Voltage-controlled filter

Claim 2

An audio frequency filter comprising in cascade in the order recited, a first operational amplifier, a two quadrant first multiplier, a second integrating operational amplifier having two inputs, a second two quadrant multiplier, a third integrating operational amplifier, a source of audio signal applied to a first input of said first operational amplifier, means for applying multiplicative voltage factors respectively to both said first multiplier and said second multiplier, means for feeding back output from said second operational amplifier to a second input of said first operational amplifier, means for feeding back output from said third operational amplifier to said first input of said first operational amplifier, and means for deriving separate audio signal outputs from each of said operational amplifiers.

Claim 3

The combination according to claim 1, wherein said first operational amplifier is a unity gain phase inverter.

Claim 4

The combination according to claim 1, wherein the multiplicative voltage factors of said multipliers are the same.

Claim 5

The combination according to claim 1, wherein said multiplicative voltage factors are dc voltages.

Claim 6

The combination according to claim 1, wherein said means for feeding back output from said second and third operational amplifiers to an input of said first operational amplifier include negative feedback loops.

Claim 7

The combination according to claim 5, wherein said operational amplifiers have transfer functions such that said filter has selectively high pass, low pass and band pass characteristics variable as a function of said multiplicative voltages in respect to cut-offs or center frequency.

Claim 8

The combination according to claim 5, wherein each of said operational amplifiers includes a non-phase inverting and a phase inverting input terminal, and wherein the output of each of said amplifiers is proportional to the differences of the voltages applied to said terminals.

Claim 9

The combination according to claim 7, wherein said first operational amplifier includes a positive and a negative input terminal, said first input being a negative input terminal, and wherein said second and third operational amplifiers each includes one grounded terminal.

Claim 10

An audio frequency amplifier, comprising an audio signal input terminal, a first different operational amplifier having resistive feedback and having a first and a second input terminal, means connecting said signal input terminal to said first input terminal, a first multiplier connected in cascade with the output terminal of said first differential operational amplifier, a second operational amplifier having a further input terminal connected to the output of said first multiplier, said second operational amplifier having a capacitive feedback, means deriving a voltage from the output of said second operational amplifier, means applying a variable fraction of said voltage to said second input terminal, a second multiplier connected in cascade with said second operational amplifier, a third operational amplifier connected in cascade with said second multiplier, said third operational amplifier having an output terminal, a negative feedback circuit connected from said last Mentioned output terminal to said first input terminal, means for at will deriving output signal from any one or more of said operational amplifiers, a source of dc control voltage, and means applying said dc control voltage to both said multipliers as multiplicative factors.

Claim 11

An audio frequency amplifier, comprising an audio signal input terminal, a first different operational amplifier having resistive feedback and having a first and a second input terminal, means connecting said signal input terminal to said first input terminal, a first multiplier connected in cascade with the output terminal of said first differential operational amplifier, a second operational amplifier having a further input terminal connected to the output of said first multiplier, said second operational amplifier having a capacitive feedback, means deriving a voltage from the output of said second operational amplifier, means applying a variable fraction of said voltage to said second input terminal, a second multiplier connected in cascade with said second operational amplifier, a third operational amplifier connected in cascade with said second multiplier, said third operational amplifier having an output terminal, a negative feedback circuit connected from said last Mentioned output terminal to said first input terminal, means for at will deriving output signal from any one or more of said operational amplifiers, a source of dc control voltage, and means applying said dc control voltage to both said multipliers as multiplicative factors.

Claim 12

The combination according to claim 9, wherein said audio signal input is a tone signal and wherein said source of dc control voltage is a dc voltage representative of the pitch of said tone signal.

Claim 13

The combination according to claim 9, wherein said audio signal input is a tone signal, and wherein said source of dc control voltage is representative of amplitude of said tone signal.

Claim 14

The combination according to claim 9, wherein said audio signal input is a tone signal and wherein said dc control voltage is representative of a tonal characteristic of said tone signal.