JPH04273705A - Pop noise reduction circuit - Google Patents

Abstract

PURPOSE:To surely eliminate a pop noise and to prevent malfunction from occurring by detecting the disconnection of a power switch at high speed, and applying mute to a signal. CONSTITUTION:This circuit is provided with a mute circuit 4 which applies the mute to an input audio signal SIG at need, a comparator 6 which compares two terminal voltages of the power switch POW and turns on a mute signal MUTE when the difference of them exceeds a constant value, and a speaker SP driven by a power IC 3. When potential at a point A goes less than that at a point B, a transistor T3 is turned on. and the mute signal MUTE is turned on. Since such condition is established immediately after the switch POW is turned off, the mute circuit 4 can be quickly operated. Also, since the voltage at the point A is equal to that at the point B by a ripple on an ACC when the switch POW is turned on, no malfunction of the mute circuit 4 can be prevented from occurring.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pop noise reduction circuit for reducing pop noise when power is turned off.

0002

2. Description of the Related Art In-vehicle audio equipment uses a power source stabilized by a regulator because output fluctuations are large when the vehicle's battery is used directly as a power source.

For example, as shown in FIG. 2, an audio circuit 2 is operated with a voltage VCC stabilized by an 8V regulator 1, and the audio circuit 2 outputs VCC/VCC as shown in FIG.
Audio signals SIG each having a maximum amplitude of VCC/2 above and below centering on VCC/2 are output.

The input of regulator 1 is power switch P.
This is an accessory power supply ACC provided through OW. The value of this ACC varies from 10 to 1 depending on the ambient temperature, load conditions, etc.
Although it fluctuates within a range of 6V, the audio circuit 2 can operate normally as long as the regulator 1 is operating.

However, when the regulator input drops to about 9V when the power switch POW is turned off,
It becomes difficult to secure VCC=8V. As a result, VCC/2 begins to fluctuate as shown by the broken line in FIG. 3, which becomes one of the causes of pop noise generation. Therefore, conventionally, a mute circuit is provided between the audio circuit 2 and the power IC 3 to prevent the pop noise from being output from the speaker SP when the power switch POW is turned off.

FIG. 4 shows an example of a conventional pop noise reduction circuit, in which 4 is a mute circuit provided before the power IC 3, and 5 is an ACC drop detection circuit.

The mute circuit 4 is composed of an npn transistor T2 and its base input resistors R22 and R23, which ground the audio signal SIG input to the power IC 3 through a resistor R21 and a DC cut capacitor C2 when necessary, and operates the transistor T2. A mute signal MUTE is input from the ACC drop detection circuit 5.

[0008] The ACC drop detection circuit 5 includes a power switch P.
The value of the ACC voltage through OW is set to the predetermined Zener voltage VZ.
When ACC falls below VZ, the npn transistor T1 is turned off and the mute signal MUTE is turned on.

That is, if ACC exceeds VZ, Zener diode ZD is turned on and supplies base current to transistor T1 (R12 and R13 are base input resistances). Therefore, the transistor T1 is turned on and the mute signal MUTE is turned off. R11 is a resistor that pulls up the collector of transistor T1 to ACC.

0010

[Problems to be Solved by the Invention] In the circuit shown in FIG. 4, it is difficult to select the Zener voltage VZ. That is, in order for the 8V regulator 1 to ensure VCC=8V, the input only needs to be about 9V. Therefore, if the base-emitter voltage VBE of the transistor T1 is 0.6 to 0.7V, the standard value of the Zener voltage VZ is about 8.2V.

However, when VZ is set to 8.2V, A
If the power switch POW is turned off when CC is at a high value (up to about 16), it takes time for the potential at point A to drop below 9V, and pop noise is generated from the power IC 3 during that time.

In order to avoid this point, the Zener voltage VZ
For example, if the voltage is set as high as 9V, if the battery becomes weak and ACC drops to around 10V, if a ripple appears on the power supply of the power IC3, the detection circuit will fail even though the ripple removal capacitor C1 is used. 5 may malfunction and turn on the mute signal MUTE.

On the other hand, if VZ is set as low as 7V, even if the above-mentioned malfunction can be prevented, muting is always delayed and is not useful for removing pop noise.

The present invention eliminates the above-mentioned absolute level type ACC voltage drop detection, and instead detects disconnection of the power switch at high speed and mutes it, thereby reliably removing pop noise and preventing malfunction. That is.

[0015]

[Means for Solving the Problems] The present invention compares the voltages across a mute circuit that mutes the audio input to a power stage that drives a speaker when necessary, and a power switch that supplies power to the power stage, and In this case, the present invention is characterized by comprising a comparison circuit that operates the mute circuit.

[0016]

[Operation] The voltages across the power switch are equal when the switch is on, but as soon as the switch starts to turn off, a potential difference occurs. This is the same whether the immediately preceding ACC voltage is high or low. Therefore, if a comparison circuit is provided that operates the mute circuit when there is a difference in the voltage across the power switch,
Pop noise when the power switch is turned off can be quickly removed, and there is no malfunction caused by ripple voltage.

[0017]

[Embodiment] FIG. 1 is a block diagram showing an embodiment of the present invention.
4 is a power IC, 4 is a mute circuit having the same configuration as in FIG. 4, which mutes the input audio signal SIG when necessary, 6
SP is a comparison circuit that compares the voltages across the power switch POW and turns on the mute signal MUTE when the difference exceeds a certain value, and SP is a speaker driven by the power IC 3.

Comparison circuit 6 compares the voltages at both ends A and B of power switch POW, and includes a pnp transistor T3 that turns on when the potential at point A (power IC side) is lower than point B (ACC side) by more than VBE. . R31 and R32 are its base input resistances. Point A is this transistor T3
The point B is connected to the base of the transistor through a diode D2 in the forward direction, and the point B is connected to the emitter of the same transistor through the diode D1 in the forward direction. The diode D1 is used to prevent backflow, and the diode D2 is used to balance the potential when D1 is used.

The potential at point A is equal to the potential at point B when the switch POW is on, and becomes lower than the potential at point B when the switch is off. This relationship does not depend on the voltage of ACC. When the potential at point A becomes VBE lower than that at point B, transistor T3 turns on, turning on mute signal MUTE. Since this condition is established immediately after the switch POW is turned off, the mute circuit 4 operates quickly to prevent pop noise from occurring.

This mute operation is not affected by the value of ACC when switch POW is turned off. Therefore, A.C.
Even if the value of C is high and it takes time to drop to a voltage that affects the output VCC of the 8V regulator, pop noise can be reliably removed. Moreover, if the switch POW is on, the voltages at points A and B are equal even if a ripple is applied to ACC, so that the mute circuit 4 will not malfunction.

Although the comparison circuit 6 can be constructed using an operational amplifier, it is cheaper to construct it using a transistor as shown in FIG. 1, and there is no need to set a reference voltage.

[0022]

[Effects of the Invention] As described above, according to the present invention, it is possible to quickly eliminate pop noise when the power switch is turned off in in-vehicle audio equipment, and there is also an advantage that malfunctions do not occur depending on the battery voltage value. have

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of the present invention.

FIG. 2 is a block diagram of an in-vehicle audio device.

FIG. 3 is an operational waveform diagram of FIG. 2;

FIG. 4 is a configuration diagram of a conventional pop noise reduction circuit.

[Explanation of symbols]

3 Power IC 4 Mute circuit 6 Comparison circuit POW power switch SP Speaker

Claims (1)

[Claims] 1. A mute circuit that mutes audio input to a power stage that drives a speaker when necessary and a voltage across a power switch that supplies power to the power stage, and when they do not match, operates the mute circuit. A pop noise reduction circuit characterized by comprising a comparison circuit for reducing noise.