GB2102225A

GB2102225A - Automatic volume control device for acoustic apparatus mounted in vehicle

Info

Publication number: GB2102225A
Application number: GB08218415A
Authority: GB
Inventors: Yoshiyuki Sato; Norio Minami
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1981-06-29
Filing date: 1982-06-25
Publication date: 1983-01-26
Also published as: DE3224078A1; JPS583305A; GB2102225B

Abstract

An automatic volume control device for controlling the sound volume of an acoustic apparatus mounted in a vehicle includes a microphone (8) for receiving ambient sounds and generating electric signals with levels corresponding to the levels of the ambient sounds, a first filter (9) for receiving the electric signals from the microphone and for passing the electric signals with a first predetermined frequency band, first volume control signal generator (11,30,32) for rectifying the output electric signal from the first filter to generate a first sound volume control signal, and a sound volume controller (23) for controlling an output sound volume of a sound reproducing system in accordance with levels of the first sound volume control signal. The automatic volume control device further includes a second filter (36) for passing only the electric signal with a second predetermined frequency band contained in the first predetermined frequency band among the electric signal from the microphone, and a second volume control signal generator (37,38) for rectifying the output electric signal from the second filter to generate a second sound volume control signal, whereby the sound volume controller corrects the output sound volume of the sound reproducing system in accordance with levels of the second sound volume control signal. <IMAGE>

Description

SPECIFICATION Automatic volume control device for acoustic apparatus mounted in vehicle The present invention relates to an automatic volume control device for an acoustic apparatus mounted in a vehicle such as a car radio and a car stereo which are used in a place where ambient noise is greatly varied.

In the case where music, news and others are heard on a car stereo, car radio, or the like in a motor car, the masking of a listening sound such as music or news by ambient noise becomes a problem. Sound to be listened such as music or news is hereinafter referred to a listening sound.

That is, in some cases, a listening sound which can be heard in a quiet room without any disturbance, may be masked by ambient noise.

For example, in the case where such a listening sound is heard in a passenger car, as shown in Fig. 1, the noise level in the car is increased as the speed of the car becomes higher, and the noise level is equal to or greater than a level of listening sound which is kept when the listening sound is heard in the quiet room. In the quiet room, the listening sound is kept, on the average, at a level of about 60 to 65 dB(A).

Further, as shown in Fig. 2, it is required to change the level of listening sound by more than 10 dB when the noise level in the car varies greatly.

Fig. 2 shows an average (ordinary) relation between noise level and audible level of listening sound in the case where a listening sound is heard at a noisy place. In other words, in order to hear a listening sound in a car, it is required to always adjust a volume controller in accordance with changes in noise level so that the listening sound is easy to hear. When such adjustment is not made, the level of listening sound becomes too low to hear the listening sound, or becomes so high that the listening sound is clamorous.

However, since it is undesirable from the standpoint of safety driving to adjust sound volume while driving a motor car, man drives the car while feeling that the listening sound is low in level and is hard to hear. Accordingly, an acoustic apparatus is earnestly desired in which the level of listening sound is automatically changed in accordance with noise level and thus listening sound easy to hear is always obtained. In one example of conventional automatic volume control methods, noise is picked up directly by a microphone, and the volume of sound is adjusted by a control signal corresponding to a level of the detected noise.

A typical example of a car stereo unit equipped with an automatic volume control device according to the above-mentioned method is disclosed in British Patent Application No.

8014417 filed May 1,1980 (corresponding to West German Patent Application No.3017312.7 filed May 6, 1 980). Fig. 3 is a block diagram showing such a conventional acoustic apparatus of cassette tape recorder type mounted in a car.

In Fig. 3, reference numeral 1 designates a reproducing magnetic head, 2 a reproducing preamplifier, 3 a variable-gain amplifier, 4 a potentiometer for manually adjusting sound volume 5, an output amplifier, 6 a speaker, 7 an automatic volume control (AVC) circuit, 8 a microphone for detecting noise, 9 a low-pass filter, 10 an amplifier, 11 a diode for envelope detection, 1 2 a control circuit for controlling the gain of the amplifier 3, and 13 and 14 variable resistors for adjusting the gain of the control circuit 12.

Next, explanation will be made on the operation of the circuit shown in Fig. 3.

When a reproducing operation is started after a magnetic tape has been set in the car stereo unit, an audio signal reproduced by the reproducing magnetic head 1 is supplied to the output amplifier 5 through the pre-amplifier 2, variablegain amplifier 3 and potentiometer 4 to be subjected to power amplification, and then outputted from the speaker 6 in the form of a reproduced sound. The volume of the reproduced sound can be freely controlled by adjusting the potentiometer 4.

On the other hand, simultaneously with the above operation, ambient noise and others are picked up by the microphone 8 to be converted into an electric signal, only a low frequency component of which is taken out by the low-pass filter 9. The low frequency component is amplified by the amplifier 10 to a predetermined level, converted by the diode 11 into a d.c. signal, and then supplied to the control circuit 12. The control circuit 12 generates a control signal V, for automatically controlling sound volume, in accordance with the above d.c. signal, and the control signal V, is supplied to the gain control signal input terminal of the variable-gain amplifier 3 to control the gain of the amplifier 3. The control signal V, is supplied to the amplifier 3 so that the gain of the amplifier 3 is increased as the level of the control signal V, is higher.

Accordingly, when the level of ambient noise becomes high, the level of the output signal of the low-pass filter 9 becomes high, and thereby the level of the control signal V, is made high.

Accordingly, the gain of the amplifier 3 is enhanced to increase the volume of sound reproduced by the speaker 6. Thus, the reduction of signal-to-noise ratio due to a rise in noise level is cancelled.

Further, when the level of ambient noise becomes low, the level of the control signal V, is made low, and the gain of the amplifier 3 is lowered to decrease the volume of sound reproduced by the microphone 6. Thus, a listening sound is obtained which is easy to hear.

The variable resistor 13 serves to vary the input-to-output characteristic of the control circuit 12 so that the inclination of a line showing variation of the gain of the amplifier 3 with the level of ambient noise is freely changed as shown in Fig. 4. The variable resistor 14 serves to set that level of ambient noise at which the gain of the amplifier 3 begins to change, to various values such as shown in Fig. 5. Accordingly, by adjusting the variable resistors 1 3 and 14, the operation characteristic of the AVC circuit 7 can be freely changed, and therefore the circuit 7 can perform an optimum operation in accordance with a condition under which the car stereo unit is operated.

As mentioned above, in a car stereo unit equipped with the conventional AVC circuit shown in Fig. 3, the volume of reproduced sound is automatically controlled in accordance with the level of ambient noise. Accordingly, even when the noise level becomes high, the reproduced sound is prevented from being hard to hear.

Further, even when the ambient noise is low in level, the reproduced sound never becomes too clamorous. Thus, it is possible to hear the reproduced sound at optimum volume.

The above-mentioned AVC circuit 7 includes the low-pass filter 9 to detect, as noise, only a low frequency component of an acoustic signal picked up by the microphone 8, and the low frequency component thus detected is used to form the control signal V1. The above operations are carried out for the following reasons. The reproduced sound emitted from the speaker 6 is also picked up by the microphone 8. If the AVC circuit 7 does not include the low-pass filter 9, the AVC circuit 7 also responds to the reproduced sound from the speaker 6, and therefore performs a positive feedback operation. Thus, there is a fear of the AVC circuit 7 getting into an operational state which has no connection with noise level.

The low-pass filter 9 is provided to prevent the above-mentioned unfavorable operation. In other words, in sound instruments such as a car radio and a car stereo, the spectrum of sound reproduced by the speaker 6 is mainly distributed in a middle frequency range of the audio frequency band, and therefore the greater part of sound energy is concentrated in the middle frequency range. On the other hand, the spectrum of ambient noise is distributed not only in the whole audio frequency band but also on the outside thereof. Accordingly, when the cut-off frequency of the low-pass filter 9 is set to a value in a range of about 50 to 100 Hz, the level of output signal of the filter 9 is considered to mainly correspond to the level of ambient noise. Thus, the above-mentioned conventional AVC circuit can perform a right operation.

However, in the case where a sound instrument, for example, a car radio or car stereo is operated in special conditions, even if a noise level felt in listener's ears is low, the abovementioned conventional AVC circuit detects and responds to noise, and the volume of sound reproduced by the speaker is made higher than an appropriate value.

For example, when a motor car equipped with the above-mentioned sound instrument runs on a road put in a special state, for example, an unpaved, graveled road, noise is detected by the AVC circuit and the volume of reproduced sound is abnormally increased, notwithstanding a listener in the car hardly feels ambient noise.

Thus, the sound instrument is put in an unfavorable operational state.

An object of the present invention is to provide an AVC circuit which can control the volume of reproduced sound in exact accordance with noise felt in listeners's ears.

In order to attain the above object, according to the present invention, a sound pressure level at a first frequency range included in a specified region of an audio frequency band and a sound pressure level at a second frequency range included in a low frequency part of the first frequency range are detected, to automatically control the volume of sound reproduced by a sound instrument, in accordance with the sound pressure level at the first frequency range, and to correct the volume of the reproduced sound in accordance with the sound pressure level at the second frequency range.

The present invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: Fig. 1 is a characteristic diagram showing a relation between the running speed of a motor car and the level of running noise produced in the car; Fig. 2 is a characteristic diagram showing an average relation between the level of noise and the audible level of a listening sound in the case where the listening sound is heard at a noisy place; Fig. 3 is a block diagram showing a typical example of a conventional AVC device for acoustic apparatus mounted in vehicles; Figs. 4 and 5 are characteristic diagrams for explaining the operation of the AVC device shown in Fig. 3;; Fig. 6 is a characteristic diagram showing an example of a 1/3-octave band spectrum of running noise produced in a passenger car; Fig. 7 is a characteristic diagram showing respective examples of noise spectrum and listening sound spectra obtained in a passenger car; Fig. 8 is a characteristic diagram showing frequency spectra of noise obtained in a passenger car when the car is put in various running states; Fig. 9 is a block diagram showing a typical embodiment of an AVC device according to the present invention which is suitable for use in an acoustic apparatus mounted in vehicles; Figs. 10 and 11 are graphs for explaining the operation of the variable-gain amplifier 30 shown in Fig. 9; Fig. 12 is a circuit diagram of the variable-gain amplifier 30 shown in Fig. 9; ; Fig. 1 3 is a circuit diagram of the variable-gain amplifier 23 and control circuit 32 shown in Fig.

9; and Figs. 14 and 1 5 are block diagrams showing other embodiments of the present invention.

Now, the present invention will be explained below in detail, by reference to the drawings.

First, let us analyse the noise produced in a passenger car, on the basis of the characteristics which were obtained by actual measurement and are shown in Figs. 1 and 2. In this case, a 1/3octave band analysis is used for the reasons that the noise in the car is distributed in the form of a continuous spectrum, and that a marked line spectrum is scarcely found. Fig. 6 shows an example of the 1/3-octave band measurement of the noise in the room of a running car.

The following can be said from the analyses.

The energy of the noise inside the closed room of a running car is contained mostly in a lowfrequency region thereof and energy density decreases with the frequency. This tendency is generally true for all kinds of cars and for all vehicle speeds.

Next, a relation between noise and listening sound will be explained. Fig. 7 diagrammatically shows a relation between a frequency characteristic of noise obtained in a passenger car and frequency characteristics of listening sound obtained in the car. In Fig. 7, reference symbol A designates a noise spectrum, and B, and B2 spectra of listening sound. In more detail, reference symbol Br designates a spectrum of listening sound in the case where a listening sound is reproduced at an ordinary level in the passenger car kept at an ordinary running state, and B2 a spectrum of a maximum permissible listening sound in the case where the listening sound is reproduced at an intentionally raised level.Referring to Fig. 7, there exists a frequency range in which the level of noise is higher than the level of listening sound in any case, that is, a frequency range lower than a frequency f2 at which the noise spectrum A and the spectrum B2 of the maximum permissible listening sound intersect. According to experiments, that frequency component of a listening sound heard in a motor car which exists in the frequency range lower than the frequency f2, is far smaller than a frequency component of noise existing in the above frequency range. The frequency f2 lies in a range from 50 to 100 Hz, for example, is equal to 80 Hz.Accordingly, when a signal outputted from the microphone is supplied to the low-pass filter having a cut-off frequency nearly equal to the frequency f2, the output signal of the low-pass filter is mainly formed of noise, and is scarcely affected by the listening sound and conversation.

Fig. 8 shows frequency characteristics of noise obtained in a motor car when the car runs on roads which are put in different states. In Fig. 8, a curve N, designates a noise spectrum obtained when the car runs at a low speed on a bad road such as a graveled road, N2 a noise spectrum obtained when the car runs at an intermediate speed on a paved road, and N3 a noise spectrum obtained when the car runs at a high speed on an express way.

As is apparent from Fig. 8, when a frequency f, is set in a low frequency region of the audio frequency band so that the frequency f1 is lower than the frequency f2, that frequency component of the noise spectrum N, which exists-in a frequency range lower than the frequency f, is greater than respective frequency components of the noise spectra N2 and N3 which exist in the above frequency range. That is, in the frequency range below the frequency f1, noise obtained in the case where the car runs on a bad road such as a graveled road is higher in level than noise obtained in other cases. The frequency f, lies in a range of about 10 to 30 Hz, i.e. a minimum audible frequency.

On the other hand, the hearing sensation of human being becomes dull as the frequency of sound is lower in a frequency range below 100 Hz, and sound in a frequency range below the frequency f, is substantially inaudible.

Accordingly, in the case where the car runs on the graveled road, since the greater part of noise lies in the frequency range below the frequency f, as shown by the noise spectrum N1, a listener in the car scarcely feels noise in his ears, notwithstanding that frequency component of noise which exists in a frequency range below the frequency f2 and is outputted from the low-pass filter in the form of an electric signal, has a high level. In view of the above-mentioned facts, according to the present invention, the volume of a reproduced sound emitted from the speaker is controlled in accordance with the frequency component of noise detected by the microphone which exists in a frequency range below the frequency f2, and is corrected in accordance with the frequency component of noise existing in a frequency range below the frequency f,.

Fig. 9 is a block diagram showing a typical embodiment of an automatic volume control device according to the present invention which is suitable for use in sound instruments mounted in vehicles and is now applied to a cassette type car stereo.

In Figs. 3 and 9, elements and circuits having the same or equivalent functions are given the same reference numerals.

In Fig. 9, reference numeral 1 designates a reproducing magnetic head, 2 a reproducing preamplifier, 4 a potentiometer for manually adjusting sound volume, 5 an output amplifier, 6 a speaker, and 1 7 an AVC circuit according to the present invention. The AVC circuit 1 7 includes a microphone 8 for detecting noise, a low-pass filter 9, an amplifier 30, a diode 11 for envelope detection, a variable-gain amplifier 23, a control circuit 32 for controlling the gain of the amplifier 23, variable resistors 33 and 34 for controlling the gain of the control circuit 32, and a gain control circuit 35 for controlling the gain of the amplifier 30. The cut-off frequency f2 of the lowpass filter 9 is put in a range of about 50 to 100 Hz, for example, is made equal to 80 Hz.The gain control circuit 35 is connected to the microphone 8, and includes a iow-pass filter 36, an amplifier 37 and a diode 38 for envelope detection.

In the present embodiment, the amplifier 30 has a control signal input terminal 31 to control the maximum amplitude level of the output signal of the amplifier 30 by a control signal V2 which is supplied from the gain control circuit 35 to the control signal input terminal 31.

The low-pass filter 36 has a cut-off frequency f1 which is lower than the cut-off frequency f2 of the low-pass filter 9. The cut-off frequency f, is put in a range of about 20 to 30 Hz, and is made equal to 20 Hz in the present embodiment. The output of the low-pass filter 36 is amplified by the amplifier 37 to a predetermined level, and then converted by the diode 38 into the d.c. control signal V2 which is applied to the control signal input terminal 31 of the amplifier 30.Accordingly, when the amplitude level of a signal S supplied from the low-pass filter 9 to an input signal receiving terminal of the amplifier 30 exceeds a predetermined value, the amplitude of the output signal of the amplifier 30 is clipped in accordance with the level of the control signal V2, and thus the output signal has a maximum amplitude level in accordance with the level of the control signal V2 as shown in Fig. 10. In Fig. 10, maximum amplitude levels Lr, L2 and L3 for the output signal of the amplifier 30 correspond to a sufficiently high level, an intermediate level and a fairly low level of the control signal V2, respectively.In other words, when the level of the output signal S of the low-pass filter 9 exceeds a predetermined value, the output of the amplifier 30 is suppressed in accordance with the output level of the lowpass filter 36, and the amplitude level of the output of the amplifier 30 becomes low as the output level of the low-pass filter 36 is higher.

As a result, the gain of the variable-gain amplifier 30 depends upon the level of the output signal S of the low-pass filter 9 in such a manner as shown in Fig. 11. When the level of the control signal V2 takes values V2C, V26 and V2A (where V2C < V2B < V2A) in a continuous manner, a signal S - gain of amplifier 30 characteristic is continuously varied, and the maximum gain of the amplifier 30 becomes equal to values a, b and c.

For example, in the case where the level of the control signal V2 is low and has the value V2c, the gain of the amplifier 30 is limited to the value c for the signal Shaving levels above a level S1. The value c of the gain of the amplifier 30 may be made correspond to a maximum audible level of listening sound.

The diode 11 makes smooth the output signal of the amplifier 30 to form a d.c. signal. The control circuit 32 supplies a control signal V corresponding to the above d.c. signal to the variable-gain amplifier 23 to control the gain thereof.

A circuit configuration of each of the variablegain amplifiers 23 and 30 and control circuit 32 will be explained later in detail.

Next, the operation of the present embodiment will be explained.

When a reproducing operation is started after a magnetic tape has been set in the car stereo, an audio signal reproduced by the reproducing magnetic head 1 is supplied to the output amplifier 5 through the pre-amplifier 2, variablegain amplifier 23 and potentiometer 4, to be subjected to power amplification, and then outputted from the speaker 6 in the form of a reproduced sound. The volume of the reproduced sound can be freely controlled by adjusting the potentiometer 4.

On the other hand, simultaneously with the above operation, ambient noise and others are picked up by the microphone 8 to be converted into an electric signal. Only that low frequency component of the electric signal which exists in a frequency range below the frequency f2, is taken out by the low-pass filter 9, and then amplified by the amplifier 30 to a predetermined level.

Further, another low frequency component of the above electric signal which exists in a frequency range below the frequency f1 is taken out by the low-pass filter 36. This low frequency component is amplified by the amplifier 37, and then smoothed by the diode 38 to form the control signal V2. The control signal V2 is supplied to the amplifier 30 to control the gain thereof in such a manner as shown in Fig. 11. The output of the amplifier 30 is detected by the diode 11 to form a d.c. signal, which is supplied to the control circuit 32. The control circuit 32 generates the control signal V1 for automatically controlling sound volume, in accordance with the above d.c.

signal. The control signal V1 is applied to a gain control signal input terminal 24 of the variablegain amplifier 23 to control the gain thereof. The control signal V1 is supplied to the amplifier 23 so that the gain of the amplifier 23 is increased as the level of the control signal V1 is higher.

Accordingly, in the case where ambient noise detected by the microphone contains a very small amount of low frequency component in a frequency range below the frequency f1, the level of the control signal V2 is low, and the gain of the amplifier 30 can be increased to the maximum value c shown in Fig. 11.

Further, when the level of ambient noise becomes high, the level of the output signal of the low-pass filter 9 and the level of the control signal V1 are made high, and thereby the gain of the variable-gain amplifier 23 is enhanced to increase the volume of sound reproduced by the speaker 6.

Thus, the reduction of signal-to-noise ratio due to a rise in noise level is cancelled.

Further, when the level of ambient noise becomes low, the level of the output signal of the low-pass filter 9 and the level of the control signal V1 are made low, and therefore the gain of the variable-gain amplifier 23 is lowered to decrease the volume of sound reproduced by the speaker 6.

Thus, a listening sound is obtained which is easy to hear.

On the other hand, in the case where ambient noise contains a large amount of low frequency component in the frequency range below the frequency f1, the level of the control signal V2 is low, and the maximum gain of the amplifier 30 is limited to less than the value a. Therefore, the volume of sound reproduced by the speaker 6 is restricted. Thus, the volume of the reproduced sound is adjusted so that the reproduced sound is easy to hear.That is, when the motor car runs on a bad road such a graveled road, the level of the control signal V2 from the low-pass filter 36 becomes high as compared with the level of the output signal S of the low-pass filter 9, and the gain of the amplifier 30 depends upon the output signal S in such a manner as indicated by a line V2A in Fig. 11. That is, when the level of the output signal S exceeds a value 53 the gain of the amplifier 30 is limited to a value a. Accordingly, even if the level of the output signal S of the lowpass filter 9 further increases in such a state, the volume of the reproduced sound is limited to a predetermined value. Thus, the volume of the reproduced sound is made correspond to a noise level felt in listeners's ears.

Further, when the motor car runs on an express way, the level of the output signal S of the lowpass filter 9 becomes high as compared with the level of the control signal V2, and the gain of the amplifier 30 depends upon the output signal S in such a manner as indicated by a line V2c in Fig.

11. That is, the gain of the amplifier 30 is increased with the level of the output signal S until the level of the signal S reaches a value S1, and takes a high value c when the level of the signal S reaches the value S1. Accordingly, the volume of the reproduced sound is increased in accordance with a rise in noise level felt in listener's ears, and thus a favorable s/N ratio is maintained.

Further, when the motor-car runs on a paved road, the gain of the amplifier 30 depends upon the output signal S in such a manner as indicated by a line V26 in Fig. 11. Thus, in any case, the volume of the reproduced sound is controlled in accordance with the noise level felt in listener's ears.

As is evident from the above explanation, according to the present embodiment, even if a motor car equipped with a sound instrument such as a car radio or car stereo is put in any running state, the volume of reproduced sound is automatically controlled in accordance with the noise level felt in listener's ears all the time, that is, volume control can be carried out in exact accordance with the noise level felt in listener's ears.

Fig. 12 shows an example of the circuit configuration of the variable-gain amplifier 30.

variable-gain amplifier 30 shown in Fig. 12 includes an operational amplifier 40 and transistors 42, 44 and 46. The non-inverting input terminal of the operational amplifier 40 is connected to the output side of the low-pass filter 9, to receive the signal S. The base of the transistor 46 receives the control signal V2.

When the control signal V2 is applied to the base of the transistor 46 through the control signal input terminal 31, a current flowing through a resistor 48 is determined in accordance with the control signal V2. Accordingly, a potential V49 of a junction 49, that is, the bias potential at the base of the transistor 44 is determined in accordance with the control signal V2.

Accordingly, an emitter potential V, of the transistor 44 varies with the control signal V2, and controls the base current of the transistor 42.

Therefore, the saturation level of the amplifier 40 for various levels of the signal S is controlled by the emitter potential V44. That is, when the control voltage V2 is increased, the potentials V49 and V44 are both decreased. Thus, the saturation level of the amplifier 40 is lowered as indicated by the level L1 in Fig. 10, or as indicated by the line V2A in Fig. 11.

Fig. 13 shows an example of the circuit configuration of the variable-gain amplifier 23 and control circuit 32. Referring to Fig. 13, the amplifier 23 includes a differential amplifier, which is made up of a pair of transistors 60 and 62 and resistors 64 and 65, and a transistor 58.

Further, the control circuit 32 includes a differential amplifier, which is made up of a pair of transistors 50 and 52 and resistors 54, 33 and 34, and a voltage divider 56.

The base of the transistor 58 receives the output signal of the pre-amplifier 2, and the base of the transistor 50 receives the d.c. signal through the diode 11. A d.c. voltage V11 applied from the diode 11 to the base of the transistor 50 is compared with a reference voltage V52 applied to the base of the transistor 52. Thus, a potential V55 of a junction 55 is determined in accordance with the magnitude of the voltage V,1. The potential V55 is applied to the base of the transistor 60. A voltage V56 which is obtained by dividing a supply voltage Vcc by the voltage divider 56, is applied to the base of the transistor 62.An output voltage V23 of the amplifier 23 varies with the voltages V55 and V56. In more detail, when the d.c. voltage V11 is increased to become higher than the reference voltage V52, the transistor 50 becomes conductive, and the potential V55 of the junction 55 is decreased.

When the potential V55 becomes lower than the potential V55, the transistor 62 is made conductive. On the other hand, when the d.c.

voltage V11 is decreased to become lower than the reference voltage V52, the transistor 52 becomes conductive, and the potential V55 becomes higher than the potential V55. Thus, the transistor 60 is made conductive. That is, in the case where the level of ambient rioise is very high (that is, the d.c. voltage V11 takes a maximum value), the transistor 62 is put in the ON-state, and the transistor 60 is put in the OFF-state.

When respective resistance values of the resistors 64, 65 and 66 are expressed by R,, R65 and R66, the gain of the amplifier 23 in the above case is given by (R64+R65)/R66. Further, in the case where the noise level is very low (that is, the d.c. voltage V11 takes a minimum value), the transistor 62 is put in the OFF-state, and the transistor 60 is put in the ON-state. The gain of the amplifier 23 in this case is given by R65/R65. As mentioned above, the degree of conduction of each of the transistors 60 and 62 varies with noise level, and the gain of the amplifier 23 varies in a range from R8s/R58 to (R64+R6s)/R66 in accordance with noise level.

The variable resistor 33 serves to vary the input-to-output characteristic of the control circuit 32 so that the inclination of a line showing variation of the gain of the amplifier 23 with the level of ambient noise is freely changed as shown in Fig. 4. The variation of gain with noise level shown in Fig. 4 is large as a resistance value of the resistor 33 is larger. The variable resistor 34 serves to set that level of ambient noise at which the gain of the variable-gain amplifier 23 begins to change, to various values such as shown in Fig.

5. Accordingly, by adjusting the variable resistors 33 and 34, the operation characteristic of the AVC circuit 1 7 can be freely changed, and therefore the circuit 1 7 can perform an optimum operation in accordance with a condition under which the car stereo is operated. The operation characteristics of the AVC circuit may be adjusted by varying the gain of the amplifier 30 in place of adjusting the resistors 33 and 34 of the amplifier 32.

In the present embodiment, the low-pass filters 9 and 36 are employed. However, these low-pass filters may be replaced by band-pass filters. In more detail, the low-pass filter 9 may be replaced by a band-pass filter having a pass band of 5 to 90 (or 100) Hz, and the low-pass filter 36 may be replaced by a band-pass filter having a pass band of 5 to 20 (or 30) Hz.

Further, though the output side of the microphone 8 is connected to a parallel combination of the low-pass filters 9 and 36 in the present embodiment, the filters 9 and 36 may be connected in series as shown in Fig. 14.

In the present embodiment, the output of the diode 11 controls the control circuit 32, and the output of the diode 38 controls the amplifier 30.

Alternatively, a circuit shown in Fig. 1 5 may be used in which the output of the diode 11 controls the pre-amplifier 2 through the control circuit 32, and the output of the diode 38 controls the amplifier 23 through another control circuit 32'.

In the foregoing explanation, an acoustic apparatus of the monaural system has been shown only for the convenience of explanation.

Needless to say, an actual circuit part between the magnetic head 1 and speaker 6 has two channels, that is, a stereo phonic system is employed in this circuit part. The control signal V is applied commonly to respective variable-gain amplifiers 23 of these channels. It is needless to say that the present invention is applicable to an acoustic apparatus of the monaural system.

Further, it is a matter of course that the present invention is applicable not only to a cassette type car stereo but also to a car stereo for receiving FM and AM broadcasting.

As has been explained in the foregoing, according to the present invention, by adding a relatively simple control circuit, the volume of reproduced sound can be automatically controlled in exact accordance with the noise level felt in listener's ears. That is, there is provided an automatic gain control circuit which can eliminate the drawbacks of the prior art, and which can put a car radio, car stereo or the like in a favorable operation state to always make optimum the S/N ratio of sound heard in a vehicle.

Claims

1. An automatic volume control device for controlling the sound volume of an acoustic apparatus mounted in a vehicle, comprising: a microphone for receiving ambient sounds and generating electric signals with levels corresponding to the levels of said ambient sounds; first filter means for receiving said electric signals from said microphone and for passing the electric signals with a first predetermined frequency band; first volume control signal generating means for rectifying the output electric signal from said first filter means to generate a first sound volume control signal; and sound volume control means for controlling an output sound volume of a sound reproducing system in accordance with levels of said first sound volume control signal, wherein said automatic volume control device further comprises; second filter means for passing only the electric signal with a second predetermined frequency band contained in said first predetermined frequency band among the electric signal from said microphone, and second volume control signal generating means for rectifying the output electric signal from said second filter means to generate a second sound volume control signal, whereby said sound volume control means corrects the output sound volume of the sound reproducing system in accordance with levels of said second sound volume control signal.

2. An automatic volume control device according to Claim 1, wherein said sound volume control means increases the output sound volume of said sound reproducing system in accordance with the increase of the level of said first sound volume control signal, and decreases the maximum value of the output sound volume of said sound reproducing system in accordance with the increase of the level of said second sound volume control signal.

3. An automatic volume control device according to Claim 1 or 2, wherein said first frequency band is set to be a lower frequency area of an audio frequency band, and said second frequency band is set to be a lower frequency area of said first frequency band.

4. An automatic volume control device according to Claim 3, wherein said first frequency band is set to be lower than a frequency at which a maximum listening sound spectrum crosses a noise spectrum, and said second frequency band is set to be lower than a minimum audible frequency.

5. An automatic volume control device according to Claim 4, wherein said first filter means is a low-pass filter with a cut-off frequency in a range of about 50-100 Hz, and said second filter means is a low-pass filter with a cut-off frequency in a range of about 10-30 Hz.

6. An automatic volume control device according to Claim 4, wherein said first and second filter means are band-pass filters for passing the electric signals having frequencies in ranges of about 5-100 Hz and 5-30 Hz, respectively.

7. An automatic volume control device constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in Figures 9 to 15 of the accompanying drawings.