Patent Publication Number: US-2009227297-A1

Title: Mobile telephone device

Description:
TECHNICAL FIELD 
     The present invention relates to a cellular telephone apparatus. In particular, the invention relates to a cellular telephone apparatus in which a single sound-producing body serves as both of a receiver for reproducing a speech sound and a speaker for reproducing a ringer tone. 
     BACKGROUND ART 
     Conventionally, in cellular telephone apparatuses, it is a common practice to reproduce a ringer tone with a speaker at the time of call arrival and to reproduce a speech sound with a receiver during a call. 
       FIG. 7  is a block diagram showing the circuit configuration of a conventional cellular telephone apparatus. In this cellular telephone apparatus  700 , reception signals received by an antenna  701  are supplied via a duplexer  702  to a receiving circuit  703 , where a desired reception channel signal is selected. The selected signal is sent to a demodulator  704 . 
     The demodulator  704  performs digital demodulation processing, error correction processing, etc. If the reception signal is a ringing signal, a microprocessor  710  activates a speaker amplifier power source  740 , sets a prescribed ringer tone in a ringer tone source  723 , and causes it to generate a ringer tone signal. The ringer tone signal is supplied via a digital lowpass filter to a D/A converter  725 , where it is converted into an analog signal. Its volume is then set to a prescribed value by a volume setting device  726 . A speaker amplifier  727  current-amplifies the volume-set ringer tone signal and drives a speaker  729  via a high-pass filter  728 . 
     On the other hand, if reception data demodulated by the demodulator is a speech signal, the microprocessor  710  activates a receiver amplifier power source  741 . The demodulator  704  sends the reception signal to a decoder  721 , where it is subjected to prescribed decoding processing and thereby converted into speech data. The speech data is supplied via a digital lowpass filter  730  to a D/A converter  731 , where it is converted into an analog signal. Its volume is then set to a prescribed value by a volume setting device  732 . A receiver amplifier  733  current-amplifies the volume-set speech signal and drives a receiver  735  via a high-pass filter  734 . 
     A microphone  750  converts a voice into a transmission speech signal, which is then amplified to a prescribed signal level by a microphone amplifier  751  and converted into a digital signal by an A/D converter  752 . The transmission speech signal as converted into the digital signal by the A/D converter  752  is encoded by an encoder  722 , subjected to digital modulation in a modulator  705 , processed so as to be carried by a signal having a prescribed channel frequency by a transmitting circuit  706 , and transmitted from the antenna  701  via the duplexer  702 . 
     In general, each of the speaker  729  and the receiver  735  is a dynamic speaker which is composed of a fixed magnet and a voice coil in which a coil is attached to a vibration plate or a ceramic speaker using a piezoelectric device. 
     The functions of the ringer tone source  723  as incorporated in recent cellular telephone apparatuses are equivalent to those of a sound source incorporated in karaoke equipment, such as 40 chords. And the ringer tone reproduction frequency band of the ringer tone source  723  is as wide as 100 Hz to tens of kilohertz. On the other hand, the speech sound reproduction band is standardized so as to fall approximately within a range of hundreds of hertz to several kilohertz (depends on the communication scheme) and hence is narrower than the ringer tone reproduction band. 
     Speakers for ringer tone reproduction need to produce a large-volume tone. Therefore, for example, a dynamic speaker having equivalent impedance of 8Ω is employed. Where a ceramic speaker is used, a necessary volume is secured by applying a voltage of about 9 V to it. 
     On the other hand, speakers for speech sound reproduction are not required to produce a sound of as large a volume as speakers for ringer tone reproduction are required. Therefore, a dynamic speaker having equivalent impedance of 32Ω is employed. Where a ceramic speaker is used for speech sound reproduction, a voltage of about 9 V need not be applied to it unlike in the case where it is used for ringer tone reproduction. 
     The speaker amplifier  727  for driving the speaker  729  and the receiver amplifier  733  for driving the receiver  735  are amplifiers having different characteristics because of the above-described differences in reproduction bandwidth and the speaker (load) to drive. The speaker amplifier  727  is an amplifier which is large in circuit current because it is required to have a wider bandwidth and a higher drive-current-providing ability than the receiver amplifier  733 , for the following reason. In general, to reduce the cross-over distortion, class AB amplifiers are used as speaker amplifiers. Although amplifiers for driving a receiver having equivalent impedance of 32Ω and amplifiers for driving a speaker having equivalent impedance of 8Ω both perform class AB operation, the bias voltage of the output-stage transistor of the former is set small and that of the latter is set large to accord with the difference in load current. The circuit current consumed in the amplifier itself is optimized in this manner. 
     Furthermore, the frequency characteristics of dynamic speakers and ceramic speakers (loads) have great influence on the drive circuit. Dynamic speakers are inductive loads and ceramic speakers are capacitive loads. Therefore, ceramic speakers decrease in impedance and become heavier loads as the drive frequency increases. Therefore, to use a ceramic speaker for ringer tone reproduction, it is necessary to assume a load operating at 20 kHz and to set a bias voltage for class AB amplification, which results in a tendency that the circuit current consumed by the amplifier itself increases. 
     In the circuit configuration of the conventional cellular telephone apparatus, the receiver amplifier power source  741  is turned off during reproduction of a ringer tone and the speaker amplifier power source  740  is turned off during reproduction of a speech sound, which enables operation with an optimum current consumption in each of the reproduction of a ringer tone and the reproduction of a speech sound. 
     Cellular telephone apparatuses are known in which for their miniaturization a common speaker and amplifier are used for both of the reproduction of a ringer tone and the reproduction of a speech sound (refer to Patent document 1, for example). 
     In cellular telephone apparatuses of this type, when the volume of a ringer tone indicated by an output signal of a ringer tone output circuit exceeds a reference volume continuously, a ringer tone volume control circuit controls the increase rate of the volume exceeding the reference volume. This makes it possible to use a single speaker as both of a speaker for reproduction of a ringer tone and a speaker for reproduction of a speech sound and to thereby realize miniaturization of a cellular telephone apparatus. 
     Patent document 1: JP-A-2002-185571 
     DISCLOSURE OF THE INVENTION 
     Problems to Be Solved by the Invention 
     However, in the above conventional cellular telephone apparatuses, the common speaker is merely used as both of the speaker for reproduction of a speech sound and the speaker for reproduction of a ringer tone and no consideration is given to the difference between the reproduction frequency band for the reproduction of a speech sound and that for the reproduction of a ringer tone. Therefore, it is impossible to secure a necessary reproduction frequency band for the reproduction of a ringer tone while satisfying the standard of the reproduction frequency band for the reproduction of a speech sound. Furthermore, the current consumption during reproduction of a speech sound is made higher than in the other conventional case where a speech sound and a ringer tone are reproduced by independent sets of a drive circuit and a sound-producing body. 
     In the above cellular telephone apparatuses, the common sound-producing body is required to be used for both of the purpose of producing a large-volume ringer tone and the purpose of producing a small-volume speech sound. Therefore, a dynamic speaker having equivalent impedance of 8Ω is used as the common speaker. If a ceramic speaker is used as the common speaker, a voltage of about 9 V needs to be applied to it, which necessitates a booster circuit. As a result, the drive circuit of the cellular telephone apparatus disclosed in Patent document 1 consumes much more power during a call than the other conventional circuit configuration. 
     The present invention has been made in view of the above circumstances in the art, and an object of the invention is therefore to provide a cellular telephone apparatus which employs a single sound-producing body as both of a receiver for reproducing a speech sound and a speaker for reproducing a ringer tone and which can secure a necessary reproduction frequency characteristic for each of the reproduction of a speech sound and the reproduction of a ringer tone. 
     Means for Solving the Problems 
     The invention provides a cellular telephone apparatus in which a single sound-producing body is used as both of a sound-producing body for reproducing a speech sound and a sound-producing body for reproducing a ringer tone and a drive circuit for driving the sound-producing body is a single drive circuit, comprising a drive circuit switching unit which switches a drive form of the drive circuit, and a control unit which controls the switching of the drive form of the drive circuit. 
     This configuration enables switching between different drive forms and hence makes it possible to use a single sound-producing body as a receiver for reproducing a speech sound and a speaker for reproducing a ringer tone. 
     The drive form that is switched by the drive circuit switching unit includes BTL drive output and single drive output. 
     This configuration enables switching between the BTL drive output and the single drive output and hence makes it possible to drive the drive circuit properly in, for example, both of a case that a large volume is necessary and a case that it is not necessary. 
     The control unit causes switching of the drive form of the drive circuit so that BTL drive output is selected as the drive form of the drive circuit during reproduction of a ringer tone and single drive output is selected as the drive form of the drive circuit during reproduction of a speech sound. 
     With this configuration, during reproduction of a ringer tone, a large-volume tone can be produced by BTL-output driving. During reproduction of a speech sound, whereas a sufficiently large volume to reproduce a speech sound can be secured by single-output driving, the current consumed by the drive circuit can be reduced and the usable time for calls of the cellular telephone apparatus can thereby be elongated by suspending one of the BTL outputs of the drive circuit. 
     The invention also provides a cellular telephone apparatus in which a single sound-producing body is used as both of a sound-producing body for reproducing a speech sound and a sound-producing body for reproducing a ringer tone and a drive circuit for driving the sound-producing body is a single drive circuit, comprising a power circuit output state switching unit which switches an output state of a power circuit for the drive circuit for driving the sound-producing body between boosted voltage output and non-boosted voltage output, the sound-producing body using a piezoelectric device, and a control unit which controls the switching of the output state of the power circuit for the drive circuit for driving the sound-producing body. 
     In this configuration, a booster circuit that is provided in the power circuit for the drive circuit is activated when a large volume is necessary and switching is made to a non-boosting circuit when a large volume is not necessary. This configuration thus makes it possible to reduce the current consumed by the power circuit for the drive circuit and to thereby elongate the usable time for calls of the cellular telephone apparatus. 
     The control unit performs a control so that the power circuit output state is made the boosted voltage output during reproduction of a ringer tone and it is made the non-boosted voltage output during reproduction of a speech sound. 
     With this configuration, a booster circuit that is provided in the power circuit for the drive circuit is activated during reproduction of a ringer tone in which a large volume is required, whereby a voltage that is a little more than 10 V can be applied to the drive circuit. During reproduction of a speech sound, the booster circuit which is provided in the power circuit for the drive circuit is disabled and a voltage that is high enough to reproduce a speech sound and is provided in the power circuit is output. As a result, during reproduction of a speech sound, the current consumed in the power circuit for the drive circuit can be reduced and hence the usable time for calls of the cellular telephone apparatus can be elongated. 
     The cellular telephone apparatus according to the invention further comprises a bias voltage switching unit which switches a bias voltage of an output-stage transistor of the drive circuit for driving the sound-producing body depending on which of a ringer tone and a speech sound is to be reproduced. 
     In this configuration, the bias voltage of the output-stage transistor of the drive circuit is switched depending on which of a ringer tone and a speech sound is to be reproduced. Therefore, during reproduction of a ringer tone, the output-stage transistor of the drive circuit can be given so high a voltage that cross-over distortion does not occur even when the drive frequency is about 20 kHz. High sound quality can thus be secured. During reproduction of a speech sound, the bias voltage which is applied to the output-stage transistor of the drive circuit is reduced to such a degree that cross-over distortion does not occur at several kilohertz, whereby the current consumed by the drive circuit itself can be reduced and hence the usable time for calls of the cellular telephone apparatus can be elongated. 
     The control unit switches a signal pass-band characteristic of the drive circuit for driving the sound-producing body or a preprocessing circuit of the drive circuit depending on which of a ringer tone and a speech sound is to be reproduced. 
     With this configuration, the high-frequency signal passage characteristic can be switched depending on which of a ringer tone and a speech sound is to be reproduced. As a result, a passage characteristic having an upper limit of several kilohertz can be secured during reproduction of a speech sound and a passage characteristic having an upper limit of a little more than 10 kHz can be set during reproduction of a ringer tone. Therefore, a necessary reproduction frequency characteristic can be satisfied for each of the reproduction of a speech sound and the reproduction of a ringer tone though the common drive circuit and sound-producing body are employed. 
     The control unit switches a signal pass-band characteristic of a post-processing circuit of the drive circuit for driving the sound-producing body depending on which of a ringer tone and a speech sound is to be reproduced. 
     With this configuration, the low-frequency signal passage characteristic can be switched depending on which of a ringer tone and a speech sound is to be reproduced. As a result, a passage characteristic having a lower limit of hundreds of hertz can be secured during reproduction of a speech sound and a passage characteristic having a lower limit of about 100 kHz can be set during reproduction of a ringer tone. Therefore, a necessary reproduction frequency characteristic can be satisfied for each of the reproduction of a speech sound and the reproduction of a ringer tone though the common drive circuit and sound-producing body are employed. 
     ADVANTAGES OF THE INVENTION 
     The invention can provide a cellular telephone apparatus which employs a single sound-producing body as both of a receiver for reproducing a speech sound and a speaker for reproducing a ringer tone and which can secure a necessary reproduction frequency characteristic for each of the reproduction of a speech sound and the reproduction of a ringer tone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a cellular telephone apparatus according to a first embodiment of the invention. 
         FIG. 2  shows detailed circuit configurations of a speaker amplifier and a highpass filter circuit of the cellular telephone apparatus according to the first embodiment of the invention. 
         FIG. 3  is a flowchart for description of the operation of the cellular telephone apparatus according to the first embodiment of the invention. 
         FIG. 4  is a block diagram of a cellular telephone apparatus according to a second embodiment of the invention. 
         FIG. 5  shows detailed circuit configurations of a speaker amplifier, a lowpass filter circuit, and a booster circuit/non-boosting circuit of the cellular telephone apparatus according to the second embodiment of the invention. 
         FIG. 6  is a flowchart for description of the operation of the cellular telephone apparatus according to the second embodiment of the invention. 
         FIG. 7  is a block diagram showing the configuration of a conventional cellular telephone apparatus. 
     
    
    
     DESCRIPTION OF SYMBOLS 
     
         
           100 ,  400 : Cellular telephone apparatus 
           101 : Antenna 
           102 : Duplexer 
           103 : Receiving circuit 
           104 : Demodulator 
           105 : Modulator 
           106 : Transmitting circuit 
           110 ,  410 : Microprocessor 
           121 : Decoder 
           122 : Encoder 
           123 : Ringer tone source 
           130 : Digital lowpass filter 
           131 : D/A converter 
           132 : Volume setting device 
           133 ,  433 : Speaker amplifier 
           134 : Highpass filter 
           135 ,  435 : Speaker 
           140 : Power circuit 
           150 : Microphone 
           151 : Microphone amplifier 
           152 : A/D converter 
           430 : Digital highpass filter 
           434 : Lowpass filter 
           440 : Booster circuit/non-boosting circuit 
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Cellular telephone apparatuses according to embodiments of the present invention will be hereinafter described with reference to the drawings. 
     First Embodiment 
       FIG. 1  is a block diagram of a cellular telephone apparatus according to a first embodiment of the invention. 
     As shown in  FIG. 1 , the cellular telephone apparatus  100  is equipped with an antenna  101  which converts reception radio waves into reception electric signals (hereinafter referred to as “reception signals”) and converts a transmission electric signal (Thereinafter referred to as “transmission signal”) into transmission radio waves, a duplexer  102  which supplies, to a receiving circuit  103 , the reception signals coming from the antenna  101  and supplies, to the antenna  101 , the transmission signal coming from a transmitting circuit  106 , the receiving circuit  103  which amplifies the reception signals and selects a desired frequency channel, a demodulator  104  which demodulates the reception signal of the frequency channel selected by the receiving circuit  103  into a digital signal and converts the digital signal into reception speech data, reception communication data, and reception control information data, a modulator  105  which modulates and converts transmission communication data, transmission control information data, and transmission speech data into a transmission signal, a transmitting circuit  106  which superimposes the transmission signal on a carrier wave having a desired frequency channel and amplifies a resulting transmission signal, and a microprocessor  110  which controls the individual sections of the cellular telephone apparatus  100 . 
     The cellular telephone apparatus  100  according to this embodiment is also equipped with a decoder  121  which decodes and converts the reception speech data into a reception speech signal, an encoder  122  which encodes and converts a transmission speech signal into the transmission speech data, and a ringer tone source  123  which generates and outputs a ringer tone or melody according to an instruction from the microprocessor  110 . 
     The cellular telephone apparatus  100  according to this embodiment is further equipped with a digital lowpass filter  130  which performs high-frequency bandwidth limitation processing on the ringer tone signal coming from the ringer tone source  123  or the reception speech signal coming from the decoder  121 , a D/A converter  131  which converts the ringer tone signal or the reception speech signal into an analog signal, a volume setting device  132  which sets the volume of the ringer tone signal or the reception speech signal to a prescribed value, a speaker amplifier  133  which amplifies the ringer tone signal or the reception speech signal and drives a speaker  135 , a highpass filter circuit  134  which performs low-frequency bandwidth limitation processing on the ringer tone signal or the reception speech signal that is output from the speaker amplifier  133 , the speaker  135  which reproduces a ringer tone or a reception speech sound, a power circuit  140  for the speaker amplifier  133 , a microphone  150  which converts a voice to be transmitted into an electric signal, a microphone amplifier  151  which amplifies the transmission speech signal coming from the microphone  150 , and an A/D converter  152  which converts the resulting transmission speech signal into a digital signal. 
     Although not shown in the figure, the cellular telephone apparatus  100  also has a memory in which control programs, ringer melody data, etc. are stored, key switches to be used for manipulating the cellular telephone apparatus, and a display unit. 
     Next, main sections of the cellular telephone apparatus  100  will be described. The digital lowpass filter  130 , which performs high-frequency bandwidth limitation processing on a ringer tone signal coming from the ringer tone source  123  or a reception speech coming from the decoder  121 , is composed of two digital lowpass filters  130   a  and  130   b  having different frequency passage characteristics. The circuit configuration is such that one of the two digital lowpass filters  130   a  and  130   b  having the different frequency passage characteristics is chosen in accordance with a selection signal  160  supplied from the microprocessor  110 . 
     For example, of the two digital lowpass filters  130   a  and  130   b  having the different frequency passage characteristics, the one digital lowpass filter  130   a  is a lowpass filter whose cutoff frequency is 3 kHz and the other digital lowpass filter  130   b  is a lowpass filter whose cutoff frequency is 20 kHz. 
     The speaker amplifier  133 , which amplifies a ringer tone signal or a reception speech signal and drives the speaker  135 , is a BTL (balanced transformerless)—output, class AB amplifier. The circuit configuration is such that one of the BTL outputs is disabled in accordance with a selection signal  161  supplied from the microprocessor  110 . 
     The highpass filter circuit  134 , which performs low-frequency bandwidth limitation processing on a ringer tone signal or a reception speech signal that is output from the speaker amplifier  133 , is composed of two highpass filters  134   a  and  134   b  having different frequency passage characteristics. The circuit configuration is such that one of the two highpass filters  134   a  and  134   b  having the different frequency passage characteristics is chosen in accordance with a selection signal  162  supplied from the microprocessor  110 . For example, of the two digital highpass filters  134   a  and  134   b  having the different frequency passage characteristics, the one highpass filter  134   a  is a highpass filter whose cutoff frequency is 300 Hz and the other highpass filter  134   b  is a highpass filter whose cutoff frequency is 100 Hz. Furthermore, the circuit configuration is such that one of the input terminals of the speaker  135  is terminated to the ground when the highpass filter  134   a  having the cutoff frequency of 300 Hz is chosen. 
     In this embodiment, the speaker  135 , which reproduces a ringer tone or a reception speech sound, is a dynamic speaker. 
       FIG. 2  shows detailed circuit configurations of the speaker amplifier  133  and the highpass filter circuit  134 . 
     As shown in  FIG. 2 , the speaker amplifier  133  has a preamplifier  133   a , a preamplifier  133   b  which phase-inverts an output of the preamplifier  133   a , a push-pull circuit  133   c  which current-amplifies the output of the preamplifier  133   a  and performs class AB operation, and a push-pull circuit  133   d  which current-amplifies an output of the preamplifier  133   b  and performs class AB operation. 
     The circuit operation of the preamplifier  133   b  is stopped by a selection signal  161 . Furthermore, the output transistors of the push-pull circuit  133   d  are turned off by the selection signal  161 . In a state that the operation of the preamplifier  133   b  and the push-pull circuit  133   d  is suspended by the selection signal  161 , almost no circuit current flows through the preamplifier  133   b  and the push-pull circuit  133   d.    
     The highpass filter circuit  134  has analog switches  134   a  and  134   b  and capacitors  134   c ,  134   d , and  134   e  which form a highpass filter when combined with the impedance of the speaker  135 . 
     Switching between the contacts of each of the analog switches  134   a  and  134   b  can be made in accordance with a selection signal  162 . The constants of the capacitors  134   c  and  134   e  are set so that the cutoff frequency of the highpass filter becomes about 100 Hz in a state that the analog switch  134   a  is switched to a contact  134   a   1  and the analog switch  134   b  is switched to a contact  134   b   1 . 
     On the other hand, in a state that the analog switch  134   a  is switched to a contact  134   a   2  and the analog switch  134   b  is switched to a contact  134   b   2 , one of the input terminals of the speaker  135  is terminated to the ground and a single drive form is thereby established. The constant of the capacitor  134   d  is set so that the cutoff frequency of the highpass filter becomes about 300 Hz. 
     Such things as voltage drops across the analog switches  134   a  and  134   b  are negligible as long as they have small on-resistance values. 
     The operation of the above-configured cellular telephone apparatus will be described below with reference to  FIG. 3 . First, the microprocessor  110  receives reception control information from the demodulator  104  (step S 301 ). If the reception control information is ringing information, the microprocessor  110  sets the signal pass-band to a pass-band of from 100 Hz to 20 kHz (step S 302 ). At this step, the microprocessor  110  chooses the characteristic of the lowpass filter  130   b  as the frequency characteristic of the digital lowpass filter  130  in accordance with a selection signal  160 . At the same time, the microprocessor  110  renders the preamplifier  133   b  and the push-pull circuit  133   d  of the speaker amplifier  133  in an active state in accordance with a selection signal  161 . Furthermore, at the same time, the microprocessor  110  switches the analog switches  134   a  and  134   b  of the highpass filter circuit  134  to the respective contacts  134   a   1  and  134   b   1  in accordance with a selection signal  162 , and thereby sets the output form (circuit configuration) of the speaker amplifier  133  to the BTL output form and sets the signal pass-band to a pass-band of from 100 Hz to 20 kHz. 
     Then, the microprocessor  110  reproduces a ringer melody and thereby notifies the user of the cellular telephone apparatus  100  about arrival of a call (step S 303 ). At this step, the microprocessor  110  sends ringer melody data (stored in the memory (not shown)) to the ringer tone source  123  and instructs the ringer tone source  123  to generate a ringer melody. The ringer melody data that is output from the ringer tone source  123  is high-frequency-limited at 20 kHz by the digital lowpass filter  130 , converted into an analog signal by the D/A converter  131 , subjected to volume setting to a prescribed value by the volume setting device  132 , current-amplified by the speaker amplifier  133 , and subjected to cutting of a low-frequency component of 100 Hz or less by the highpass filter circuit  134 . The thus-processed ringer melody data reaches the speaker  135 , which reproduces a ringer melody. 
     Notified about arrival of a ringing call by ringer tone output of the speaker  135 , the user of the cellular telephone apparatus  100  depresses a call start key among the key switches (not shown) which are connected to the microprocessor  110  (step S 304 ). 
     The microprocessor  110  detects the depression state of the call start key, and instructs the ringer tone source  123  to stop the generation of the ringer melody. At the same time, the microprocessor  110  sends, to the modulator  105 , transmission control information indicating reception of the ringing call and informs a base station about the reception of the ringing call via the transmitting circuit  106 , the duplexer  102 , and the antenna  101  (step S 305 ). 
     The base station receives the transmission control information indicating reception of the ringing call (step S 306 ) and transmits, to the cellular telephone apparatus  100 , control information for rendering a speech channel open (step S 307 ). 
     The cellular telephone apparatus  100  receives the control information for rendering a speech channel open (step S 308 ). At this step, the control information for rendering a speech channel open is communicated to the microprocessor  110  via the antenna  101 , the duplexer  102 , the receiving circuit  103 , and the demodulator  104 . 
     Receiving the control information for rendering a speech channel open, the microprocessor  110  sets the signal pass-band to a pass-band of from 300 Hz to 3 kHz (step S 309 ). At this step, the microprocessor  110  sends, to the decoder  121  and the encoder  122 , instructions for rendering them in an active state. At the same time, the microprocessor  110  chooses the characteristic of the lowpass filter  130   a  as the frequency characteristic of the digital lowpass filter  130  in accordance with a selection signal  160 . At the same time, the microprocessor  110  renders the preamplifier  133   b  and the push-pull circuit  133   d  of the speaker amplifier  133  in a halt state in accordance with a selection signal  161 . Furthermore, at the same time, the microprocessor  110  switches the analog switches  134   a  and  134   b  of the highpass filter circuit  134  to the respective contacts  134   a   2  and  134   b   2  in accordance with a selection signal  162 , and thereby sets the output form (circuit configuration) of the speaker amplifier  133  to the single output and sets the signal pass-band to a pass-band of from 300 kHz to 3 kHz. 
     Then, the microprocessor  110  sends speech channel opening completion information to the modulator  105  as transmission control information and thereby informs the base station about completion of opening of a speech channel via the transmitting circuit  106 , the duplexer  102 , and the antenna  101  (step S 310 ). 
     The base station receives the transmission control information indicating the completion of opening of a speech channel (step S 311 ) and transmits speech data to the cellular telephone apparatus  100  (step S 312 ). 
     The cellular telephone apparatus  100  receives the speech data (reception speech data) (step S 313 ) and reproduces the reception speech data (step S 314 ). At this step, the reception speech data is supplied to the decoder  121  via the antenna  101 , the duplexer  102 , the receiving circuit  103 , and the demodulator  104 . The decoder  121  decodes the thus-supplied reception speech data into a reception speech signal and sends the latter to the digital lowpass filter  130 . The reception signal is high-frequency-limited at 3 kHz by the digital lowpass filter  130 , converted into an analog signal by the D/A converter  131 , subjected to volume setting to a prescribed value by the volume setting device  132 , current-amplified by the speaker amplifier  133 , and subjected to cutting of a low-frequency component of 300 Hz or less by the highpass filter circuit  134 . The thus-processed reception signal reaches the speaker  135 , which reproduces a voice. 
     The dynamic range of a case that the output form of the speaker amplifier  133  is set to the single output is 6 dB narrower than that of a case that it is set to the BTL output. However, since the difference between the maximum volume of a ringer tone and that of a speech sound is more than or equal to 10 dB, a dynamic range that is wide enough to reproduce a speech sound properly can be secured even if the output form of the speaker amplifier  133  is set to the single output. 
     In the above-described cellular telephone apparatus according to the first embodiment of the invention, the single sound-producing body is used as both of the sound-producing body for reproducing a speech sound and the sound-producing body for reproducing a ringer tone, the drive circuit for driving the sound-producing body is the single drive circuit, and the drive form of the drive circuit is switched between the BTL drive output and the single drive output depending on which of a speech sound and a ringer tone is to be reproduced. As a result, a ringer tone can be reproduced at a large volume by driving the dynamic speaker having equivalent impedance of 8Ω in the BTL output form. 
     On the other hand, a speech sound can be reproduced at a sufficiently large volume by switching the drive form to the single output. Since one of the BTL outputs of the drive circuit is disabled, the current consumption of the drive circuit can be reduced and hence the usable time for calls of the cellular telephone apparatus can be elongated. 
     Since the signal pass-band characteristic of the drive circuit for driving the sound-producing body or the preprocessing circuit of the drive circuit is switched depending on which of a speech sound and a ringer tone is to be reproduced, the high-frequency signal passage characteristic can be switched depending on which of a speech sound and a ringer tone is to be reproduced. As a result, a passage characteristic having an upper limit of several kilohertz can be set during a call (conversation) and a passage characteristic having an upper limit of a little more than 10 kHz can be set during reproduction of a ringer tone. This provides an advantage that a necessary reproduction frequency characteristic can be satisfied for each of the reproduction of a speech sound and the reproduction of a ringer tone, though the common drive circuit and sound-producing body are employed. 
     Furthermore, since the signal pass-band characteristic of the post-processing circuit of the drive circuit for driving the sound-producing body is switched depending on which of a speech sound and a ringer tone is to be reproduced, the low-frequency signal passage characteristic can be switched depending on which of a speech sound and a ringer tone is to be reproduced. As a result, a passage characteristic having a lower limit of hundreds of hertz can be set during a call (conversation) and a passage characteristic having a lower limit of about 100 kHz can be set during reproduction of a ringer tone. This provides an advantage that a necessary reproduction frequency characteristic can be satisfied for each of the reproduction of a speech sound and the reproduction of a ringer tone, though the common drive circuit and sound-producing body are employed. 
     In this embodiment, the high-frequency pass-band is switched by means of the digital lowpass filter  130 . Alternatively, the same function can be implemented by incorporating an active lowpass filter in the speaker amplifier  133  and switching the cutoff frequency by switching the filter constants of the active lowpass filter. 
     Furthermore, although this embodiment employs a class AB amplifier as the speaker amplifier  133 , the same advantages can be obtained even if a class D amplifier is employed. 
     Second Embodiment 
       FIG. 4  is a block diagram showing the configuration of a cellular telephone apparatus according to a second embodiment of the invention. The cellular telephone apparatus according to the second embodiment is different in configuration from the cellular telephone apparatus according to the first embodiment shown in  FIG. 1  in that the microprocessor  110  is replaced by a microprocessor  410  whose functions are somewhat different from the functions of the former, the digital lowpass filter  130  is replaced by a digital highpass filter  430 , the speaker amplifier  133  is replaced by a speaker amplifier  433 , the highpass filter  134  is replaced by a lowpass filter  434 , the speaker  135  is replaced by a speaker  435 , and the amplifier power source  140  is replaced by a booster circuit/non-boosting circuit  440 . Components having the same components in the cellular telephone apparatus  100  according to the first embodiment shown in  FIG. 1  are given the same reference symbols as the latter and will not be described below. 
     The microprocessor  410  controls the individual sections of the cellular telephone apparatus  400 . The digital highpass filter  430  performs low-frequency band limitation processing on a ringer tone signal coming from the ringer tone source  123  or a reception speech signal coming from the decoder  121 , and is composed of two digital highpass filters having different frequency passage characteristics. The circuit configuration is such that one of the two digital highpass filters having the different frequency passage characteristics is chosen in accordance with a selection signal  460  supplied from the microprocessor  410 . Of the two digital highpass filters having the different frequency passage characteristics, one highpass filter  430   a  is a highpass filter whose cutoff frequency is 300 Hz and the other highpass filter  430   b  is a highpass filter whose cutoff frequency is 100 Hz. 
     The speaker amplifier  433  amplifies a ringer tone signal or a reception speech signal and drives the speaker  435 , and is a BTL-output, class AB amplifier. The circuit configuration is such that the bias voltage of the transistors constituting output-stage push-pull circuits can be switched in accordance with a selection signal  461  supplied from the microprocessor  110 . 
     The lowpass filter  434  performs high-frequency bandwidth limitation processing on a ringer tone signal or a reception speech that is output from the speaker amplifier  433 , and is composed of two lowpass filters having different frequency passage characteristics. The circuit configuration is such that one of the two lowpass filters having the different frequency passage characteristics is chosen in accordance with a selection signal  462  supplied from the microprocessor  410 . Of the two lowpass filters having the different frequency passage characteristics, one lowpass filter  434   a  is a lowpass filter whose cutoff frequency is 20 kHz and the other lowpass filter  130   b  is a lowpass filter whose cutoff frequency is 3 kHz. 
     The speaker  435  reproduces a ringer tone or a reception speech sound and, in this embodiment, is a ceramic speaker. 
     The booster circuit/non-boosting circuit  440  is a power circuit for the speaker amplifier. The circuit configuration is such that one of the output of a 9-V booster circuit and a battery output voltage is output in accordance with the selection signal  461  supplied from the microprocessor  410 . 
       FIG. 5  shows detailed circuit configurations of the speaker amplifier  433 , the lowpass filter circuit  434 , and the booster circuit/non-boosting circuit  440 . 
     The speaker amplifier  433  has a preamplifier  433   a , a preamplifier  433   b  which phase-inverts an output of the preamplifier  433   a , a push-pull circuit  433   c  which current-amplifies the output of the preamplifier  433   a  and performs class AB operation, and a push-pull circuit  433   d  which current-amplifies an output of the preamplifier  133   b  and performs class AB operation. 
     The circuit configuration is such that the bias voltage of the output transistors constituting each of the push-pull circuits  433   c  and  433   d  can be switched between a bias voltage- 1  and a bias voltage- 2  in accordance with a selection signal  461 . 
     The bias voltage  1  is so high a voltage that cross-over distortion does not occur when the speaker  435  as the load of the speaker amplifier  433  is driven at a drive frequency of 3 kHz. The bias voltage- 2  is so high a voltage that cross-over distortion does not occur when the speaker  435  as the load of the speaker amplifier  433  is driven at a drive frequency of 20 kHz. Since the speaker  435  is a ceramic speaker which is a capacitive load, its impedance is larger at 20 kHz than at 3 kHz. Therefore, the bias voltage- 2  is higher than the bias voltage  1 . The current consumed in each push-pull circuit itself is smaller when the bias voltage  1  is chosen than when the bias voltage- 2  is chosen. 
     The lowpass filter circuit  434  is composed of analog switches  434   a  and  434   b  and resistors  434   c ,  434   d ,  434   e , and  434   f  which constitute an RC filter when combined with the capacitive impedance of the speaker  435 . Switching between the contacts of each of the analog switches  434   a  and  434   b  can be made in accordance with a selection signal  462 . The resistance values of the resistors  434   c  and  434   e  are set so that the cutoff frequency of the lowpass filter becomes about 20 kHz in a state that the analog switch  434   a  is switched to a contact  434   a   1  and the analog switch  434   b  is switched to a contact  434   b   1 . On the other hand, the resistance values of the resistors  434   d  and  434   f  are set so that the cutoff frequency of the lowpass filter becomes about 3 kHz in a state that the analog switch  434   a  is switched to a contact  434   a   2  and the analog switch  434   b  is switched to a contact  434   b   2 . The analog switches  434   a  and  434   b  have small on-resistance values and hence such things as voltage drops across them are negligible. 
     The booster circuit/non-boosting circuit  440  has a boosted voltage output circuit  440   a  which boosts an input battery voltage and outputs a resulting voltage of 9 V, a non-boosted voltage output circuit  440   b  which outputs the battery voltage, and a switch  440   c  which switches between the output of the boosted voltage output circuit  440   a  and the output of the non-boosted voltage output circuit  440   b  and outputs a resulting voltage. The circuit configuration is such that when the switch  440   c  is switched so as to choose the output of the non-boosted voltage output circuit  440   b , the boosted voltage output circuit  440   a  suspends its boosting operation. 
     The operation of the above-configured cellular telephone apparatus  400  will be described below with reference to  FIG. 6 . 
     First, the microprocessor  410  receives reception control information from the demodulator  104  (step S 601 ). If the reception control information is ringing information, the microprocessor  410  sets the signal pass-band to a pass-band of from 100 Hz to 20 kHz (step S 602 ). At this step, the microprocessor  410  chooses the characteristic of the highpass filter  430   b  as the frequency characteristic of the digital highpass filter  430  in accordance with a selection signal  460 . At the same time, the microprocessor  410  sets, to the above-mentioned bias voltage- 2 , the bias voltage of the transistors constituting each of the push-pull circuits  433   c  and  433   d  of the speaker amplifier  433  and chooses the output of the boosted-voltage output circuit  440   a  as the output of the switch  440   c  of the power circuit  440  in accordance with a selection signal  461 . Furthermore, at the same time, the microprocessor  410  switches the analog switches  434   a  and  434   b  of the lowpass filter circuit  434  to the respective contacts  434   a   1  and  434   b   1  in accordance with a selection signal  462 . With these operations, the microprocessor  410  gives the speaker amplifier  433  an ability to drive the speaker  435  at 20 kHz, supplies a power source voltage of 9 V to the speaker amplifier  433 , and sets the signal pass-band to a pass-band of from 100 Hz to 20 kHz. 
     Operations that are performed after setting the signal pass-band to the pass-band of from 100 Hz to 20 kHz, that is, steps S 303 -S 308 , are the same as in the first embodiment (see  FIG. 3 ) and hence will not be described. 
     Receiving the reception control information for rendering a speech channel open at step S 308 , the microprocessor  410  sets the signal pass-band to a pass-band of from 300 Hz to 3 kHz (step S 609 ). At this step, the microprocessor  410  sends, to the decoder  121  and the encoder  122 , instructions for rendering them in an active state. At the same time, the microprocessor  410  chooses the characteristic of the highpass filter  430   a  as the frequency characteristic of the digital highpass filter  430  in accordance with a selection signal  460 . At the same time, the microprocessor  410  sets, to the above-mentioned bias voltage- 1 , the bias voltage of the transistors constituting each of the push-pull circuits  433   c  and  433   d  of the speaker amplifier  433  and chooses the output of the non-boosted-voltage output circuit  440   b  as the output of the switch  440   c  of the power circuit  440  in accordance with a selection signal  461 . Furthermore, at the same time, the microprocessor  410  switches the analog switches  434   a  and  434   b  of the lowpass filter circuit  434  to the respective contacts  434   a   2  and  434   b   2  in accordance with a selection signal  462 . With these operations, the microprocessor  410  gives the speaker amplifier  433  an ability to drive the speaker  335  at 3 kHz, reduces the current consumed by the speaker amplifier  433  itself, supplies a power source voltage of the battery voltage level to the speaker amplifier  433 , making a setting for suspending the boosting operation of the boosted voltage output circuit  440   a , and sets the signal pass-band to a pass-band of from 100 Hz to 3 kHz. 
     Operations that are performed after setting the signal pass-band to the pass-band of from 100 Hz to 3 kHz, that is, steps S 310 -S 314  are the same as in the first embodiment (see  FIG. 3 ) and hence will not be described. 
     The dynamic range of a case that the power source voltage of the speaker amplifier  433  is set at 3.2 V which is the minimum voltage (battery voltage) is about 9 dB narrower than that of a case that it is set at 9 V. However, since the difference between the maximum volume of a ringer tone and that of a speech sound is more than or equal to 10 dB, a dynamic range that is wide enough to reproduce a speech sound properly can be secured even if the power source voltage of the speaker amplifier  433  is set at 3.2 V which is the minimum voltage (battery voltage). 
     In the above-described cellular telephone apparatus according to the second embodiment of the invention, the single sound-producing body is used as both of the sound-producing body for reproducing a speech sound and the sound-producing body for reproducing a ringer tone, the drive circuit for driving the sound-producing body is the single drive circuit, a piezoelectric device is used in the sound-producing body, and the output state of the power circuit for the drive circuit for driving the sound-producing body is switched between the boosted voltage output and the non-boosted voltage output depending on which of a speech sound and a ringer tone is to be reproduced. As a result, the booster circuit which is provided in the power circuit for the drive circuit is activated during reproduction of a ringer tone in which a large volume is required, whereby a voltage that is a little more than 10 V can be applied to the drive circuit. During reproduction of a speech sound, the booster circuit which is provided in the power circuit for the drive circuit is disabled and a voltage that is high enough to reproduce a speech sound and is provided in the power circuit is output. As a result, during reproduction of a speech sound, the current consumed in the power circuit for the drive circuit can be reduced and hence the usable time for calls of the cellular telephone apparatus can be elongated. 
     In this embodiment, the low-frequency pass-band is switched by means of the digital highpass filter  430 . Alternatively, the same function can be implemented by incorporating an active highpass filter in the speaker amplifier  433  and switching the cutoff frequency by switching the filter constants of the active highpass filter. 
     Furthermore, although this embodiment employs a class AB amplifier as the speaker amplifier  433 , the same advantages can be obtained even if a class D amplifier is employed. 
     The invention has been described above in detail by using the particular embodiments. However, it is apparent to those skilled in the art that various changes and modifications are possible without departing from the spirit and scope of the invention. 
     This application is based on Japanese Patent Application No. 2004-235208 filed on Aug. 12, 2004, the disclosure of which is incorporated by reference herein. 
     INDUSTRIAL APPLICABILITY 
     The invention provides the advantages that a single sound-producing body can be used as both of a receiver for reproducing a speech sound and a speaker for reproducing a ringer tone and that a necessary reproduction frequency characteristic can be secured for each of the reproduction of a speech sound and the reproduction of a ringer tone. As such, the invention is useful when applied to cellular telephone apparatuses etc.