JP3092237B2 - Digital gain variable 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 digital gain variable device applied to a digital-to-analog converter called a 1-bit digital-to-analog converter.

[0002]

2. Description of the Related Art Conventionally, a gain variable device for adjusting the level of an output audio signal is provided around a digital-to-analog converter for converting a digital audio signal into an analog audio signal. FIG. 2 is a diagram showing an example of the digital audio signal. In FIG. 2, reference numeral 1 denotes a digital audio signal output terminal. And
The gain setting signal (digital data) obtained at the gain setting signal input terminal 3 is supplied to the digital multiplier 2 to multiply the digital audio signal level data by the gain setting signal. Then, the multiplied output is supplied to the digital / analog converter 4, and the digital / analog converter 4 converts the digital audio signal into an analog audio signal. Then, the analog audio signal converted by the digital / analog converter 4 is
It is supplied to an analog audio signal output terminal 5.

In this way, before the digital audio signal is converted into an analog signal, the gain is adjusted, and the gain is adjusted only by changing the value of the gain setting signal. Is done.

As another configuration, for example, as shown in FIG. 3, an analog audio signal output from a digital / analog converter 4 is converted into an inverting input of an operational amplifier 6 forming an output circuit via a predetermined resistor. And the non-inverting input terminal of the operational amplifier 6 is grounded. Then, the inverting input terminal side of the operational amplifier 6 is connected to one end of a plurality of semiconductor switches 7a, 7b # 7i composed of transistors and the like. In this case, the gain setting signal of each bit obtained at the input terminals 8a, 8b ‥‥ 8i of the gain setting signal of a plurality of bits is transmitted to the respective semiconductor switches 7a, 7b
$ 7i is supplied to the control terminal. The other end of each of the semiconductor switches 7a, 7b # 7i is commonly connected through resistors 9a, 9b # 9i having different resistance values, and this connection point is connected to the output terminal of the operational amplifier 6. I do.
Then, the output terminal of the operational amplifier 6 is connected to the analog audio signal output terminal 5.

By doing so, the input terminals 8a, 8
The resistance value of the resistor connecting the inverting input terminal side and the output terminal side of the operational amplifier 6 changes depending on the connection state of the semiconductor switches 7a and 7b # 7i according to the gain setting signal obtained at b ‥‥ 8i. Then, the gain of the analog audio signal obtained at the output terminal 5 changes.

By the way, when such a variable gain device is configured, there is a possibility that the output analog audio signal is deteriorated. That is, as shown in FIG. 2, when digitally attenuating by a digital multiplier, the dynamic range of the digital-to-analog converter is limited, so that the distortion increases as the amount of attenuation increases. In addition, as shown in FIG. 3, when the gain is switched by an analog switch such as a semiconductor switch, the distortion rate deteriorates due to the non-linearity of the characteristics of the analog switch, and the sound quality deteriorates.

In order to solve this problem, the present applicant has previously proposed in Japanese Patent Application No. Hei 2-274709 a variable gain device of this type which does not deteriorate the sound quality.

[0008] This variable gain device will be described.
In this example, gain adjustment is performed using a 1-bit digital-to-analog converter as the digital-to-analog converter. First, the 1-bit digital-to-analog converter will be described. This 1-bit digital-to-analog converter obtains a pulse signal whose number or width changes as a converted output, and supplies an output whose number or width changes to a low-pass filter. By averaging, an analog audio signal is obtained. In this case, the pulse waveform output from the digital / analog converter has either a high level or a low level, and the pulse waveform that changes in number according to the input digital data is pulse number modulation ( PN
M), and those in which the width of the pulse waveform changes are called pulse width modulation (PWM). According to the digital-to-analog converter of such a system, distortion generated at the time of conversion can be suppressed to a minimum, and a good analog audio signal without distortion can be converted.

FIG. 4 shows an overall configuration of a variable gain device applied to a device using the 1-bit digital-to-analog converter. In FIG. 4, 1
Reference numeral 1 denotes a digital audio signal input terminal. The digital audio signal obtained at the digital audio signal input terminal 11 is supplied to a 1-bit digital / analog converter 12. The pulse signal converted and output by the digital / analog converter 12 is supplied to the plurality of logic gates 13a, 13b # 13i. As the logic gate, various gate elements such as an AND gate, a tri-state gate, and a flip-flop can be considered. In the following description, the logic gate will be described as an AND gate.

In the figure, 14a, 14b１４14i are:
A gain setting signal input terminal is shown.
Each bit data of the gain setting signal of a plurality of bits is supplied to 14b ‥‥ 14i. In this case, the gain setting signal is supplied from a control circuit (not shown) of the audio equipment in which the digital-to-analog converter is incorporated, and only some of the bits are set to the high level signal “1” according to the set gain. , And the other bits are set to the low level signal “0”. And input terminals 14a, 14b
The gain setting signal of each bit obtained at # 14i is supplied to logic gates 13a and 13b # 13i. Then, respective logic gates 13a, 13b #
13i is connected to a resistor 2 having a different resistance value.
0a, 20b ‥‥ 20i. The other ends of the resistors 20a, 20b # 20i are commonly connected to the inverting input terminal of the operational amplifier 21. Then, the non-inverting input terminal of the operational amplifier 21 is grounded, and the inverting input terminal and the output terminal of the operational amplifier 21 are connected to each other.
Connected by a resistor 22.

The output terminal of the operational amplifier 21 is connected to a low-pass filter 23, and the pulse signal supplied from the operational amplifier 21 is averaged by the low-pass filter 23 to obtain an analog audio signal. To supply.

According to the configuration shown in FIG.
The digital audio signal converted into the pulse signal by the analog converter 12 is applied to logic gates 13a, 13b # 13.
The pulse-converted audio signal is output only from the logic gate to which the high-level signal “1” is supplied as the gain setting signal. That is, the low level signal “0” is used as the gain setting signal.
Is always supplied with a low-level signal "0" regardless of the state of the pulse signal supplied from the digital / analog converter 12 side. And
From the logic gate to which the high-level signal “1” is supplied as the gain setting signal, a logical product output that becomes the high-level signal “1” when the pulse signal is the high-level signal “1” is obtained. Do not change the signal state of the signal.

Therefore, the signal is converted into a pulse signal via a resistor (either of the resistors 20a, 20b # 20i) connected to the logic gate to which the high level signal "1" is supplied as the gain setting signal. The audio signal is supplied to the operational amplifier 21 side, and the level of the pulse signal (that is, the potential of the high-level signal “1”) is adjusted according to the resistance value of the connected resistor. For this reason, the audio signal obtained by averaging by the low-pass filter 23 changes according to the resistance value of the connected resistor, and the gain adjustment is performed by selecting the resistor to be connected by the gain setting signal. It can be carried out.

The gain adjustment performed in this manner is 1
Only the level adjustment of the output pulse of the bit digital / analog converter 12 is performed, and the switching of the resistor itself is also performed by the logical operation by the logic gate, and the distortion does not change due to the adjustment of the gain. A good analog audio signal can be obtained.

[0015]

However, such a gain adjusting device has a disadvantage that the DC level of the analog audio signal obtained at the output terminal 24 fluctuates due to the gain adjustment. That is, the DC level V _ODC of the output analog signal is expressed by the following equation.

[0016]

## EQU1 ## V _ODC = − (R _f / R _H ) V _DC

Here, R _f is the resistance value of the resistor 22 as a feedback resistor of the amplifier 21, and R _H is the resistance value of the resistor 20.
a, the resistance value of the resistor selected by the gate element within 20b ‥‥ 20i, and _VDC is the DC component of the output signal of the 1-bit digital / analog converter 12.

As can be seen from this equation, by changing the connected resistors 20a, 20b ‥‥ 20i in order to change the gain, the resistance value R _H in the equation changes, and
Even if the DC component V _DC of the output signal of the analog converter 12 is constant, the DC component V _ODC of the output analog signal fluctuates. Normally, the DC component V _DC of the output signal of the digital / analog converter 12 is relatively high in potential near the midpoint of the amplitude of the input digital signal (for example, 2.5 V), and the DC component V _{ODC of the} output analog signal is relatively high. Also increases the fluctuation range.

In view of the foregoing, it is an object of the present invention to suppress a change in a DC component of an output analog signal in a gain variable device of this type.

[0020]

According to the present invention, as shown in FIG. 1, for example, the output gain of a 1-bit digital-to-analog converter 12 in which the number or width of output pulses changes in response to a digital input signal. In the digital gain varying device for changing the gain, the output of the digital / analog converter 12 is supplied to one input of a plurality of logic gate elements 13a, 13b # 13i, and a gain setting control signal is supplied to each logic gate element 13a. , 13b ‥‥ 13i to the other inputs of the respective logic gate elements 13a, 13b.
The output of {13i is connected to one end of resistors 20a, 20b {20i, and the respective resistors 20a, 20b}
The other end of # 20i is commonly connected, a signal obtained at this connection point is supplied to the inverting input terminal of the operational amplifier 21, and the DC component of the output of the digital / analog converter 12 is supplied to the non-inverting terminal of the operational amplifier 21. Side input terminal, the amplified output of the operational amplifier 21 is supplied to the low-pass filter 23,
An analog output signal whose gain is adjusted from the output of the low-pass filter 23 is obtained.

[0021]

In this manner, the DC component of the output of the digital-to-analog converter 12 is supplied to the operational amplifier 21, so that even if the connected resistor (that is, the resistance value) changes, the operational amplifier 21 Thus, the fluctuation of the DC component is canceled, and an analog signal having a constant DC component is obtained as the output of the low-pass filter 23.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIG.

In this embodiment, the present invention is applied to the above-described digital gain varying device shown in FIG. 4, and is configured as shown in FIG. In FIG. 1, portions corresponding to those in FIG. 4 are denoted by the same reference numerals, and detailed description thereof will be omitted.

In the present embodiment, as shown in FIG. 1, the pulse signal output from the digital-to-analog converter 12 is supplied to a plurality of logic gates 13a, 13b # 13i, and also supplied to an amplifier 15. The output of the amplifier 15 is supplied to the non-inverting input terminal of the operational amplifier 21 via the resistor 16. Further, the connection point between the resistor 16 and the operational amplifier 21 is grounded via the capacitor 17. By doing so, a low-pass filter is formed by the resistor 16 and the capacitor 17, and the output of the amplifier 15 is rectified and supplied to the operational amplifier 21. In this case, the cut-off frequency of the low-pass filter including the resistor 16 and the capacitor 17 is determined by the digital-to-analog converter 1
2 is set to a frequency sufficiently lower than the pulse signal output.

The other parts are configured similarly to the digital gain variable device shown in FIG.

According to the structure shown in FIG. 1, similarly to the digital gain variable device shown in FIG. 4, a resistor (resistor) connected to a logic gate to which a high level signal "1" is supplied as a gain setting signal is provided. Vessels 20a, 20b @ 20i
) Is supplied to the operational amplifier 21 via the pulse signal, and the level of the pulse signal (that is, the high-level signal "") is determined according to the resistance of the connected resistor. 1 ″ potential) is adjusted. For this reason, the audio signal obtained by averaging by the low-pass filter 23 changes according to the resistance value of the connected resistor, and the gain adjustment is performed by selecting the resistor to be connected by the gain setting signal. It can be carried out.

In this embodiment, the pulse signal output from the digital / analog converter 12 is converted into a direct current by the resistor 16 and the capacitor 17 constituting a low-pass filter, and this direct current component is converted into an operational amplifier 21 as an inverting amplifier.
Is supplied to the non-inverting input terminal of the
At 1, the fluctuation of the DC component is removed. This will be described with reference to the following expression indicating the DC component of the circuit shown in FIG.

[0028]

(Equation 2)

Here, V _{off +} is a DC component supplied to the non-inverting input terminal of the operational amplifier 21. Since the DC component V _{off +} is a DC component of the pulse signal output from the digital-to-analog converter 12, it is equal to the DC component V _DC of the output signal of the digital-to-analog converter 12. Therefore,
In the equation (2), the term of (V _{off +} + V _DC ) becomes 0,
The DC level of the output analog signal becomes V _ODC = V _{off +} ,
Regardless of the connection state of the resistors 20a, 20b ‥‥ 20i (ie, no matter how the gain is adjusted), the digital
The DC component V _DC of the output signal of the analog converter 12 directly becomes the DC level V _ODC of the output analog signal. Therefore, if the _{DC component VDC} of the output signal of the digital / analog converter 12 is constant, the DC component of the analog audio signal obtained at the output terminal 24 does not fluctuate due to gain adjustment.

[0030]

According to the variable gain device of the present invention, the gain is adjusted by controlling the level of the output pulse of the 1-bit digital-to-analog converter by controlling the logic gate element, and the distortion is deteriorated with a simple configuration. A good analog audio signal whose gain has been adjusted can be obtained, and the DC component of the output analog audio signal can be kept constant even if this gain adjustment is performed.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing one embodiment of the present invention.

FIG. 2 is a configuration diagram illustrating an example of a conventional variable gain device.

FIG. 3 is a configuration diagram illustrating an example of a conventional variable gain device.

FIG. 4 is a configuration diagram showing an example of a conventional gain variable device to which the present invention is applied.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Digital audio signal input terminal 12 1-bit type digital / analog converter 13a, 13b @ 13i Logic gate 14a, 14b @ 14i Gain setting signal input terminal 15 Amplifier 16 Rectifier resistor 17 Rectifier capacitor

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-10411 (JP, A) JP-A-58-210706 (JP, A) JP-A-4-150508 (JP, A) JP-A 63-104 279607 (JP, A) JP-A-60-114040 (JP, A) JP-A-1-292913 (JP, A) JP-A-4-172823 (JP, A) (58) Fields investigated (Int. ⁷ , DB name) H03M 1/00-1/88 H03G 1/00-3/18

Claims (1)

(57) [Claims] 1. A digital gain variable device for changing the output gain of a 1-bit digital-to-analog converter in which the number or width of output pulses changes in response to a digital input signal, the output of said digital-to-analog converter Is supplied to one input of a plurality of logic gate elements, a gain setting control signal is supplied to the other input of each of the logic gate elements, and the output of each of the logic gate elements is connected to one end of a resistor. The other end of each resistor is connected in common, a signal obtained at this connection point is supplied to an inverting input terminal of an operational amplifier, and the DC component of the output of the digital-to-analog converter is calculated. Supply to the non-inverting input terminal of the amplifier, supply the amplified output of the operational amplifier to the low-pass filter, and adjust the gain from the output of the low-pass filter. Digital gain variable device for obtaining a converted analog output signal.