JP2010252025A - External sound input circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an external sound input circuit for: suppressing noise entering around from GND as the configuration of a differential input circuit; determining that an external sound apparatus is not connected when input is absent or small; and reducing the influence of induction noise on a line on a negative side as a single-end input circuit. <P>SOLUTION: The external sound input circuit includes: the differential input circuit 3 provided with a non-inverting terminal and an inverting terminal for inputting sound signals; a level detection part 2 for detecting the signal level of an input line connected to the non-inverting terminal and the inverting terminal; and a control part 1 for switching the single-end input circuit and the differential input circuit based on the output of the level detection part 2. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an external audio input circuit that inputs an audio signal from an external audio device such as a portable audio device.

  In this type of external audio input circuit, a configuration of a differential input circuit using an operational amplifier is generally used in order to perform GND isolation and reduce noise. Further, for example, in Patent Document 1, when the audio signal level is lower than a predetermined level, the circuit is switched to perform a low output power amplification operation, and when the audio signal level is higher than the predetermined level, the circuit is switched to produce a large output. A power amplifier that performs a power amplification operation is disclosed.

JP-A 63-10906

However, since the differential input circuit has a high impedance on the negative side line, there is a problem that when the external audio device is not connected, noise jumps on the negative side line and inductive noise is likely to occur. When an external audio device is connected, the negative line is connected to the GND of the external audio device, so that the impedance is reduced and the influence of inductive noise on the negative line is reduced.
As a method for eliminating the influence of inductive noise on the differential input circuit, it is generally known to use a twisted cable as a cable.
However, when the wiring inside the vehicle is a shielded wire instead of a twisted wire, or when the user uses a shielded wire for a connection cable of an external audio device, it is affected by the induction noise on the negative side line.
In addition, the power amplifier disclosed in Patent Document 1 reduces the power loss by switching the bias voltage, and no consideration is given to reducing the influence of noise.

  The present invention has been made to solve the above problems, and when the audio signal input is nothing or small, it is determined that an external audio device is not connected, and the operational amplifier input circuit is configured as a single-ended input circuit. As an external audio input circuit that reduces the effect of inductive noise on the negative line, and isolates GND as a differential input circuit configuration at the time of audio signal input, and reduces the effect of noise coming from GND The purpose is to provide.

  An external audio input circuit according to the present invention includes a differential input circuit having a non-inverting terminal and an inverting terminal for inputting an audio signal, and a level for detecting a signal level of an input line connected to the non-inverting terminal and the inverting terminal. A detection unit; a first switch provided between the input line connected to the inverting terminal and the ground; a second switch provided between the output line connected to the output terminal of the differential input circuit and the ground; A third switch provided on an output line connected to the output terminal of the differential input circuit, and a control unit for switching between the single-ended input circuit, the differential input circuit and the circuit based on the output of the level detection unit, It is equipped with.

The external audio input circuit of the present invention can easily switch between a single-ended input circuit and a differential input circuit according to the signal level of the input audio signal, so the negative side when no external audio device is connected It is possible to reduce the influence of noise induced to the line and noise that circulates from GND at the time of voice input from an external voice device, and there is an effect that high-quality voice with less noise can be provided to the user.

2 is a configuration diagram of an external audio input circuit in Embodiment 1. FIG. 6 is a flowchart for explaining the operation of the external audio input circuit according to the first embodiment. FIG. 5 is a diagram summarizing combinations of switch ON and OFF, with the input level (V) of the audio signal + as the horizontal axis and the input level (V) of the audio signal − as the vertical axis.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
Embodiment 1 FIG.
FIG. 1 is a diagram illustrating a configuration of an external audio input circuit, which includes a control unit 1, a level detection unit 2, and an operational amplifier 3. Bias cut capacitors 6a and 6b and voltage dividing resistors 7a and 7b are provided in the input lines of the audio signal input + and the audio signal input − of the differential input circuit of the operational amplifier 3, and input terminals (non-inverted) of the audio signal input + Terminal) is supplied with a midpoint potential (Va) 4 through a parallel circuit of a voltage dividing resistor 8a and a noise removing capacitor 9a.

  A voltage dividing resistor 8b and a noise removal capacitor 9b are connected in parallel between the input terminal (inverting terminal) of the sound signal input − of the operational amplifier 3 and the output terminal of the operational amplifier. Further, a voltage dividing resistor 10a, a bias cut capacitor 11, and a switch SW3 (5c) as a third switch are provided in series at the output terminal of the operational amplifier 3. A switch SW1 (5a) as a first switch is directly connected between the input line of the audio signal input − and GND. In addition, a switch SW2 (5b) as a second switch is connected between the connection point of the voltage dividing resistor 10a and the bias cut capacitor 11 at the output terminal of the operational amplifier 3 and GND via the voltage dividing resistor 10b. . The switch 5c is connected to an output circuit such as a speaker or an amplifier (not shown).

  The bias cut capacitors 6a and 6b have the same constant (C1), the voltage dividing resistors 7a and 7b have the same constant (R1), the voltage dividing resistors 8a and 8b have the same constant (R2), and the noise removal capacitors 9a and 9b have The same constant (C2), the voltage dividing resistors 10a and 10b are the same constant (R3), the bias cut capacitor 11 is the constant (C3), and a power supply voltage (Vcc) 12 is supplied to the operational amplifier 3.

Next, the operation will be described with reference to the flowchart of FIG.
It is determined whether the user has selected an external audio input mode (AUX, etc.) (step ST1). If YES, SW3 is turned on to cancel the mute control (step ST2), and the audio signal input + side The signal level of (Point 1) is detected by the level detector 2 (Step ST3), and then the signal level of (Point 2) on the audio signal input side is detected by the level detector 2 (Step ST4). Upon receiving the detection result notification, the controller 1 controls the switches (SW1, SW2) according to the detected level (step ST5).

  Based on the signal level detected by the level detector 2, the control unit 1 controls the switching of the switches 5a, 5b, and 5c to switch between the single-end input circuit and the differential input circuit. In the external audio input mode (AUX or the like), level detection is repeated periodically, and control is always performed to the optimum circuit (step ST6). If the determination in step ST1 is NO, the switch SW3 is turned off and the mute circuit is continued (step ST7).

FIG. 3 is a diagram summarizing the combinations of the signal level of the audio signal input +, the signal level of the audio signal input−, and ON / OFF of each switch.
It is a point when the output format of the external audio device is single-ended output (no audio signal input-input). 1 and points. 2 signal levels are detected and controlled as follows according to the level.

1) Point. 1 detection level is 0 or more and less than V1, and points. 2 detection level is 0 or more and less than V2 <State 1>
When there is no or small audio signal input, it is determined that the external device is not connected.
Configure a single-ended input circuit. This reduces the influence of the induction noise on the negative line.
At this time, switch SW1: ON, switch SW2: OFF, and switch SW3: ON are set. V1 and V2 are set to small values.

2) Point. 1 detection level is V1 or higher and points. 2 detection level is less than V2
<State 2>
It is determined that the signal level is high to some extent, the external audio device is connected and there is an audio input. Therefore, a differential input circuit is configured to reduce noise that wraps around from GND. Even in the configuration of the differential input circuit, since the negative line is connected to the GND of the device, the impedance is reduced, and the influence of inductive noise on the negative line is reduced. At this time, the switch 5a is set to OFF, the switch 5b is set to OFF, and the switch 5c is set to ON.
When the audio signal input + is Vin and the audio signal input − is 0, the operational amplifier output level in the circuit configuration of FIG. 1 is Vin × (R2 / R1) in the differential input configuration.

On the other hand, even when a single-end input circuit is configured, Vin × (R2 / (R1 + R2)) × ((R1 + R2) / R1 = Vin × (R2 / R1), which is the same gain as that when a differential input circuit is configured. The switch SW2 is always kept OFF.
When the audio signal of the external audio device is a differential output (the audio signal input − is an input in which the phase of the audio signal input + is inverted, the level is the same). 1 and points. 2 signal levels are detected and controlled as follows according to the detected level.

1) Point. 1 detection level is 0 or more and less than V1, and points. 2 detection level is 0 or more and less than V2 <State 1>
<State 1>
When there is no or small audio signal input, it is determined that the external device is not connected.
Configure a single-ended input circuit. This reduces the influence of the induction noise on the negative line.
At this time, switch SW1: ON, switch SW2: OFF, and switch SW3: ON are set. V1 and V2 are set to small values.

2) Point. 1 detection level is V1 or higher and points. 2 detection level is V2 or higher
<State 3>
The signal level is high to some extent, and an external audio device is connected and there is an audio input. Therefore, a differential input circuit is configured to reduce noise that wraps around from GND. Even in the configuration of the differential input circuit, the negative line is connected to the GND of the device, so that the impedance is small and the influence of inductive noise is reduced. At this time, switch SW1: OFF, switch SW2: ON, and switch SW3: ON are set.
When the audio signal input + is Vin and the audio signal input − is −Vin, the level of the operational amplifier output in the circuit configuration of FIG. 1 is 2Vin × (R2 / R1) in the differential input circuit configuration.

On the other hand, when the single-ended input circuit is configured, Vin × (R2 / (R1 + R2)) × ((R1 + R2) / R1 = Vin × (R2 / R1), and the gain when the differential input circuit is configured is increased by 6 dB. Therefore, in order to eliminate the uncomfortable feeling when switching due to the gain difference, at the time of differential input, the switch SW2 is turned on, the gain is lowered by 6 dB (1/2), and the gain in the single-ended input configuration When switch SW2 is turned on during differential input,
2Vin × (R2 / (R1 + R2)) × ((R3 + 2R3) = Vin × (R2 / R1).

  According to the first embodiment, the single-ended input circuit, the differential input circuit, and the circuit are simply switched and controlled in accordance with the signal level of the input audio signal. This can reduce the influence of inductive noise and noise that circulates from GND at the time of voice input, and can provide a user with high-quality voice with less noise.

  Since the GND potential of the vehicle frequently fluctuates, noise is likely to occur, and because the vehicle has a long internal wiring, inductive noise is likely to occur when an external audio device is not connected. However, by applying the external audio input circuit of the present invention to a vehicle, it is possible to effectively reduce noise and inductive noise coming from GND.

Further, when the vehicle travels, the wiring connected to the voice input circuit is also vibrated by the vibration of the vehicle, and the induction noise to the wiring changes when the external voice device is not connected. Therefore, the vibration detection sensor 13 is provided, and the vibration value detected by the vibration detection sensor 13 is supplied to the control unit 1, and the control unit 1 switches between the single input circuit and the differential input circuit based on the supplied vibration value. The input level V1 of the audio signal is changed.
When vibration is large, induced noise is likely to occur, so the value of V1 is increased. When vibration is small, induced noise is less likely to occur and the level is decreased, so the value of V1 is decreased. With this configuration, according to the environment to which the present invention is applied, it is possible to more effectively realize the induction noise when the external audio device is not connected. The same configuration can be adopted when applied to a place where vibrations other than the vehicle are generated.

  DESCRIPTION OF SYMBOLS 1 Control part, 2 level detection part, 3 operational amplifier, 4 midpoint potential, SW1-SW3 (5a-5c) switch, 6a, 6b, 11 Bias cut capacitor, 7a, 7b, 8a, 8b, 10a, 10b Voltage dividing resistor , 9a, 9b Noise elimination capacitor, 12 power supply voltage, 13 vibration detection sensor.

Claims (4)

A differential input circuit having a non-inverting terminal and an inverting terminal for inputting an audio signal;
A level detection unit for detecting a signal level of an input line connected to the non-inverting terminal and the inverting terminal;
A first switch provided between the input line connected to the inverting terminal and the ground;
A second switch provided between the output line connected to the output terminal of the differential input circuit and the ground;
A third switch provided in an output line connected to the output terminal of the differential input circuit;
An external audio input circuit comprising a single-end input circuit, a differential input circuit, and a control unit for switching the circuit based on the output of the level detection unit.   When the voice input is a single input, the control unit sets the first switch to ON, the second switch to OFF, and the third switch to ON based on the output of the level detection unit. 2. The external audio input circuit according to claim 1, wherein the differential input circuit and the circuit are switched by turning off the switch, turning off the second switch, and turning on the third switch.   When the audio input is a differential input, the control unit sets the first switch to ON, the second switch to OFF, and the third switch to ON based on the output of the level detection unit. 2. The external audio input circuit according to claim 1, wherein the differential input circuit and the circuit are switched by turning off the second switch, turning on the second switch, and turning on the third switch.   A vibration detection sensor for detecting vibration is provided, and the control unit changes an input level for switching the circuit to a single-ended input circuit or a differential input circuit based on a vibration value detected by the vibration detection sensor. The external audio input circuit according to any one of claims 1 to 3.

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