Circuit for preventing crosstalk

Abstract: An operational amplifier (OP1) is applied with an audio signal at a non-inverting terminal thereof for amplifying the audio signal. A crosstalk cancelling circuit (1) is provided for reducing the output of the operational amplifier (OP1) to a level of crosstalk generated at the non-inverting terminal. The output of the crosstalk cancelling circuit (1) is applied to an inverting terminal of the operational amplifier (OP1) so as to cancel any crosstalk.

Description: The present invention relates to a circuit for preventing crosstalk between adjacent circuits such as acoustic circuits. Heretofore, various circuits have been proposed for preventing reduction of sound quality caused by crosstalk. One of the methods for preventing crosstalk is reduction of the length of a signal line. A long signal line causes the stray capacity to increase, which may generate crosstalk, causing distortion of an audio signal. Fig. 5 shows a conventional audio amplifier. The audio amplifier has an operational amplifier (hereinafter called opamp) OP1 for the left stereo channel and an operational amplifier (hereinafter called opamp) OP2 for the right stereo channel. The non-inverting terminal of the opamp OP1 is connected to an input terminal Li of the left stereo channel (hereinafter called L-channel) through a variable voltage dividing resistor VR1 and a source changeover switch SW1. The inverting terminal of the opamp OP2 is connected to a gain setting feed-back resistor voltage dividing circuit Ra1 and Rb1. The non-inverting terminal of the opamp OP2 is connected to an input terminal Ri of the right stereo channel (hereinafter called R-channel) through a variable voltage dividing resistor VR2 and a source changeover switch SW2. The inverting terminal of the opamp OP2 is connected to a gain feed-back setting resistor dividing circuit Ra2 and Rb2. Audio signals from the input terminals Li and Ri are amplified by opamps OP1 and OP2 and output from output terminals Lo and Ro. In such an audio amplifier, stray capacities occur, for example stray capacities C1 and C2 between the output terminal Ro and a point a and between the output terminal Lo and a point b. Consequently, crosstalk is generated between the output terminals Lo and Ro. Crosstalk becomes remarkable when a high frequency signal is included in the audio signal. Crosstalk reduces the sound quality of the output sound. Prior art crosstalk preventing circuits are known e.g. from JP-A-54-41103, JP-A-57-162103, and JP-A-60-98800. In particular JP-A-54-41103, and JP-A-60-98800 disclose the implementation of low-pass filters in the crosstalk cancelling circuits. An object of the present invention is to provide a circuit which may prevent generation of crosstalk, in order to ensure a good quality of the output data. This object is achieved by the features of Claim 1. Further advantageous embodiments are specified in the dependent claims. A circuit for preventing crosstalk according to the present invention comprises first and second input terminal means for receiving a first signal and a second signal, respectively, a first operational amplifier having a first input terminal and a second input terminal, opposite to the first input terminal, the first input terminal being electrically connected to the first input terminal means, a crosstalk cancelling circuit provided between the second input terminal means and the second input terminal for applying a component of the second signal to the second input terminal thereby cancelling crosstalk generated between the first input terminal means and the first input terminal. The circuit further comprises a second operational amplifier connected to second input terminal means, and the crosstalk cancelling circuit is provided between an output terminal of the second operational amplifier and the second input terminal of the first operational amplifier, wherein the crosstalk cancelling circuit comprises an attenuator for reducing the output of the second operational amplifier. According to one aspect of the invention, the crosstalk cancelling circuit has an amplifier for amplifying the second signal so as to cancel crosstalk. According to a further aspect of the invention, there are provided a plurality of signal sources, and a plurality of crosstalk cancelling circuits for amplifying input signals from the signal sources, respectively. A plurality of select switches are provided for selecting one of the sources as an input signal for the first operational amplifier and for selecting one of the crosstalk cancelling circuits connected to another source than the selected source. Other objects and features of this invention will readily be apparent from the following description with reference to the accompanying drawings. Fig. 1 is a circuit of an audio amplifier having a crosstalk preventing circuit according to the present invention; Fig. 2 is a circuit showing a second embodiment of the present invention; Fig. 3 is a circuit showing a third embodiment of the present invention; Fig. 4 shows a fourth embodiment of the present invention; and Fig. 5 is a circuit showing a conventional audio amplifier. Referring to Fig. 1, the audio amplifier comprises an opamp OP1 for the L-channel and an opamp OP2 for the R-channel. The non-inverting terminal of the opamp OP1 is connected to the input terminal Li for the L-channel through a variable resistor VR1. The inverting terminal of the opamp OP1 is connected to the gain setting resistors R1 and R2 and to an output of a crosstalk cancelling circuit 1 for generating a cancellation signal. The non-inverting terminal of the opamp OP2 is connected to the input terminal Ri for the R-channel through a variable resistor VR2. The inverting terminal of the opamp OP2 is connected to the gain setting resistors R3 and R4 and to an output of a crosstalk cancelling circuit 2 for generating another cancellation signal. The crosstalk cancelling circuit 1 comprises resistors R8, R9 for dividing the output voltage of the opamp OP2, a capacitor C3 for passing an alternating current component, a buffer amplifier BA2 for preventing the change of characteristics with repect to th opamp OP1, a variable resistor VR4 being connected to the capacitor C3 for automatically changing the gain of the buffer amplifier BA2 in accordance with the change of the resistance of the variable resistor VR2, and a resistor R10 for restricting the output current of the buffer amplifier BA2. Similarly, a crosstalk cancelling circuit 2 comprises resistors R5, R6 for dividing the output voltage of the opamp OP1, a capacitor C2 for passing an alternating current component, a buffer amplifier BA1 for preventing the change of characteristics with respect to the opamp OP2, a variable resistor VR3 being connected to the capacitor C3 for changing the gain of the buffer amplifier BA1 corresponding to the resistance of the variable resistor VR1, and a resistor R7 for restricting the output current of the buffer amplifier BA1. In each crosstalk cancelling circuit 2(1), the resistance R1(R3) is set to be equal to the resistance R5(R8) and the resistance R2(R4) is set to be equal to resistance R6(R9), so that the output voltage of the circuit 2(1), that is the voltage of the cancellation signal CS becomes equal to the quotient obtained by dividing the output voltage LV of the opamp OP1 by the gain G of the opamp, expressed as follows: CS = LV/G. In other words, the level of the cancellation signal is 1/G of the output voltage LV. Furthermore, the capacitance C2(C3) is set to a value equal to a stray capacity C4. In operation, the L-channel signal input from the input terminal Li is amplified by the opamp OP1 and output from the output terminal Lo. Similarly, the R-channel signal is amplified by the opamp OP2 and output from the output terminal Ro. The output of the opamp OP1 is fed to the crosstalk cancelling circuit 2. In the circuit 2, the output LV is reduced to a value equal to the quotient of the output divided by the gain (LV/G) which is regulated by resistors R5, R6, the capacitor C2 and the variable resistor VR3 and applied to the inverting terminal of the opamp OP2 as the cancellation signal. On the other hand, the output of the opamp OP2 is fed to the crosstalk cancelling circuit 1. In the circuit 1, the output LV is reduced to the value equal to the quotient of the output divided by the gain (LV/G) which is regulated by the resistors R8, R9, the capacitor C3 and the variable resistor VR4 and applied to the inverting terminal of the opamp OP1 as the cancellation signal. The cancellation signal is a high frequency signal corresponding to the crosstalk signal. The cancellation signal is applied to the opposite input of the opposite channel opamp of the audio amplifier. Thus, the crosstalk signal is cancelled by the cancellation signal, so that the desired signal is output undistorted from each output terminal. Fig. 2 shows the second embodiment of the present invention where the same components as the first embodiment are identified with the same reference numerals as Fig. 1. The non-inverting terminal of the opamp OP1 for the L-channel is connected to the input terminal Li through the variable resistor VR1. The inverting terminal of the opamp OP1 is connected to the gain setting resistors R1 and R2 and to a crosstalk cancelling circuit 3 comprising an attenuator R8, VR6 for generating a first cancellation signal. The non-inverting terminal of the opamp OP2 for the R-channel is connected to the input terminal Ri through the variable resistor VR2. The inverting terminal of the opamp OP2 is connected to the gain setting resistors R3 and R4 and to a crosstalk cancelling circuit 4 comprising an attenuator R6, VR5 for generating a second cancellation signal. The crosstalk cancelling circuit 3 comprises the variable resistor VR6 and the resistor R8 for dividing or attenuating the output voltage of the opamp OP2, and a capacitor C3. The resistance of the variable resistor VR6 changes corresponding to the resistance of the variable resistor VR2. The crosstalk cancelling circuit 4 comprises the variable resistor VR5, the resistor R6 for dividing the output voltage of the opamp OP1, and the capacitor C2. The resistance of the variable resistor VR5 corresponds to the resistance of the variable resistor VR1. In operation, the L-channel signal input from the input terminal Li is amplified by the opamp OP1 and output from the output terminal Lo. Similarly, the R-channel signal is amplified by the opamp OP2 and output from the output terminal Ro. The output of the opamp OP1 is fed to the crosstalk cancelling circuit 4. In the circuit 4, the output LV is attenuated by resistors VR5, R6 to a predetermined level and applied to the inverting terminal of the opamp OP2 via capacitor C2 as the cancellation signal. On the other hand, the output of the opamp OP2 is fed to the crosstalk cancelling circuit 3. In the circuit 3, the output LV is attenuated by resistors VR6 and R8 to a predetermined value equal to the quotient obtained by deviding the output by the attenuating level, and applied via capacitor C3 to the inverting terminal of the opamp OP1 as the cancellation signal. In the second embodiment, each crosstalk cancelling circuit comprises an attenuator, thereby omitting any buffer amplifier. Accordingly, the circuit is simplified in construction and can be produced at low cost compared with the first embodiment. Referring to Fig. 3 showing the third embodiment, the non-interminal of the opamp OPI is connected to the input terminal Li through the variable resistor VR1. The inverting terminal of the opamp OP1 is connected to the gain setting resistors R1 and R2 and to a crosstalk cancelling circuit 5. The non-inverting terminal of the opamp OP2 for the R-channel is connected to the input terminal Ri through the variable resistor VR2. The inverting terminal of the opamp OP2 is connected to the gain setting resistors R3 and R4 and to a crosstalk cancelling circuit 6. The crosstalk cancelling circuit 5 comprises an opamp OP4 having a gain setting resistor R10 and a variable feed-back resistor VR8 corresponding to the variable resistor VR2, a current restricting resistor R13, and the capacitor C3. The non-inverting terminal of the opamp OP4 is connected to the input terminal Ri. The crosstalk cancelling circuit 6 comprises an opamp OP3 having a gain setting resistor R9 and a variable feed-back resistor VR7 corresponding to the variable resistor VR1, a current restricting resistor R12 and the capacitor C2. The non-inverting terminal of the opamp OP3 is connected to the input terminal Li. In operation, the L-channel signal input from the input terminal Li is amplified by the opamp OP1 and output from the output terminal Lo. Similarly, the R-channel signal is amplified by the opamp OP2 and output from the output terminal Ro. Furthermore, the input signal from the terminal Li is applied to the opamp OP3 of the crosstalk cancelling circuit 6. On the other hand, the input signal from the terminal Ri is applied to the opamp OP4 of the crosstalk cancelling circuit 5. The opamp OP3 of the crosstalk cancelling circuit 6 amplifies the L-channel input signal to a predetermined level which is applied to the inverting terminal of the opamp OP2 through the resistor R12 and the capacitor C2. Furthermore, the opamp OP4 amplifies the R-channel input signal of a predetermined level which is applied to the inverting terminal of the opamp OP1 through the resistor R13 and the capacitor C3. Thus, L-channel and R-channel signals are converted to the cancellation signals by the opamps OP3 and OP4 without being amplified by the audio amplifiers OP1 and OP2. Referring to Fig. 4 showing the fourth embodiment, the non-inverting terminal of the opamp OP1 is selectively connected to one of three signal sources S1 to S3 through the variable resistor VR1 and a select swith SW3. The gain setting resistors R1 and R2, and a filter 7 comprising a resistor R and a capacitor C are connected to the inverting terminal of the opamp OP1. The inverting terminal of the opamp OP1 is selectively connected to one of three crosstalk cancelling circuits 8 to 10 through select switches SW4 and SW5 and the filter 7. The select switches SW4 and SW5 are automatically operated in accordance with operation of the select switch SW3. The crosstalk cancelling circuits 8, 9, 10 comprise opamps OP5, OP6 and OP7 having gain setting resistors R14, R15, R16 and variable feed-back resistors VR9, VR10, VR11, respectively. The resistance of each variable resistor is automatically changed with the change of the resistance of the variable resistor VRI. The opamps OP5 to OP7 amplify the source signal from the sources S1 to S3 to produce cancellation signals, respectively. When a terminal b of the select switch SW3 is selected, terminals c of the select switches SW4 and SW5 are automatically selected. Thus, the source signal from the source S2 is applied to the non-inverting terminal of the opamp OP1 through the variable resistor VR1, and the cancellation signals from the crosstalk cancelling circuits 8 and 10 are applied to the inverting terminal through the filter 7. When a terminal a or c of the switch SW3 is selected, a terminal b or a of the switch SW4 and a terminal a or b of the switch are selected. In the fourth embodiment, the cancellation signal is obtained from a source which is not selected for the audio amplifier. Consequently, any crosstalk caused by a stray capacity generated between the non-inverting terminal of the audio opamp and the not selected switch can be prevented. Although the present invention is described in connection with audio amplifiers, the present invention can also be applied to other circuits such as a video amplifier. While the presently preferred embodiments of the present invention have been shown and described, it is to be understood that the disclosure is for the purpose of illustration only and that various changes and modifications may be made without departing from the scope of the invention as claimed in this specification.

Claims: crosstalk preventing circuit comprising: first and second input signal means (Li, Ri) for receiving a first signal and a second signal respectively; first operational amplifier (OP1) having a first input terminal (+) and a second input terminal (-), the first input terminal (+) being electrically connected to the first input signal means (Li); crosstalk cancelling circuit (1, 3, 5) provided between the second input signal means (Ri) and the second input terminal (- of OP1) for applying a component of the second signal to said second input terminal (-), thereby cancelling any crosstalk generated between the second input signal means (Ri) and the first input terminal (+ of OP1), characterized in that said component is proportional to the level of the second signal, and said crosstalk cancelling circuit includes high-pass filter means (C2, C3, C), the capacity of which is about equal to a stray capacity (C4) generated between the first and second input signal means (Li, Ri), for limiting the component of the second signal to a high-frequency component corresponding to the crosstalk-signal. The circuit according to claim 1 further comprising a second operational amplifier (OP2, OP4) connected with a first input terminal (+) to the second input signal means (Ri), wherein the crosstalk cancelling circuit (3, 5) is provided between an output (Ro) of the second operational amplifier (OP2, OP4) and the second input terminal (-) of the first operational amplifier (OP1). The circuit according to claim 1 or 2, wherein the crosstalk cancelling circuit (3) further comprises an attenuator (VR6, R8) for reducing the output of the second operational amplifier (OP2). The circuit according to any one of claims 1 to 3, wherein the crosstalk cancelling circuit (1) comprises an amplifier (BA2; OP5-7) for amplifying the second signal so as to cancel the crosstalk. The circuit according to any one of claims 1 to 4 comprising: a plurality of signal sources (S1-S3), a plurality of crosstalk cancelling circuits (8-10) for amplifying input signals from the signal sources (S1-S3) respectively, and a plurality of select switches (SW3-SW5) for selecting one of the sources as an input signal for the first operational amplifier (OP1) and for selecting one of the crosstalk cancelling circuits (8-10) connected to another sources (S1-S3) than the selected source. The circuit according to claim 4, further comprising a first variable resistor (VR1) provided between the first input signal means (Li) and the first input terminal (+) of the first operational amplifier (OP1) for adjusting the level of the input signal, and a second variable resistor (VR4) for controlling the gain of the amplifier (BA2), and the resistance of the second variable resistor (VR4) being set a value corresponding to a resistance of the first variable resistor (VR1).

Inventors: HIRAOKA HARUO, SUGAWARA KAZUAKI, HIRAOKA, HARUO, SUGAWARA, KAZUAKI, HIRAOKA, HARUO, C/O OHMORI KOUJOU, SUGAWARA, KAZUAKI, C/O OHMORI KOUJOU
Applicants: PIONEER ELECTRONIC CORP, PIONEER ELECTRONIC CORPORATION
Application year: 1990
Main classification: H03F3
IPC codes: H03F1, H04S7, H03F3