Abstract:
The method and apparatus are provided for several audio signal processing apparatuses to share at least one audio output device. Several audio signals sent from the audio signal processing apparatuses are received. The DC levels of the audio signals are shifted to a predetermined value. One of the adjusted audio signals is then selected for signal-transmitting to the audio output device. Afterwards, the DC level of the selected audio signal is re-adjusted to the predetermined value.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation of U.S. patent application Ser. No. 10/822,921, filed on Apr. 13, 2004, now U.S. Pat. No. 6,934,398. 

   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The invention relates to a switching device and, in particular, to a device for switching audio signals and the method thereof. 
   2. Related Art 
   With the rapid development in information technology, computers and their peripherals become very popular. Computer users often use the mouse and keyboard to control the computers. Through the monitors or speakers, the computer users can monitor the state of the computers. Sometimes a user may have more than one computer to process different types of things. Traditionally, each computer is equipped with one set of input/output (IO) peripheral devices, including the keyboard, mouse, monitor, and speakers. However, this is a waste of money and space if one has several computers. 
   On the other hand, large system businesses or enterprise internal networks often involve tens to thousands servers. Each server needs a monitor, a keyboard and a mouse to for management. In practice, one rarely needs to use these IO peripherals of the servers. Most of the time, the servers do not need to be controlled by the manager. In this situation, it is totally unnecessary, costly, and wasting the space to have a set of IO peripheral devices for each server. 
   Therefore, a switching device that enables one to use one set of IO peripheral devices to manage several computers has been proposed to solve this problem. The use of a switching device does not only save the cost, it also solve the space and compatibility problems. 
   However, conventional switching devices do not have a good performance in switching audio signals. For example, the DC levels of audio signals output from different sound cards may vary. This may result in sound blast during the switching. Moreover, if the switching device also process other IO peripheral devices that consume high power at the same time, the DC level in the switching device will float as a result of the huge power consumption elsewhere. This will generate the problem of audio interference. 
   Moreover, conventional switching devices often use mechanical relays to switch. However, the mechanical switching device has a limited lifetime. It is likely to have spark during the switching. Therefore, it may cause damages to the switching device or even hurt the devices inside the computer. 
   SUMMARY OF THE INVENTION 
   An objective of the invention is to provide a method for switching audio signals to adjust the DC level of audio signals. Therefore, the DC levels of the audio signals on both ends of the multitasking switch are remained fixed. This avoids sound blasts caused by a level difference when switching the audio signals or an interference problem due to other high power loads. 
   Another objective of the invention is to provide a device for switching audio signals. It uses a DC level filter circuit and a DC level adjusting circuit along with a chip to switch audio signals. In addition, to avoid sound blasts and interference, the lifetime of the disclosed switching device is longer. It also prevents the production of sparks or burst waves that damages the computer devices. 
   In accord with the above objectives, the invention provides a method for switching audio signals and the device thereof. An audio signal output device is shared by a plurality of first audio signal processing devices. The disclosed method first receives a plurality of first audio signals sent from the plurality of first audio signal processing devices and adjusts the DC levels of the first audio signals to a first predetermined value to obtain a plurality of second audio signals. Afterwards, one of the second audio signals is selected. The DC level of the selected second audio signal is adjusted to the first predetermined value to obtain a third audio signal. 
   The disclosed device includes a plurality of first pre-processing devices connecting to the first audio signal processing devices, a first multitasking switch and at least one post-processing device. Each of the first pre-processing devices receives a first audio signal from the connected first audio signal processing device and adjusts its DC level to a first predetermined value to obtain a second audio signal. The first multitasking switch receives the second audio signals from the first pre-processing devices and selects one of them for output. The first post-processing device receives the selected second audio signal and adjusts its DC level to the first predetermined value to obtain a third audio signal. 
   According to a preferred embodiment of the invention, each of the first pre-processing devices contains a first DC level filter circuit and a first DC level adjusting circuit. The first DC level filter circuit removes the DC level of the first audio signal. The first DC level adjusting circuit receives the first audio signal with the DC level removed from the first DC level filter circuit and adjusts the DC level of the DC-level-filtered first audio signal to the first predetermined value. 
   The first post-processing device contains a second DC level adjusting circuit and a second DC level filter circuit. The second DC level adjusting circuit receives the second audio signal from the first multitasking switch and adjusts the DC level of the second audio signal to the first predetermined value. The second DC level filter circuit receives the DC-level-adjusted second audio signal from the second DC level adjusting circuit and removes the DC level of the DC-level-adjusted second audio signal. The final signal is output to the audio signal output device. 
   The first DC level filter circuit contains a first capacitor, and the second DC level filter circuit contains a second capacitor. The first DC level adjusting circuit contains a first resistor and a second resistor. One end of the first resistor is in electrical communications with a high level; the other end of the first resistor is in electrical communications with one end of the second resistor; and the other end of the second resistor is in electrical communications with a low level. The second DC level adjusting circuit contains a third resistor and a fourth resistor. One end of the third resistor is in electrical communications with a high level; the other end of the third resistor is in electrical communications with one end of the fourth resistor; and the other end of the fourth resistor is in electrical communications with a low level. The other end of the first resistor further electrically connects to the first DC level filter circuit and the first multitasking switch. The other end of the third resistor also electrically connects to the second DC level filter circuit and the first multitasking switch. 
   Moreover, in the preferred embodiment of the invention, the first resistor and the third resistor have the same resistance, and the third resistor and the fourth resistor have the same resistance. The first multitasking switch is a multitasking switch chip. The high level is provided by a voltage adjuster. The low level is a ground level. When the first multitasking switch is a positive-voltage multitasking switch chip, the resistance of the first resistor is less than that of the second resistor and the resistance of the third resistor is less than that of the fourth resistor. When the first multitasking switch is a positive-negative-voltage multitasking switch chip, the resistance of the first resistor is equal to that of the second resistor and the resistance of the third resistor is equal to that of the fourth resistor. 
   According to another embodiment of the invention, the disclosed audio signal switching device can enable a plurality of second audio signal processing devices to share at least one audio signal input device. In this case, the audio signal switching device further contains a second pre-processing device, a second multitasking switch, and a plurality of second post-processing devices. The second pre-processing device connects to the audio signal input device to receive a fourth audio signal from the audio signal input device. It further adjusts the DC level of the fourth audio signal to a second predetermined value to obtain a fifth audio signal. The second multitasking switch receives the fifth audio signal from the second pre-processing device. The second multitasking switch selects the second post-processing devices to input the fifth audio signal. After each of the second post-processing devices receives the fifth audio signal, the DC level of the fifth audio signal is adjusted to the second predetermined value to obtain a sixth audio signal. 
   The disclosed switching device is formed by adding a set of DC level filter circuit and DC level adjusting circuit on both ends of a multitasking switch. After an audio signal enters the switching device, its DC level is adjusted. After the audio signal passes through the multitasking switch, its DC level is adjusted again so that the DC levels of the audio signal on both ends of the multitasking switch are kept fixed. This can avoid the production of sound blasts because of the level difference during audio signal switching or sound interference due to other high-power loads. The invention can use a multitasking switch chip to switch the audio signals. This can elongate the lifetime of the switching device and prevent the production of sparks or burst waves that hurt the devices. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the invention will become apparent by reference to the following description and accompanying drawings which are given by way of illustration only, and thus are not limitative of the invention, and wherein: 
       FIG. 1  is a schematic view of a preferred embodiment of the invention in practice; 
       FIG. 2  is a flowchart of a preferred embodiment of the disclosed method; 
       FIG. 3  is a schematic view of the audio signal switching device in  FIG. 1 ; 
       FIG. 4  is a schematic view of a preferred embodiment of the disclosed KVM switch in the audio signal switching device; 
       FIG. 5  is a schematic view of another embodiment in practice; and 
       FIG. 6  is a schematic view of a part of the audio signal switching device in  FIG. 5 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   As shown in  FIG. 1 , the audio signal processing devices  106   a ,  106   b ,  106   c ,  106   d  share at least one audio signal output device  102  using the disclosed audio signal switching device  104 . The audio signal processing devices  106   a ,  106   b ,  106   c ,  106   d  can be sound cards, musical instrument digital interface (MIDI) devices, stereos, sound sources, or televisions. The audio signal output device  102  can be speakers, headphones, microphones, amplifiers, or even the input terminal of other audio signal processing devices. 
     FIG. 2  shows the flowchart of a preferred embodiment of the disclosed method. The audio signal switching device  104  uses a multitasking switch to select one of the audio signal processing devices  106   a ,  106   b ,  106   c ,  106   d . The selected audio signal processing device uses the audio signal output device  102  to output audio signals. The disclosed method first receives the audio signals sent from the audio signal processing devices  106   a ,  106   b ,  106   c ,  106   d  (step  202 ). The DC levels of the audio signals are filtered (step  204 ). The DC levels of the filtered audio signals are adjusted (step  206 ). After the audio signals pass through the multitasking switch (step  208 ), the DC level of the selected audio signal is adjusted again (step  216 ). Afterwards, the DC level of the re-adjusted audio signal is filtered. 
     FIG. 3  is a schematic view of a part of the audio signal switching device in  FIG. 1 . As described above, the disclosed audio signal switching device connect several audio signal processing devices and at least one audio signal output device. In order to clearly explain the contents of the invention, we only draw one audio signal processing device  106  and one audio signal output device  102  on both ends of the audio signal switching device  104 . 
   The disclosed audio signal switching device  104  contains several pre-processing devices, each of which connects to an audio signal processing device. We use only one set of pre-processing device  306  and audio signal processing device  106  to demonstrate the essence of the invention in  FIG. 3 . The audio signal switching device  104  also contains at least one post-processing device  302  for connecting to the audio signal output device  102 . It uses the multitasking switch  304  to control the switches between the preprocessing device  306  and the post-processing device  302 . Therefore, only a particular audio signal processing device  106  can use the audio signal output device  102  to output audio signals. 
   After the audio signal enters the audio signal switching device  104  from the audio signal processing device  106 , the first DC level filter circuit  342  uses the capacitor  348  to remove the DC level of the audio signal. Afterwards, the first DC level adjusting circuit  332  adjusts the DC level of the audio signal. In this embodiment, the first DC level adjusting circuit  332  contains a first resistor  336  and a second resistor  334 , using the partial voltage principle of resistors to adjust the DC level of the audio signal. One end of the first resistor  336  is in electrical communications with a high level  356 , the other end of the first resistor  336  is in electrical communications with one end of the second resistor  334 , and the other end of the second resistor  334  is in electrical communications with a low level  354 . 
   The DC level adjusted audio signal is sent to the post-processing device  302  after pass through the path assigned by the multitasking switch  304 . In the post-processing device  302 , its DC level is adjusted again by the second DC level adjusting circuit  312 . Likewise, the second DC level adjusting circuit  312  contains a third resistor  316  and a fourth resistor  314 , using the partial voltage principle of resistors to adjust the DC level of the audio signal. One end of the third resistor  316  is in electrical communications with the high level  356 , the other end of the third resistor  316  is in electrical communications with one end of the fourth resistor  314 , and the other end of the fourth resistor  314  is in electrical communications with the low level  354 . Finally, the DC level re-adjusted audio signal passes through the second DC level filter circuit  322 , using the capacitor  328  to filter out the DC level of the audio signal. 
   The sound blast occurring in the conventional audio signal switching device is simply caused by the fact that the DC level of an audio signal on both ends of the multitasking switch are different when switching among different pre-processing devices and post-processing devices. The invention utilizes the combination of two DC level filter circuit and DC level adjusting circuit  342 / 332  and  322 / 312  to fix the DC levels on both ends of the multitasking switch  304 . This avoids the production of sound blasts during audio signal switching. 
   In the preferred embodiment, the first resistor  336  and the third resistor  316  are both in electrical communications with the same high level  356  (3.3V), and the second resistor  334  and the fourth resistor  314  are both in electrical communications with the same low level  354  (ground). Therefore, the resistances of the first resistor  336  and the third resistor  316  are the same, while those of the second resistor  334  and the fourth resistor  314  are the same. This configuration makes the DC levels of the audio signal on both ends of the multitasking switch  304  fixed. 
   In order to elongate the lifetime of the switching device and to avoid the generation of sparks or burst waves that damage the device, the multitasking switch  304  in the preferred embodiment is a multitasking switch chip. The multitasking switch chip can be a positive-voltage multitasking switch chip driven by a positive voltage or a positive-voltage multitasking switch chip driven by both positive and negative voltages. The positive-voltage multitasking switch chip has a smaller volume. Along with the disclosed audio signal DC level adjusting function, it can be used in a small-size simple audio signal switching device. The positive-negative-voltage multitasking switch chip has a better sound quality and can avoid the crosstalk problem. It is suitable for high-price high-quality audio signal switching devices. 
   When the multitasking switch  304  is a positive-voltage multitasking switch chip, the resistance of the first resistor  336  is less than that of the second resistor  334 , and the resistance of the third resistor  316  is less than that of the fourth resistor  314 . According to a preferred embodiment of the invention, the resistances of the first resistor  336  and third resistor  316  are 5.6 k.OMEGA. and those of the second resistor  334  and the fourth resistor  314  are 10 k.OMEGA. This configuration fixes the DC level of the audio signal on both ends of the multitasking switch  304  to about 2V. 
   When the multitasking switch  304  is a positive-negative-voltage multitasking switch chip, the resistance of the first resistor  336  is equal to that of the second resistor  334 , and the resistance of the third resistor  316  is equal to that of the fourth resistor  314 . According to another preferred embodiment of the invention, the resistances of the first resistor  336 , the second resistor  334 , third resistor  316 , and the fourth resistor  314  are all 10 k.OMEGA.). This configuration fixes the DC level of the audio signal on both ends of the multitasking switch  304  to about 0V. 
   In the current preferred embodiment, the disclosed audio signal switching device is combined inside a keyboard-video-mouse (KVM) switch. The KVM switch enables a user to use one set of several sets of IO peripheral devices to manage several computers.  FIG. 4  is a schematic view of a preferred embodiment of a KVM switch with the disclosed audio signal switching device. The KVM switch  404  switches among multiple computers  406  and at least one user  402 . In this embodiment, to avoid the loads of other high-power devices (e.g. an optical mouse) that cause fluctuations in the level of the KVM switch and therefore the interference sound problem, the invention is powered by a voltage regulator whose low level is the ground level. 
     FIG. 5  shows a schematic view of another embodiment in practice. In this embodiment, in addition to using the audio signal switching device  504  to make an audio signal output device  102  selectively receive audio signals sent from the audio signal processing devices  106   a ,  106   b ,  106   c ,  106   d , the user can further use the same audio signal switching device  504  to control an audio signal input device  502 , such as a microphone or a pre-processing device of the audio signal input device. Thus, another audio signal can be input to the audio signal processing devices  106   a ,  106   b ,  106   c ,  106   d  or some other audio signal processing device  106 . 
     FIG. 6  shows a part of the audio signal switching device in  FIG. 6 . In the following, we only explain the part of audio signal input by a user. Other parts are the same as the audio signal switching device in  FIG. 3 . As described above, the disclosed audio signal switching device connects several audio signal processing devices and at least one audio signal input device. In order to concentrate on the essence of the invention, we only draw an audio signal processing device  106  and an audio signal input device  502  on both ends of the audio signal switching device  504 . 
   The audio signal switching device  504  according to the invention has several post-processing devices, each of which connects to an associated audio signal processing device. We show only one set of post-processing device  602  and audio signal processing device  106  for demonstration purposes. The audio signal switching device  504  also contains at least one pre-processing device  606  for connections with the audio signal input device  502 . The multitasking switch  604  is used to control the path switch between the pre-processing device  606  and the post-processing device  602 . This enables the audio signal input device  502  to input an audio signal to some audio signal processing device  106 . 
   The audio signal enters the audio signal switching device  504  via the audio signal input device  502 , such as a microphone. As the audio signals received by normal microphones are generally very weak, we use a high level  656  with a resistor  662  to amplify them. Afterwards, the first DC level filter circuit  642  uses the capacitor  648  to remove the DC level in the audio signal. 
   The first DC level adjusting circuit  632  further adjusts the DC level of the audio signal. In the current embodiment, the first DC level adjusting circuit  632  contains a first resistor  636  and a second resistor  634 , using the partial voltage principle of resistors to adjust the DC level of the audio signals. One end of the first resistor  636  is in electrical communications with a high level  656 , the other end of the first resistor  636  is in electrical communications with one end of the second resistor  634 , and the other end of the second resistor  634  is in electrical communications with a low level  654 . 
   After passing the path assigned by the multitasking switch  604 , the DC level adjusted audio signal is sent to the post-processing device  602  associated with some audio signal processing device  106 , where its DC level is adjusted again by the second DC level adjusting circuit  612 . Likewise, the second DC level adjusting circuit  612  contains a third resistor  616  and a fourth resistor  614 , using the partial voltage principle of resistors to adjust the DC level of the audio signals. One end of the third resistor  616  is in electrical communications with the high level  656 , the other end of the first resistor  636  is in electrical communications with one end of the fourth resistor  614 , and the other end of the fourth resistor  614  is in electrical communications with the low level  654 . Finally, the DC level re-adjusted audio signal passes through the second DC level filter circuit  622 , whose capacitor  628  removes the DC level of the audio signal. 
   Likewise, when the multitasking switch  604  is a positive-voltage multitasking switch chip, the resistance of the first resistor  636  is less than that of the second resistor  634 , and the resistance of the third resistor  616  is less than that of the fourth resistor  614 . According to a preferred embodiment of the invention, the resistances of the first resistor  636  and the third resistor  616  are both 15 k.OMEGA. The resistances of the second resistor  634  and the fourth resistor  614  are both 27 k.OMEGA. 
   When the multitasking switch is a positive-negative-voltage multitasking switch chip, the resistance of the first resistor  636  is equal to that of the second resistor  634 , and the resistance of the third resistor  616  is equal to that of the fourth resistor  614 . According to another preferred embodiment of the invention, the resistances of the first resistor  636 , the second resistor  634 , third resistor  616 , and the fourth resistor  614  are all 10 k.OMEGA. 
   While the invention has been described by way of example and in terms of the preferred embodiment, 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.