 shows how the relays 56, 66 of the first detector 16 and those (55<1>, 66<1>) of the second detector 17 are associated with the send unit 18. When the input to the link is below the first predetermined level, all the detector relays are operated and relay 71 pulls up to the battery (not shown) connected to terminal 70, so that a potential of + 20 V. (constituting the low-level monitoring signal) is extended from terminal 74 via lowpass filter 19 to the phantom. When the input level rises above the first predetermined level, the relays 66, 66<1> release but 56, 56<1> remain operated, and the consequent relapse of 71 connects- 20 V. (the neutral signal) from terminal 79 to the phantom. Finally, if the input level exceeds the second predetermined level, relays 56, 56<1> are released, whereupon relay 76 pulls up and connects + 80 V. (the high-level signal) from terminal 81 to the phantom. Test equipment at receiving end. This comprises two identical detectors 22, 23 (Figs. 1, 4) which indicate to the receive unit 21 whether the output level at terminals 25, 26 is below or above the third predetermined level. In the receive unit 21 the monitoring signals are applied to three relays 82, 83 and 84 (Fig. 4) in series, which are such that the low-level, neutral, and high-level signals operate 84, 83, and 82 and 84 respectively. With the output at terminals 25, 26 below the third predetermined level the contacts 86, 87 associated with detectors 22, 23 respectively are in the positions shown in Fig. 4, and if the low-level signal is received, relays 93 and 96 come up to battery (not shown) at terminals 94 and 97 respectively, so that both contacts 98 are open. If, however, the highlevel signal is received, their operating circuits are broken at front 82, and contacts 98 are closed, thus initiating any desired switching operation, e.g. changing over to a reserve line. In a similar manner, when the output is above the third predetermined level (contacts 86, 87 changed over), the reception of the high-level signal operates 93 and 96, but the reception of the low-level signal does not. Reception of the neutral signal, however, ensures that relays 93 and 96 are operated whenever the output level may be, so that the monitoring apparatus cannot respond to a fault on the link. Faulty detector at sending end. If the two detectors 16, 17 give different indications of the range in which the input lies, then neither of relays 71 and 76 (Fig. 4) will be operated, and the neutral signal will be transmitted. Thus the relays 93 and 96 are operated and the monitoring apparatus gives no indication should the link be faulty. Faulty detector at receiving end. If the two detectors 22, 23 give different indications to the receive unit 21, then if the monitoring signal received is either the low-level or the highlevel one, one of the relays 93 and 96 will be operated so that no indication can be given if the link is faulty. Modifications. The direct current monitoring signals may be replaced by alternating current signals the frequencies of which are outside the band occupied by the intelligence signals, thus enabling monitoring of radio-links to be performed. The closure of the two contacts 98 (Fig. 4) may also operate a relay at the receiving end of the link which opens the phantom circuit, thus giving an alarm at the sending end.