Patent GB-1449825-A (GB)
Title: Audio amplifier systems inhibition of corrosion
IPC: H03G3/32
Filed: 1974-06-25
Abstract: 1449825 Gain control TELEVISION RESEARCH Ltd 26 June 1974 [28 March 1973 25 June 1974] 14951/73 and 28053/74 Heading H4R In an amplifier system in which the gain is controlled in accordance with the ambient noise level means having a short initial time-constant are provided to prevent the gain changing except during periods when the input signal falls below a predetermined level. Audio signal passes via buffer amplifier 1, Fig. 1 and variable gain amplifier 8 to the output: the signal level at buffer amplifier 1 is detected by switching amplifier 2 which controls inhibitor 3. The sound level is measured by means of microphone 4 and amplifier,.5, rectified by detector 6 and applied to store 7: whenever switching amplifier 2 detects a signal above the predetermined level it causes inhibitor 3 to clamp the output from store 7 which controls the gain of amplifier 8, so that the gain depends only on the second level when the signal is below the predetermined level. In addition the output from store 7 is passed to ancilliary equipment 9, e.g. recording means and may be used, e.g. in a store for determining periods of peak business. A timer 10 may also be provided whereby the incoming signal is removed at periodic intervals to allow the gain to be set in accordance with the noise level, the arrangement being such that at every measuring period the timer is re-set. Preferably the time-constant of detector 6 is increased during quiet periods, so that a sudden burst of noise does not affect the gain, while the store may be digital.  Switching amplifier and inhibitor circuits. Signal is applied via buffer amplifier 1, Fig. 2 to the base of transistor 12, rendering it conductive, so that capacitors 13, 15 are not charged: during quiet periods transistor 12 becomes non-conductive and capacitor 13 charges via resistor 14. Capacitor 15 also charges via resistor 14 and adjustable resistor 16, but both capacitors discharge rapidly via transistor 12 when the signal level increases. Level detectors 17, 18 provide control outputs for switches 24, 25 (Fig. 3) and for quad latch 26, converter 21 and gate 30 (Fig. 4) respectively. Ambient noise level detector. The output from microphone 19, Fig. 3 is-amplified in amplifier 20 and rectified by a voltage-doubler rectifier to charge capacitors 22, 23. When level detector 17 (Fig. 2) provides an output corresponding to signal above the predetermined level, switches 24, 25 which may be solid state devices, are closed, giving the rectifying circuit a short time constant: the voltage applied to analogue-todigital converter 21 then rapidly follows changes in noise level. With signals below the predetermined level, switches 24, 25 are open, giving a long time-constant. Converter 21 provides a 4 bit digitally coded signal corresponding to its analogue input, changing by one level at a time at a slow predetermined rate: whena signal is present, however, change is prevented by means of a control signal from detector 18 (Fig. 2). The output from converter 21 is fed to the store (Fig. 4): an auxiliary analogue to digital converter may be provided to feed the ancilliary equipment. Store. The coded output from analogue to digital converter 21 (Fig. 3) is fed to quad latch 26, Fig. 4 which provides an output (A) following the input whenever a control signal from detector 18 (Fig. 2) is present; but gives a constant output when it is absent: the complement (#A) is also produced. Output (A) is fed to quad AND-OR gate 27 and output (A) to quad adder 28. A digitally coded signal (B) is fed from "set maximum level" switch(es) to both quad adder 28 and gate 27, adder 28 providing an output wherever signal (A) corresponds to a lower level than does signal (B): this output is fed together with its complement to the control terminals of quad AND OR gate 27. In gate 27 signal (A) is selected as output (C) when it is less than signal (B), but otherwise (B) is selected: thus signal (C) comprises again control signal varying under control of ambient noise level until it reaches a preset maximum beyond which its value is fixed. Signal (C) if fed to a further quad AND-OR gate 29 together with a signal (D) from a "set emergency announcement level" switch, signal (D) being the digital code corresponding to the desired gain for such announcements. Signal (C) is normally selected but is over-ridden by (D). Output (E) is fed to the variable gain amplifier (Fig. 5). Variable gain amplifier. Signal (E) (Fig. 4) is applied to a quad AND-OR gate 30, Fig. 5 which may select a minimum gain setting (signal F) for the amplifier 33 during sensing periods, under the control of detector 18 (Fig. 2): the digital output of gate 30 controls electronic switches 31, e.g. field-effect transistors, which short-circuit resistors 32 in the feedback path of amplifier 33. Alternatively switches may be provided which shoot out resistors in one leg of an input attenuator.