Abstract:
A method and apparatus to control output power of a digital amplifier capable of automatically determining an impedance of an earphone or headphone connected to a portable audio device and outputting a power that is optimized according to the impedance. The apparatus is usable with a digital amplification device and includes a signal processing unit to convert a pulse signal into a direct current (DC) voltage by power-amplifying the pulse signal, an impedance measuring unit to connect a resistance device between the signal processing unit and a speaker unit and to detect a voltage value applied to the resistance device and the speaker unit, and a micom unit to generate a pulse signal having a predetermined duty cycle in the signal processing unit if the speaker unit is connected to the device, to determine an impedance of the speaker unit based on the voltage value detected by the impedance measuring unit, to control a power amplification by providing a voltage level control signal to the signal processing unit according to the determined impedance, and to control an audio level in an audio reproduction mode accordingly.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 10-2005-0105066, filed on Nov. 3, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present general inventive concept relates to a portable audio reproduction apparatus, and more particularly, to a method and apparatus to control an output power of a digital amplifier capable of automatically determining an impedance of an earphone or headphone connected to a portable audio device and enabling a power output to be optimized for the earphone or headphone. 
     2. Description of the Related Art 
     Recently, users of portable audio devices, such as MP3 players and portable multimedia players (PMPs), have demanded high quality sound. Also, the portable audio devices use digital audio amplifiers for low power consumption, smaller size, and high quality sound. Generally, A class, B class, AB class, and D class are used for digital audio amplifiers. Among these digital amplifiers, D class amplifier can reduce degradation of amplification efficiency occurring in the A, B, and AB class amplifiers. The D class amplifier converts an audio signal into a pulse width modulation (PWM) signal and switches the signal. In addition, a variety of earphones and headphones ranging from a 16Ω impedance earphone offered as a bundle item to a 120Ω impedance high-end headphone are being used in the portable audio devices. 
     Accordingly, in order to support the variety of earphones and/or headphones, an output capacity of a portable audio device should be large enough and a magnitude of an output signal should be set differently with respect to a type of the earphone and/or headphone being used in the portable audio device. 
       FIG. 1  is a block diagram illustrating a conventional digital amplification apparatus. 
     Referring to  FIG. 1 , a digital audio signal is output to an earphone/headphone  180  through a PWM unit  130 , a power switching unit  150 , and a low-pass filter  160 . 
     An analog to digital converter (ADC) unit  120  converts a sound input to a microphone  122  into a digital signal in order to record the sound in a recorder. A microcomputer (micom) unit  110  generates different control signals and an ADC control signal to control respective blocks and units. A power supply unit  140  provides a predetermined direct current (DC) voltage (fixed V DC ) to the power switching unit  150 . 
     However, there occurs a large difference in magnitude (volume) of sound that a user hears according to a magnitude of an impedance of the earphone/headphone  180  connected to an output end of an amplifier. In general, an output power of an earphone or headphone at the magnitude of volume that a user hears most conveniently ranges from 5 mW to 7 mW. 
     For example, with reference to a 16Ω impedance headphone, a magnitude of a signal at the output end of the amplifier at which an output power of about 6 mW is obtained is 0.310 Vrms when the output signal is measured in a root square mean (RMS). 
     Accordingly, since most audio devices employ a 16Ω impedance earphone, an output level of the amplifier is set to 0.310 Vrms in order to output the output power of about 6 mW (approximately volume level 20). 
     However, since the output level of the amplifier is still set to 0.310 Vrms even when a 120Ω impedance high-end headphone is connected, an actual output power of the headphone may be limited to only about 0.8 mW. Accordingly, even though a user sets an identical volume level, if the impedance of the earphone/headphone  180  is increased, the sound output from the headphone/earphone  180  may become too soft to hear. 
     Similarly, if the volume level is set to deliver an output of about 6 mW with a 120Ω impedance high-end headphone, and the amplifier connection is then switched to a 16Ω impedance headphone, the user hears a very loud sound of about 44.7 mW and the user&#39;s ears may be hurt.  FIGS. 2A and 2B  are tables comparing headphone impedances, power consumptions in a headphone, and audible sound pressures with respect to magnitudes of signals output by the amplifier. 
     Therefore, according to the conventional digital amplification apparatus, unless the magnitude of the amplifier is appropriately adjusted with respect to the characteristics of the earphone or headphone impedance connected to the apparatus, the user may either not be able to hear the sound being amplified or may hear the sound at a decibel level that is too loud. 
     SUMMARY OF THE INVENTION 
     The present general inventive concept provides a method and apparatus capable of controlling output power of a digital amplifier in an audio device according to a measured impedance of an earphone or headphone connected to the digital amplifier such that an appropriate output level of the digital amplifier is determined. 
     Additional aspects of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept. 
     The foregoing and/or other aspects of the present general inventive concept may be achieved by providing an apparatus to control output power of a digital amplifier in a digital audio amplification device, the apparatus including a signal processing unit to power amplify a pulse signal and to convert the pulse signal into a direct current (DC) voltage, an impedance measuring unit to connect a resistance device between the signal processing unit and a speaker unit and to detect a voltage value applied to the resistance device and the speaker unit, and a micom unit to generate a pulse signal having a predetermined duty cycle in the signal processing unit if the speaker unit is connected to the device, to determine an impedance of the speaker unit based on the voltage value detected by the impedance measuring unit, to control power amplification by providing a voltage level control signal to the signal processing unit according to the determined impedance, and to control an audio level in an audio reproduction mode accordingly. 
     The foregoing and/or other aspects of the present general inventive concept may also be achieved by providing a digital amplifier usable in a portable audio device, the amplifier including a signal processing unit having a variable power supply unit to output a variety of voltages, the signal processing unit receiving an audio signal and amplifying the audio signal based on a DC voltage currently output by the variable power supply unit, an impedance measuring unit to measure an impedance value of any speaker unit connected to the portable audio device and to output an impedance detection signal, and a micom unit to receive the impedance detection signal and to control the variable power supply unit to update the DC voltage currently output based on the measured impedance value indicated by the impedance detection signal. 
     The foregoing and/or other aspects of the present general inventive concept may also be achieved by providing a portable audio device, including a digital amplifier to amplify an audio signal based on a power supply level of a power supply, and a power supply control unit to vary the power supply level of the power supply based on an impedance of at least one speaker unit connected to the digital amplifier. 
     The foregoing and/or other aspects of the present general inventive concept may also be achieved by providing a portable audio device, including an output port, an audio unit to output an audio signal, and an amplifier to receive the audio signal from the audio unit and to amplify the audio signal based on an impedance value detected at the output port. 
     The foregoing and/or other aspects of the present general inventive concept may also be achieved by providing a portable audio device, including a digital amplifier to amplify an audio signal based on a power supply level of a power supply, and an apparatus to control an output power of the digital amplifier by varying the power supply level of the power supply based on an impedance of at least one speaker unit connected to the digital amplifier. The apparatus includes a signal processing unit to power amplify a pulse signal and to convert the pulse signal into a direct current (DC) voltage, an impedance measuring unit to connect a resistance device between the signal processing unit and the at least one speaker unit and to detect a voltage value applied to the resistance device and the at least one speaker unit, and a micom unit to generate a pulse signal having a predetermined duty cycle in the signal processing unit if the at least one speaker unit is connected to the portable audio device, to determine the impedance of the at least one speaker unit based on the voltage value detected by the impedance measuring unit, to control a power amplification by providing a voltage level control signal to the signal processing unit according to the determined impedance, and to control an audio level in an audio reproduction mode accordingly. 
     The foregoing and/or other aspects of the present general inventive concept may also be achieved by providing a portable audio device, including a signal processing unit having another terminal to generate a voltage signal in a headphone impedance measuring mode, a terminal to be connected to an external headphone, an impedance measuring unit to be selectively connected between the terminal and the another terminal to generate one of a first value and a second value according to the voltage signal corresponding to a characteristic of the external headphone, and a processor unit to control the signal processing unit to generate one of a first audio reproducing signal and a second audio reproducing signal according to the one of the first value and the second value. 
     The foregoing and/or other aspects of the present general inventive concept may also be achieved by providing a method of controlling output power of a digital audio amplification device, the method including if a speaker unit is connected to the digital audio amplification device, setting an impedance measuring mode and generating an arbitrary pulse signal, converting the arbitrary pulse signal into a DC voltage, applying the DC voltage to a resistance device connected to the speaker unit and detecting a voltage between the resistance device and the speaker unit, determining an impedance of the connected speaker unit based on the detected voltage, and adjusting a level of audio being reproduced by controlling switching power-amplification with respect to the determined impedance of the speaker unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a block diagram illustrating a conventional digital amplification apparatus; 
         FIGS. 2A and 2B  are tables comparing headphone impedances, power consumptions in a headphone, and audible sound pressures with respect to magnitudes of signals; 
         FIG. 3  is a block diagram illustrating an apparatus to control an output power of a digital amplifier according to an embodiment of the present general inventive concept; 
         FIG. 4  illustrates a measuring circuit in a headphone impedance measuring mode according to an embodiment of the present general inventive concept; 
         FIG. 5  illustrates a result of a simulation of the headphone impedance measuring circuit of  FIG. 4 ; 
         FIG. 6  is a flowchart illustrating a control method of a micom unit of the output power controlling apparatus of  FIG. 3 , according to an embodiment of the present general inventive concept; and 
         FIG. 7  illustrates a portable audio player according to an embodiment of the present general inventive concept. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present general inventive concept by referring to the figures. 
       FIG. 3  is a block diagram illustrating an apparatus to control an output power of a digital amplifier according to an embodiment of the present general inventive concept. 
     The output power controlling apparatus of  FIG. 3  includes a microcomputer (micom) unit  310 , a signal processing unit  320 , an impedance measuring unit  340 , and a headphone  350 . Here, the headphone  350  may be replaced by a variety of speaker units including different types of headphones, earphones, and speakers. The output power controlling apparatus of  FIG. 3  may be included in an audio device or a portable audio player. 
     The signal processing unit  320  includes a pulse width modulation (PWM) unit  322 , a power supply unit  323 , a power switching unit  324 , and a low-pass filter  326 . The impedance measuring unit  340  includes a voltage measuring unit  342  including a first switch (SW 1 ), a second switch (SW 2 ), and a test resistor (R T ), and an analog to digital converter (ADC) unit  344 . 
     The signal processing unit  320  may be the digital amplifier and power-amplifies an arbitrary pulse signal having a predetermined duty cycle into a DC voltage when operating in a headphone impedance measuring mode. When operating in an audio reproduction mode, the signal processing unit  320  extracts an audio signal from a PWM signal generated from a digital audio signal by comparing the audio signal and a carrier signal. 
     The impedance measuring unit  340  connects the test resistor (R T ) between the signal processing unit  320  and the headphone  350  in the headphone impedance measuring mode, and detects a voltage value applied between the test resistor (R T ) and the headphone  350 . 
     The micom unit  310  switches between the headphone impedance measuring mode and the audio reproduction mode according to whether the headphone  350  is connected to the output power controlling apparatus and the digital amplifier (i.e., a digital amplification device). In particular, in the headphone impedance measuring mode, the micom unit  310  controls the signal processing unit  320  to generate the arbitrary pulse signal with the predetermined duty cycle, determines an impedance of the headphone  350  based on the voltage value detected at the test resistor (R T ) by controlling an impedance measuring switch of the impedance measuring unit  340 , and generates a voltage level control signal to control a switching power level of the signal processing unit  320  according to the determined impedance of the headphone  350 . A method of sensing a connection between the headphone  350  and the impedance measuring unit  340  used by the micom unit  310  may be performed using a known technology, for example, by using changes in a voltage level of an output end of the headphone  350 . Other methods including mechanical contact may also be used to sense the connection between the headphone  350  and the impedance measuring unit  340 . 
     Referring to  FIG. 3 , the operation of the apparatus to control the output power of a digital amplifier according to an embodiment of the present general inventive concept will now be explained in more detail. 
     The micom unit  310  outputs an ADC control signal and a switch control signal to the ADC unit  344  and the first and second switches (SW 1 , SW 2 ), respectively, according to whether the output power controlling apparatus is in the headphone impedance measuring mode or the audio reproduction mode. First, if the connection of the headphone  350  is sensed, the micom unit  310  sets the ADC unit  344  and the first and second switches (SW 1 , SW 2 ) to the impedance measuring mode using the ADC control signal and the switch control signal. That is, in the impedance measuring mode, according to the switch control signal, the first and second switches (SW 1 , SW 2 ) connect an output end (terminal) (a) of the low-pass filter unit  326  and an input end (terminal) (a) of the headphone  350  to both ends (terminals) (b) of the test resistor (RT). In the audio reproducing mode, the ADC unit  344  inputs a microphone signal of the microphone  302 . If the operation mode is changed from the audio reproducing mode to the headphone impedance measuring mode, the ADC unit  344  converts a voltage value detected at a test point into a digital signal. The test point may be disposed between the test resistor (RT) and the second switch (SW 2 ). 
     At the same time, the micom unit  310  outputs a test pulse generation control signal to the PWM unit  322 . If the test pulse generation control signal and a mode control signal from the micom unit  310  are received by the PWM unit  322 , the PWM unit  322  outputs the pulse signal with the predetermined duty cycle to the power switching unit  324  for a predetermined time. Then, the power switching unit  324  performs power switching with the pulse signal with the predetermined duty cycle (or duty ratio) output from the PWM unit  322  with respect to the voltage supplied by the power supply unit  323 . The low-pass filter unit  326  includes a coil (L) and a capacitor (C) and converts the pulse signal with the predetermined duty ratio into a DC voltage. Then, the DC voltage output from the low-pass filter unit  326  is applied to the test resistor (R T ) connected to the headphone  350 . Accordingly, the ADC unit  344  converts the voltage value detected between the test resistor (R T ) and the headphone  350  (i.e., at the test point) into a voltage value in a digital form. The digital voltage value is a headphone impedance detection signal, and the ADC unit  344  outputs the headphone impedance detection signal to the micom unit  310 . The micom unit  310  extracts the voltage value from the headphone impedance detection signal, and determines the impedance of the headphone  350  from the voltage value according to a preset table. The preset table may be stored in a memory (not illustrated) and may include a plurality of impedances corresponding to a plurality of voltage values. For example, if the voltage value is in a range of 0.04˜0.05V, the micom unit  310  determines that the impedance of the headphone  350  is 32Ω. If the voltage value is in a range of 0.02˜0.03V, the micom unit  310  determines that the impedance is 16Ω. 
     Then, the micom unit  310  generates a voltage level control signal to control a magnitude of a DC voltage of the power supply unit  323  based on the impedance of the headphone  350  as determined by the impedance measuring unit  340 . Accordingly, the power supply unit  323  outputs a variable DC voltage according to the voltage level control signal generated by the micom unit  310 . For example, if the detected impedance is small, the micom unit  310  outputs the voltage level control signal to control the power supply unit  323  to output the DC voltage to have a small magnitude. Similarly, if the detected impedance is large, the micom unit  310  outputs the voltage level control signal to control the power supply unit  323  to output the DC voltage to have a large magnitude. Then, once the magnitude of the DC voltage is adjusted, the micom unit  310  outputs a control signal to each component of the output power controlling apparatus (e.g., the PWM unit  322 , the power supply unit  323 , the first and second switches (SW, SW 2 ), and the ADC unit  344 ) to switch from the headphone impedance measuring mode to the audio reproduction mode. That is, in the audio reproducing mode, the first and second switches (SW 1 , SW 2 ) connect the output end (a) of the low-pass filter unit  326  with the input end (a) of the headphone  350  according to the switch control signal of the micom unit  310 . The PWM unit  322  compares the audio signal and the carrier signal and generates a PWM signal. The power switching unit  324  amplifies the voltage and current of the PWM signal output from the PWM unit  322  with respect to the DC voltage supplied by the power supply unit  323 . The low-pass filter unit  326  extracts the audio signal to be reproduced by removing a high frequency component of the PWM signal output from the power switching unit  324 . Accordingly, by controlling a switching power level of the power switching unit  324 , the level of the audio signal input to the headphone  350  is adjusted. 
     In another embodiment, when it is difficult to adjust the magnitude of the DC voltage of the power supply unit  323 , the micom unit  310  outputs a digital signal processor (DSP) control signal to control a level of volume appropriate for the detected impedance of the headphone  350  to a DSP unit  370 . Accordingly, the DSP unit  370  adjusts the volume level according to this DSP control signal. 
       FIG. 4  illustrates a measuring circuit in a headphone impedance measuring mode according to an embodiment of the present general inventive concept. 
     Referring to  FIG. 4 , a source unit (‘SOURCE’) is modeling an output signal of a digital amplifier and generates a pulse wave with a predetermined duty cycle. An L/C filter unit  420  removes a high frequency component of the pulse wave through inductive L and capacitive C components, passes only a signal in an audio band, and converts the input signal with the predetermined duty cycle into a signal that approximates a DC signal. The DC signal is provided to a test resistor (R T ) and the headphone impedance  430 . Accordingly, at a voltage detection point  440 , an appropriate voltage is detected according to a ratio of resistance values (i.e., the headphone impedance and the test resistor R T ). The impedance  430  of the headphone is measured by this detected voltage value. Here, since almost only a DC component remains in the voltage passing through the L/C filter unit  420 , the headphone impedance  430  component can be modeled using a circuit diagram with only a resistance component. 
       FIG. 5  illustrates a result of a simulation of the headphone impedance measuring circuit of  FIG. 4 .  FIG. 5  shows voltage values measured at voltage detection point  440  for impedances  430  of 16Ω, 32Ω, 60Ω, and 100Ω. 
     Referring to  FIG. 5 , the voltage value at the voltage detection point  440  varies with respect to the impedance  430  of the headphone. Accordingly, the micom unit  310  can identify the impedance  430  of the headphone through a preset table or the like, from the voltage value detected differently with respect to the impedance  430  of the headphone. For example, if the voltage value is in a range of 0.04˜0.05V, it can be determined that the impedance of the headphone  350  is 32Ω. If the voltage value is in a range of 0.02˜0.03V, it can be determined that the impedance is 16Ω. 
       FIG. 6  is a flowchart illustrating a control method of the micom unit  310  of the output power controlling apparatus of  FIG. 3 , according to an embodiment of the present general inventive concept. 
     First, if a power is turned on, the output power controlling apparatus is initialized to the audio reproducing mode in operation  610 . 
     Then, it is determined (i.e., sensed) whether a headphone (e.g.,  350  in  FIG. 3 ) or earphone is connected to the output power controlling apparatus in operation  620 . A method of sensing a connection of the headphone  350  (i.e., whether a headphone or earphone is connected to the output power controlling apparatus) may be performed by using a known technology, for example, by using changes in a voltage level of an output end of the headphone  350 . 
     If the connection of the headphone  350  to the output power controlling apparatus is sensed, the audio reproducing mode is switched to the headphone impedance measuring mode in operation  630 . 
     A test pulse signal with a predetermined duty cycle is then generated in the headphone impedance measuring mode in operation  640 . 
     Then, the headphone impedance is determined using a preset table based on a voltage value measured between the headphone  350  and a test resistor (e.g., R T ) to which a DC current is applied in operation  650 . 
     Then, with respect to a magnitude of the impedance of the headphone  350 , the magnitude of the DC voltage supplied to a switching power end is adjusted in operation  660 . The headphone impedance measuring mode is switched to the audio reproducing mode in operation  670 . 
       FIG. 7  illustrates a portable audio player (device)  700  according to an embodiment of the present general inventive concept. The portable audio player  700  includes a digital audio unit  705 , a digital amplifier  703 , an output port  701 , and at least one speaker unit  702 . The digital amplifier  703  includes an output power control apparatus  704 . The output power control apparatus  704  may be similar to the output power control apparatus of  FIG. 3  (described above). The digital audio unit  705  provides digital audio to the digital amplifier  703  such that the digital amplifier  703  amplifies the digital audio and outputs a signal to drive the at least one speaker unit  702 . The at least one speaker unit  702  is connected to the portable audio player  700  via the output port  701 . The output power control apparatus  704  operates between an impedance measuring mode (described above) and an audio reproduction mode (also described above) to vary a DC power supply applied to the at least one speaker unit  702  based on a measured impedance of the at least one speaker unit  702 . Accordingly, an optimum sound level can be produced by the portable audio player  700  using a variety of different speaker units having a variety of different impedances. The at least one speaker unit  702  may be a speaker, a headphone, an earphone, etc. 
     The present general inventive concept can be embodied as computer readable codes on a computer readable recording medium. The computer readable recording medium is any data storage device that can store data which can be thereafter read by a computer system. Examples of the computer readable recording medium include read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, and carrier waves (such as data transmission through the Internet). The computer readable recording medium can also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. 
     According to embodiments of the present general inventive concept as described above, an impedance of an earphone or headphone connected to an audio device using a digital amplifier is automatically determined and an appropriate output level can be determined with respect to the impedance. 
     Accordingly, the embodiments of the present general inventive concept utilizes characteristics of a portable audio device and a digital amplifier such that an impedance of an earphone or headphone can be effectively and accurately measured and a resultant optimum sound level can be output without adding a separate unit or increasing material costs. 
     Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.