JP3815836B2 - Audio signal amplifier circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an audio signal amplifier circuit used for a telephone, for example.
[0002]
[Prior art]
Many telephones, for example, answering machines, have a speakerphone function so that a hands-free call can be made, or a call recording function to leave a message when the user is away. Since the required amplification characteristics of the audio signal are different between the speakerphone operation and the call recording operation, two audio amplifiers are provided for each operation as shown in FIG.
[0003]
In FIG. 8, the output destination of the input audio signal is switched by the switch 11. During the recording operation, the signal is amplified by the amplifier 12 and then adjusted by an ALC (Automatic Level Control) unit 13 so that the output level becomes constant. The output of the ALC unit 13 is input to a DSP (Digital Signal Processor) 15. When the speakerphone is in operation, the audio signal is amplified by the amplifier 14 and then input to the DSP 15. The DSP 15 is for performing a process for hands-free call using a speakerphone, a process for recording a call (answering machine), and the like.
[0004]
In a conventional telephone, when a DSP is used for call recording and speakerphone processing, gain setting and correction are required according to the state, and the audio signal is switched by the changeover switch 11 as shown in FIG.
[0005]
The recording level is corrected by a method of making the output level constant by using an IC for ALC function or the like. ALC is a method of amplifying a small sound to a certain amount.
[0006]
[Problems to be solved by the invention]
In the prior art described above, two systems of audio amplifiers and switches for call recording operation and speakerphone operation must be provided, and the structure is complicated.
[0007]
In addition, in the conventional ALC, even a small sound with a poor S / N such as a sound from a telephone line is amplified with a high amplification factor, so that the noise is excessively amplified and difficult to hear. For example, Japanese Patent Laid-Open No. 8-8669 discloses a device in which a level conversion means is provided in the input stage in order to adjust the input level when a signal exceeding the reference value of the ALC circuit is input. However, the problem of amplifying noise with a high amplification factor for a small sound whose S / N is not so good has not been solved.
[0008]
The present invention has been made to solve the above-described problems, and an object thereof is to provide an audio signal amplifier circuit that does not require a plurality of audio amplifier circuits and that can simplify the signal path and the configuration of the apparatus. . It is another object of the present invention to provide an audio signal amplifying circuit that can arbitrarily set a gain control width and can naturally record without excessively suppressing an increase or decrease in volume.
[0009]
[Means for Solving the Problems]
An audio signal amplifying circuit according to the present invention is an audio signal amplifying circuit that operates in either a first operation mode or a second operation mode having different amplification characteristics, the amplifier amplifying an input audio signal, An output level detection unit for detecting a level of an output signal of the amplifier; an amplification rate control unit for controlling the amplification rate of the amplifier based on a detection result of the output level detection unit; and the amplification rate in the second operation mode. And a control unit that stops control of the amplification factor by the control unit.
[0010]
The first operation mode is, for example, a telephone call recording operation mode of the telephone, and the second operation mode is, for example, a speakerphone operation mode of the telephone.
[0011]
In the audio signal amplifier circuit according to the present invention, the amplification factor control unit controls the amplification factor so that the output level is substantially constant between a predetermined first operating point and a second operating point. is there.
[0012]
The first operating point is determined based on, for example, a monitoring level of an input signal, and the second operating point is determined based on, for example, a recording level.
[0013]
In the audio signal amplifier circuit according to the present invention, the gain control unit is connected in series to the first resistor and the second resistor that divide the level of the input audio signal, and the second resistor, and the output level detection unit And an active element that changes the division ratio of the audio signal.
[0014]
The audio signal amplification circuit according to the present invention is based on the control signal of the control unit, and the amplification of the amplifier is performed so that the difference between the amplification factor of the first operation mode and the amplification factor of the second operation mode is averaged. And a second gain control unit for controlling the rate.
[0015]
Averaging the amplification factor means reducing the difference in output signal for each operation mode so that the subsequent processing of the audio signal amplifier circuit can be performed appropriately. Even if the amplification factor is different for each operation mode, the overall average value of the input-output characteristics is reduced.
[0016]
In the audio signal amplifier circuit according to the present invention, the second gain control unit averages the difference from the gain by attenuating the output of the amplifier in the first operation mode.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Embodiment 1 of the Invention
FIG. 1 shows a functional block diagram of an apparatus according to Embodiment 1 of the present invention. In this figure, a gain control unit 1 receives an input signal and performs predetermined amplification processing or attenuation processing to set the input signal to an appropriate level. The audio amplifier (audio AMP) 2 amplifies the output of the gain control unit 1 and outputs the amplified output as an ALC (Automatic Level Control) output signal to a DSP (Digital Signal Processor) (not shown). In a telephone to which the apparatus according to the first embodiment of the present invention is applied, a DSP performs processing for a hands-free call using a speakerphone, call recording (answering machine) processing, line signal such as busy signal and busy tone signal. This is for performing monitoring processing. The detection amplifier (AMP) 3 amplifies the ALC output signal and supplies it to the output detection unit 4. The output detection unit 4 detects the ALC output signal and inputs this detection output to the gain control unit 1. A microcomputer (MICON) 6 generates a control signal for the gain control unit 1. The operation of the gain control unit 1 will be described later.
[0018]
FIG. 2 is a specific circuit configuration example of the apparatus of FIG. The main components of the gain control unit 1 are resistors R1 and R2 and a transistor Q1. The transistor Q1 functions as a variable resistor. Therefore, the level of the input signal of the audio amplifier 2 is determined by the division ratio between the resistance value R1 and the resistance value (R2 + Q1). The transistor Q1 is directly controlled by the signal detected by the output detector 3, and the voltage division ratio is changed. Eventually, the output level of the audio amplifier 2 can be controlled by the output level of the output detector 4 in FIG. Here, the resistor R2 is a limiting resistor, so that even if the resistance value of the transistor Q1 is small (ideally zero), the division ratio does not fall below a certain limit. Therefore, the gain control unit 1 works to keep the output constant only in the set range. The operation of the gain control unit 1 can be turned on / off by turning on / off the transistor Q1 with the control signal CONT of the microcomputer 6. For this reason, as described later, it is possible to cope with both the recording state and the speakerphone state without changing the audio signal path.
[0019]
The gain control setting range can be freely set by a series resistor R2 inserted in the transistor Q1. In this example, it is set to 20 dB.
[0020]
When gain control is performed, that is, when the ALC is ON, the input-output characteristic is shown in the curve 101 of FIG. 3, and when the ALC is OFF, the input-output characteristic is shown in the curve 103, respectively. A dotted curve 102 is a characteristic when there is no resistance R2.
[0021]
Next, the operation of the apparatus according to the first embodiment of the present invention will be described. The device according to the first embodiment of the present invention performs a recording operation for recording an audio signal from a line to function as an answering machine and a speakerphone operation for a hands-free call using a speaker and a microphone. Different operations have different performance requirements. The apparatus according to the first embodiment of the present invention is configured so as to be compatible with any operation.
[0022]
<Recording operation>
This device is characterized in that a resistor R2 is inserted in series with the gain control unit of the ALC so that the ALC does not work intentionally above a certain value. In other words, it should be called pseudo ALC. The insertion of the resistor R2 in this way is not so much theoretically preferable in that sound distortion is generated. However, in the case of a telephone, unlike a case of music equipment, a very small distortion is allowed.
[0023]
In essence, the ALC desirably functions so that the output level is constant over a wide range of input signal levels. However, there are the following problems when the telephone is set to a normal ALC operation.
[0024]
(1) The gain becomes very high, especially in the case of a sound with a poor S / N from the beginning, such as a sound from a telephone line, resulting in excessive amplification of noise, making it difficult to hear.
[0025]
(2) The DSP monitors the busy tone and busy tone, and controls the operation according to the level. If the gain linearity is lost by the ALC function, the level detection function necessary for the DSP operation is also adversely affected.
[0026]
In order to avoid these (1) and (2), it is conceivable to provide a plurality of audio signal systems and amplify them with different gains as necessary. However, this makes the configuration complicated, increases the number of parts, and further increases the signal level. A switching circuit and its control circuit are required.
[0027]
Therefore, in the apparatus according to the first embodiment of the present invention, the range of ALC operation is narrowed while sacrificing the distortion rate to some extent, and the gain is increased (attenuation only in the case of a small input, that is, a sound that occurs when the line is far away). The circuit is configured to reduce the amount).
[0028]
The operation will be described based on the characteristic diagram of FIG.
[0029]
When the input level is −45 dBm or less, the characteristic of the curve 101 is linear in which the output increases as the input increases in this range. This is because the detection output of the output detection unit 4 is low and the transistor Q1 is in the off state. Therefore, the attenuation amount of the gain control unit 1 is the lowest, and the input signal is input to the audio amplifier 2 via the resistor R1 without being attenuated so much.
[0030]
In this range when the input level is from −45 dBm to −25 dBm, the characteristic of the curve 101 is a flat characteristic that the output is almost constant despite the increase in input. This is because, when the level of the input voice falls within the ALC operation range, the transistor Q1 is turned on by the detection output of the output detection unit 4, and the attenuation increases according to the level of the input signal.
[0031]
When the input level is −25 dBm or more, the characteristic of the curve 101 becomes linear again in this range. If the input level exceeds the ALC operating range, the resistance value of the transistor Q1 becomes smaller than that of the resistor R2, the output ratio of the output detector 4 increases, and even if the resistance value of the transistor Q1 is further reduced, the division ratio is This is because it does not change much. The division ratio at this time is determined by the resistors R1 and R2. Thus, the attenuation amount of the gain control unit 1 becomes equal to or less than a certain value (MAX) by the resistor R2.
[0032]
Since the gain does not become very high in the region where the input level is low as described above, the noise is not excessively amplified with respect to the sound having a poor S / N like the sound from the telephone line from the beginning. It becomes easy to hear.
[0033]
Incidentally, in the example of FIG. 3, the ALC range is from −45 dBm to −25 dBm of the input level, but this is an example. The upper limit (point A in FIG. 3) and the lower limit (point B in FIG. 3) of the ALC range are determined from the following viewpoints.
[0034]
Point A is a point indicating the detection limit of the line input level of the DSP. Since the linearity of the gain is necessary for the DSP level detection function to operate normally, the ALC range is set so that the point A is slightly outside.
[0035]
Point B is a point indicating the optimum level of recording. In order to keep the recording state well, the ALC range is set so that the point B is slightly outside.
[0036]
By the way, when the value of the resistor R2 is changed, the characteristics change as shown in FIG. 4 with the second operation mode as a reference. Therefore, the ALC range can be set to a desired range by adjusting the value of the resistor R2. In FIG. 4, when the resistance value is reduced, the ALC range is expanded as shown by a curve 101a, and conversely, when the resistance value is increased, the ALC range is reduced as shown by a curve 101c. Therefore, a desired ALC range can be obtained by appropriately adjusting the value of the resistor R2.
[0037]
<Speakerphone operation>
Considering only the case of recording, a sufficient effect can be obtained even with the characteristic of the curve 101 in FIG. 3, but a linear amplification characteristic is desirable when the speakerphone is operated. In the apparatus according to the first embodiment of the present invention, the speakerphone operation is performed with the same audio circuit to simplify the configuration and reduce the number of parts. That is, during the speakerphone operation, the transistor Q1 of the gain control unit 1 is directly turned on by the control signal CONT from the microcomputer 6. Then, the attenuation amount of the gain control unit 1 is fixed to MAX, and the ALC does not function. Therefore, the input-output characteristic is as shown by the curve 103 in FIG. 3, and a linear amplification characteristic is obtained.
[0038]
As described above, the recording operation and the speakerphone operation can be switched under the control of the microcomputer 6. Therefore, even when the audio circuit is shared, both the recording operation and the speakerphone operation can be appropriately performed. In practice, it is considered that the recording operation and the speakerphone operation are not performed at the same time. Since the recording operation is generally performed during an answering machine operation, while the speakerphone operation is performed during a normal conversation, these operations are generally performed in separate cases.
[0039]
Embodiment 2 of the Invention
As can be seen from the characteristics 101 and 103 in FIG. 3, when the input level is low (−25 dBm or less), there is a relatively large difference in amplification characteristics between the characteristics 101 during recording operation and the characteristics 103 during speakerphone. It can be seen that there is a maximum of about 20 dB. If the position of the point A in FIG. 3 is appropriately set, it is considered that there is not much influence, but if the level difference due to correction becomes extremely large, it is difficult to optimally set the monitoring input level during level monitoring by the DSP. It is.
[0040]
Therefore, a separate gain switching means is provided outside the ALC loop, and is configured to be switched simultaneously by the control signal of the gain control unit 1 of the ALC. By controlling the characteristics when ALC is off (curve 101 in FIG. 3) to be close to the average of the characteristics when ALC is on (curve 103 in the figure), the level of the ALC output signal falls within the optimum input level range of the DSP. Set to fit.
[0041]
A device according to a second embodiment of the present invention will be described with reference to FIGS. 5 and 6, the same or corresponding parts as those in FIGS. 1 and 2 are denoted by the same reference numerals.
[0042]
FIG. 5 is a functional block diagram of the apparatus according to the second embodiment of the present invention. The gain switching unit 5 switches the gain based on the control signal of the microcomputer 6.
[0043]
FIG. 6 is an example of a specific configuration of the gain switching unit 5. The transistor Q2 constituting the gain switching circuit 5 is turned on / off simultaneously with the transistor Q1 of the gain control unit 1. When the transistor Q1 is on, that is, when the ALC function is off, the transistor Q2 is off and the output of the audio amplifier 2 is output as it is. Therefore, the input-output characteristic 103 of the first embodiment (FIG. 3) and the characteristic 103 of the second embodiment (FIG. 7) are substantially the same. On the other hand, when the transistor Q1 is off, that is, when the ALC function is on, the transistor Q2 is on. At this time, the output of the audio amplifier 2 is divided by the resistors R3 and R4 of the gain switching unit 5. The operation of this part is substantially the same as the operation of the gain control unit 1 described above. Since the value of the resistor R5 is considerably larger than both the resistors R3 and R4, the influence of the resistor R5 can be ignored.
[0044]
Since the output of the audio amplifier 2 is divided by the resistors R3 and R4 at a predetermined division ratio, the overall gain is lowered. For example, the characteristic is as shown by the curve 104 in FIG. Compared with the curve 101 of FIG. 3, the gain of the curve 104 of FIG. 7 is lowered by about 10 dB as a whole. In FIG. 3, a gain difference of 20 dB at maximum occurs due to the on / off of ALC, but this becomes 10 dB at maximum in FIG. Thus, if the maximum value of the gain difference due to the on / off of ALC is to be suppressed, about half of the maximum value of the gain difference when there is no gain switching unit 5 is reduced with the attenuation amount of the gain switching unit 5. You can see that
[0045]
When the gain is corrected by the gain switching unit 5, a small sound (about −25 dBm or less in FIG. 7) is louder and a louder sound (about in FIG. 7) than when the gain control is off (characteristic 103 in FIG. 7). On the contrary, it works to make it smaller. In the gain switching unit 5, amplification can be performed with optimal gain distribution by turning off the gain control during speakerphone and turning on the gain control during recording.
[0046]
As described above, according to the apparatus of the second embodiment of the present invention, since it is not necessary to provide a plurality of audio amplifier circuits, it is not necessary to switch the audio signal path, and the signal path and the configuration of the apparatus can be simplified. Further, the transistor Q1 of the gain control unit 1 can simultaneously realize the gain control function and the ALC on / off function, thereby reducing the number of components. Furthermore, since the gain control width can be arbitrarily set by appropriately setting the value of the resistor R2, recording can be naturally performed without excessively increasing or decreasing the volume.
[0047]
In addition, although the case where it applied to a telephone was mentioned as an example in said description, it cannot be overemphasized that it can apply also to another use. For example, the present invention can be applied to general audio equipment such as headphone stereo and AV equipment.
[0048]
【The invention's effect】
As described above, according to the present invention, an audio signal amplifier circuit that operates in either the first operation mode or the second operation mode having different amplification characteristics, the amplifier for amplifying an input audio signal; An output level detection unit for detecting a level of an output signal of the amplifier; an amplification factor control unit for controlling an amplification factor of the amplifier based on a detection result of the output level detection unit; and the amplification in the second operation mode. And a control unit that stops the control of the amplification factor by the rate control unit, and can operate in a plurality of different operation modes. The signal path and the configuration of the device can be simplified.
[0049]
According to the present invention, the gain control unit controls the gain so that the output level is substantially constant between the predetermined first operating point and the second operating point. Therefore, it is possible to realize natural recording or telephone call without excessively suppressing increase / decrease in volume.
[Brief description of the drawings]
FIG. 1 is a functional block diagram of an apparatus according to a first embodiment of the invention.
FIG. 2 is a specific circuit configuration example of the apparatus according to the first embodiment of the invention.
FIG. 3 is an input-output characteristic diagram when the ALC function of the apparatus according to the first embodiment of the invention is on and when it is off.
FIG. 4 is a diagram for explaining a method of adjusting an ALC range in the apparatus according to the first embodiment of the invention.
FIG. 5 is a functional block diagram of an apparatus according to a second embodiment of the invention.
FIG. 6 is a specific circuit configuration example of the apparatus according to the second embodiment of the present invention.
FIG. 7 is an input-output characteristic diagram when the ALC function of the device according to the second embodiment of the invention is on and when it is off.
FIG. 8 is a functional block diagram of a conventional apparatus.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Gain control part 2 Audio amplifier 3 Detection amplifier 4 Output detection part 5 Gain switching part 6 Control microcomputer

Claims (2)

An audio signal amplifier circuit that operates in either a first operation mode or a second operation mode having different amplification characteristics,
An amplifier for amplifying the input audio signal;
An output level detector for detecting the level of the output signal of the amplifier;
An amplification factor control unit for controlling the amplification factor of the amplifier based on the detection result of the output level detection unit;
A control unit for stopping control of the gain by the gain control unit in the second operation mode;
With
The gain control unit controls the gain so that the output level is substantially constant between a predetermined first operating point and a second operating point;
An audio signal amplifier circuit characterized by the above. Wherein the first operating point is determined based on monitoring the level of the input signal, the second operating point audio signal amplifier circuit according to claim 1, characterized in that it is determined based on the recording level.

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