Patent GB-1398732-A (GB)
Title: Signal processing system
IPC: H03G9/18
Filed: 1972-07-24
Abstract: 1398732 Compandors; active filters SONY CORP 24 July 1972 [24 July 1971] 34535/72 Headings H4R and H3U A dynamic noise limiter, e.g. for a tape recorder, has a variable filter 17 (Fig. 1) connected to the output of an amplifier 12. As shown, the filtered output is applied to an output terminal 21. In an alternative position of a switch 16 the filter 17 is connected to form a negative feedback loop to feed back information signals filtered in the variable filter 17. The output signal is then at terminal 13. The overall frequency characteristic provided by the variable filter 17 at terminal 21 with switch 16 as shown is converse to that at terminal 13 which switch 16 in its other position. The input circuit of the variable filter 17 comprises an emitter follower 26 (Fig. 2). A subcircuit 29 comprises a pair of capacitors 32, 33 connected in parallel with resistors 27, 28 as a voltage divider. A transistor 36 has its emitter collector circuit in parallel with a resistor 37 and its base connected to a control circuit through terminal 38 The maximum impedance between a capacitor 34 and ground, when transistor 36 is cut off, is limited by resistor 37. An output transistor 39 is provided. Subcircuit 29, together with resistors 27, 28, is a high pass filter whose frequency response is varied by the impedance of the emitter-collector circuit of transistor 36. For low amplitude control signals transistor 36 has relatively low conductivity. Transistor 36 becomes more conductive as the amplitude of control signals applied to terminal 38 increases. Alternative circuits for the transistor 36 and resistor 37 are given (Figs. 3A to 3B, not shown). When recording signals on tape 23 (Fig. 1) a switch arm 16 is in contact with terminal R and the output voltage of amplifier is modified by variable filter 17 and applied to recording head 22. Curve a (Fig. 4) applies when the amplitude of the input signal, and therefore of the control signal, is low. Curve b applies when the amplitude of the input signal, and therefore of the control signal, is high. At intermediate signal levels the response curve is between a and b. During playback, however, terminal 11 receives the incoming signal from a playback head and switch arm 16 is in contact with terminal P. The feedback loop of amplifier 12 includes not only a resistor 14 but the variable filter 17, along with a compensating amplifier 19 and attenuator 20. The effect is to provide an overall frequency characteristic (Fig. 5) converse to that of Fig. 4. In recording and playback attenuator 20, variable filter 17 and amplifier 19 have substantially unity gain. Detailed circuitry (Fig. 7, not shown), shows a switch 16<SP>1</SP>, ganged to switch 16, by which according to the mode of operation of a tape recorder either connects a playback head (29) or a microphone (52) to the main amplifier 12 through a further amplifier (52). The input of the control amplifier 18 is taken directly from the emitter of transistor 36. Control amplifier 18 includes three transistor stages 54, 55, 56) the last of which is connected as an emitter follower to supply input signals to a band elimination filter (53) for eliminating specific signals such as a 19 kHz pilot frequency. A low pass filter (57) connects two stages (56 to 54) to boost high frequency response. The output of the band elimination filter (53) is rectified and connected to the base of transistor 36. The variable filter is claimed in Specification 1,398,733.