Patent Publication Number: US-2011058692-A1

Title: Audio output devices

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an audio output device, and more particularly to an audio output device which eliminates noises. 
     2. Description of the Related Art 
       FIG. 1  shows a conventional audio output device  1 . A power source  10  provides a 12V voltage. Two lower voltages are obtained by converting the 12V voltage, by two regulators  11  and  12 , into a 5V voltage and a 3.3V voltage. The 3.3V voltage is provided to a processor  13  and a digital-to-analog converter (DAC)  14 , while the 5V voltage is provided to an amplifier  15 . Assume that the audio output device  1  is working, and a speaker  16  is operating to produce source according to analog signals from the amplifier  15 . When the power source  10  is turned off and stops providing the 12V voltage, for example when the electric plug of the audio output device  1  is pulled out of the socket, pump noise is produced through the amplifier  15  and the speaker  16 , degrading auditory effect. 
     Thus, it is desired to provide an audio output device which can reduce pump noise when a power source of the audio output device is turned off. 
     BRIEF SUMMARY OF THE INVENTION 
     An exemplary embodiment of an audio output device comprises a power source, a controller, a signal generating circuit, and a first amplifier. The power source provides a supply voltage signal. The controller receives the supply voltage signal. The controller further compares the supply voltage signal with a threshold voltage signal and generates a control signal according to the comparison result. The signal generating circuit generates a first analog signal. The first amplifier receives the first analog signal and generates a first amplified signal according to the control signal. 
     Another exemplary embodiment of an audio output device comprises a power source, a controller, a signal generating circuit, and a first amplifier. The power source provides a supply voltage signal. The signal generating circuit generates a first analog signal. The first amplifier receives the first analog signal. The controller receives the supply voltage signal. The controller further compares the supply voltage signal with a threshold voltage signal and generates a control signal according to the comparison result. When a level of the supply voltage signal is higher than a level of the threshold voltage signal, the first amplifier generates a first amplified signal according to the control signal. When the level of the supply voltage signal is lower than the level of the threshold voltage signal, the first amplifier stops generating the first amplified signal according to the control signal. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  shows a conventional audio output device; 
         FIG. 2  shows an exemplary embodiment of an audio output device; 
         FIG. 3  shows an exemplary embodiment of the amplifier in  FIG. 2 ; 
         FIG. 4  shows an exemplary embodiment of the controller in  FIG. 2 ; and 
         FIG. 5  shows another exemplary embodiment of an audio output device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     Audio output devices are provided. In an exemplary embodiment of an audio output device in  FIG. 2 , an audio output device  2  comprises a power source  20 , regulators  21  and  22 , a controller  23 , a signal generating circuit  24 , an amplifier  25 , and a speaker  26 . The power source  20  provides a supply voltage signal VS 20 . In the embodiment, when the power source  20  is turned on (for example, when the electric plug of the audio output device  2  is put into a socket), the supply voltage signal VS 20  has a level of 12V. The regulator  21  receives the supply voltage signal VS 20  of 12V and generates a voltage signal VS 21  of 5V. The regulator  22  receives the voltage signal VS 21  of 5V and generates a voltage signal VS 22  of 3.3V. The voltage signal VS 21  of 5V is provided to the amplifier  25 , and the voltage signal VS 22  of 3.3V is provided to the signal generating circuit  24 . The controller  23  also receives the supply voltage signal VS 20 . The controller  23  compares the supply voltage signal VS 20  with a threshold voltage signal VTH and generates a control signal S 23  according to the comparison result for controlling the amplifier  25 . The level of the threshold voltage signal VTH is set to be lower than the level of the supply voltage signal VS 20 . In the embodiment, the level of the threshold voltage signal VTH is between 12V and 0V. In a preferred embodiment, the level of the threshold voltage signal VTH is between 12V and 5V. 
     Referring to  FIG. 2 , in the embodiment, the signal generating circuit  24  comprises a processor  240  and a digital-to-analog converter (DAC)  241  which both receive the voltage signal VS 22  of 3.3V. The processor  240  generates a digital signal S 240 . The DAC  241  receives the digital signal S 240  and converts the digital signal S 240  to an analog signal S 241 . The amplifier  25  receives the analog signal S 241 . In the embodiment, whether the amplifier  25  amplifies the analog signal S 241  to generate an amplified signal S 25  is determined according to the control signal S 23  from the controller  23 . The detailed operations of the controller  23  and the amplifier  25  will be described in the following. 
     When the power source  20  is turned on, the power source  20  provides the supply voltage signal VS 20  of 12V. Since the level of the supply voltage signal VS 20  is higher than the level of the threshold voltage signal VTH, the controller  23  de-asserts the control signal S 23  by comparing the supply voltage signal VS 20  and the threshold voltage signal VTH. At this time, the amplifier  25  amplifies the analog signal S 241  to generate the amplified signal S 25  according to the de-asserted control signal S 23 . The speaker  26  receives the amplified signal S 25  and produces sound according to the amplified signal S 25 . 
     Referring to  FIG. 3 , in the embodiment, the amplifier  25  may comprises a mute control unit  30  and an amplifying unit  31 . When receiving the de-asserted control signal S 23 , the mute control unit  30  is disabled, so that the amplifying unit  31  can amplify the analog signal S 241  to generate the amplified signal S 25  for the speaker  26 . 
     When the power source  20  is turned off, the power source  20  stops providing the supply voltage signal VS 20  of 12V, for example when the electric plug of the audio output device  2  is pulled out of the socket. In this situation, the level of the supply voltage signal VS 20  is reduced from 12V to 0V. In the duration of that the level of the supply voltage signal VS 20  is reduced from 12V to 0V, when the level of the supply voltage signal VS 20  is lower than the level of the threshold voltage signal VTH, the controller  23  asserts the control signal S 23  by comparing the supply voltage signal VS 20  and the threshold voltage signal VTH. At this time, the amplifier  25  stops amplifying the analog signal S 241  and generating the amplified signal S 25  according to the asserted control signal S 23 . Thus, the speaker  26  does not receive any signal from the amplifier  25  and does not produce sound. 
     Referring to  FIG. 3 , when receiving the asserted control signal S 23 , the mute control unit  30  is enabled, so that the amplifying unit  31  is disabled by the mute control unit  30  and stops generating the amplified signal S 25  for the speaker  26 . 
     According to the above described embodiment, the threshold voltage signal VTH is set between 12V and 0V. In the duration of that the level of the supply voltage signal VS 20  is reduced from 12V to 0V, before the level of the supply voltage signal VS 20  becomes 0V, the controller  23  directs the asserted control signal S 23  for the amplifier  25  to stop generating the amplified signal S 25 , and the speaker  26  does not produce any more sound. Thus, when the level of the supply voltage signal VS 20  is at 0V, the speaker  26  does not produce pump noise. 
       FIG. 4  shows an exemplary embodiment of the controller  23  of  FIG. 2 . Referring to  FIG. 4 , the controller  23  comprises a comparison unit  40  and a voltage dividing unit  41 . The comparison unit  40  has a first input terminal for receiving the supply voltage signal VS 20  and a second input terminal for receiving the threshold voltage signal VTH. The comparison unit  40  compares the supply voltage signal VS 20  with the threshold voltage signal VTH and generates the control signal S 23  to the amplifier  25 . The voltage dividing unit  41  is coupled between a voltage source VDD and a ground GND and provides the threshold voltage signal VTH. 
     Referring to  FIG. 4 , the voltage dividing unit  41  comprises a plurality of resistors and a plurality of switches. In the embodiment, four resistors R 1 -R 4  and three switches SW 1 -SW 3  are given as an example. The resistors R 1 -R 4  are coupled in series between the voltage source VDD and the ground GND. In the embodiment, the voltage source VDD provides a voltage signal which has the same level as the supply voltage signal VS 20  when the power source  20  is turned on. In other words, the voltage source VDD provides a voltage signal of 12V. The resistors R 1 -R 4  divide the voltage signal of 12V and generate three voltage signals having different levels respectively at the connection nodes N 40 -N 42  between the resistors R 1 -R 4 . Each of the switches SW 1 -SW 3  is coupled to one connection node between the two adjacent resistors and the second input terminal of the comparison unit  40 . For example, the switch SW 1  is coupled between the connection node N 40  between the adjacent resistors R 1 -R 2  and the second input terminal of the comparison unit  40 . Among the switches SW 1 -SW 3 , one switch is tuned on at a time. The voltage signal at the corresponding connection node is transmitted to the second input terminal of the comparison unit  40  to serve as the threshold voltage signal VTH. For example, when the switch SW 2  is turned on, the voltage signal at the connection node N 41  serves as the threshold voltage signal VTH. In the embodiment, the turned-on and turned-off states of the switches SW 1 -SW 3  can be controlled by the processor  240 . Which switch is tuned on at a time is determined by system requirements and/or the specification of the power source  20 . 
     In the embodiment of  FIG. 2 , the signal generating circuit  24  may comprises a plurality of DACs. In the following, two DACs are given as an example. As shown in  FIG. 5 , in addition to the DAC  241 , the signal generating circuit  24  further comprises a DAC  241 _ 1  which is also supplied by the voltage signal VS 22  of 3.3V. The DAC  241 _ 1  is coupled to the processor  240 . The processor  240  further generates a digital signal S 240 _ 1  for the DAC  241 _ 1 . The DAC  241 _ 1  converts the digital signal S 240 _ 1  to an analog signal S 241 _ 1 . For the two DACs in the signal generating circuit  24 , the audio output device  2  further comprises one set comprising an amplifier  25 _ 1  and a speaker  26 _ 1  for the DAC  241 _ 1 . The amplifier  25 _ 1  and the speaker  26 _ 1  perform the same operations as the amplifier  25  and the speaker  26  respectively. The control signal S 23  is used to control both of the amplifiers  25  and  25 _ 1 . 
     When the power source  20  is turned on to provide the supply voltage signal VS 20  of 12V, both of the amplifiers  25  and  25 _ 1  amplify the analog signal S 241  and S 241 _ 1  to generate amplified signals S 25  and S 25 _ 1  according to the de-asserted control signal S 23 , respectively. The speakers  26  and  26 _ 1  receive the amplified signals S 25  and S 25 _ 1  and produces sound according to the amplified signals S 25  and S 25 _ 1 , respectively. 
     In the duration of that the level of the supply voltage signal VS 20  is reduced from 12V to 0V, when the level of the supply voltage signal VS 20  is lower than the level of the threshold voltage signal VTH, both of the amplifiers  25  and  25 _ 1  stop amplifying the analog signal S 241  and S 241 _ 1  and generating the amplified signals S 25  and S 25 _ 1  according to the asserted control signal S 23 , respectively. The speakers  26  and  26 _ 1  do not receive any signal from the amplifiers  25  and  25 _ 1  and thus do not produce sound. Thus, when the level of the supply voltage signal VS 20  is at 0V, the speakers  26  and  26 _ 1  do not produce pump noise. 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.