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Automatic Loudness Control Circuit

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Automatic Loudness Control Circuit Europaisches Patentamt J) European Patent Office 0 Publication number: 0 287 057 Office europeen des brevets A2 © EUROPEAN PATENT APPLICATION © Application number: 88105872.1 © IntCI.4: H03G 5/16 © Date of filing: 13.04.88 ® Priority: 17.04.87 JP 58832/87 © Applicant: SANYO ELECTRIC CO., LTD. 17.04.87 JP 95574/87 18, Keihanhondori 2-chome 01.07.87 JP 164719/87 Moriguchi-shi Osaka-fu(JP) © Date of publication of application: @ Inventor: Kato, Masami 19.10.88 Bulletin 88/42 510-8, Shimokoizumi Oizumi-cho Ora-gun Gunma(JP) © Designated Contracting States: Inventor: Horikoshi, Katsu OE FR GB 513-193, Nishitakane-cho Tatebayashi-shi Gunma(JP) © Representative: Glawe, Delfs, Moll & Partner Patentanwalte Postfach 26 01 62 Liebherrstrasse 20 D-8000 MUnchen 26(DE) Automatic loudness control circuit. © An automatic loudness control circuit for auto- matically controlling the magnitude of boosting of the low-frequency component of an audio signal in ac- cordance with the output signal level of a power amplifier 4 for driving a loudspeaker 5. The low- frequency component of the output signal from an audio signal source 1 is boosted by a low frequency boosting circuit 2 to prepare a low-frequency boost signal, which is then added to the output signal of the source 1, and the resulting signal is fed to the power amplifier 4. The magnitude of boosting by the boosting circuit 2 is controlled according to the out- ^put level of the power amplifier 4 and is therefore ^increased as the output level lowers. Consequently, the circuit assures optimum loudness control at all [Jj times, permitting the loudspeaker 5 to produce dy- Qnamic sounds. r> oo CL LU Xerox Copy Centre I 3 287 057 AUTOMATIC LOUDNESS CONTROL CIRCUIT and 103 into the tone control circuit when the FIELD OF THE INVENTION circuit is to be fabricated in the form of an IC, consequently offsetting the advantage to be ob- The present invention relates to automatic tained by constructing the circuit as an IC. loudness control, circuits for use in acoustic appara- 5 tus such as radio receivers and stereophonic re- production systems for automatically optimally ad- SUMMARY OF THE INVENTION justing the frequency characteristics of an audio signal before feeding the signal to a loudspeaker. An object of the present invention is to provide w an automatic loudness control circuit which is adapted to detect the output level of an audio BACKGROUND OF THE INVENTION signal and to automatically adjust the frequency characteristics of the signal in accordance with the Conventional acoustic apparatus are provided detected level so as to obtain an optimum tone at with bass control and treble control knobs on the rs all times. operation panel which are . manipulated for adjust- Another object of the invention is to provide an ment to realize the desired tone. automatic loudness control circuit a major portion Fig. 6 shows a tone control circuit heretofore of which can be fabricated into an IC. proposed for effecting tone control in this manner Another object of the invention is to provide an (Unexamined Japanese Patent Publication SHO 59- 20 automatic loudness control circuit which is adapted 212012). to selectively feed to a loudspeaker one of an With this circuit, the signal applied to an input output signal subjected to loudness control accord- terminal 100 is fed to a low-pass filter 101, a band- ing to the level of an audio signal and an output pass filter 102 and a high-pass filter 103. The signal not subjected to loudness control without output signals from the low-pass filter 101 and the 25 permitting occurrence of any noise due to a high-pass filter 103 are passed through variable change-over from one signal to the other. gain amplifiers 104 and 105, respectively, and fed Still another object of the invention is to pro- to an adder 106 along with the output signal from vide an automatic loudness control circuit for con- the band-pass filter 102. The audio signal obtained trolling the loudness characteristics of an audio at an output terminal 1 07 is sent to a loudspeaker 30 signal according to the output level thereof with via a power amplifier. high accuracy, the circuit being normally operable A bass control signal is applied to a control despite some variations in the power supply volt- signal input terminal 108 to adjust the gain of the age. amplifier 104 for bass control, while a treble control Still another object of the present invention is signal is applied to another control signal input 35 to provide an automatic loudness control circuit terminal 109 to control the gain of the amplifier 105 wherein there is not occurred any clipping in the for treble control. boosting circuit provided for obtaining the best To reproduce an acoustically dynamic sound, it loudness characteristics even when a power supply is generally effective to boost the low-frequency of low voltage like 1 .5V is applied. component of the audio signal as the level of the 40 Other objects of the invention will become ap- signal lowers. With the conventional circuit of Fig. parent from the description of embodiments given 6, however, the bass control signal and the treble later. control signal are prepared by adjusting the cor- The automatic loudness control circuit of the responding knobs on the operation panel, so that present invention comprises a low frequency boost- every time the level of the audio signal varies, 45 ing circuit for extracting a low-frequency compo- there arises a need for tone adjustment to realize nent from the output signal of an audio signal optimum frequency characteristics, thus entailing source and boosting the low-frequency component the problem of a cumbersome adjustment proce- to prepare a low-frequency boost signal, an addi- dure. tion circuit for adding the low-frequency boost sig- The tone control circuit of Fig. 6 includes three 50 nal to the ouput signal from the audio signal filters, i.e., the low-pass filter 101, band-pass filter source, a power amplifier for feeding the output 102 and high-pass filter 103 which require capaci- signal of the addition circuit upon amplification, a tors of large capacity. This presents extreme dif- level detection circuit for detecting the output level ficulties in incorporating the three filters 101, 102 of the power amplifier, and a boosting control cir- cuit for controlling the magnitude of boosting by 2 3 0 287 057 4 the low frequency boosting circuit in accordance power amplifier 4 is detected by a level detection with the detection signal of the level detection circuit 6. A boosting control circuit 7 controls the circuit. The boosting control circuit prepares a con- magnitude of boosting by the low frequency boost- trol signal for increasing the magnitude of boosting ing circuit 2 in accordance with the detection signal by the low frequency boosting circuit as the output 5 from the level detection circuit 6. The boosting level lowers and feeds the control signal to the low control circuit 7 prepares a control' signal for in- frequency boosting circuit. creasing the magnitude of boosting by the low Accordingly, the low frequency boosting circuit frequency boosting circuit 2 as the output level of greatly boosts the low-frequency component of the the power amplifier 4 lowers to feed the control audio signal source when the output level lowers w signal to the low frequency boosting circuit 2. and feeds the resulting signal to the addition cir- Fig. 2 shows the basic automatic loudness cuit. Conversely, when the output level rises, the control circuit of the invention shown in Fig. 1, as boosting circuit boosts the low-frequency compo- adapted for use in a stereophonic audio signal nent of the audio signal by a small magnitude and reproduction system. The circuit of Fig. 2 com- feeds the resulting signal to the addition circuit. rs prises the circuit of Fig. 1 serving as a left signal The audio signal to be fed to the loudspeaker channel and a right signal channel arranged in from the power amplifier has frequency characteris- parallel to the left channel. The right signal channel tics wherein the low-frequency component is boost- comprises an audio signal source 1a, an addition ed to a greater magnitude as the output level circuit 3a, a power amplifier 4a and a loudspeaker lowers, causing the loudspeaker to produce a dy- 20 5a, and shares the low frequency boosting circuit namic sound. 2, the level detection circuit 6 and the boosting control circuit 7 with the left signal channel. The automatic loudness control circuit shown in BRIEF DESCRIPTION OF THE DRAWINGS Fig. 2 for stereophonic audio signal reproduction 25 will be described in greater detail with reference to Fig. 1 is a block diagram showing the basic the specific example shown in Fig. 3. In the circuit construction of an automatic loudness control cir- of Fig. 3, the left signal channel and the right signal cuit; channel operate the same, so that a description will Fig. 2 is a block diagram showing the basic be given of the left signal channel only, and the construction of an automatic loudness control cir- 30 right signal channel will not be described. Each cuit for use in a stereophonic reproduction system; circuit component of the right signal channel will be Fig. 3 is a circuit diagram showing a specific designated by the same reference number as the example of automatic loudness control circuit in corresponding component of the left channel, with detail; the adscript a attached to the reference number. Fig. 4 is a circuit diagram showing a level 35 The audio signal from the audio signal source control circuit and a level detection circuit specifi- 1 is fed to a first buffer amplifier 11 via a pream- cally; plifier 9 and a capacitor 10, these components thus Fig.
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