Abstract:
To avoid adverse effect on sound quality when level of audio signal is raised, in a speaker driving circuit comprising ultralow range emphasizing means for emphasizing an ultralow range and a high-pass filter, switching means for removing the high-pass filter from an audio signal path when an audio signal to be supplied to a speaker is below a medium level, and inserting the high-pass filter in the audio signal path when an audio signal to be supplied to the speaker is above the medium level is provided.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a speaker driving circuit suitably used for driving a relatively small speaker such as a speaker incorporated in a television receiver.  
           [0003]    2. Description of the Related Art  
           [0004]    In a television receiver, for example, a relatively small speaker is used because the space is limited so that bass of an ultralow region cannot be reproduced well generally. In addition, human auditory characteristics vary according to levels of sound, and have a tendency as follows. As a sound level becomes low, sensitivity for an ultralow range is also lowered. Accordingly, when a sound level is low, there is a problem that it is difficult to hear ultralow bass.  
           [0005]    In order to solve the above problem that it is hard to hear ultralow bass with its sound level being low, there has hitherto been known a method of using a so-called loudness control circuit, which makes frequency characteristics of a speaker driving circuit variable corresponding to a reproducing level of a sound level. By the loudness control circuit, frequency characteristics of a speaker driving circuit is changed to be relatively flat when a reproducing level is high, and a low range of frequency characteristics of the speaker driving circuit is made to boost when a reproducing level is low, When the reproducing level is low, a high range is also emphasized by the loudness control circuit. FIG. 5 is a circuit diagram showing an example of a loudness control circuit.  
           [0006]    In explaining according to FIG. 5, reference numeral  1  in FIG. 5 denotes an audio signal input terminal for receiving an audio signal, this audio signal input terminal  1  is connected to a one end of a coupling capacitor  2 , while the other end of this coupling capacitor  2  is grounded by way of a series circuit of capacitors  3 ,  4  and a resistor  5 , and a connection mid point of the coupling capacitor  2  and capacitor  3  is grounded through a variable resistor  6 , and a grounding mid point of the capacitors  3  and  4  is connected to an intermediate point of a resistor element  6   a  of this variable resistor  6 .  
           [0007]    An audio signal obtained at a movable element  6   b  of this variable resistor  6  is supplied into a power amplifier  8  through a coupling capacitor  7 , and an audio signal obtained at the output side of this power amplifier  8  is supplied into a voice coil of a speaker  9 , and the diaphragm of this speaker  9  is oscillated to emit a sound.  
           [0008]    As shown in FIG. 6, frequency characteristics of an audio signal supplied from the power amplifier  8  to the speaker  9  is flat from the ultralow range to the high range when the reproducing level is high. When the reproducing level is low, the ultralow range and the high range is boosted. Therefore, irrespective of a high or low level of reproduction, ultralow range sound can be heard. Furthermore, the frequency characteristics extend to the ultralow range so that bass of high quality can be obtained without phase loss of the signal.  
           [0009]    However, with a relatively small speaker such as a speaker incorporated in a television receiver, when the level of audio signal is raised and the speaker  9  is driven by the ultralow range signal of high level, the diaphragm of the speaker  9  itself does oscillates, but cannot drive the air sufficiently. So, adverse effects are caused on the sound quality.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention is devised in the light of the above problems, and it is an object thereof to avoid adverse effects on the sound quality even if the level of the audio signal is raised.  
           [0011]    The present invention presents a speaker driving circuit comprising a volume adjusting means for adjusting an output level of an audio signal supplied to a speaker and an ultralow bass emphasizing circuit frequency characteristics of which is variable corresponding to an adjusted state of the volume adjusting means, which further comprises level detecting means for detecting an adjusted state of the volume adjusting means and ultralow bass component reducing means for, when it is detected by the level detecting means that the volume adjusting means is adjusted to make an output level of the audio signal become higher than a specified level, reducing an ultralow bass component of the audio signal.  
           [0012]    According to the present invention, as far as the audio signal is below the medium level, the speaker is driven by a signal extended in the frequency characteristic to the ultralow range, the bass of high quality is obtained without phase loss, but when the audio signal is above the medium level, since the ultralow range is cut off by the high-pass filter, clip or the like does not occur, and the diaphragm of the speaker does not oscillate in the ultraslow range, so that there is no adverse effect on the sound quality. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a block diagram showing an example of an embodiment of a speaker driving circuit of the invention;  
         [0014]    [0014]FIG. 2 is a diagram for explaining FIG. 1;  
         [0015]    [0015]FIG. 3 is a block diagram showing another example of the embodiment of the speaker driving circuit of the invention;  
         [0016]    [0016]FIG. 4 is a diagram for explaining FIG. 3;  
         [0017]    [0017]FIG. 5 is a block diagram showing a prior art of a speaker driving circuit; and  
         [0018]    [0018]FIG. 6 is a diagram for explaining FIG. 5. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    An embodiment of a speaker driving circuit of the invention is explained by referring to FIG. 1 and FIG. 2. In FIG. 1, the parts corresponding to FIG. 5 are identified with the same reference numerals.  
         [0020]    In FIG. 1, reference numeral  1  denotes an audio signal input terminal for receiving an audio signal, this audio signal input terminal  1  is connected to a one end of a coupling capacitor  2 , while the other end of this coupling capacitor  2  is grounded by way of a series circuit of capacitors  3 ,  4  and a resistor  5 , and a connection mid point of the coupling capacitor  2  and capacitor  3  is grounded through a variable resistor  6 , and a grounding mid point of the capacitors  3  and  4  is connected to an intermediate point of a resistor element  6   a  of this variable resistor  6 . The capacitor  3 , capacitor  4 , resistor  5 , and variable resistor  6  comprise the loudness control circuit.  
         [0021]    In this example, a movable element  6   b  of this variable resistor  6  is connected to a movable contact point  10   c  of a changeover switch  10  through the coupling capacitor  7 , and one fixed contact point  10   a  of this changeover switch  10  is connected to one fixed contact point  10   a  of a changeover switch  12 , and other fixed contact point  10   b  of this changeover switch  10  is connected to another fixed contact point  12   b  of the changeover switch  12  through a high-pass filter  11  for cutting off the ultralow range.  
         [0022]    An audio signal obtained at a movable contact point  12   c  of this changeover switch  12  is supplied to a power amplifier  8  through a coupling capacitor  13 , and an audio signal obtained at the output side of this power amplifier  8  is supplied to a voice coil of a speaker  9 , thereby oscillating the diaphragm of the speaker  9  to emit a sound.  
         [0023]    In this embodiment, a level detecting variable resistor  14  is provided. That is, a power source terminal  15  for obtaining a positive DC voltage of +V is grounded through this level detecting variable resistor  14 , and the movable element  14   a  of this level detecting variable resistor  14  is interlocked with the movable element  6   b  of the variable resistor  6  for audio level adjustment.  
         [0024]    In this case, a detection voltage level Vs depending on the position of the movable element  6   b  of the variable resistor  6  for audio level adjustment, i.e., an audio volume adjusted state, is obtained at the movable terminal  14   a  of the level detecting variable resistor  14 .  
         [0025]    This detection voltage level Vs obtained at the movable element  14   a  of the level detecting variable resistor  14  is applied to a non-inverting input terminal+of an operational amplifier  16  for composing a comparator.  
         [0026]    The power source terminal  15  is grounded through a series circuit of resistors  17  and  18  for division, so that the voltage obtained at the connection mid point of the resistors  17  and  18  may be voltage Vc equal to the detection voltage level Vs obtained at the movable element  14   a  of the level detecting variable resistor  14  when the level of the audio signal obtained at the movable element  6   b  of the variable resistor  6  may be a medium level, for example, the level applied to the speaker  9  is −30 dB.  
         [0027]    The voltage Vc obtained at the connection mid point of the resistors  17  and  18  is applied to an inverting input terminal−of the operation amplifier  16 . Therefore, on the output side of the operational amplifier  16 , when the detection voltage level Vs obtained at the movable element  14   a  of the level detecting variable resistor  14  ranges from 0 V to voltage Vc, that is, the level of audio signal supplied to the voice coil of the speaker  9  is below the medium level, for example, below −30 dB, it is low level “0”, and when the detection voltage level Vs obtained at the movable element  14   a  is above voltage Vc, that is, when the level of audio signal supplied to the voice coil of the speaker  9  is above the medium level, for example, above −30 dB, it is high level “1”.  
         [0028]    In the embodiment, by the output of the operational amplifier  16 , the movable contact points  10   c  and  12   c  of the changeover switches  10  and  12  are controlled. That is, when the output side of the operational amplifier  16  is low level “0”, the movable contact points  10   c  and  12   c  of the changeover switches  10  and  12  are connected to the fixed contact points  10   a  and  12   a  of the changeover switches  10  and  12 , and the high-pass filter  11  is inserted into the audio signal path.  
         [0029]    On the other hand, when the output side of the operational amplifier  16  is high level “1”, the movable contact-points  10   c  and  12   c  of the changeover switches  10  and  12  are connected to the other fixed contact points  10   b  and  12   b , and the high-pass filter  11  is inserted the audio signal path.  
         [0030]    Therefore, the frequency characteristic of the audio signal supplied to the voice coil of the speaker  9  is as shown in FIG. 2, and when the audio signal level is below the medium level, for example, below −30 dB (indicated by a broken line), the characteristic is emphasized in the ultralow range, and at this time since the voice coil of the speaker  9  is driven by an audio signal extended in the frequency characteristic to the ultralow range, the bass of high quality is obtained without phase loss.  
         [0031]    When the level of the audio signal supplied in the voice coil of the speaker  9  is above the medium level, for example, above −30 dB, the frequency characteristic of the audio signal is as shown in FIG. 2, that is, the ultralow range is cut off by the high-pass filter  11 , and hence clip or the like does not occur, and moreover since the diaphragm of the speaker  9  does not oscillate in the ultralow range, there is no adverse effect on the sound quality.  
         [0032]    As explained above, according to the embodiment, when the audio signal is below the medium level, for example, below −30 dB (containing an ordinary audio level in television receiver), the ultralow range is emphasized, and the voice coil of the speaker  9  is driven by an audio signal extended in the frequency characteristic to ultralow range, and therefore the bass of high quality is obtained without phase loss, and when the audio signal is above the medium level, for example, above −30 dB, the ultralow range is cut off by the high-pass filter  11 , and hence clip or the like does not occur, and moreover since the diaphragm of the speaker  9  is not oscillated in the ultralow range, there is no adverse effect on the sound quality.  
         [0033]    [0033]FIG. 3 shows another example of the embodiment of the speaker driving circuit of the invention. In FIG. 3, the same parts corresponding to those in FIG. 1 are identified with same reference numerals, and detailed description is omitted. In this example in FIG. 3, unlike the example in FIG. 1, the ultralow range is not emphasized when the level of an audio signal is extremely low.  
         [0034]    In FIG. 3, an audio signal input terminal  1  is connected to a one end of a coupling capacitor  2 , the other end of this coupling capacitor  2  is connected to a fixed contact point  20   a  of a changeover switch  20  through a capacitor  3 , a movable contact point  20   c  of this changeover switch  20  is grounded by way of a series circuit of a capacitor  4  and a resistor  5 , a connection mid point of the capacitor  4  and resistor  5  is connected to the other fixed contact point  20   b  of the changeover switch  20 , the connection mid point of the coupling capacitor  2  and capacitor  3  is grounded through a variable resistor  6 , and the movable contact point  20   c  of this changeover switch  20  is connected to an intermediate point of a resistor element  6   a  of the variable resistor  6 . The movable element  6   a  of the variable resistor  6  is connected to the movable contact point  10   c  of the changeover switch  10 .  
         [0035]    In this case, when the movable contact point  20   c  of the changeover switch  20  is connected to one fixed contact point  20   a , the same as shown in FIG. 1, a loudness control circuit for emphasizing the ultralow range is inserted in the audio signal path, and the ultralow range is emphasized, and when the movable contact point  20   c  of this changeover switch  20  is connected to another fixed contact point  20   b , the audio signal supplied to the audio signal input terminal  1  is supplied directly to the variable resistor  6 , so that the ultralow range is not emphasized.  
         [0036]    Also in the example shown in FIG. 3, a power source terminal for obtaining a positive DC voltage of +V is grounded through a level detecting variable resistor  14 , and the movable element  14   a  of this level detecting variable resistor  14  is interlocked with the movable element  6   b  of the variable resistor  6  for audio level adjustment. In this case, at the movable element  14   a  of the level detecting variable resistor  14 , a detection voltage level Vs depending on the level of the audio signal obtained at the movable element  6   b  of the variable resistor  6  for audio level adjustment is obtained.  
         [0037]    In this example, the detection voltage level Vs depending on the audio signal level obtained at the movable element  14   a  of the level detecting variable resistor  14  is applied to a non-inverting input terminal+of an operational amplifier  21  and an inverting input terminal−of an operational amplifier  22  for composing a comparator.  
         [0038]    Also in the example, by applying a voltage VH at a fixed voltage input terminal  21   a , of which the level of an audio signal obtained by the movable element  6   b  of the variable resistor  6  is equal to a medium level, for example, the level supplied to the speaker  9  is equal to the detection voltage level Vs obtained at the movable element  14   a  of the level detecting variable resistor  14  corresponding to −30 dB, the voltage VH obtained at this fixed voltage input terminal  21   a  is applied to the inverting input terminal−of the operational amplifier  21 .  
         [0039]    Further in the example, by applying a voltage VL at a fixed voltage input terminal  22   a , of which the level of an audio signal obtained at the movable element  6   b  of the variable resistor  6  is glow, for example, the level supplied to the speaker  9  is equal to the detection voltage level Vs obtained at the movable element  14   a  of the level detecting variable resistor  14  corresponding to −55 dB, the voltage VL obtained at this fixed voltage input terminal  22   a  is supplied to the non-inverting input terminal+of the operational amplifier  22 .  
         [0040]    Therefore, at outputs X 1  and X 2  of the operational amplifiers  21  and  22 , when the detection voltage level Vs obtained at the movable element  14   a  of the level detecting variable resistor  14  shown in Table 1 is 0 V to VL, output X 1  is low level “0” and output X 2  is high level “1”, and when the detection voltage level Vs is VL to VH, output X 1  is low level “0” and output X 2  is low level “0”, and further when the detection voltage level Vs is VH to +V, output X 1  is high level “1” and output X 2  is low level “0”.  
                           TABLE 1                                   Vs   0-VL-VH- + V                           X 1     “0” “0” “1”           X 2     “1” “0” “0”                      
 
         [0041]    In this example, the outputs X 1  and X 2  of the operational amplifiers  21  and  22  are supplied to a switch control circuit  23  through resistors. This switch control circuit  23  controls to change over the movable contact points  20   c ,  10   c , and  12   c  of the changeover switches  20 ,  10  and  12  depending on the outputs X 1  and X 2  of the operational amplifiers  21  and  22 .  
         [0042]    That is, the movable contact point  20   c  of this changeover switch  20  is connected to the other fixed contact point  20   b  until the detection voltage level Vs obtained at the movable element  14   a  of the level detecting variable resistor  14  becomes the voltage VL, and when this detection voltage level Vs exceeds the voltage VL, the movable element  20   c  is connected to one fixed contact point  20   a , and at this time the loudness control circuit is inserted in the audio signal path, and the ultralow range is emphasized.  
         [0043]    The movable contact points  10   c  and  12   c  of the changeover switches  10  and  12  are connected to the fixed contact points  10   a  and  12   a  respectively until the detection voltage level Vs obtained at the movable element  14   a  of the level detecting variable resistor  14  reaches the voltage VH, and at this time the high-pass filter  11  is not inserted in the audio signal path.  
         [0044]    When the detection voltage level Vs exceeds the voltage VH, the movable contact points  10   c  and  12   c  are connected to the other fixed contact points  10   b  and  12   b , and the high-pass filter  11  is inserted in the audio signal path at this time, and the ultralow range is cut off. The example in FIG. 3 is the same as the configuration in FIG. 1 in all other respects.  
         [0045]    The example in FIG. 3 is thus composed, and the frequency characteristic of the audio signal supplied to the voice coil of the speaker  9  is as shown in FIG. 4, and it is a flat characteristic when the audio signal level is low, for example, below −55 dB, and hence noise is not emphasized. When this audio signal level is higher than this low level and lower than the medium level, for example, around −55 dB to −30 dB, the ultralow range is emphasized in this frequency characteristic, and the voice coil of the speaker  9  is driven by the audio signal extended in the frequency characteristic to the ultralow range, so that the bass of high quality is obtained without phase loss.  
         [0046]    When the level of the audio signal supplied to the voice coil of the speaker  9  is higher than the medium level, for example, more than −30 dB, the frequency characteristic of the audio signal is as shown in FIG. 4, and the ultralow range is cut off by the high-pass filter  11 , and clip or the like does not occur, and the diaphragm of the speaker  9  is not oscillated in the ultralow range, so that no adverse effect is caused on the sound quality.  
         [0047]    Therefore, it is easily understood that the same action and effect as in the example in FIG. 1 are obtained also in the example in FIG. 3.  
         [0048]    The foregoing examples are analog compositions, a digital composition may be also formed by using DSP (digital signal processor) and others.  
         [0049]    The invention is not limited to the illustrated examples alone, but may be changed and modified within the scope of the invention.  
         [0050]    As explained above, according to the present invention, when the audio signal is below the medium level, for example, below −30 dB (a containing ordinary audio level in a television receiver), the ultralow range is emphasized, and the voice coil of the speaker is driven by an audio signal extended in the frequency characteristic to the ultralow range, and therefore the bass of high quality is obtained without phase loss, and when the audio signal is above the medium level, for example, above −30 dB, the ultralow range is cut off by the high-pass filter, and hence clip or the like does not occur, and moreover since the diaphragm of the speaker is not oscillated in the ultralow range, there is no adverse effect on the sound quality.  
         [0051]    Having described preferred embodiments of the present invention with reference to the accompanying drawings, it is to be understood that the present invention is not limited to the above- mentioned embodiments and that various changes and modifications can be effected therein by one skilled in the art without departing from the spirit or scope of the present invention as defined in the appended claims.