Patent GB-655688-A (GB)
Title: Improvements in and relating to apparatus for monitoring the transmission of electric signals
Inventor: RANTZEN HENRY BARNATO
IPC: H04B3/46
Filed: 1948-06-22
Abstract: 655,688. Monitoring systems. BRITISH BROADCASTING CORPORATION. June 22, 1948, No. 16795. [Class 40 (iv)] [Also in Group XL (c)] The transmission of signals between two points in a transmission system is automatically monitored by providing two capacitors which are charged under the control of the signals at the two points respectively, means for preventing rapid discharge of the capacitors, and indicating, alarm, or switching means responsive to a difference in the voltages across the two capacitors exceeding a predetermined value. As shown in Fig. 1, a transmission channel 10 which may comprise a landline, one or more radio links, or a combination of landline and radio links extends between the points A and B. Each of these points is coupled by an amplifier (11, 19) to a rectifier (12, 20) and if the signal is of radio frequency it is demodulated before being applied to the amplifier. The rectified signal is then applied to an integrator (13, 21) whose output is coupled by a modulator (14, 22), amplifier (15, 23), bandpass filter (16, 24), and an amplifier-demodulator (17, 26) to an alarm device 18 which is differential in operation. In the case of B, the output of band-pass filter 24 is passed over a landline 25 and a second filter 24<SP>1</SP> to the amplifier-demodulator 26. The amplifiers, rectifiers and integrators 11, 12, 13 and 19, 20, 21 are shown in detail in Fig. 2. The input terminals 27, 28 of the amplifier are coupled to input transformer 29 through a shaping network 30 attenuating the lower frequencies and thence to triode 31 the output of which is applied to a second triode 32. Two negative feedback paths are provided ; the first which includes capacitor 33, resistor 34, capacitor 35 and cathode resistor 36 functions at all levels of applied signal and assuming the values of 34 and 36 to be R1 and R2 gives a gain of approximately (R1+R2)/R2. The second feedback path comprises the rectifiers 37, 38 and the resistors 39, 40 which form part of a network for biassing 37, 38. This network includes resistors 41, 42 in series with 39, 40 connected between a positive potential source 43 and ground. The rectifiers 37, 38 are therefore of high impedance until the input rises to a predetermined level at which the feedback voltage exceeds the voltage drop across 39 and 40, whereupon 37, 38 pass current and additional negative feedback is applied. If the values of 39 and 40 are each R3, the gain then becomes [R2+R1R3/(R1+R3)]/R2. The output of triode 32 is applied to a full-wave rectifier comprising metal rectifiers 53, 54 connected through resistors 55, 56 to charge a capacitor 57, the leak resistor 58 having a high resistance compared with the charging path resistance, so that capacitor 57 acts as an integrator. Further rectifiers 59 and 62 in series with resistors 60 and 63 respectively are connected across the capacitor 57 and are biassed over terminals 61 and 64 so as to be non-conductive until predetermined levels are reached. The resulting input-output curve is shown in Fig. 4, rectifier 59 producing a substantially straight portion between 48 and 65, and rectifier 62 the almost vertical portion extending upwards from the point 65. The alarm device 18 is arranged to operate on a level difference of 3 db., the shape of the Fig. 4 curve ensuring that noise which is just audible when the signal level is low will trip the alarm only when the signal level falls to level 67 (where signal plus noise is at level 45). The alarm will operate to change in transmission equivalent, frequency response distortion, or cross-talk. The alarm device may be replaced by an indicator or a switching arrangement which switches in an alternative channel or alternative components such as amplifiers, radio transmitters or receivers. Specifications 605,464 and 653,099, [both in Group XL (c)], are referred to.