Patent Publication Number: US-2009220106-A1

Title: Audio signal processing circuit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an audio signal processing circuit which processes an audio signal. 
     2. Description of the Related Art 
     Electronic equipment such as a CD player, an audio amplifier, a car stereo, a mobile radio, a mobile audio player, and the like having a function which reproduces an audio signal ordinarily has a volume for adjusting a sound, an equalizer which adjusts frequency characteristics, a tone control function, and the like. The volume and the equalizer are controlled by changing amplitude of the audio signal. 
     For example, the tone control is performed by correcting the band of an audio signal to be processed and synthesizing the corrected signal with the original audio signal. 
     [Patent Document 1] Japanese Patent Application Laid-Open Publication No. 2005-117489 
     [Patent Document 2] Japanese Patent Application Laid-Open Publication No. 2005-217710 
     [Patent Document 3] Japanese Patent Application Laid-Open Publication No. 2004-222077 
     [Patent Document 4] Japanese Patent Application Laid-Open Publication No. 11-340759 
     [Patent Document 5] Japanese Patent Application Laid-Open Publication No. 2003-283262 
     When a cut-off frequency and a Q value are discretely switched in a stage in which a band is corrected at the time the tone control is performed, an audio signal is made discontinuous and the amplitude thereof is abruptly changed, from which a problem arises in that noise (which is called shock noise) is output from an electroacoustic conversion device such as a speaker, a headphone, and the like. 
     Further, when two audio signals are mixed, one of the audio signals is turned on and the other of them is turned off, an audio signal obtained by mixing the above audio signals is abruptly changed and shock noise is generated thereby. 
     SUMMARY OF THE INVENTION 
     A general purpose of the present invention, which was made in view of the problem according to the present invention, is to provide an audio signal processing circuit which suppresses noise. 
     An audio signal processing circuit of an embodiment of the present invention has a first amplifier which amplifies a first audio signal and a synthesizer circuit which synthesizes an output signal of the first amplifier with a second audio signal. The first amplifier is configured to seamlessly switch a first state in which the first amplifier amplifies a signal with a set gain and a second state in which the first amplifier outputs a fixed bias voltage. 
     According to the embodiment, noise of an output signal of the synthesizer circuit can be suppressed by shifting the first amplifier to the second state before the timing at which discontinuous points and an abrupt change occur to the first audio signal. In the specification, “amplification” is a concept including an attenuation in a case that a gain is 1 and further a case that the gain is smaller than 1 in addition to a case that the gain is larger than 1. 
     An audio signal processing circuit of an embodiment may further have a signal processing unit which is disposed upstream of the first amplifier, subjects a third audio signal to a predetermined signal process, and outputs the third audio signal as the first audio signal. When a state of the signal processing unit is switched, the first amplifier may be changed from the first state to the second state, the state of the signal processing unit may be switched in the second state, and subsequently the first amplifier may be switched from the second state to the first state. 
     The signal processing unit may be a filter. In this case, noise, which is caused when a cut-off frequency, a band, a Q value, and the like of the filter are discretely switched, can be suppressed. 
     The signal processing unit may be input with the same signal as the second audio signal as the third audio signal. 
     The first amplifier may be a variable gain amplifier. When a gain of the first amplifier is switched from a present value to a target value, the first amplifier may be changed from the first state to the second state, the gain of the first amplifier may be switched from the present value to the target value in the second state, and subsequently the first amplifier may be changed from the second state to the first state. 
     The audio signal processing circuit may be a mixer circuit which mixes the first audio signal with the second audio signal. In this case, noise, which is caused when the first audio signal is turned on and off, can be suppressed. 
     The first amplifier may include an amplifier which amplifies the first audio signal by a set gain, a bias voltage generating circuit which outputs the predetermined bias voltage, and a soft switching circuit which receives an output of the amplifier at a first input terminal, receives the bias voltage at a second input terminal, and gently shifts an output of the soft switching circuit from one of the input terminals to the other input terminal. In this case, a first state in which the first input terminal is selected can be smoothly switched to a second state in which the second input terminal is selected. 
     It is to be noted that any arbitrary combination or rearrangement of the above-described structural components and so forth is effective as and encompassed by the present embodiments. Moreover, this summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features. 
     A noise can be suppressed according to the audio signal processing circuit of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which: 
         FIG. 1  is a block diagram showing an arrangement of an audio signal processing circuit according to a first embodiment; 
         FIG. 2  is a block diagram showing an example of an arrangement of a first amplifier; 
         FIGS. 3A and 3B  are time charts showing operating conditions of the audio signal processing circuit of  FIG. 1  and an audio signal processing circuit of a comparative example; 
         FIG. 4  is a block diagram showing an arrangement of an audio signal processing circuit according to a modification of the first embodiment; and 
         FIG. 5  is a block diagram showing an arrangement of an audio signal processing circuit according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described based on preferred embodiments which do not intend to limit the scope of the present invention but exemplify the invention. All of the features and the combinations thereof described in the embodiment are not necessarily essential to the invention. 
     In the specification, “a state in which a member A is connected to a member B” also includes a case in which the member A is directly and physically connected to the member B and a case in which the member A is indirectly connected to the member B through other member which does not affect an electrically connected state. Likewise, “a state in which a member C is interposed between the member A and the member B” also includes a case in which the member A is indirectly connected to the member C or the member B is connected to the member C through other member which does not affect an electrically connected state, respectively in addition to a case in which the member A is directly connected to the member C or the member B is directly connected to the member C. 
     First Embodiment 
       FIG. 1  is a block diagram showing an arrangement of an audio signal processing circuit  10   a  according to a first embodiment. 
     The audio signal processing circuit  100   a  subjects an input audio signal Sin to a predetermined signal process and outputs an output audio signal Sout. Specifically, the input audio signal Sin is branched to two subsignals, and one of the subsignals is subjected to the signal process and output after it is synthesized with the other subsignal. A tone control is exemplified as an example of the signal process. 
     The audio signal processing circuit  100   a  has a synthesizer circuit  10 , a first amplifier  20 , a controller  22 , and a signal processing unit  30 . 
     The first amplifier  20  amplifies a first audio signal S 1  input thereto. The synthesizer circuit  10  has two inputs, and an output signal S 1 ′ from the first amplifier  20  is input to one of the inputs and a second audio signal S 2  is input to the other input. The second audio signal S 2  is the input audio signal Sin of the audio signal processing circuit  100   a.  The synthesizer circuit  10  synthesizes the first audio signal S 1 ′ with the second audio signal S 2 . Synthesization means an addition process, a subtraction process, an average process, a partial process, and the like. 
     In the first embodiment, the synthesizer circuit  10  is an addition circuit which uses an operational amplifier  12 . The synthesizer circuit  10  has input terminals P 1  and P 2 , synthesizes the signals S 2  and S 1 ′ input thereto, respectively, and outputs a synthesized signal from an output terminal P 3 . The synthesizer circuit  10  has the operational amplifier  12  and first to fourth resistors R 1  to R 4 . The third resistor R 3  is interposed between an inverted input terminal and the output terminal of the operational amplifier  12 , and the fourth resistor R 4  is interposed between the inverted input terminal and a fixed voltage terminal (ground terminal). The first resistor RI is interposed between a non-inverted input terminal and the input terminal P 1  of the operational amplifier  12 , and the second resistor R 2  is interposed between the non-inverted input terminal and the input terminal P 2  of the operational amplifier  12 . 
     The synthesized signal shown by the following expression is output from the output terminal P 3  of the synthesizer circuit  10 . 
         S out=( R 2· S 1′+ S 2· R 1)/( R 1+ R 2)×( R 3+ R 4)/ R 4 
     Note that the synthesizer circuit  10  may be simply composed of a partial circuit and other addition circuit using a resistor. 
     The first amplifier  20  is arranged such that a first state φ 1  which has a set gain and a second state φ 2  which outputs a fixed bias voltage Vbias can be seamlessly switched. The states φ 1  and φ 2  of the first amplifier  20  are controlled by the controller  22 . 
     The audio signal processing circuit  100   a  is basically arranged as described above. Subsequently, a basic operation of the audio signal processing circuit  100   a  will be described. 
     It is assumed now that the first amplifier  20  is set to the first state φ 1 , and the first audio signal S 1  is in a stable state. Before the timing at which discontinuous points and an abrupt change occur to the first audio signal S 1 , the controller  22  gently and seamlessly changes the first amplifier  20  from the first state φ 1  to the second state φ 2 . After the first audio signal S 1  is stabilized, the first amplifier  20  is changed from the second state φ 2  to the first state φ 1  again. 
     According to the basic arrangement of the audio signal processing circuit  100   a,  since an abrupt change of the first audio signal S 1  does not affect the output signal Sout, noise can be suppressed. Further, since the second audio signal S 2  is continuously output as the output audio signal Sout, uncomfortable feeling and unnaturalness can be reduced in audibility. 
     The arrangement of the audio signal processing circuit  10   a  will be further described. The signal processing unit  30  is disposed upstream of the first amplifier  20  to subject a third audio signal S 3  to a predetermined signal process and outputs the third audio signal S 3  as the first audio signal S 1 . 
     When a state of the signal processing unit  30  is switched, the controller  22  changes the first amplifier  20  from the first state φ 1  to the second state φ 2  before the state of the signal processing unit  30  is switched. Then, the state of the signal processing unit  30  is switched in the second state φ 2 . Subsequently, the signal processing unit  30  is switched from the second state φ 2  to the first state φ 1 . 
     When the audio signal processing circuit  10   a  performs a tone control process, the signal processing unit  30  is arranged as a filter which can discretely switch frequency characteristics. A band-pass filter (BPF), a low-pass filter (LPF), a high-pass filter (HPF), and a band elimination filter (BEF) are exemplified as the filter. The frequency characteristics means a center frequency (or a cut-off frequency) fo, the Q value, and the like. It is needless to say that the process performed by the signal processing unit  30  is not limited to the above process, and the signal processing unit  30  can be applied to various signal processes. 
     In the audio signal processing circuit  100   a  having the tone control function, the signal processing unit  30  is input with the same signal as the second audio signal S 2 , i.e., the input signal Sin as the third audio signal S 3 . The signal processing unit  30  amplifies a specific frequency of the input signal Sin or eliminates an unnecessary band from the input signal Sin and outputs it as the first audio signal S 1 . 
       FIG. 2  is a block diagram showing an example of an arrangement of the first amplifier. The first amplifier  20  includes an amplifier  24 , a bias voltage generating circuit  26 , and a soft switching circuit  28 . 
     The amplifier  24  amplifies the first audio signal S 1  by a set gain. The bias voltage generating circuit  26  outputs a predetermined bias voltage Vbias. Both the amplifier  24  and the bias voltage generating circuit  26  may be arranged as inverted amplifiers or as non-inverted amplifiers each making use of an operational amplifier. When the tone control is performed, a tone gain can be adjusted by arranging the amplifier  24  as a variable gain amplifier. 
     The soft switching circuit  28  receives an output signal of the amplifier  24  at a first input terminal IN 1  and receives the bias voltage Vbias at a second input terminal IN 2 . The soft switching circuit  28  gently shifts an output OUT thereof from one of the input terminals to the other input terminal in response to a control signal from the controller  22 . Various known switching circuits can be used as the soft switching circuit  28 . 
     The audio signal processing circuit  100   a  is arranged as described above in its entirety. Subsequently, an operation of the audio signal processing circuit  100   a  will be described. 
       FIGS. 3A and 3B  are time charts showing operating conditions of the audio signal processing circuit of  FIG. 1  and an audio signal processing circuit of a comparative example. 
     A process of the comparative example will be described first referring to  FIG. 3B  to clarify an advantage of the present invention. In the process of the comparative example, the frequency characteristics of a signal processing unit  30  are set to a certain state before a time t 0 . When it is indicated to change the frequency characteristics at the time t 0 , an output of an audio signal processing circuit is gently switched to a mute state. After the output of the audio signal processing circuit is made to the mute state, the frequency characteristics of the signal processing unit  30  or the tone gain thereof is switched to different state at a time t 1 . Thereafter, the mute state is released at a time t 2 , and the audio signal Sout is output again. Since the audio signal is muted each time the state of the tone control is switched, a user feels uncomfortably. 
     In the comparative example, a mute circuit is additionally required and a micro processor (host processor), which controls the audio signal processing circuit, must output a control signal to the audio signal processing circuit as well as must output the control signal also to the mute circuit, from which a problem arises in that a load on the micro processor is increased. 
     Subsequently, an operation of the audio signal processing circuit  100   a  according to the first embodiment will be described referring to  FIG. 3A . The frequency characteristics and the tone gain of the signal processing unit  30  are set to certain states before a time t 0 , and the first amplifier  20  is set to the first state φ 1  before the time t 0 . When it is indicated to change the frequency characteristics or the tone gain at the time t 0 , the first amplifier  20  is gently switched from the first state φ 1  to the second state φ 2 . Thereafter, when the first amplifier  20  is switched to the second state φ 2  at the time t 1 , the frequency characteristics or the tone gain of the signal processing unit  30  is instantly switched to a different state. Thereafter, the first amplifier  20  is shifted from the second state φ 2  to the first state φ 1  again. 
     As described above, in the audio signal processing circuit  100   a  according to the first embodiment, even if the state of the signal processing unit  30  or the gain (tone gain) of the amplifier  24  in the first amplifier  20  is instantly switched, the output audio signal Sout is not muted, and the input audio signal Sin is continuously output. Accordingly, since the user is unlike to be aware of the control for switching the tone, the value of the audio signal processing circuit employed in an audio device can be improved. 
       FIG. 4  is a block diagram showing an arrangement of an audio signal processing circuit  100   b  according to a modification of the first embodiment. 
     The audio signal processing circuit  100   b  has an inverted amplifier  40 , a first switch SW 1 , and a second switch SW 2  in addition to the arrangement of the audio signal processing circuit  100   a  of  FIG. 2 . A signal processing unit  30  is a filter which can switch frequency characteristics. The inverted amplifier  40  inverts the phase of an output audio signal Sout. The first switch SW 1  is interposed between an input terminal  102  of the input audio signal Sin and an input terminal of the signal processing unit  30 . The second switch SW 2  is interposed between an output terminal of the inverted amplifier  40  and the input terminal of the signal processing unit  30 . 
     The controller  22  switches turned-on/turned-off states of the first switch SW 1  and the second switch SW 2  in addition to that it switches the frequency characteristics of the signal processing unit  30  and switches between the first state φ 1  and the second state φ 2  of the first amplifier  20 . 
     When the first switch SW 1  is turned on, since it is equivalent with the audio signal processing circuit  100   a  of  FIG. 1 , the frequency band set by the signal processing unit  30  is boosted. On the contrary, when the second switch SW 2  is turned off, the frequency band set by the signal processing unit  30  is cut. 
     The controller  22  sets the first amplifier  20  to the second state φ 2  before the timing at which the input signal of the first amplifier  20  is made discontinuous or before the timing at which the gain of the amplifier  24  in the first amplifier  20  is switched and returns the first amplifier  20  to the first state φ 1  after the input signal is stabilized. The input signal of the first amplifier  20  is made discontinuous when the frequency characteristics of the signal processing unit  30  are switched or when the first switch SW 1  and the second switch SW 2  are switched from a turned-on state or a turned-off state. 
     According to the modification of  FIG. 4 , occurrence of shock noise can be preferably prevented at the timing at which a specific frequency band is switched from a boosted state to a cut state and vice versa in addition to the advantage of the audio signal processing circuit  100   a  of  FIG. 1 . 
     Second Embodiment 
     The first embodiment has explained about the signal processing circuit for performing the tone control. The audio signal processing circuit  100   c  according to a second embodiment relates to a mixer circuit for mixing a first input audio signal Sin 1  with a second input audio signal Sin 2 . 
       FIG. 5  is a block diagram showing an arrangement of the audio signal processing circuit  100   c  according to the second embodiment. 
     The audio signal processing circuit  100   c  has a synthesizer circuit  10 , a first amplifier  20 , and a controller  22 . 
     The first amplifier  20  amplifies the first input audio signal Sin 1  by a predetermined gain and outputs it to the synthesizer circuit  10 . The first amplifier  20  can switch between the first state φ 1  and the second state φ 2  likewise the first embodiment. The controller  22  switches between the first state φ 1  and the second state φ 2  of the first amplifier  20 . 
     To change a mixing ratio, the first amplifier  20  is arranged as a variable gain amplifier so that it can discretely switches a gain. The controller  22  also switches the gain of the first amplifier  20 . In this case, an amplifier  24  in the first amplifier  20  is preferably arranged as a variable gain amplifier. 
     The synthesizer circuit  10  synthesizes the second input audio signal Sin 2  with an amplified first input audio signal Sin 1 ′. 
     An operation of the audio signal processing circuit  100   c  arranged as described above will be described. 
     When the gain of the first amplifier  20  is switched from a present value to a target value, first, the first amplifier  20  is switched from the first state φ 1  to the second state φ 2 . Subsequently, in the second state φ 2 , the gain of the first amplifier  20  is switched from the present value to the target value. Then, the synthesizer circuit is switched from the second state φ 2  to the first state φ 1 . 
     According to the audio signal processing circuit  100   c  of  FIG. 5 , occurrence of a shock sound (noise) can be suppressed when the mixing ratio is switched. Further, since the second input audio signal Sin 2  is continuously output, i.e., since audio is not disconnected, mixing can be switched seamlessly while reducing unnaturalness in audibility. 
     As a modification of the audio signal processing circuit  100   c  of  FIG. 5 , a second amplifier similar to the first amplifier  20  may be also disposed to the second input audio signal Sin 2  side. In this case, the mixing ratio can be changed by changing the amplitudes of both the first input audio signal Sin 1  and the second input audio signal Sin 2 . 
     While the preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the appended claims.