Abstract:
An audio circuit has an function of removing noise signals before sending audio signals to a sounder. The circuit includes a mute detector for detecting a mute portion of the audio signals, an audio signal amplifier for amplifying the audio signals, and a noise canceler for canceling noise signals mixing with the audio signals by utilizing the noise signals picked up by the mute detector in the mute portion of the audio signals.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an audio circuit for outputting sounds to the outside through a speaker for example. 
   2. Description of the Related Art 
   Recent personal computers are often provided with a display unit made of a liquid crystal panel.  FIG. 2  illustrates a display unit of a typical desktop personal computer. The illustrated display unit  2  includes a liquid crystal display  3  arranged centrally at the front portion of the display unit  2 . The display  3  is flanked by a pair of speakers  4   a ,  4   b  for outputting sounds to the outside. Generally, the speakers  4   a ,  4   b  are connected to an audio circuit (not apparent in  FIG. 2 ) built in a housing of the display unit  2 . The audio circuit feeds audio signals to the speakers  4   a ,  4   b . Recently, the audio circuit generally utilizes a Hi-Fi system for providing high-fidelity sound to promote the so-called audio-visualization in the personal computers. 
   The liquid crystal display  3  generally utilizes a cold cathode ray tube as a back light source driven by an inverter. In controlling the lighting of the cold cathode ray tube by the inverter, pulse signals having a predetermined duty cycle are fed to the cold cathode ray tube. The brightness of the liquid crystal display  3  is adjusted by varying the duty cycle of the pulse signals. 
   As shown in  FIG. 3 , the audio circuit  1  includes a signal input  11  for inputting audio signals to a signal amplifier  12  for amplifying the audio signals. The signal amplifier  12  is connected to the speakers  4   a ,  4   b . The signal amplifier  12  is connected to a ground line  6  to which an inverter  5  of the display unit  2  is connected. 
   In controlling the lighting of the cold cathode ray tube at the inverter  5 , current fluctuates with an amplitude of about 1–2A in response to the pulse signals. Such current fluctuation causes noises to be fed to the signal amplifier  12  through the ground line  6 . Therefore, noise-laden audio signals are outputted to the speakers  4   a ,  4   b , which makes the listener feel unpleasant. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide an audio circuit capable of outputting high quality sound by eliminating noise signals. 
   Another object of the present invention is to provide an audio-visual apparatus, typically a computer system, which incorporates such an audio circuit. 
   According to a first aspect of the present invention, there is provided an audio circuit for outputting audio signals to a sounder. The audio circuit comprises a mute detector for detecting a mute portion of the audio signals, an audio signal amplifier for amplifying the audio signals, and a noise canceler for canceling noise signals mixing with the audio signals by utilizing the noise signals picked up by the mute detector in the mute portion of the audio signals. 
   Preferably, the noise signals are mixed with the audio signals from another electric circuit connected to the audio signal amplifier through a common ground line. 
   Preferably, the noise canceler comprises a noise detector connected to said another electric circuit through the ground line for detecting non-mixing noise signals from said another electric circuit, and the noise canceler combines the non-mixing noise signals with the noise-laden audio signals for canceling the mixing noise signals by the non-mixing noise signals. 
   Preferably, the noise canceler further comprises a noise amplifier for amplifying the non-mixing noise signals in accordance with a level of the mixing noise signals detected by the mute detector in the mute portion of the audio signals and for inverting a phase of the amplified non-mixing noise signals. 
   Preferably, the noise canceler further comprises an adder for adding the phase-inverted non-mixing noise signals from the noise amplifier and the noise-laden audio signals from the signal amplifier. 
   According to a second aspect of the present invention, there is provided an audio-visual apparatus which comprises an audio source for supplying audio signals, an audio signal amplifier connected to the audio source for amplifying the audio signals, a sounder connected to the audio signal amplifier for generating sounds in response to the audio signals, and a display connected to the audio signal amplifier through a common ground line, wherein noise signals from the display mixes with the audio signals through the ground line. The apparatus further comprises a mute detector connected to the audio source and the sounder for detecting a mute portion of the audio signals, and a noise canceler for canceling the mixing noise signals by utilizing the noise signals picked up by the mute detector in the mute portion of the audio signals. 
   Preferably, the noise canceler comprises a noise detector connected to the display through the ground line for detecting non-mixing noise signals from the display, and the noise canceler combines the non-mixing noise signals with the noise-laden audio signals for canceling the mixing noise signals by the non-mixing noise signals. 
   Preferably, the noise canceler further comprises a noise amplifier for amplifying the non-mixing noise signals in accordance with a level of the mixing noise signals detected by the mute detector in the mute portion of the audio signals and for inverting a phase of the amplified non-mixing noise signals. 
   Preferably, the noise canceler further comprises an adder for adding the phase-inverted non-mixing noise signals from the noise amplifier and the noise-laden audio signals from the signal amplifier. 
   Other features and advantages of the present invention will become clearer from the detailed description given below with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing an audio circuit and its peripheral circuit embodying the present invention; 
       FIG. 2  is a perspective view showing a typical display unit; and 
       FIG. 3  is a block diagram showing a prior art audio circuit and its peripheral circuit. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  is a schematic block diagram of an audio circuit and its peripheral circuit embodying the present invention. The audio circuit  1  may be incorporated in a display unit  2  of a personal computer, as shown in  FIG. 2 . The display unit  2  is provided with a liquid crystal display  3  at the front portion thereof. The display  3  is flanked by a pair of speakers  4   a ,  4   b . In this embodiment, each of the speakers  4   a ,  4   b  is a so-called slim speaker having a vertically elongated configuration. The speakers  4   a ,  4   b  include a diaphragm, a voice coil and a magnet unit (not shown). 
   The liquid crystal display  3  is a back light type display which comprises a light source (not shown) such as a cold cathode ray tube. The display unit  2  includes an inverter  5  for driving the cold cathode ray tube by an inverter system. The inverter is connected to a ground line  6  to which the audio circuit  1  is also connected. 
   The audio circuit  1  includes a signal input  11  for inputting audio signals, an audio signal amplifier  12  connected to the signal input  11  for amplifying the audio signals, and a signal adder  13  connected to the audio signal amplifier  12 . The audio circuit  1  further includes a noise detector  14  connected to the ground line  6  for detecting noise signals passing through the ground line  6 , a noise amplifier  15  for amplifying the noise signals detected by the noise detector  14  for output to the signal adder  13 . The audio circuit  1  further includes a mute detector  16  connected to the output side of the signal input  11  and to the output side of the signal adder  13  for detecting mute portions included in the audio signals. 
   The signal input  11  may comprise a connector for example. Audio signals generated in the main unit of e.g. a personal computer (not shown) are inputted to the signal input  11 . The audio signals include not only musical signals but also voice signals. 
   The audio signal amplifier  12 , which may include peripheral devices in addition to the main amplifying unit, amplifies the audio signals inputted through the signal input  11  with an appropriate gain. 
   The noise detector  14  is connected to the ground line  6  to detect the noise signals generated at the inverter  5  and passing through the ground line  6 . 
   The noise amplifier  15  amplifies the noise signals detected by the noise detector  14  with the gain determined by the mute detector  16 . The noise amplifier  15  also functions to invert the phase of the noise signals (180° phase shift) before the signals are fed to the signal adder  13 . 
   The signal adder  13  adds the audio signals outputted from the audio signal amplifier  12  and the noise signals outputted from the noise amplifier  15 . The output from the signal adder  13  is fed to the speakers  4   a ,  4   b.    
   The mute detector  16  detects mute portions in the audio signals inputted through the signal input  11 . Thus, the mute detector  16  determines the level of the noise signals mixed in the mute portions of the audio signals amplified by the audio signal amplifier  12 . The level of the actual audio signals in the mute portions is almost zero, and only the noise signals prevail in the mute portions for allowing easy determination of the noise levels. The mute detector  16  sets, based on the level of the noise signals thus determined, a gain for amplification of the noise signals and supplies the gain setting to the noise amplifier  15 . The noise signals are cancelled in the manner as described hereinafter. 
   The operation of the audio circuit  1  according to the present invention will be described below in detail. 
   First, audio signals for music for example are inputted through the signal input  11  to the signal amplifier  12  for amplification with a predetermined gain. Meanwhile, noise signals generated at the inverter  5  of the display unit  2  are also inputted to the signal amplifier  12  through the ground line  6 . Thus, the noise signals are mixed with the audio signals at the signal amplifier  12  and amplified together with the audio signals. On the other hand, the noise signals passing through the ground line  6  are detected by the noise detector  14 . 
   The mute detector  16  detects a mute portion included in the audio signals inputted through the signal input  11 . The mute portion may be an intermission between different pieces of music or during a single piece of music for example. 
   The audio signals mixed with the noise signals at the signal amplifier  12  are inputted via the signal adder  13  to the mute detector  16 . While the mute detector  16  is detecting a mute portion in which the noise signals prevail, it picks up only the amplified noise signals to determine the gain level for the mixing noise signal amplification. 
   Based on the determined level of the mixing noise signals, the mute detector  16  determines an appropriate gain level for the non-mixing noise signals detected by the noise detector  14  and feeds a gain setting signal to the noise amplifier  15 . 
   Based on the gain setting signal, the noise amplifier  15  adjusts the gain level of the non-mixing noise signals detected by the noise detector  14  so that the level of the non-mixing noise signals provided by the noise amplifier  15  coincides with the level of the mixing noise signals provided by the signal amplifier  12 . The noise amplifier  15  also inverts the phase of the non-mixing noise signals. 
   Subsequently, the non-mixing noise signals whose phase is inverted are fed to the signal adder  13  for adding to the audio-noise signal mixture amplified by the signal amplifier  12 . As a result, the mixing noise signals contained in the audio signals are cancelled by the phase-inverted non-mixing noise signals. Therefore, the audio signals containing no noise signals are outputted to the speakers  4   a ,  4   b.    
   In this way, with the audio circuit  1  according to the present invention, the level of the noise signals mixing with the audio signals is measured when the mute detector  16  detects a mute portion in the audio signals. Then, the non-mixing noise signals having been inverted in phase and amplified to the same extent as the mixing noise signals are added to the mixing noise signals for cancellation. Therefore, the audio circuit  1  according to the present invention can output audio signals which contain no noise signals and therefore have a high quality. 
   The present invention is not limited to the above-described embodiment. For example, the audio circuit  1  of the present invention may be used for any apparatus in which noises are transmitted through a ground line.