Patent GB-1387026-A (GB)
Title: Compressing and/or expanding circuit
IPC: H03G9/02
Filed: 1972-04-18
Abstract: 1387026 Compandors NIPPON VICTOR KK 18 April 1972 [24 April 1971] 17955/72 Heading H4R A signal compressing/expanding circuit comprises a first circuit including a first frequency characteristic changing circuit having a specific fixed frequency response characteristic which reinforces the level of an input signal in a predetermined frequency band and a second frequency characteristic charging circuit including a control element the resistance of which is varied in accordance with a voltage applied thereto and having a frequency response characteristic variable between a characteristic complementary with said frequency response characteristic of said first frequency characteristic changing circuit and a flat characteristic, the arrangement further comprising an amplifier connected in series with the first circuit and including in a negative feedback loop thereof a second circuit consisting of third and fourth frequency characteristic changing circuits which are substantially the same as the first and second frequency chareteristic changing circuits respectively. Fig. 6 shows a compandor including a change over switch 53, 54 for changing the function of the circuit from a compressor to an expandor. Such a circuit may be employed in a tape recorder in which case the switch 53, 54 is associated with the record/playback controls of the recorder. Circuits 11, 13 are the first and second circuits respectively and circuits 20a, 21a, 20b and 21b are the first, second, third and fourth frequency characteristic changing circiuts respectively. In Fig. 3 curve J illustrates the fixed characteristics of circuits 20a and 20b whilst the variable curve K represents the variable response of circuits 21a, 21b. When the circuit is switched to the "expander" function, transistor Qb of circuit 21b in the negative feedback loop of amplifier 12 has its base coupled to a control circuit 55 which derives a control signal from the signal level at input terminal 50 such that the frequency response of the second circuit 13 is dependent on the input signal level and frequency. Transistor Qa in the second frequency characteristic changing circuit 21a is then connected to a constant voltage source which is of such a value that the frequency response of circuits 21a and 20a are complementary thereby resulting in circuit 11 having an overall flat frequency response characteristic. The operator of the whole circuit is such that for signals in a prescribed frequency range below a predetermined amplitude level, the frequency response of circuit 13 reinforces the signal level thereby increasing the negative feedback of amplifier 12 and decreasing its gain. When the switch is in the "compand" position the circuits 11 and 13 interchange functions, i.e. circuit 13 is arranged to have a flat frequency characteristic whereas circuit 11 reinforces the signal level in a prescribed frequency range. The control circuit output then is connected to the base of transistor Qa in circuit 21a and the control circuit input is obtained from terminal 51 at the amplifier output. The circuit operation is then such that the gain of amplifier 12 is increased for signals below a predetermined level in the prescribed frequency band. Filter 56 and limiter 58 in the control circuit ensure that the control voltage is only applied to transistor Qa or Qb for signals in the required amplitude and frequency ranges.