Patent GB-669483-A (GB)
Title: Improvements in and relating to apparatus for monitoring the transmission of electric signals
IPC: H03F1/54
Filed: 1949-03-09
Abstract: 669,483. Monitoring apparatus; automatic volume control. BRITISH BROADCASTING CORPORATION, PEACHEY, F. A., and RUSSELL, C. GUNN-. March 8, 1950 [March 9, 1949], No. 6489/49. Class 40 (iv). [Also in Group XL (c)] In apparatus for automatically monitoring the transmission of signals between two points in a transmission system of the type disclosed in Specification 655,688, the link between the two points over which the comparison voltage is transmitted is provided with automatic gain control to prevent inaccuracies being introduced by changes in the transmission equivalent of the link. As shown in Fig. 1, the system to be monitored comprises amplifiers 10, 12 and a line 11. Signals from the point 13 pass via an amplifier 14 to a modulator 15 and the resulting modulated carrier passes over a link circuit 16 to the vicinity of the second point 17. Signals from this point 17 pass via circuits 18, 19 identical with 14, 15, respectively, to a demodulator 20 and those from the link 16 to an identical demodulator 21, the unidirectional outputs of the two demodulators being applied to a differential indicator 22. The circuit 23 applies a control voltage dependent upon the difference between the peak outputs of 20 and 21 to an automatic gain control device 24. Amplifiers 14, 18. Each of these is as shown in Fig. 2. The input is connected to an amplifier 25 whose output is applied through two parallel paths 26, 27 to output terminals 28. The amplifier 25 may be as described in Specification 653,099, [Group XL (c)], and has reduced gain for applied voltages exceeding a predetermined level. The path 27 includes a limiter 29 and an amplifier 30 similar to 25 but having reduced gain for applied voltages exceeding a second predetermined level. The outputs of paths 26, 27 are applied to rectifier and integrator circuits 31, 32 respectively. Operation is as follows: for input voltage levels between zero and level 34 (Fig. 3), the output increases linearly, the voltage developed across resistor 32a in circuit 32 back-biasing rectifiers 35, 36 in circuit 31 to render path 26 inoperative. When level 34 is reached the limiter 29 (Fig. 2) becomes operative and at the same time the gain of amplifier 30 is automatically reduced so that substantially complete limiting takes place, as shown by the vertical portion of curve 33 (Fig. 3). At level 37 however, the voltage developed across resistor 31a equals that across resistor 32a and the path 26 becomes operative. However, as the output from this path is applied in opposition to that from path 27, increase in input voltage causes a decrease in output voltage as shown by the upper portion of curve 33 (Fig. 3) and at input level 38 the output is zero. The characteristic is chosen so that normal signals being monitored frequently swing the input voltage from 0 to 37 (Fig. 3) so that the output voltage then has peaks of predetermined amplitude and the peak amplitude at the receiving end of link 16 (Fig. 1) is a measure of the loss of the link irrespective of the frequency at which the signal varies. Automatic gain control, Fig. 7. An output from the demodulator 20 is applied to the winding 49 of relay 23 and operates it whenever the output is within 1¢ db. of the peak voltage. The outputs of the two demodulators are thus connected in series opposition to the A.G.C. device 24. Normally the peak outputs of the demodulators are equal and the only bias applied to valve 52 is that appearing across the resistor 53. However, if the transmission equivalent of the link 16 changes, a voltage is developed across the condenser 51 whose magnitude and sign depend upon that change. The time constant of discharge of 51 is of the order of minutes. A change in the transmission equivalent of the system 10, 11, 12 (Fig. 1) causes the indicator 22 to respond since relay 23 does not operate if the output of demodulator 20 is not within 1¢ db. of the normal peak value. The modulators 15, 19 (Fig. 1) are preferably of the type described in Specification 649,612, [Group XL (c)].