JP2009171058A - Amplifier circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an amplifier circuit that prevents a noise signal from being continuously amplified for output when amplifying an input signal while changing a gain so that the amplitude of an output signal approaches a prescribed reference value, and can perform control so that the amplitude of the output signal does not change unnaturally. <P>SOLUTION: The amplifier circuit detects the amplitude of the output signal outputted by an amplifier having variable gain, changes the gain of the amplifier so that the amplitude of the output signal approaches the prescribed reference value based on the detected amplitude of the output signal, determines whether the input signal inputted to the amplifier is a signal to be amplified or a noise signal, and attenuates the output signal, while increasing an attenuation factor gradually when it is determined that the input signal is a noise signal. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an amplifier circuit that amplifies an input signal while changing the gain of the amplifier so that the amplitude of the output signal approaches a predetermined reference value.

For example, there is an amplifier circuit such as a so-called ALC (Automatic Level Control) circuit that amplifies an electric signal representing sound collected by a microphone while adjusting the gain so that the amplitude approaches a predetermined reference value (for example, Patent Document 1). In such an amplifier circuit, the gain is increased when the amplitude of the output signal output from the amplifier is lower than a predetermined reference value, and conversely, the gain is decreased when the amplitude is higher than the predetermined reference value. Thus, the gain of the amplifier is controlled.
JP 2006-41732 A

  In the conventional technique, when gain control is performed even when a signal to be amplified (amplification target signal) is not input, a noise signal (noise signal) having a small amplitude approaches a predetermined reference value. Will be amplified. As a result, there arises a problem of noise pumping in which a signal that should not be necessary is amplified with a maximum gain and output as a signal having a large amplitude. Thus, for example, a circuit that cuts the output of a signal while it is determined that the amplification target signal is not input has been proposed. However, in this method, the amplitude of the output signal suddenly becomes 0, or the output signal suddenly starts to output a signal having a large amplitude from the state where the amplitude of the output signal is 0. May change unnaturally.

  Further, as a method other than the above-described method, a circuit that holds the gain of the amplifier so as to maintain the gain immediately before it is determined that the amplification target signal is not input has been proposed. However, in this method, when a signal for amplification with a relatively small amplitude is input and, as a result, the state in which the input signal is amplified with a relatively large gain is changed to a state in which a noise signal is input, this comparison is performed. A large gain is maintained. For this reason, depending on how the amplitude of the input signal changes, it may not be possible to prevent a state in which the noise signal is continuously amplified and output.

  The present invention has been made in view of the above circumstances, and one of its purposes is to reduce noise when the input signal is amplified while changing the gain so that the amplitude of the output signal approaches a predetermined reference value. An object of the present invention is to provide an amplifier circuit that can prevent a state in which a signal is continuously amplified and output, and can control the amplitude of an output signal so that it does not unnaturally change.

  An amplifier circuit according to the present invention for solving the above problems is based on an amplifier with variable gain, amplitude detection means for detecting the amplitude of an output signal output from the amplifier, and the amplitude of the detected output signal. Gain control means for changing the gain of the amplifier so that the amplitude of the output signal approaches a predetermined reference value; determination means for determining whether the input signal input to the amplifier is an amplification target signal or a noise signal; And a signal attenuating unit for attenuating the output signal while gradually increasing the attenuation rate when the determining unit determines that the input signal is a noise signal.

  According to the present invention, in an amplifier circuit that amplifies an input signal while changing the gain so that the amplitude of the output signal approaches a predetermined reference value, when the input signal is determined to be a noise signal, the attenuation factor By gradually increasing the output signal and attenuating the output signal, it is possible to prevent the amplified noise signal from being continuously output while controlling the amplitude of the output signal so that it does not change unnaturally.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  As shown in FIG. 1, an amplifier circuit 2 according to an embodiment of the present invention includes a variable gain amplifier 4, a register 6, an A / D converter 8, a gain control unit 10, a signal determination unit 12, and a digital And a volume circuit 14. For example, the amplifier circuit 2 according to the present embodiment is an audio codec circuit that receives a voltage signal representing sound collected by a microphone or the like as an input signal and converts the input signal into digital data having a substantially constant volume level. Used.

  The variable gain amplifier 4 is a programmable gain amplifier or the like, accepts a voltage signal input from the outside as an input signal Vi, and outputs an output signal Vo obtained by amplifying the input signal Vi to the A / D converter 8. . The gain (amplification factor) G of the variable gain amplifier 4 is expressed in units of dB (decibel), and is a value proportional to the common logarithm of the ratio of the output signal Vo to the input signal Vi.

  The gain G of the variable gain amplifier 4 changes based on a control signal input from the outside of the variable gain amplifier 4. In the present embodiment, the control signal is a digital signal indicating a discrete value, and the gain G changes discretely according to this digital signal.

  Specifically, the variable gain amplifier 4 in this embodiment converts the input signal Vi with a gain G corresponding to the stored value Dr stored in the register 6. The register 6 is a memory for storing digital information, and stores a stored value Dr in the present embodiment. The stored value Dr is updated by the gain controller 10 as will be described later. Then, of the stored value Dr, for example, a value represented by a predetermined upper bit corresponding to the integer part is input to the variable gain amplifier 4 as the value of the gain G.

  The gain G may be a negative value. When the gain G has a negative value, the variable gain amplifier 4 attenuates the input signal Vi and outputs an output signal Vo having a smaller amplitude.

  The A / D converter 8 is a circuit that converts an analog signal into a digital signal, detects the output signal Vo output from the variable gain amplifier 4, converts it to a digital value (hereinafter referred to as an output value Do1), and outputs it. . Specifically, the A / D converter 8 samples the output signal Vo every predetermined time, and outputs the result as an output value Do1. The predetermined time in this case is determined by, for example, a clock signal supplied from the outside.

  The gain control unit 10 is configured by, for example, a logic circuit and receives an output value Do1 output by the A / D converter 8 every predetermined time as an input. A digital value representing the amplitude (level) of the output signal Vo (hereinafter referred to as an amplitude value Dl) is calculated by performing digital detection processing on the signal represented by the output value Do1. Further, the gain control unit 10 performs control to change the gain G of the variable gain amplifier 4 so that the amplitude value Dl approaches the predetermined reference value Da. As a specific example, the gain control unit 10 first compares the amplitude value Dl with the reference value Da. When the amplitude value Dl is larger than the reference value Da, control is performed to change the gain G in a direction to decrease the gain G, and when the amplitude value Dl is smaller than the reference value Da, control to change the gain G is performed.

  In the present embodiment, the gain control unit 10 gives a positive addition value ΔDr when the amplitude value Dl is smaller than the reference value Da, and conversely when the amplitude value Dl is larger than the reference value Da. Is added to the stored value Dr stored in the register 6, respectively. As described above, the variable gain amplifier 4 performs amplification with the gain G corresponding to the stored value Dr. Therefore, the gain control unit 10 can change the gain G so that the amplitude value Dl approaches the predetermined reference value Da by updating the stored value Dr.

  The process of adding the addition value ΔDr to the stored value Dr is executed, for example, every time the A / D converter 8 samples the output signal Vo and outputs a new output value Do1 to the gain control unit 10. Thereby, for example, while the amplitude of the output signal Vo is lower than a predetermined reference value, the process of the gain control unit 10 adding the positive addition value ΔDr to the stored value Dr is repeated every predetermined time. As a result of repeatedly performing this addition process, when the value represented by the integer part of the stored value Dr increases due to carry, the gain G of the variable gain amplifier 4 also increases.

  The signal determination unit 12 is configured by, for example, a logic circuit or the like, and similarly to the gain control unit 10, receives the output value Do1 output by the A / D converter 8 every predetermined time as an input, and outputs based on the output value Do1. An amplitude value Dl representing the amplitude of the signal Vo is calculated. Based on the amplitude value D1, it is determined whether the input signal Vi input to the variable gain amplifier 4 is an amplification target signal or a noise signal.

  Here, the amplification target signal is a signal whose amplitude satisfies a predetermined condition and is a target of amplification by the amplifier circuit 2. That is, it is an object of the amplifier circuit 2 according to this embodiment to amplify and output this signal so that its amplitude approaches a predetermined reference value. On the other hand, the noise signal is an input signal that does not include such an amplification target signal, and this noise signal is a signal that does not need to be amplified. Note that the determination processing by the signal determination unit 12 is also repeatedly executed at predetermined time intervals (for example, every time the A / D converter 8 samples the output signal Vo and outputs a new output value Do1 to the gain control unit 10). Is done.

  Specifically, for example, the signal determination unit 12 determines whether the input signal Vi is an amplification target signal or a noise signal by comparing the magnitude between the amplitude value Dl and a predetermined threshold value. That is, if the amplitude value D1 is smaller than a predetermined threshold value, the input signal Vi is determined to be a noise signal, and otherwise, the input signal Vi is determined to be an amplification target signal. Further, when the period in which the amplitude value Dl is smaller than a predetermined threshold continues for a predetermined period or longer, it may be determined that the input signal Vi has changed from the amplification target signal to the noise signal.

  Alternatively, the signal determination unit 12 calculates an estimated value De of the amplitude of the input signal Vi based on the amplitude value Dl and the gain G of the variable gain amplifier 4, and calculates the estimated value De and a predetermined threshold value. By comparing, it may be determined whether the input signal Vi is an amplification target signal or a noise signal. In this case, the signal determination unit 12 not only receives the output value Do1 from the A / D converter 8 as an input, but also refers to the stored value Dr stored in the register 6, for example, to convert the output value Do1 into the A / D The value of the gain G of the variable gain amplifier 4 when the converter 8 outputs is acquired. Since the amplitude value Dl is a digital value representing the amplitude of the output signal Vo obtained by amplifying the input signal Vi with the gain G, the amplitude value Dl is based on the amplitude value Dl and the value of the gain G. The estimated value De representing the amplitude can be calculated as a digital value. For example, when the estimated value De is smaller than a predetermined threshold value, the signal determination unit 12 determines that the input signal Vi is a noise signal. Similarly to the example in which the amplitude value D1 itself is compared with a predetermined threshold value, when the estimated value De continues for a predetermined period or longer for a period smaller than the predetermined threshold value, the input signal Vi is changed from the amplification target signal to the noise signal. It may be determined that it has changed. As described above, although the estimated value De is used to determine whether the target signal is an amplification target signal or a noise signal, the circuit scale becomes larger compared with the case where the determination is made based only on the amplitude value Dl. For example, when the gain G of 4 can take a large value, it is possible to make a determination in accordance with the actual amplitude of the input signal Vi.

  Further, as described above, when the amplitude value Dl or the estimated value De is compared with a predetermined threshold, the signal determination unit 12 uses two types of values (that is, the first threshold T1) as the predetermined threshold depending on the scene. And the second threshold value T2) may be properly used. Specifically, for example, the signal determination unit 12 determines that the input signal Vi is the amplification target signal at the previous determination, and determines whether the input signal Vi changes from the amplification target signal to the noise signal in the current determination. When determining, the amplitude value D1 or the estimated value De is compared with the first threshold value T1. Conversely, the input signal Vi is determined to be a noise signal at the time of the previous determination, and when determining whether or not the input signal Vi changes from the noise signal to the amplification target signal in the current determination, the amplitude value Dl. Alternatively, the estimated value De is compared with the second threshold value T2. Here, a relationship of T1 <T2 is established between the first threshold value T1 and the second threshold value T2. By doing so, it is possible to avoid a state in which the determination result frequently fluctuates due to a slight fluctuation in the amplitude of the input signal Vi.

  The digital volume circuit 14 is composed of, for example, a logic circuit and receives an output value Do1 output from the A / D converter 8 as an input. Then, according to the determination result of the signal determination unit 12, the output value Do1 is attenuated and output as a new digital value (hereinafter referred to as output value Do2). That is, the output value Do1 is converted into a smaller value according to a given attenuation rate A, and is output as the output value Do2. This output value Do2 becomes the output of the entire amplifier circuit 2. The timing at which the digital volume circuit 14 attenuates the output value Do1 and the attenuation rate A at the time of attenuation are determined according to the determination result of the signal determination unit 12 as described below.

  Specifically, the digital volume circuit 14 outputs the output value Do1 as it is as the output value Do2 without being attenuated while the signal determination unit 12 determines that the input signal Vi is the amplification target signal. However, when the signal determination unit 12 determines that the input signal Vi has changed from the amplification target signal to the noise signal, the amplification factor A is gradually increased while increasing the attenuation factor A (that is, expressed by a negative value with dB as a unit). While gradually decreasing the rate value), the output value Do1 is attenuated. As a specific example, the digital volume circuit 14 attenuates the output value Do1 while increasing the attenuation rate A by a predetermined value (for example, 1) in dB units at predetermined time intervals. Thereby, when the input signal Vi changes from the amplification target signal to the noise signal, the output value Do2 is equal to the output value Do1, but the output value Do2 does not change even if the amplitude of the output signal Vo does not change with time. The amplitude of becomes smaller. As a result, the amplitude of the noise signal amplified to a certain level by the variable gain amplifier 4 can be output while gradually changing it to a small value instead of suddenly having the minimum value.

  Further, as described above, the digital volume circuit 14 gradually increases the attenuation rate A while it is determined that the input signal Vi is a noise signal. As a result, the attenuation rate A reaches a predetermined maximum value Amax. Thereafter, a predetermined minimum value (for example, 0) may be output as the output value Do2, regardless of the amplitude of the output value Do1. In this way, the amplification circuit 2 gradually reduces the amplitude of the output value Do2 while the noise signal is being input, and then outputs a predetermined minimum value as an output for the noise signal that is originally an unnecessary signal. Can be output.

  Further, the digital volume circuit 14, when the signal determination unit 12 determines that the input signal Vi has changed from the noise signal to the amplification target signal, gradually decreases the attenuation factor A (that is, negative in dB as a unit). The output value Do1 may be attenuated and output as the output value Do2 until the attenuation rate A becomes 0 (while gradually increasing the amplification factor represented by the value of. Specifically, for example, immediately after the input signal Vi changes from the noise signal to the amplification target signal, the digital volume circuit 14 attenuates the output value Do1 with a predetermined maximum value Amax, and then sets the attenuation rate A every predetermined time. The output value Do1 is attenuated while decreasing by a predetermined value (for example, 1) in dB units. When the attenuation rate A returns to 0, the output value Do1 is not attenuated and is output as it is as the output value Do2. Thereby, even when the input signal Vi changes from the noise signal to the amplification target signal, the amplifier circuit 2 does not suddenly output a signal having a large amplitude, but gradually changes the output signal to a large value, It is possible to return to a signal having an amplitude corresponding to the predetermined reference value Da.

  FIG. 2 shows the change in the amplitude of the input signal Vi, the change in the estimated value De calculated by the signal determination unit 12, the change in the attenuation factor of the digital volume circuit 14, and the amplification circuit 2 when such control is performed. It is a figure which shows the correspondence between the change of the amplitude of output value Do2 which outputs. In this figure, the uppermost graph shows the change in the amplitude of the input signal Vi. As shown in the figure, the input signal Vi changes to a signal having a small amplitude (noise signal) at time t1, and thereafter, at time t2. The signal again changes to a signal having a large amplitude (amplification target signal). Further, the second graph from the top shows the time change of the estimated value De calculated by the signal determination unit 12 in accordance with the change of the input signal Vi.

  Further, the third graph from the top shows the change in the attenuation factor A in the digital volume circuit 14. As shown in this graph, after the time t1, when the signal determination unit 12 determines that the input signal Vi has changed from the amplification target signal to the noise signal, the digital volume circuit 14 changes the attenuation rate A from 0 to the maximum value. It gradually increases to Amax. When the signal determination unit 12 determines that the input signal Vi has changed from the noise signal to the amplification target signal after time t2, the digital volume circuit 14 gradually decreases the attenuation rate A from the maximum value Amax to 0. Yes. The lower graph shows the change in the amplitude of the output value Do2 (that is, the output signal of the amplifier circuit 2). As shown in this graph, after time t1, the noise signal amplified by the variable gain amplifier 4 is gradually attenuated by the digital volume circuit 14, and thereafter the output of the amplifier circuit 2 is a predetermined minimum value (here, 0). )It has become. Further, after time t2, the output signal of the amplifier circuit 2 gradually increases from 0, and then is converted to a signal of substantially the same level as before time t1.

  According to the amplifier circuit 2 according to the present embodiment described above, when the input signal Vi is determined to be a noise signal, the output value Do1 representing the output signal Vo is attenuated while gradually increasing the attenuation factor A. By doing so, it is possible to prevent the noise signal from being amplified and continuously output while controlling the amplitude of the output value Do2 that is finally output to be unnaturally changed.

  The embodiment of the present invention is not limited to the above-described embodiment. For example, in the above description, after the output signal Vo is converted into the output value Do1 by the A / D converter 8, the digital volume circuit 14 attenuates the output value Do1, but the present invention is not limited to this. The output signal may be attenuated in the state of the voltage signal by a gain amplifier or the like.

1 is a circuit diagram illustrating a schematic configuration example of an amplifier circuit according to an embodiment of the present invention. It is a figure which shows an example of the signal which the amplifier circuit which concerns on embodiment of this invention outputs according to an input signal.

Explanation of symbols

  2 amplifying circuit, 4 variable gain amplifier, 6 register, 8 A / D converter, 10 gain control unit, 12 signal determination unit, 14 digital volume circuit.

Claims (6)

A variable gain amplifier;
Amplitude detecting means for detecting the amplitude of the output signal output from the amplifier;
Gain control means for changing the gain of the amplifier based on the detected amplitude of the output signal so that the amplitude of the output signal approaches a predetermined reference value;
Determining means for determining whether an input signal input to the amplifier is an amplification target signal or a noise signal;
Signal attenuating means for attenuating the output signal while gradually increasing the attenuation rate when the determining means determines that the input signal is a noise signal;
An amplifier circuit comprising: The amplifier circuit according to claim 1,
The amplification circuit according to claim 1, wherein the signal attenuating means gradually increases the attenuation rate until a predetermined maximum value is reached, and then sets the amplitude of the output signal to a predetermined minimum value. The amplifier circuit according to claim 1 or 2,
The signal attenuating means attenuates the output signal until the attenuation rate becomes 0 while gradually reducing the attenuation rate when the input signal changes from a noise signal to an amplification target signal. circuit. The amplifier circuit according to any one of claims 1 to 3,
The determination means determines whether the input signal is an amplification target signal or a noise signal by comparing the amplitude of the output signal with a predetermined threshold based on the detected amplitude of the output signal. An amplifier circuit characterized by the above. The amplifier circuit according to any one of claims 1 to 3,
The determination means compares the input signal amplitude estimated value calculated based on the detected output signal amplitude and the gain of the amplifier with a predetermined threshold value, thereby comparing the input signal. An amplification circuit characterized by determining whether a signal is an amplification target signal or a noise signal. The amplifier circuit according to claim 4 or 5,
When determining whether the input signal changes from an amplification target signal to a noise signal, the determination unit uses a first threshold as the predetermined threshold, and the input signal changes from the noise signal to the amplification target signal. When determining whether or not to change, an amplifying circuit characterized in that determination is performed using a second threshold value larger than the first threshold value as the predetermined threshold value.

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