1. Field of the Invention
The present invention relates to an automatic volume control for a sound reproducing apparatus used in noisy environments.
2. Description of the Related Art
In an apparatuses possessing sound output means, such as a television set, radio or a tape recorder, it has been attempted to automatically control the volume depending on the level changes in the ambient noise. As for the sound reproducing apparatus used in noisy environments, such as a car radio or a car stereo equipment, it has been attempted to automatically raise the reproducing volume depending on the level of the ambient noise so as to maintain a favorable listening condition even in a noisy situation.
Typical apparatuses are as follows: (a) an apparatus in which a mixed sound of an ambient noise with a speaker sound is detected by a microphone, and the level difference in the detected signal from the signal out of a sound signal output circuit is calculated to detect the level of the ambient noise, thereby to control the gain of the sound signal output circuit; (b) an apparatus in which the ambient noise is detected only when no sound is produced from the loudspeaker so as to obtain a sound level corresponding to the ambient noise thereby to control the volume by maintaining the same level when the loudspeaker produces sound; (c) an apparatus where an ambient noise is detected by picking up a mixed sound of the ambient noise with a speaker sound by a microphone, produces a signal imitating the speaker sound from the signal outputted from the sound signal output circuit by using an adaptive filter, then subtracting the adaptive filter output signal from the microphone output signal, thereby to control the volume depending on the ambient noise level.
As for the apparatus (a), adequate control by detecting the level of the ambient noise signal component separately from the mixed signal of the ambient noise with speaker sound is difficult because the speaker sound component detected by the microphone is greatly different from the original electric signal due to phase delay of the speaker sound, frequency response of loudspeaker, and room reverberation characteristic.
As for the apparatus (b), it is far from practical. When used in a place where the ambient noise changes suddenly or periodically while the loudspeaker is producing sound, and in particular if the sound is continuous such as with music, the apparatus (b) is almost impossible to control because the loudspeaker rarely stops producing sound.
As for the apparatus (c), it functions normally when the sound source is monaural, but has difficulty detecting the ambient noise in case of stereo. Since the sum of the right and left signals is used as the input to the adaptive filter, the apparatus (c) is affected by the positions of the microphone and the loudspeakers, or shape of the room.
FIG. 1 is a block circuit diagram showing an example of a conventional automatic volume controlling apparatus of the (c) type proposed by the present inventors in the Japanese Patent Application Laid-Open No. Hei. 5-259779 (1993). In the figure, numeral 101 denotes a sound source, 104, 105, 106 denote amplifiers, 107, 108 denote loudspeakers, 109 denotes a microphone, 110, 111, 112 denote A/D converters, 113 denotes an adaptive filter, 115 denotes a subtracter, 116 denotes a level judging device for level converting the A/D converted signal and comparing the level with the reference levels 118, 120 denote signal converters for level converting the outputs of the subtracter 115 and A/D converter 112, respectively, and 121 denotes an adder for summing the outputs of the A/D converters 110, 111.
In this apparatus, by using the adaptive filter 113, an imitative loudspeaker signal added with a characteristic closely approximating a signal on route from the loudspeakers 107, 108 to the microphone 109 is canceled from the output of the microphone 109. The level of the ambient noise is obtained as the output of the signal converter 120 when a signal is not produced from the sound source 101. When a signal is produced from the sound source 101, the estimated value of the ambient noise level and the level of the mixed sound of the speaker sound with ambient noise are obtained as the outputs of the signal converters 118, 120, respectively.
FIG. 2 is a graph showing an example of the result obtained in the apparatus shown in FIG. 1, in which A indicates the level of the mixed sound of the ambient noise with speaker sound obtained by the microphone, B indicates the level of the ambient noise detected by the conventional apparatus, and C indicates the level of the ambient noise obtained by the microphone when no sound is produced by the loudspeaker. Herein, the ambient noise is imitatively reproduced by the loudspeaker and added to the speaker sound.
As clear from FIG. 2, there is a slight difference between the level B of the detected ambient noise and the level C of the ambient noise obtained when the speaker sound is not produced, so that the ambient noise is not detected correctly.
Moreover, in this prior art, although the speaker sound level can be controlled depending on the ambient noise, the volume of the speaker sound is not considered. In this kind of the automatic volume controlling apparatus, the volume is changed relative to the ambient noise level to suit the human auditory characteristic. The volume amplification degree is changed according to the level of the reproduced sound of the sound producing apparatus. That is, the higher the level of the reproduced sound from the sound producing apparatus, the smaller the volume should be amplified in response to the same ambient noise level.
One proposal is to use an automatic volume controlling apparatus capable of varying the amplification degree relative to the noise level signal depending on the volume level set by a volume changing means. FIG. 3 is a block diagram showing a conventional automatic volume controlling apparatus disclosed in the Japanese Patent Publication No. Hei. 3-13762 (1991). In the diagram, numeral 360 denotes a voltage controlled amplifier (VCA), 361 denotes a power amplifier, 362 denotes a loudspeaker, 363 denotes a noise detection circuit, 364 denotes volume controlling means, 365 denotes an adder, 366 denotes volume changing means, and 367 denotes volume setting means.
In this constitution, a sound signal is passed through the VCA 360 and power amplifier 361 and is reproduced by the loudspeaker 362. The amplification rate of the VCA 360 is controlled by the output of the adder 365. To the adder 365 are inputted the noise level signal detected by the noise detection circuit 363 via the level controlled by the volume controlling means 364, and the output from the volume setting means 376 for producing a volume level signal corresponding to the desired volume of the user on receiving a volume increase or decrease signal from the volume changing means 366. In this constitution, the volume is controlled in accordance with the noise detected by the noise detection circuit 363. The volume controlling means 364 changes the amplification degree against the noise level signal corresponding to the set volume at the time of receiving the output from the volume changing means 366. More specifically, as shown by a symbol h2 in FIG. 4, when the sound level (e2) is high, the amplification degree (g2) against the noise level is lowered to reduce the volume increase against the noise increase. As shown by a symbol h1, by contrast, when the sound level (e1) is low, the amplification degree (g1) against the noise level is raised to increase the volume increase against the noise increase, so that the volume control against the ambient noise more highly suits the auditory sense.
However, the speaker sound is actually heard, so that when the volume amplification degree is controlled depending on the volume set by the volume changing means 366, or when the volume amplification degree is controlled depending on the output from the volume controlling means 364, it is further necessary to control the volume amplification degree depending on the performance of the loudspeaker to be used. Besides, since the gain of the sound signal against the noise level is determined by representing the sound signal level with the set volume, it is impossible to avoid shortage or excess of auditory volume due to a difference in recording levels of music sources, or level changes in phrases of music.
Moreover, in the automatic volume controlling apparatus as shown in FIG. 1, the signal from the microphone 109 and the input signal to the adaptive filter 113 are usually digital signals from the A/D converter 112, but at this time since the input from the microphone to the A/D converter 112 must be within a specific range, the microphone output level must be controlled every time to the optimum value depending on the installed positions of the loudspeakers 107, 108 and microphone 109.
Incidentally, the sense of shortage of the volume due to the noise is mainly caused by masking of the sound signal by the noise. The sound signal component to be masked is larger in the frequency range of a larger noise component, and smaller in the frequency range of a smaller noise component. For example, the automobile running noise has a larger component at lower frequencies and a smaller component at higher frequencies.
Therefore, if the reproduced volume is uniformly controlled without regard to the frequency of the sound signal, as in the conventional apparatus, the effect is insufficient in the frequency range with larger noise components, but is excessive in the frequency range with smaller noise components. When such mere gain increase is done for the automobile running noise, sensation of the volume shortage in the low frequency range and sensation of the volume excess in the high frequency range are unavoidable.
As a solution, a frequency characteristic can be compensated by boosting the low frequency range of the sound signal or attenuating the high frequency range. This solution assumes the spectrum of the noise, and it may not be a sufficient solution if the noise spectrum varies.