JPH09321561A - Loudness control circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a loudness control circuit that compensates a high frequency sound region while keeping the S/N excellent at all times not interlocked with the master VR. SOLUTION: The loudness control circuit is made up of a differential signal detection circuit 2 that detects a differential signal between the left and right channel input signals, an HPF 3 passing only a high frequency component of the differential signal, a detection circuit 4 detecting the output of the HPF 3, and high frequency compensation circuits 5L, 5R provided to each channel to emphasized a high frequency sound region only when an output level of a detection signal is a prescribed level or over. Since the high frequency compensation is made only when a high frequency component is included in the differential signal, the S/N is not deteriorated and since the differential signal is used, the sense of spread in the sound field is emphasized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a loudness circuit, and is applied to an audio system having a low loudness level such as a cassette tape recorder with a radio and a micro component, and can be applied to a high range without deteriorating the S / N ratio. The present invention relates to a circuit configuration for compensating for lack of feeling.

[0002]

2. Description of the Related Art Generally, "loudness" means the loudness of a sound, but it is not the physical loudness of sound pressure but the loudness of sound that can be perceived by the auditory sense, and depends on the sound pressure and frequency. When the frequency is constant, the loudness increases when the sound pressure rises. However, even if the sound pressure is the same, the loudness changes when the frequency is changed, and particularly the loudness decreases in the low / high range. Therefore, conventionally, an audio circuit often incorporates a loudness circuit for compensating for the lack of a feeling of low and high frequencies.

For example, there is a general loudness circuit shown in FIG. 4 (A). In this loudness circuit, the master VR (R2
A filter whose frequency characteristic changes as shown in FIG. 4 (B) is constructed by using the center tap of 2), and the master VR
By lowering the volume with (R22), the low and high frequencies are emphasized, and by raising the volume, a flat frequency response is secured.

In addition, regarding only the bass range, a bass enhancing circuit as shown in FIG. 5 has been proposed (Japanese Patent Laid-Open No. 7-321).
No. 581). The bass boosting circuit is a secondary active high-pass filter (secondary ACT-
HPF) 52, an output level detection circuit 53 that detects and outputs the output signal of the power amplifier 51, and an output level detection circuit 53
And a control circuit 54 that controls the amount of positive feedback of the secondary ACT-HPF 52 in accordance with the detection output level of the secondary ACT-HPF 52. Functionally, the secondary ACT-HPF5
The boost selectivity is controlled by 2 to enhance the bass range when the output level is low. Therefore, as shown in the frequency characteristic of FIG. 6, the boost selectivity is controlled by setting the cutoff frequency fc of the secondary ACT-HPF 52 to 69 Hz, and (a) when the output level is high, and when the output level is in the middle.
(b), if it is low, change the frequency characteristics as shown in (c),
It is possible to enhance the bass range with a smooth frequency characteristic that does not cause dips or the like.

[0005]

By the way, the loudness circuit of FIG. 4 emphasizes the low and high frequencies when the volume is turned down. However, a cassette tape recorder with a radio, a micro component and a CD with a low sound pressure are used. Because the sound pressure is low when playing earphones on a player, the master VR (R2
In 2), the volume is often raised and listened to.
The frequency characteristic of (B) becomes flat, which causes a lack of bass and treble. That is, the maximum level of sound pressure varies greatly depending on the audio system, and the loudness circuit of FIG. 4 cannot compensate for the low / high range when the volume is raised. There will be a lack of feeling.

On the other hand, the bass boosting circuit shown in FIG. 5 does not change the frequency characteristic in association with the adjustment state of the master VR like the loudness circuit shown in FIG.
Since the amount of boost in the low frequency range is controlled according to the output level of, the optimum compensation of the low frequency range is always possible. However, since only the low frequency range is naturally emphasized only by the low frequency range boosting circuit, the sense of lack of the high frequency range is promoted. As a countermeasure, a high-frequency emphasizing circuit as shown in FIG. 7 (A) is inserted before the bass boosting circuit shown in FIG.
It is conceivable that the high frequency range is compensated by the frequency characteristic of (B), but the high frequency range enhancing circuit always enhances the high frequency range regardless of the presence or absence of the input, so that the S / N ratio becomes extremely poor. In particular, noise is very conspicuous in the pianissimo part of the music. In that case, the noise in the low frequency range also increases, but the noise in the low frequency range is not so noticeable to the auditory sense, and rather the high frequency range called 1 / f noise becomes a problem.

Therefore, the present invention realizes a loudness circuit capable of compensating the high range without deteriorating the S / N ratio, and by combining with a bass enhancing circuit as shown in FIG. It was created for the purpose of providing an audio system capable of emphasized reproduction.

[0008]

SUMMARY OF THE INVENTION The present invention is directed to a difference signal detection circuit for detecting a difference between left and right two-channel audio input signals, and a high pass filter for passing only the high frequency component of the difference signal detected by the difference signal detection circuit. A filter, a detection circuit that detects the output signal of the high-pass filter, and a left and right two-channel audio signal system are provided. When the level of the detection signal output by the detection circuit is equal to or higher than a certain level, the high level of each audio input signal is detected. The present invention relates to a loudness circuit including a high frequency compensating circuit that emphasizes only a band component.

According to the loudness circuit of the present invention, when the difference signal of the left and right two channels of the audio input signal contains a certain amount or more of the high frequency component, the high frequency component of the audio input signal is detected by the high frequency compensation circuit. Automatically emphasizes and compensates for lack of treble. In this loudness circuit, the master VR
Since the treble range is emphasized only when the difference signal includes the treble range, the treble range can be compensated irrespective of the volume adjustment state without interlocking with the adjustment state. That is, even in a simple audio system in which the volume is often increased and listened to, the high-frequency compensating circuit operates only when a high-frequency component is included, unlike the loudness circuit in FIG. Therefore, the S / N ratio does not decrease. Therefore, in combination with the bass boosting circuit as shown in FIG. 5, the bass and treble can be emphasized in a good balance regardless of whether the output level is high or low.

In the present invention, the difference signals of the left and right channels are detected, and the high frequency component contained in the difference signal is used for controlling the high frequency compensation circuit. Here, the reason for using the difference signal is to prevent the circuit from malfunctioning due to the harmonics of the vocal sound signal, and to emphasize the high range with a monaural signal, which causes annoying noise.
It is based on the fact that the enhancement of the high frequency range with the high frequency components having different L / R can realize a sense of sound field expansion.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the loudness circuit of the present invention will be described in detail below with reference to FIGS. First, FIG. 1 shows a block circuit diagram of a stereo-audio reproduction system to which a loudness circuit according to the present invention is applied. In the figure, a circuit that belongs to a stage subsequent to the VOL circuits 1L and 1R corresponds to the bass boost circuit shown in FIG. 5, and the output level detection circuit 53 (LR) is used for the power amplifier 5 of each of the L and R channels.
The output levels of 1L and 51R are detected, and the secondary ACT-HPF 52L, 5 corresponding to the detected output levels of the respective control circuits 54L and 54R.
By controlling the boost selectivity of 2R, low range compensation is performed when the output level is low.

This embodiment is characterized by a circuit configuration which belongs to the preceding stage of the VOL circuits 1L and 1R, and the circuit includes a difference signal detection circuit 2 for detecting a difference signal of input audio signals for each of the L and R channels. , HPF3 that removes the low-frequency component of the difference signal output by the difference signal detection circuit 2 and passes only the high-frequency signal component that is an index for high-frequency compensation, and the detection circuit 4 that detects the output signal of HPF3, It is composed of high-frequency compensating circuits 5L and 5R that have the function of emphasizing the high frequency range of the input signal when the audio signals of the L and R channels are input and the signal level after detection by the detection circuit 4 is above a certain threshold. Has been done.

A concrete circuit configuration centering on the above characteristic portion is shown in FIG. Here, the difference signal detection circuit 2 removes the DC component of the audio signal of each channel with C1 and C2 and subtracts the Rch signal from the Lch signal with the configuration of the differential amplifier circuit using the operational amplifier OP1. Is output to HPF3.

HPF3 is a Butterworth type second-order ACT-HPF using an operational amplifier OP2, and has a cutoff frequency fc.
= 1 / [2π√ {C5 ・ (R5 + R8) ・ C6 ・ R6}], selectivity Q = √
{C5 ・ (R5 ＋ R8) ・ C6 ・ R6} / {(1-A) ・ C6 ・ R6 ＋ C5 ・ (R5 + R8) ＋ C6 ・
(R5 + R8)}, for example C5 = C6 = 200 pF, R5 =
By setting 8.2 kΩ, R6 = 220 kΩ, R8 = 10 kΩ and the gain A of the operational amplifier OP2 to 1, fc = 12.6 k
H and Q are set to 1.74. The detection circuit 4 half-wave rectifies and smoothes the output of the HPF 3 and outputs it to the control signal generation unit of the high frequency compensation circuit 5L.

The high frequency compensating circuit 5L comprises a control signal generating section and a variable filter section. The control signal generation unit has a configuration in which the detection signal is input to the base of TR1 via the parallel circuit of C11 and R11 and the base resistor R12, and the collector of TR1 is connected to the base of TR2, which speeds up the operation. C11, which is equivalent to a capacitor, shortens the rise / fall time of the detection signal to shape the pulse wave, and when the level of the detection signal exceeds a certain value, TR1 turns on and TR2 turns off. Is configured. Further, as shown in the figure, the variable filter section comprises a passive filter with R17, R18, R19 and C13, C12, and both ends of R18 are n-channel type F
The FET 1 is connected to the source and drain of ET1 and is turned on / off by the control signal of the control signal generator, so that the high frequency enhancement mode and the flat mode can be selectively obtained. In this embodiment, R17 = 4.7
kΩ, R18 = 220kΩ, R19 = 3.3kΩ, C13 = 10μF, C12
= 3900 pF, the cut-off frequency fc becomes HPF of about 10.5 kHz in the high frequency boost mode when FET1 is ON,
A flat frequency characteristic can be obtained in the flat mode when the FET1 is off. Of course, the high frequency compensating circuit 5R on the R channel side also has the same circuit configuration.

Next, the high frequency compensation operation in the circuit of this embodiment will be described. First, the difference signal detection circuit 2 always detects the difference signal obtained by subtracting the R channel input signal from the L channel input signal, and only the high frequency component of the difference signal is detected by the detection circuit 4 based on the above characteristics of HPF3. Detection output. Then, in the high frequency compensating circuits 5L and 5R, when the detection output level E of the high frequency component included in the difference signal is smaller than the constant threshold Eth, TR1 of the control signal generating unit is turned off and its collector The voltage rises, the base current flows through TR2, and TR2 is turned on. However, when TR2 is turned on, its collector voltage drops and FET1 of the variable filter section
Is turned off. As a result, the variable filter R18
Since it is used as it is as a circuit element, the variable filter section functions in flat mode with flat frequency characteristics.

On the other hand, when the detection output level E becomes equal to or higher than the constant threshold value Eth, TR1 of the control signal generating section is turned on, the collector voltage of the control signal is lowered, the base current stops flowing to TR2 and TR2 is turned off. But TR2 is OFF
In this state, the collector voltage becomes high and the FET1 of the variable filter section is turned on. As a result, R18 of the variable filter section is short-circuited, and the variable filter section enters the high frequency enhancement mode and functions as the HPF having the above characteristics.

Therefore, the variable filter sections of the high frequency compensating circuits 5L and 5R selectively take two frequency characteristics as shown in FIG. 2 according to the comparison relationship between the output level E of the detection circuit 4 and the threshold level Eth to obtain the difference signal. The high frequency range of the input audio signal is emphasized and output to the VOL circuits 1L and 1R only when the high frequency component included in is higher than a certain level, and is output with a flat frequency characteristic otherwise. Since the control is performed regardless of the adjustment state of the master VR and the high frequency range is compensated, unlike the loudness circuit of FIG. 4, even when the volume is increased, the lack of sense of the high frequency range does not occur. Further, since the compensating operation is performed only in the time period when the difference signal contains a constant high frequency component, the S / N ratio is not deteriorated. Incidentally, V
The bass boost circuit shown in FIG. 5 is provided after the OL circuits 1L and 1R, and the bass boost circuit does not operate in accordance with the output level of the power amplifiers 51L and 51R without interlocking with the adjustment state of the master VR. Enhance. Therefore, according to the circuit configuration of the present embodiment, it is possible to always realize a well-balanced enhancement of the low and high range.

By the way, even if there is a high frequency component in the pianissimo part of the music and the like, when high frequency compensation is performed, noise also increases and the S / N ratio deteriorates. This is the reason why the constant threshold level Eth is provided in the present embodiment, and when the input signal becomes extremely small as in the pianissimo portion, noise is conspicuous without applying high frequency compensation. To prevent.

Further, in this embodiment, the high frequency component level of the difference signal of the input audio signals for each of the L and R channels is used for determining whether or not the high frequency compensation control is performed. The method using the difference signal seems unnatural from the viewpoint of stereo reproduction, but it is rational from the following viewpoints. (1) Vocal sound contains harmonic components, and if the high frequency components of each channel are detected separately, unnecessary high frequency compensation will be applied by the harmonic components. Therefore, in the present embodiment, the signal of the vocal sound, which is generally sound-image-localized in the front, is canceled by taking the difference signal of each channel so that unnecessary high frequency compensation operation does not occur. (2) When a high frequency component is detected in monaural sound, the high frequency components of both channels are always added, so high frequency compensation is applied continuously and the high frequency range is emphasized. On the contrary, the annoyance becomes noticeable. (3) When the high frequency compensation is performed with the components of the left and right channels different from each other as in the present embodiment, it is extremely convenient because the feeling of the expansion of the sound field is emphasized.

[0021]

The loudness circuit of the present invention, having the above-mentioned configuration, has the following effects. V
Regardless of the adjustment state of the master VR of the OL circuit, the high-frequency compensation is automatically applied only when the high-frequency component exceeds a certain level in the difference signal of the left and right channels. Even in the system, it is possible to always reproduce with a good S / N ratio without causing a feeling of lack of high-pitched sound. Further, when the input signal level is extremely low as in the pianissimo part of the music, it is possible to prevent high frequency compensation, so that it is possible to prevent noise from becoming conspicuous in a minute sound state. Further, since high frequency compensation is applied by the different components of the left and right two channels, there is an advantage that the feeling of sound field expansion is improved.

[Brief description of drawings]

FIG. 1 is a stereo to which a loudness circuit of the present invention is applied.
It is a block circuit diagram of an audio reproduction system.

FIG. 2 is a graph showing frequency characteristics of a high frequency compensation circuit in a high frequency compensation mode and a flat mode.

FIG. 3 is a specific electric circuit diagram centering on the characteristic part of the present invention in the block circuit diagram of FIG.

FIG. 4 is an electric circuit diagram (A) of a conventional loudness circuit and a graph (B) showing its frequency characteristic.

FIG. 5 is an electric circuit diagram of a bass boosting circuit disclosed in JP-A-7-321581.

6 is a graph showing frequency characteristics of the bass boost circuit of FIG.

FIG. 7 is an electric circuit diagram of a conventional high frequency emphasis circuit (A) and a graph showing its frequency characteristic.

[Explanation of symbols]

1L, 1R ... VOL circuit, 2 ... Difference signal detection circuit, 3 ... HPF, 4
… Detection circuit, 5L, 5R… High frequency compensation circuit, 51,51L, 51R… Power amplifier, 52,52L, 52R… Secondary ACT-HPF, 53,53 (LR)
… Output level detection circuit, 54, 54L, 54R… Control circuit, C1 to C1
3, C21 to C27 ... Capacitance, D1, D2, D21 ... Diode, FET1 ... n
Channel type FET, OP1, OP2, OP21 ... Operational amplifier, R1 to R1
9, R21 to R29, R31, R32… resistors, TR1, TR2, TR21… NPN type transistors.

Claims (1)

[Claims] 1. A difference signal detection circuit for detecting a difference between left and right two-channel audio input signals, a high pass filter for passing only a high frequency component of the difference signal detected by the difference signal detection circuit, and an output of the high pass filter. A detection circuit for detecting a signal and a high frequency band provided in each of the left and right two-channel audio signal systems and emphasizing only the high frequency component of each audio input signal when the level of the detection signal output from the detection circuit is above a certain level. A loudness circuit comprising a compensation circuit.