JP2590877B2 - Graphic equalizer circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a graphic equalizer circuit capable of dividing an audio signal band into several parts and independently enhancing or attenuating signal components in each band.

[Summary of the Invention]

The graphic equalizer circuit of the present invention supplies an input signal through a resistor to a non-inverting input terminal of an operational amplifier that is applying negative feedback, and supplies a pass characteristic to the non-inverting input terminal with respect to a center frequency obtained by band division. Are connected, and each of the bandpass filters is provided with a pair of voltage-current conversion circuits for converting an output signal into a current output. Then, a first voltage-current conversion circuit of the pair of voltage-current conversion circuits is connected so as to draw current from the non-inverting input terminal, and a second voltage-current conversion circuit is connected.
Since the current conversion circuit is connected so as to draw current from the inverting input terminal of the operational amplifier, when the transfer conductances of the first and second voltage-current converters are controlled differentially with each other, audio A graphic equalizer circuit that enhances or attenuates a signal component in an arbitrary selected band of the signal can be used.

[Conventional technology]

A tone control circuit that arbitrarily emphasizes or attenuates a low sound or a high sound according to the situation of a sound source or a reproduction sound field or a listener's preference is an essential circuit function of an audio device.

For such tone control, a device that divides the audio band more finely and arbitrarily attenuates or emphasizes each divided band is commonly called a graphic equalizer circuit, and cannot be adjusted by the tone control circuit. Complicated sound field correction can be made possible.

The divided band by this graphic equalizer differs depending on the type of audio equipment, but from at least three divisions,
Generally, it is divided into 5 to 7 divisions, and in some higher-grade ones, it is divided into 9 divisions.

In the past, generally, only high-end models have been adopted, but in recent years, IC technology has begun to be used in home-use portable headphone players and the like. Behind this is the demand for higher quality and higher functionality of audio devices, and the fact that relatively low-cost graphic equalizer circuits can be easily obtained by using ICs.

FIG. 6 shows an example of a circuit configuration of a graphic equalizer circuit according to the prior art. As a simple example, FIG.
A split graphic equalizer circuit is shown.

Input signal V _i in this circuit is applied to the non-inverting input of the operational amplifier 2 via the input terminal 1 resistor R _i, the output of the operational amplifier 2 is output from the output terminal 3, some of the feedback The signal is fed back to the inverting input terminal via the resistor _R0 .

Three volumes VR ₁ , VR ₂ , VR ₃ are connected between the inverting input terminal and the non-inverting input terminal of the operational amplifier 2, and the movable pieces of the volumes VR ₁ , VR ₂ , VR ₃ have one end. Are connected to series resonance circuits 4, 5, and 6 made of an LCR grounded.

FIG. 7 is a circuit diagram showing only one band for explaining the operation of this circuit.

The resistance divided by the volume is a · R _v and (1-a)
Assuming that _Rv , the series resonance circuit is connected to the inverting input of the operational amplifier 2 when a = 1 at first. When the open loop gain of the operational amplifier 2 is sufficiently large, the potential of the inverting input and the non-inverting input terminal are both V _1, and the current I _i flowing through the resistor value aR _v 0, and the an input signal V _i = V ₁ be able to.

Therefore, in this state, the impedance of the LCr series circuit is connected to the inverting input terminal, and the gain increases near the series resonance frequency f ₀ due to the series resonance. As shown in FIG. _This indicates the characteristic 11 that emphasizes the signal components near ₀ .

When a = 0, the series resonance circuit L,
C, r is connected to the non-inverting input terminal side, because provide gain for attenuating at the resonance frequency f _0, the characteristics of a characteristic 12 for attenuating at the resonance frequency f ₀ as shown in FIG. 8.

In order for the enhancement characteristic and the attenuation characteristic to be symmetrical, the input resistance _Ri and the feedback resistance _R0 need to be equal.

Now, assuming that the frequency characteristic of the circuit in the case of a = 1 when R = R _i = R ₀ is T (S) (S = jω: complex angular frequency), Here, Z (S) is the impedance of the series resonance circuit. It is. Therefore, Is defined as This equation shows that to a low-frequency (S«ω ₀₎ and a high (S»ω ₀₎ the gain becomes 1 S = omega ₀ the gain is (1 + R / r).

 Next, the frequency characteristics of the circuit when a = 0 will be described.

Since this case is current I ₀ is 0 flow resistance of the volume (1-a) R _v, operational amplifier 2 operates as a voltage Horowa shows a frequency characteristic of attenuation in the vicinity of the resonance frequency f _0. That is, And substituting ω ₀ and Q described above, This equation is completely symmetric with the above equation (4), which means that the emphasis characteristic and the attenuation characteristic of FIG. 7 are completely symmetric in decibels.

Next, considering a = 0.5, that is, the midpoint of the volume, if the open loop gain of the operational amplifier 2 is sufficiently large in this case as well, the potential difference between both ends of the volume becomes 0.
Becomes

However, since a series resonance circuit is connected to the midpoint,
I _i = I ₀ , and the following equation holds in this circuit.

V _i = V ₁ + R _i · I _i ‥‥‥ (7) V ₀ = V ₁ + R ₀ · I ₀ ‥‥‥ (8) Since R _i = R ₀ and I _i = I ₀ , V _i = V ₀ , and the transfer characteristic T (S) of this circuit is 1.

For example, in the region of 1>a> 0.5, the characteristic 11 in FIG.
A smaller emphasis characteristic is obtained, and when 0.5>a> 0, an attenuation characteristic smaller than the characteristic 12 is exhibited.

FIG. 9 shows the frequency characteristics when the graphic equalizer circuit is divided into five, that is, when the resonance frequency (Hz) is set to 100, 300, 1K, 3K, and 10K. Each volume is emphasized by five volumes. And that the attenuation can be adjusted independently.

[Problems to be solved by the invention]

As described above, the conventional graphic equalizer circuit can emphasize or attenuate an arbitrary frequency band with one operational amplifier, but has the following problems.

1) The inductance (L) is required, and the cost is high.

2) As the number of divisions increases, the inductance value increases,
Difficult to make (especially because Q is increased and the center frequency is expanded to a lower one) 3) Since a signal component flows into the volume, induced noise is picked up and S / N is deteriorated.

4) Miniaturization becomes difficult.

By the way, as a circuit that realizes the inductance element, for example, a simulated inductance circuit can be used.

Shows a series resonant circuit using FIG. 10 according simulated inductance circuit, an arithmetic circuit 21, resistors R _1, R ₂ capacitor C ₂ constitute a simulated inductance circuit, the impedance viewed to the left from the terminal 22 It shows inductance equivalently. That is, the impedance ZL (S) as viewed from the left side of the terminal 22 is Since ZL (S) becomes R ₁ when S → 0 and R ₂ when S → ∞, it can be regarded as an equivalent circuit surrounded by a dotted line in FIG. 11, and L = C ₂ · R ₁ (R ₂ -R ₁ ) ‥‥‥ (10). If R ₂選 ぶ R ₁ is selected in this circuit, L = C ₂ RR ₁ 2R ₂ ‥‥‥ (11), and a high Q series resonance circuit is formed together with the capacitor C ₁ . At this time, the resonance frequencies ω ₀ and Q are given by the following equations.

However, even when such a simulated inductance circuit is used, there is a further problem in the case of forming an IC.

1) Even if a simulated inductance circuit is used,
To configure one band of channel, two external capacitors that are not grounded at both ends and one volume are required. If the number of divided bands increases, the number of pins of the IC increases, so miniaturization is achieved. It becomes difficult and there is little merit in implementation.

2) The resonance frequency is set by the built-in resistance of the IC and the external capacitance. However, the accuracy of the resistance value inside the IC is poor, so that the characteristics of the equalizer vary widely.

3) Since a signal current flows through the volume, it is easy to pick up noise, and it is difficult to use a DC control method.

[Means for solving the problem]

The present invention has been made to solve such a problem, and an input signal is connected to a non-inverting input via a first resistor, and an output signal is connected to an inverting input terminal via a second resistor. The operational amplifier being fed back, N bandpass filters connected to the non-inverting input terminal (or inverting input terminal) of the operational amplifier, and the output voltage of each bandpass filter converted into a current output 1 Pair voltage-
A graphic equalizer circuit is constituted by a current converter.

[Action]

The output of each band-pass filter that passes through the center frequency of each band obtained by dividing the audio band into N is controlled by a pair of voltage-current converters to control the current values of the non-inverting input terminal and the inverting input terminal of the operational amplifier. From the above 1
By controlling the transconductance (g _m ) of the pair of voltage-current converters to change differentially with respect to each other, it is possible to individually emphasize or attenuate the signals in each divided frequency band.

In addition, since a DC control method can be adopted, a graphic equalizer circuit integrated into an IC can be easily realized, and S / N can be improved.

〔Example〕

Figure 1 is a block diagram showing the principle of a graphic equalizer circuit of the present invention, 1 denotes an input terminal for the input signal V _i, 3 an output terminal of the output signal V _0, 2 is the inverting and noninverting inputs It is an operational amplifier provided.

Reference numeral 31A denotes a band-pass filter having a characteristic of passing a center frequency (ω ₀ ) when the audio band is divided into N, as will be described later. Reference numerals 32A and 33A denote a pair of current-converting output signals of the band-pass filter. Voltage-current converter.

The transmission conductance of the first voltage-to-current converter 32A is (1-a) g _m , and the transmission conductance of the second voltage-to-current converter 33A is (1 + a) · g _m.
Is 1 <a <-1.

Incidentally, such a band-pass filter 31A and a pair of voltage-current converters 32A, 33A are N band-pass filters 31B, 31C ‥‥‥ as shown by a dotted line in the case of N division,
Each of N pairs of voltage-current converters 32B, 33B, 32C, 33C ‥‥
, The following description is based on a bandpass filter 31A and a pair of voltage-current converters 32A, 33A.
Will be described.

Assuming that the center frequency of the band-pass filter 31 is ω _{0 and the} resonance characteristic is Q, the transfer characteristic H (S) is Is represented by

Further, when the voltage of the input terminal of the operational amplifier 2 and _{V 1, V 1 = V i} -R i I i ‥‥‥ (15) I i = V i (1-a) g m · H (S) ‥ ‥‥ (16) V ₀ = V ₁ + R ₀ I ₀ 17 (17) holds. Rearranging the above formula, Also, I ₀ is To highlight the characteristics and attenuation characteristics become symmetrical, as in the prior art the need is R _i = R ₀ = R, when determining the V _0, When the transfer function T (S) = V ₀ / V _i is obtained, Get.

In this equation, if a = 1, then Comparing the equation (23) with the equation (4), R / r
Is replaced by 2R · g _m . Therefore, the amplitude characteristic is the resonance frequency ω ₀ (ω ₀ =
2πf ₀ ) and the gain at this time is (1
+ 2R · g _m ).

Next, when the equation (22) is reproduced for the state of a = -1, , And this expression is equal to the first (6) which at the R / r and 2 · Rg _m. Therefore, the amplitude characteristic is the characteristic 12 in FIG.
As can be seen from FIG. 5, the attenuation characteristic at which the gain becomes 1 / (1 + 2R · g _m ) at f ₀ is shown.

In addition, T (S) = 1 at the intermediate a = 0, which indicates a flat characteristic.

As apparent from the above description, the graphic equalizer circuit of the present invention, a voltage having two outputs differentially g _m by a band-pass filter and the coefficient a is changed - is composed of a current transformer, conventional It can be seen that the operation is equivalent to that of a conventional graphic equalizer that uses a series resonant element of the LCR and a simulated inductance circuit in which the inductance is replaced in a circuit.

Next, in order to clarify the advantages of the present invention, a more specific configuration will be described. FIG. 2 shows the principle of a band-pass filter suitable for the present invention. This method is an active filter circuit called a state variable type or a biquad type, and includes an adder and an integrator.

In this circuit, 41 is an input terminal, 42 and 43 are adders, and 44 and 45 are integrators having transfer functions of Q · ω ₀ / S and ω ₀ / (SQ), respectively. When the overall transfer function is found, Get. When comparing the above equation (14) below, except the gain is ^twice Q are the same.

FIG. 3 shows the bandpass filter of FIG. 2 in more detail.

In this figure, 51 is the input voltage V applied to the input terminal 41.
_1, the voltage to provide an output current I ₁₁ multiplied by a factor of g _m obtains the difference voltage V ₁₁ of the feedback voltage V ₂₂ - is a current converter. Its capacity C ₁₁ is connected to the output form the integrating circuit. Its voltage V
The difference voltage V ₁₂ between the feedback voltage V ₂₁ and the feedback voltage V ₂₂ is converted into a current by the voltage-current converter 52. Its output is converted into I ₁₂ , I ₁₃ , I ₁₄ current output by three voltage-current converters 52, and g
_m2 , (1−a) · g _m0 and (1 + a) · g _m0 are multiplied. I ₁₂ is integrated by the capacitor C ₁₂ becomes feedback voltage V _22. I ₁₃ and I ₁₄ are connected to the operational amplifier 2 via output terminals 53 and 54, respectively.
Are connected to the inverting input and the non-inverting input.

FIG. 4 is a diagram showing a specific configuration of FIG. The input signal applied to the input terminal 41 is converted into a current by the differential amplifier 61. The current flows into the diode pair 62.

The voltage across diode pair 62 is the emitter coupled differential pair 63
Applied between the base and further taken out as the output current I ₁₁ by the current mirror 64.

Output current I ₁₁ is integrated by the capacitor C _11, it is converted into a current by the next stage of the differential amplifier 65. The current flows into the diode pair 66. Voltage across the diode pair 66 is applied between the bases of the emitter-coupled differential pair 67, it is taken out as an output current I ₁₂ by the current mirror 68. Current I ₁₂ is capacitance C
It is integrated by ₁₂ and fed back to the differential amplifiers 61 and 65.

The voltage between both ends of the diode pair 66 is equal to the emitter-coupled differential pair 69, 70 constituting the first and second voltage-current conversion circuits.
Are also added between each base. Further, current mirrors 71 and 72 are connected to the collector of the differential pair, and the output current I
₁₃ and I ₁₄ are taken out.

Current mirrors 73 and 74 that determine the operating current of the voltage-current conversion circuit are connected to the common emitter of the emitter-coupled differential pair 69 and 70, and control currents ICX and IC are supplied from terminals 75 and 76 in opposite directions (complementary). ) To increase and decrease. Terminals 77 and 78 are positive and negative power supply terminals, respectively. The control currents IC and ICX are supplied with a DC current adjusted by, for example, a volume or the like, and set the frequency characteristic of the graphic equalizer to a desired value.

79 is a bias voltage source for applying a bias current. This first biasing current to the current mirror 80 through R _11. The current gives a bias current to the differential amplifiers 61 and 65 via Q ₁₃ , Q ₁₄ and Q ₂₅ . The bias voltage source 79 is taken out to the terminal 81, via an external resistor R _16, it flows into Q ₁₉ from the terminal 82. Further, a bias current is applied to the common emitter transistor pair 63 and 67 via Q ₂₄ and Q ₃₄ .

The circuit of FIG. 4 is a faithful implementation of the method of FIG. The capacitors C ₁₁ and C ₁₂ can be arranged inside the IC or can be externally attached. Generally, the middle and high frequency bands are built in the IC, and the low frequency band is desirably external because the capacitance value is large.

Although a state-variable band-pass filter other than this embodiment can be used, the advantage of this method is that the capacitance is grounded, which is convenient for externally attaching a capacitance. Resistor R ₁₆ to give a bias current to the transistor Q ₁₉ may have a built-in IC. However, the accuracy of the center frequency is increased by using an external device. The diode pair 62 and the emitter-coupled differential pair 63 and the diode pair 66 and the emitter-coupled differential pair 67 form a multiplying circuit. Its current gain is proportional to the ratio of the current of the current and Q ₂₅ ratio and Q ₃₄ of the current of the current and Q ₁₄ of Q _24. While the voltage of the differential amplifier 61 and 65 - the transconductance of the current converter are each defined by R _12, R ₁₃ and R _14, R _15. R
_{12, R 13, R 14,} R 15 is a built-in resistance of the IC, the absolute value accuracy and temperature characteristic is not always good.

However, the current of Q ₁₄ and Q ₂₅ from relying on R _11, current gain of the multiplier circuit is dependent on the ratio of R ₁₁ and R _16,
If the ratio of R ₁₁ and _{R 12, R 13, R 14} , R 15, R 16 is constant, the current gain dependency and multiplying circuit to the IC internal resistance of the transconductance of the differential amplifier 61 and 65 The dependence on the IC's built-in resistance cancels out, and the dependence on the absolute value is eliminated. Therefore, the transfer conductance accuracy from the inputs of the differential amplifiers 61 and 65 to I ₁₁ and I ₁₂ is improved. That is, the accuracy of the center frequency of each band is improved.

Incidentally, if the critical accuracy of the center frequency, R ₁₆ may be a built-in.

Next, the control currents ICX and IC supplied to the terminals 75 and 76 will be described with reference to FIG.

FIG. 5 shows a control voltage on the X-axis and control currents IC and I on the Y-axis.
Taking CX.

The most common control current has characteristics such that the sum is constant as shown by 91 and 92.

However, characteristics such as 93 and 94 may be used. When IC = ICX, the band shows a flat characteristic, and when IC> ICX, it has an emphasis characteristic, and when IC <ICX, it has an attenuation characteristic.

In the above embodiment, in the configuration of the present invention shown in FIG. 1, the inputs of the band-pass filters 31A, 31B, 31C,... Are connected to the non-inverting inputs of the operational amplifiers. Since the same signal is generated at the input, it may be connected to the inverting input side.

〔The invention's effect〕

As described above, the graphic equalizer circuit of the present invention uses an operational amplifier having an inverting input terminal and a non-inverting input terminal, and a plurality of band-pass filters for the divided bands. Since the signal is converted into a current output and supplied to the inverting input terminal and the non-inverting input terminal of the operational amplifier, the emphasis characteristic and the attenuation characteristic are given to the signal components in each band. The characteristics and the attenuation characteristics linearly change with respect to the linear control signal, and the sound field adjustment excellent in audibility is performed.

In addition, by adopting the DC control method, the stereo signal can be controlled by a single volume, which makes it possible to reduce the size and simplification. There is an effect of improving.

In addition, since the circuit can be formed with a grounded capacitor, there is an advantage that it can be easily integrated into an IC and variations in frequency characteristics are reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing the principle of a graphic equalizer circuit according to the present invention, FIG. 2 is a block diagram showing an embodiment of a band-pass filter, and FIG. 3 is a circuit further embodying the block diagram of FIG. FIG. 4, FIG. 4 is a circuit diagram showing an embodiment in which the circuit of FIG. 3 is formed into an IC, FIG. 5 is a graph showing an example of a change in a control signal, FIG. 6 is a circuit diagram of a conventional graphic equalizer circuit, FIG. Figure is a circuit diagram for explaining the operation of FIG. 6, the graph of the emphasis characteristic and the damping characteristic Figure 8 is for the center frequency f _0, Figure 9 is a graph showing a frequency characteristic of a 5-division graphic equalizer circuit, 10 The figure shows a simulated inductance circuit diagram, and FIG. 11 shows an equivalent circuit of FIG. In the figure, 1 is an input terminal, 2 is an operational amplifier, 3 is an output terminal,
31A, 31B, 31C ‥‥‥ are bandpass filters, 32A, 32B, 32C
‥‥‥ is the first voltage-current converter, 33A, 33B, 33C ‥‥‥
Indicates a second voltage-current converter.

Claims (3)

(57) [Claims] 1. An amplifier circuit wherein an input signal is connected to a non-inverting input terminal via an element having a resistance component and an output signal is fed back to an inverting input terminal via an element having a resistance component. N band-pass filter circuits connected to an input terminal and having a center frequency when an audio band is divided into N and having a pass characteristic; and N number of N-band pass filter circuits for converting outputs of the N band-pass filter circuits into current outputs, respectively. First and second voltage-current converter pairs, and the N first voltage-current converters are connected to a non-inverting input terminal of the amplifier circuit, and the N second voltage-current converters are connected to each other. Control means for connecting the converter to the inverting input terminal of the amplifier circuit and controlling each operating current of the N first and second voltage-current converter pairs by N control signals; The control means is Serial first voltage - current converter and the second
Wherein the transmission conductance is controlled such that the operating currents of the voltage-current converters increase and decrease in the opposite directions. 2. The graphic equalizer circuit according to claim 1, wherein N bandpass filters are connected to the inverting input terminal of the amplifier circuit. 3. A band pass filter and first and second voltages.
The current converter includes first and second differential amplifiers (61, 65) each including two transistors of the same conductivity type having a feedback resistor (R12-15) between emitter electrodes, and the first and second differential amplifiers. A first and second diode pair (62, 66) to which the output current of the differential amplifier is supplied, and a first emitter-coupled differential transistor in which a potential difference between the first diode pair is applied between base electrodes. A pair (63); a second, third, and fourth emitter-coupled differential transistor pair (67.69.70) in which a potential difference between the second diode pair is applied between base electrodes; and the first and second emitters. The first and second integration capacitors provided on the output side of the coupled differential transistor pair and (C1, C2), and the common emitter current of each of the third and fourth emitter coupled transistor pairs are controlled by a control signal (IC , ICX) according to the control means (73, 74) for increasing and decreasing in the opposite direction to each other. Claims, characterized in that there first (1) graphic equalizer circuit according to claim.