Patent GB-1346327-A (GB)
Title: Amplitude regulator for separating frequency variations and amplitude variations of electrical signals
IPC: G10L21/00
Filed: 1971-04-19
Abstract: 1346327 Automatic gain control, vocoders J A DREYFUS 19 April 1971 [4 March 1970 22 Sept 1970] 23111/71 Headings H4R and H3T [Also in Division G4] An amplifier has a forward acting automatic gain control with a further amplifier, with backward acting gain control, in its gain controlling path. Both the gain controlling path of the main amplifier and that of the further amplifier may contain filters, which can be of variable cut off, and means for controlling the rise and fall times of the gain control voltages. The gain control may be effected either by controlling the grid potential of triode valves, 123, 124, 125, 126, Fig. 3, in push-pull in the signal path, of an amplifier using a mixture of valves and transistors, or by varying the source to drain impedance of an FET 201, 202, Fig. 8, connected to shunt the signal path of the amplifier. The amplifier may be used, with appropriate filters in its forward and gain control paths, to compress, expand or shape, a signal fed to a pitch extractor, a hearing aid, a transmission circuit, or a speech recognition apparatus which indicates the information contained in a spoken message (see Division G4). Figs. 3a, 3b show one embodiment of the amplifier in which the main amplifier comprises block 107v and includes two pairs of push-pull valves having forward gain control by a voltage Ev applied to the grids via the further amplifier 107r, which also has its gain controlled by a potential Er which is derived from the output of amplifier 107r via filters 110r and lllr, emitter follower 209b, amplifier 209c, rectifier circuit and charge discharge time control circuit PM3, PM4. The output of the further amplifier 107r is applied to control the gain of amplifier 107v via further filters 110v and lllv, emitter follower 209b, amplifier, rectifier, and time constant control circuits of similar form to those of the further amplifier gain control circuit. By appropriate selection of the circuit constants a wide range of expansion or compression of an input signal may be obtained with frequency characteristic shaping as well, if required. The alternative embodiment, Fig. 8, using FET's 201, 202 connected across the signal path of the amplifiers A 1  and A 2  also includes filters Fr and Fv, and charge/discharge time constant control circuits T 11 , T 12 , in the gain controlling circuits. A pitch extractor, Fig. 13, uses the gain controlled amplifier 407, 412 of the invention for emphasizing the fundamental frequency and providing an output at the fundamental frequency which is substantially independent of input level. In this arrangement the gain control channels include a band pass filter 408 of 80 to 600 Hz, a full wave rectifier 410, and a low pass filter 411 of 0 to 36 Hz. A zero crossing detector 416 and monostable 417 delivers pulses at the fundamental frequency to the logic system 418 which is fed with outputs from a slow timer 421 and rapid timer 420 to measure the pitch period by counting and convert the resulting digital signal to analogue form at 489. A gate 491 only passes the analogue output signal if the circuit 492, 497 indicates the presence of fundamental frequency. 479 then provides various outputs, such as the total energy, formant frequencies, &c., which are displayed on an oscillograph together with the fundamental frequency.