Abstract: A noise reducing apparatus consists of a compression system and an expansion system. The compression system comprises a pre-emphasis circuit for performing a pre-emphasis operation with respect to an input signal, a first variable gain control circuit for giving a level compression characteristic to an output signal of the pre-emphasis circuit, to produce an output signal to a transmission path, a first control voltage producing circuit for dividing the output signal of the first variable gain control circuit into a plurality of bands, to produce a plurality of control voltages respectively given mutually different bands, a first weighting circuit for respectively and relatively weighting the plurality of control voltages obtained from the first control voltage producing circuit, and a first adding circuit for adding the plurality of control voltages obtained from the first weighting circuit, to obtain a first control voltage and apply this first control voltage to the first variable gain control circuit.

Abstract: A noise reduction system having a compressor and an expander. The compressor comprises series connected high and low frequency band filters of the variable frequency type, the cut-off frequencies of which are respectively controlled in response to high and low frequency components of the output of the series connected filters so that they are shifted in opposite directions to each other independently as a function of the amplitude of the respective frequency components, whereby the amplitude range of the respective frequency band is emphasized to a constant value while the frequency range of the emphasized band is varied in response to the amplitude of the respective component.

Abstract: In a noise reduction system including a compressor and an expander having a complementary frequency response characteristic to the frequency response characteristic of the compressor, the compressor includes a high frequency emphasis variable frequency filter and a low frequency emphasis variable gain filter which are connected in a series circuit between input and output terminals and which respectively vary their parameters in response to the respective high and low frequency components of the compressor output. The expander includes a high frequency de-emphasis variable frequency filter and a low frequency de-emphasis variable gain filter which are connected in a series circuit between input and output terminals and which respectively vary their parameters in response to the respective high and low frequency components of the expander input.

Abstract: A monaural signal to artificial stereo signals converting and processing circuit for headphones comprises, a circuit which converts a monaural signal into artificial stereo signals, a circuit which adds localizing information of imaginary sources to the artificial stereo signals thus obtained, and a circuit which adds imaginary indirect sounds to the artificial stereo signals added with the localizing information. Due to the addition of the indirect sounds, the sound expansion felt by a listener listening to a headphone expands to regions outside the listener's head.

Abstract: A signal processing circuit has a circuit organization having a pair of crosstalk circuits respectively including at least delay circuits each of a delay time .tau. and adapted to add or cancel crosstalk components to or in input signals in correspondence with the transfer characteristics between sound sources and the two ears of a listener. Each crosstalk circuit has means for attenuating components of frequencies represented by (2n-1/2.tau.) (where n is a positive integer).

Abstract: A phase-shifting circuit comprises a first transistor circuit having a base to which an input signal is applied, first and second resistors of equal resistance values connected mutually in series and between an emitter of the first transistor circuit and ground, a third resistor connected at one terminal thereof to the emitter of the first transistor circuit, a capacitor connected between the other terminal of the third resistor and ground, and a second transistor circuit having a base connected to said other terminal of the third resistor and an emitter connected to the junction between the first and second resistors, an output signal being led out from a collector of the second transistor circuit.

Abstract: A compression and expansion system comprises a control circuit including a variable impedance element, whose impedance is varied in accordance with a control signal applied thereto. The system has a frequency-response characteristic which varies in accordance with variation of the impedance of the variable impedance element. A control signal is formed in accordance with the level of an input signal and is applied to the variable impedance element of the control circuit thereby to vary the impedance thereof. A switching means forms passing paths in a manner such that, at the time of compressing, the input signal is supplied by way of the control circuit to a negative feedback amplifier. At the time of expanding, the input signal is supplied to the negative feedback amplifier, and, at the same time, the output of the negative feedback amplifier is fed back by way of the control circuit to the input of said negative feedback amplifier.