JP3329090B2 - Variable gain device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable gain device for varying the gain of a reproduced signal by digital signal processing in various audio devices.

[0002]

2. Description of the Related Art FIG. 6 shows a conventional variable gain device using digital signal processing. In FIG. 6, 701 and 7
02 is an analog circuit including an LPF for converting an analog signal into a digital signal and an AD converter, 703 is a DSP (digital signal processor) for realizing a process of varying the gain of a reproduced signal by digital signal processing, and 704 and 705 are digital It is an analog circuit including a DA converter and an LPF for converting a signal into an analog signal.

Next, the operation of the above conventional example will be described.
In FIG. 6, when an analog reproduction signal of a sound source such as a CD player or a tape recorder is input and converted into a digital signal, the DSP 703 performs a multiplication process on the input signal using a coefficient of 0 or more and 1 or less. To attenuate the level and convert it back to an analog signal.

As described above, the conventional variable gain device described above
By attenuating the level of the reproduced signal in the multiplication process on the digital signal, the gain of the reproduced signal can be reduced by -∞dB.
It can be varied in the range of 0 to 0 dB (0 to 1 times).

[0005]

However, in the above-mentioned conventional variable gain device, since the level is attenuated in the digital signal domain, the effective bit length of the digital signal decreases as the gain value decreases. Generally, the gain is reduced by one bit for every about 6 dB of attenuation. Also, the input digital signal to the DA converter is in a fixed-point format, and as the input level decreases, the effective bit length of the digital signal decreases. As for the conversion accuracy of the DA converter, the closer the effective bit length is to the input bit length of the DA converter, the better the performance such as distortion factor and dynamic range becomes, and the more accurate the conversion accuracy becomes. FIG. 7 shows the relationship between the gain value and the decrease in the effective bit length of the input signal to the DA converter. Therefore,
As the gain value decreases, the effective bit length of the input signal to the DA converter decreases, and the conversion accuracy deteriorates, resulting in a problem that sound quality deteriorates.

The present invention solves such a conventional problem. Even if the gain value is reduced, the conversion accuracy of the D / A converter is prevented from deteriorating. It is an object of the present invention to provide a variable gain device by digital signal processing with good sound quality.

[0007]

Means for Solving the Problems The present inventionVariable gain device
Is-A variable gain device having a gain value of ∞dB to 0 dB
So,Multiplication processing for digital audio signals
Digital signal processor with an external output port.
Controls the processor and the digital signal processor
Control device and the digital signal processor
The digital audio signal output from the
And a DA converter for converting the
Output level that can vary the level of the analog output signal of the
Variable device and an analog signal of the output level variable device
Low-pass circuit for converting
With the road,The output level variable device has a plurality of gain values.
An external output port of the digital signal processor.
The digital control signal output from the
A switch for setting one gain value from a number of gain values
And according to the gain value set by the switch,
The digital audio signal is the digital signal.
Processor and the output level varying device.
A digital signal processor for controlling the analog output signal;
The signal is the digital signal processor and the output
The output level varying device of the level varying device,
Digital audio signal and said analog output signal
Is the digital signal processor and the output level.
Attenuate each with a variable bell deviceThings.

[0008]

Therefore, according to the present invention , -∞dB to 0 dB (0
A gain variable device having a gain value of (double to 1), wherein the effective bit length of the digital signal, that is, the effective bit length of the input signal to the DA converter is reduced by limiting the amount of attenuation in the digital signal domain. And minimize D
The conversion accuracy of the A converter can be prevented from deteriorating, and the gain can be varied by digital signal processing while maintaining the good sound quality of the sound quality. By regularly changing and synchronizing the change in the gain value in the digital signal domain and the change in the output level gain value of the DA converter in the analog signal domain, a desired gain value can be ensured and there is no uncomfortable feeling in hearing. Enables smooth variable gain.

[0009] From these, a dedicated IC for varying the gain
By adding a simple analog circuit to a DSP without using a DSP, there is an effect that a sound quality deterioration in auditory sensation does not occur, and a gain variable device by digital signal processing can be provided with good sound quality possessed by a sound source.

[0010]

FIG. 1 shows the configuration of a first embodiment of the present invention.

In FIG. 1, reference numeral 101 denotes a stereo sound source such as a CD player for outputting an analog audio signal, which is converted into a digital signal by two sets of LPFs 102 and an AD converter 103. Reference numeral 104 denotes a DSP that performs digital signal processing and has an external output port that can control an external switch. The output is from the DA converter 10
The output level of the D / A converter is changed by two sets of output level changing devices 106 composed of simple analog circuits, and converted into analog audio signals by two sets of LPFs 107. From the two sets of speakers 109 to the sound source 101
Is output after the gain is varied. 110 is DS
A control device such as a microcomputer for controlling P104 operates according to information from the gain adjustment device 111 set by the user.

FIG. 2 shows an example of an analog circuit of the output level varying device 106, and its operation will be described. The circuit voltage e _i the differential input signals e _i1 and e _i2 is set the gain by differential input amplifier with a divider action and the operational amplifier by the resistor is outputted as a voltage e _o. The gain is
By turning on or off an analog switch (SW1) controlled by an external digital control signal, 2
Can be set as follows.

When SW1 is off, the gain G _OFF (= e _o /
e _i ) is: G _OFF = R ₄ / (R ₂ + R ₃ ) Equation 1 When SW1 is on, the voltage e _iON by the resistance division is input to the differential input amplifier, and the gain G _ON is G _ON = R ₁ / (R ₁ + 2R ₂ ) · R ₄ / R ₃ Equation 2 As described above, if the resistors R _{1 to} R ₄ of this circuit are determined by (Equation 1) and (Equation 2), one switch is controlled. Thus, two types of gain can be obtained.

In the first embodiment, G _OFF = 0d
B, G _ON = −12 dB gain is set, and R ₁ = 2.8 kΩ and R ₂ = R ₃ by (Equation 1) and (Equation 2).
= 10 kΩ and R ₄ = 20 kΩ.

Next, the operation of the first embodiment will be described in a specific example. In the first embodiment, the gain of the reproduced signal is-
It can be varied in the range of で dB to 0 dB (0 to 1), and the amount of change in gain is hereinafter referred to as attenuation.

In FIG. 1, the DSP 104 adjusts the gain value set by the user in the gain adjusting device 111.
The relationship between the attenuation 1 processed by multiplication by a predetermined coefficient of 0 or more and 1 or less and the attenuation 2 selected by a switch in the output level varying device 106 is shown in the following (Table 1).

[0017]

[Table 1]

The operation of the first embodiment will be described with reference to the specific examples shown in Table 1. Gain adjustment device 11
If 1 is set to -10 dB, the DSP 104
Multiplies the left and right output signals of the sound source 101 digitalized by the LPF 102 and the AD converter 103 by a coefficient of −10 dB = 0.316, and outputs the result to the DA converter 105. Also, DSP
104 is a switch in the output level varying device 106: SW
Control 1 off. As a result, the output level variable device 1
06 outputs the output of the DA converter 105 as it is (attenuates it by 0 dB). As a result, a signal whose gain is changed by −10 + 0 = −10 dB with respect to the output signal of the sound source 101 is converted into an analog audio signal by the LPF 107, and output from the speaker 109 by the amplifier 108.

Next, when the setting of the gain adjusting device 111 is changed from -10 dB to -12 dB, the control device 110
Supplies the DSP 104 with a coefficient of the multiplication process as 0 dB = 1.
0, switch (SW1) in output level varying device 106
Turn on transfer information. The DSP 104 starts a series of gain changing processes by receiving the information.

In step 1, the DSP 104 monitors the waveforms of the left and right signals whose gain is to be changed,
When the level becomes a minute level of dB or less, the coefficient of the multiplication process is updated from -10 dB = 0.316 to 0 dB = 1.0 by the update control process. Then, the left and right signals are multiplied by a coefficient of 1.0, and the result is converted to a DA converter 1
Output to 05.

Thus, it is possible to prevent the waveform from being discontinuous due to the level difference at the time of the gain change, thereby preventing generation of abnormal noise.

In step 2, starting from the coefficient update processing in step 1, the output level variable device control processing starts measuring the passage of time. This is performed in order to delay the operation of the output level varying device 106 until the result of the coefficient update is output as an analog signal, that is, the time equal to the time required for the processing of the DA converter 105.

In step 3, even when the time required for the processing of the DA converter 105 has elapsed, 0 dB is applied to the left and right signals.
= 1.0 and multiply the result by the DA converter 1
Output to 05.

In step 4, the switch SW1 in the output level varying device 106 is controlled to be turned on by the external output port of the DSP 104 at the same time as the time required for the processing of the DA converter 105 has elapsed. As a result, the output level varying device 106 outputs the output of the DA
B is attenuated.

Thus, the updating process of the coefficient of the multiplication process on the digital signal processing and the updating process of the attenuation of the output level variable device on the analog signal are synchronized with respect to the input signal from the sound source. Can be. In addition, it is possible to prevent the waveform from becoming discontinuous due to the level difference at the time of the gain change and generating abnormal noise.

By the processing of steps 1 to 4, after all,
0 + (− 12) = − 1 with respect to the output signal of the sound source 101
The 2 dB-gain-variable signal is converted to an analog audio signal by the LPF 107 and output from the speaker 109 by the amplifier 108.

FIG. 3 shows the relationship between the gain value and the reduction of the effective bits of the input signal to the DA converter.

As described above, according to the first embodiment, if the normal listening level is -24 dB or more, within this range, the effective bit length of the digital signal due to the multiplication processing of the DSP 104 is reduced to less than 2 bits. Thus, there is an effect that it is possible to suppress deterioration of the sound quality in the sense of hearing and to provide a gain variable device with good sound quality possessed by the sound source. Further, since the gain is changed near the zero cross of the signal waveform, it is difficult to recognize abnormal noise caused by waveform discontinuity due to the level difference, and a smooth gain change can be obtained.

In the first embodiment, the relationship shown in (Table 1) is used. However, the interval of the gain value set by the gain adjusting device 111, the amount of attenuation by the multiplication process, and the output level variable device 1
The amount of attenuation by 06 and the set number thereof are not limited to this example and can be set freely. For example, in the output level varying device 106, by using two switches, the amount of attenuation is 0 / -6.
Four types of / -12 / -18 may be set. In this case, if the listening level is -24 dB or more, the effective bit length of the digital signal can be reduced to less than 1 bit by the DSP multiplication process.

In step 1, both the left and right signals to be gain-varied have a level of -42 dB or less. This is to approximately detect the zero crossing of the waveform. This is an example, and the value is not limited to a value for setting a minute level. -∞dB (zero as digital data).

In step 1, the waveforms may be independently monitored for the left and right sides, and the processing of steps 1 to 4 for updating the gain of the left and right signals may be independently performed for the left and right sides. In this case, DS
P104 controls two sets of switches in the two output level variable devices 106 independently. This example is shown in FIG. Also, the signals to be gain-variable are not limited to the left and right signals of the stereo channel, and the number of channels is not limited.

Next, FIG. 5 shows the configuration of a second embodiment of the present invention. In FIG. 5, reference numeral 501 denotes a stereo sound source such as a CD player that outputs a digital signal. In contrast to the first embodiment, the LPF 102 and the AD converter 10
3 is similar except that 3 is unnecessary.

The operation of the second embodiment is described in the first
For the embodiment of the present invention, the digital output signal of the sound source 701 is
The same is true except that it is directly input to the DSP 104.

[0034]

As described above, according to the present invention, the digital
Digital audio with audio signal level attenuation
The reduction of the effective bit length of the signal can be minimized,
As a result, there is an effect that the sound quality is not deteriorated in audibility, and a gain variable device having good sound quality possessed by the sound source can be provided.

[0035]

[Brief description of the drawings]

FIG. 1 is a block diagram of a variable gain device according to a first embodiment of the present invention.

FIG. 2 is an analog circuit diagram of the output level varying device in the embodiment.

FIG. 3 is a characteristic diagram showing a relationship between a gain value and an effective bit length in the embodiment.

FIG. 4 is a block diagram when the output level varying device in the embodiment is controlled.

FIG. 5 is a block diagram of a variable gain device according to a second embodiment of the present invention.

FIG. 6 is a block diagram of a conventional variable gain device.

FIG. 7 is a characteristic diagram showing a relationship between a gain value and an effective bit length in a conventional variable gain device.

[Explanation of symbols]

101 A stereo sound source such as a CD player for outputting an analog audio signal 102 Two sets of LPFs 103 A / D converter 104 Digital signal processor (DSP) with external output port 105 DA converter 106 Output level varying device 107 composed of simple analog circuit 107 2 Two sets of LPFs 108 Two sets of amplifiers 109 Two sets of speakers 110 Control devices such as microcomputers 111 Gain adjustment devices 501 Stereo sound sources such as CD players that output digital audio signals

Claims (4)

(57) [Claims] 1. A gain having a gain value of ∞dB to 0 dB.
A variable device, which performs a multiplication process on a digital audio signal and has a digital signal processor having an external output port, a control device for controlling the digital signal processor, and a digital signal output from the digital signal processor. A D / A converter for converting an audio signal into an analog signal, an output level variable device capable of varying the level of an analog output signal of the D / A converter, and a low level converter for converting the analog signal of the output level variable device into an analog audio signal. And an output level varying device.
Said digital signal processor having a number of gain values
Digital control signal output from the external output port of
Therefore, a switch for setting one gain value from the plurality of gain values.
A gain value set by the switch
The digital audio signal is
Signal processor and variable output level device
The digital signal processor,
The log output signal is output from the digital signal processor and the digital signal processor.
And the output level varying device of the output level varying device.
The digital audio signal and the analog
The output signal is the digital signal processor and
A variable gain device for attenuating each of the output level variable devices. 2. The variable gain apparatus according to claim 1, further comprising a process for synchronizing, on an analog audio signal, a coefficient change timing of the multiplication processing by the digital processor and a change timing of a switch in the output level variable device. apparatus. 3. The variable gain device according to claim 2, further comprising a process of controlling a timing of changing the gain in accordance with a signal level for varying the gain. 4. The variable gain device according to claim 1, wherein a regularity is given to a variable gain amount by a multiplication process in a digital processor and a variable gain amount by an output level variable device. .