Patent Publication Number: US-6711270-B2

Title: Audio reproducing apparatus

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an audio reproducing apparatus for reproducing a single-channel or a multichannel audio signal by means of headphones. 
     As a multichannel audio signal, there is known a 5.1-channel digital audio signal employed in the AC-3 (so-called Dolby digital) system. The 5.1-channel digital audio signal S 5 . 1  is generated by encoding the following signals into a piece of serial data. 
     SLF: audio signal for a channel at the left front of the listener 
     SRF: audio signal for a channel at the right front of the listener 
     SCF: audio signal for a channel at the center front of the listener 
     SLB: audio signal for a channel at the left back (or on the left side) of the listener 
     SRB: audio signal for a channel at the right back (or on the right side) of the listener 
     SLFE: audio signal representing a component in a low frequency range below 120 Hz, for example. 
     The digital audio signal S 5 . 1  recorded on, for example, a DVD together with a video signal provides effects such that when the image of the video signal is reproduced, the position of a sound source in the image coincides with that of a sound image that is actually heard, and also a naturally spreading sound field is created. 
     The digital audio signal S 5 . 1  is basically intended for reproduction by means of speakers that are disposed around the listener. However, there has been contrived an audio reproducing apparatus that makes it possible to reproduce a similar reproduction sound field by means of headphones. 
     Specifically, in FIG. 3, reference numeral  200  denotes an example of such an audio reproducing apparatus. Reference numeral  100  denotes a DVD player. 
     Now, a 5.1-channel digital audio signal S 5 . 1  is extracted from the DVD player  100 . The audio signal S 5 . 1  is supplied to a decoder circuit  10  of the audio reproducing apparatus  200 , where digital audio signals SLF to SLFE for respective channels are decoded. Then, the decoded and extracted audio signals SLF and SRF are supplied via adding circuits  21  and  22  to a converting circuit  3 F, which will be described below, while the audio signals SLB and SRB are supplied via adding circuits  23  and  24  to a converting circuit  3 B. 
     Furthermore, the audio signal SCF is basically a signal to be supplied to a speaker disposed at the center front of the listener; however, a similar effect can be obtained even when the signal SCF is supplied to speakers disposed respectively at the left front and at the right front of the listener. Therefore, the signal SCF is supplied to the adding circuits  21  and  22  to be added to the audio signals SLF and SRF. In addition, the audio signal SLFE is a signal representing a low frequency range below 120 Hz, and its reproduced sound does not provide a directional feeling. Thus, the audio signal SLFE is supplied to the adding circuits  21  to  24  to be added to each of the signals SLF to SRB. Signals SLF 0  to SRB 0  are extracted from the adding circuits  21  to  24 . 
     The converting circuit  3 F is provided to convert audio signals SLF 0  and SRF 0  into audio signals that can provide a reproduction sound field similar to that obtained in speaker reproduction even when the audio signals SLF 0  and SRF 0  are to be reproduced by means of headphones. Signal processing by the converting circuit  3 F will hereinafter be referred to as virtual sound image localization processing. 
     Specifically, the converting circuit  3 F processes the original audio signals SLF 0  and SRF 0  in such a way that when output audio signals SLFX and SRFX of the converting circuit  3 F are supplied to the headphones, a reproduction sound field similar to that obtained when the audio signals SLF 0  and SRF 0  before the conversion are supplied to speakers disposed respectively at the left front and at the right front of the listener can be obtained. 
     Similarly, the converting circuit  3 B processes the original audio signals SLB 0  and SRB 0  in such a way that when output audio signals SLBX and SRBX of the converting circuit  3 B are supplied to the headphones, a reproduction sound field similar to that obtained when the audio signals SLB 0  and SRB 0  before the conversion are supplied to speakers disposed respectively at the left back and at the right back of the listener can be obtained. Incidentally, specific configurations of the converting circuits  3 F and  3 B will be described later. 
     Then, an audio signal SLFX from the converting circuit  3 F and an audio signal SLBX from the converting circuit  3 B are added together at an adding circuit  4 L to be converted into a digital audio signal SL for a left channel. Also, an audio signal SRFX from the converting circuit  3 F and an audio signal SRBX from the converting circuit  3 B are added together at an adding circuit  4 R to be converted into a digital audio signal SR for a right channel. 
     Then, these audio signals SL and SR are supplied to D/A converter circuits  5 L and  5 R, where the audio signals SL and SR are subjected to D/A conversion to become analog audio signals SL and SR. The resulting audio signals SL and SR are supplied via amplifiers  6 L and  6 R to acoustic units (electroacoustic transducer devices)  7 L and  7 R of the headphones  7  for a left and a right channel. 
     FIG. 4 shows a specific example of the converting circuit  3 F. The converting circuit  3 F is provided to realize virtual sound image localization processing for the audio signals SLF 0  and SRF 0  by convolving respective transfer functions for ranges from speakers at the left front and at the right front of the listener to both ears of the listener in the audio signals SLF 0  and SRF 0  using digital filters or by signal processing substantially similar to the above method. 
     Specifically, the audio signals SLF 0  and SRF 0  from the adding circuits  21  and  22  are supplied to an adding circuit  31 L and a subtracting circuit  31 R to form a sum signal and a difference signal. The sum signal and the difference signal are supplied to digital filters  32 L and  32 R, respectively. 
     The digital filters  32 L and  32 R are each provided with a delay circuit  321  having a plurality of stages connected in a cascade manner, a plurality of coefficient circuits  322  and adding circuits  323 . The digital filters  32 L and  32 R are of a FIR type. An input signal is supplied to a first stage of the delay circuit  321  to be sequentially delayed, and an output signal from each stage of the delay circuit  321  is supplied to a coefficient circuit  322  to be multiplied by a specified coefficient. The signals resulting from the multiplication are added to each other by an adding circuit  323 , and the result is extracted as a filter output. 
     Then, output signals from the digital filters  32 L and  32 R are supplied to a subtracting circuit  33 L and an adding circuit  33 R to form a difference signal and a sum signal. The difference signal and the sum signal are extracted as output signals SLFX and SRFX of the converting circuit  3 F. 
     Here, respective head related transfer functions for ranges from speakers at the left front and at the right front of the listener to both ears of the listener are convolved in the audio signals SLF 0  and SRF 0  by providing specified characteristics to the digital filters  32 L and  32 R. Incidentally, the converting circuit  3 B can be configured in the same manner as the converting circuit  3 F, and therefore description of the converting circuit  3 B will be omitted. 
     As described above, according to the audio reproducing apparatus  200  of FIG. 3, a reproduction sound field similar to that obtained by means of five speakers disposed at the left front, at the center front, at the right front, in the left rear, and in the right rear of the listener and a speaker for a low frequency range can be reproduced by means of the headphones  7 . 
     However, it has been found that in the case of the audio reproducing apparatus  200  of FIG. 3, reproduced sound outputted from the headphones  7  lacks low frequency sound that should be obtained from the audio signal SLFE representing a component in a low frequency range. According to a consideration given by the inventor of the present invention, it has also been found that the low frequency sound is lacking for the following reasons. 
     Now, as shown in FIG. 5, if a speaker SP 30  is disposed at a position 30° to the right of the front of a dummy head DHD, and frequency characteristics when the left ear and the right ear (microphones) of the dummy head DHD receive sound reproduced by the speaker SP 30  are measured, characteristics as shown in FIG. 6, for example, are obtained. As is clear from the result of the measurement, the level of sound received by the left ear is lower than the level of sound received by the right ear, which is even true of low frequency sound that does not provide a directional feeling (especially a frequency range corresponding to the low frequency range signal SLFE). 
     In addition, since the converting circuit  3 F in the reproducing apparatus  200  of FIG. 3 is provided so that a sound field that would be obtained by speaker reproduction can be reproduced by means of headphones, the converting circuit  3 F also reproduces such decreases in the level of the low frequency range of the signals SLF and SRF for front channels. 
     Thus, in the reproducing apparatus  200  of FIG. 3, even when the low frequency range signal SLFE is added to the audio signals SLF to SRB for respective channels by the adding circuits  21  to  24 , the level of the added low frequency range signal SLFE is lowered by the converting circuit  3 F. As a result, the level of low frequency sound reaching both ears of the listener is lowered. 
     FIG. 7 shows a result obtained when frequency characteristics of the converting circuit  3 F are measured. The result of the measurement represents frequency characteristics of output signals of the converting circuit  3 F obtained when a speaker SP 60  is disposed at a position 60° to the right of the front of a dummy head DHD, and the left ear and the right ear of the dummy head DHD receive sound reproduced by the speaker SP 60 , as shown in FIG.  5 . 
     As is also clear from the result of the measurement, the converting circuit  3 F lowers the level of low frequency ranges of the audio signals SLF and SRF. The same is true of the converting circuit  3 B. 
     Thus, even when the low frequency range signal SLFE is added to the audio signals SLF to SRB by the adding circuits  21  to  24 , the level of the added low frequency range signal SLFE is lowered by the converting circuits  3 F and  3 B. As a result, the level of low frequency sound reaching both ears of the listener is lowered. 
     SUMMARY OF THE INVENTION 
     In accordance with the above, the present invention is intended to prevent lack of low frequency sound to be obtained from an audio signal representing a component in a low frequency range when multichannel stereo reproduction is performed by means of headphones. 
     For example, an audio reproducing apparatus according to the present invention comprises a converting means for converting main audio signals included in an audio signal for a plurality of channels into audio signals that provide a reproduction sound field similar to that obtained in reproduction by means of speakers even when the audio signals are reproduced by means of headphones; and an adding means for adding an auxiliary audio signal included in the audio signal for a plurality of channels to an audio signal outputted from the converting means; wherein an audio signal outputted from the adding means is supplied to headphones. 
     Thus, an audio signal representing a component in a low frequency range is added to audio signals for respective channels without its level being lowered. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a systematic diagram showing an embodiment of the present invention; 
     FIG. 2 is a systematic diagram showing another embodiment of the present invention; 
     FIG. 3 is a systematic diagram of assistance in explaining the present invention; 
     FIG. 4 is a systematic diagram showing an embodiment of a circuit that may be used with the present invention; 
     FIG. 5 is a plan view of assistance in explaining the present invention; 
     FIG. 6 is a characteristic diagram of assistance in explaining the present invention; and 
     FIG. 7 is a characteristic diagram of assistance in explaining the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, reference numeral  300  denotes an example of an audio reproducing apparatus according to the present invention. 
     Now, a 5.1-channel digital audio signal S 5 . 1  is extracted from a DVD player  100 . The audio signal S 5 . 1  is supplied to a decoder circuit  10  of an audio reproducing apparatus  300  to be decoded. Then, digital audio signals SLF to SLFE for respective channels are extracted. 
     The decoded and extracted audio signals SLF and SRF are thereafter supplied to a converting circuit  3 F via adding circuits  21  and  22 , while audio signals SLB and SRB are supplied to a converting circuit  3 B as they are. Also, an audio signal SCF is supplied to the adding circuits  21  and  22  to be added to the audio signals SLF and SRF. 
     The converting circuits  3 F and  3 B are configured as described with reference to FIG. 4, for example. The audio signals SLF to SRB are converted into audio signals that provide a reproduction sound field similar to that obtained in reproduction by speakers even when the audio signals are reproduced by headphones  7 . 
     Then, in an adding circuit  4 L, an audio signal SLFX from the converting circuit  3 F, an audio signal SLBX from the converting circuit  3 B, and an audio signal SLFE from the decoder circuit  10  representing a component in a low frequency range are added together to be converted into a digital audio signal SL for a left channel. 
     Also, in an adding circuit  4 R, an audio signal SRFX from the converting circuit  3 F, an audio signal SRBX from the converting circuit  3 B, and an audio signal SLFE from the decoder circuit  10  representing a component in a low frequency range are added together to be converted into a digital audio signal SR for a right channel. 
     Then, these audio signals SL and SR are supplied to D/A converter circuits  5 L and  5 R, where the audio signals SL and SR are subjected to D/A conversion to become analog audio signals SL and SR. The resulting audio signals SL and SR are supplied via amplifiers  6 L and  6 R to acoustic units  7 L and  7 R of the headphones  7  for a left and a right channel. 
     Thus, according to the audio reproducing apparatus  300 , a reproduction sound field similar to that obtained by means of five speakers disposed at the left front, at the center front, at the right front, in the left rear, and in the right rear of the listener and a speaker for a low frequency range can be reproduced by means of the headphones  7 . 
     Moreover, in this case, the audio signal SLFE representing a component in a low frequency range is added to the audio signals SLFX to SRBX without being passed through the converting circuits  3 F and  3 B, and therefore the level of the audio signal SLFE is not lowered by characteristics of the converting circuits  3 F and  3 B. Thus, it is possible to output low-frequency sound obtained from the low frequency range signal SLFE at its original level. 
     In addition, in order to achieve this effect, it suffices only to configure the reproducing apparatus in such a way that the low frequency range signal SLFE is supplied to the adding circuits  4 L and  4 R, and it is not necessary to add special characteristics to the converting circuits  3 F and  3 B. 
     A reproducing apparatus  300  as shown in FIG. 2 is configured in such a way that the level of components in a low frequency range included in audio signals SLF to SRB as well as an audio signal SLFE from a decoder circuit  10  will not be lowered by converting circuits  3 F and  3 B. 
     Specifically, audio signals SLF to SRB outputted from the decoder circuit  10  are supplied to circuits in the next stage, as in the reproducing apparatus  300  of FIG. 1, while a low frequency range signal SLFE outputted from the decoder circuit  10  is supplied to adding circuits  4 L and  4 R via an adding circuit  86 . 
     The audio signals SLF to SRB outputted from the decoder circuit  10  are also supplied to low-pass filters  81  to  85  respectively, where audio signals representing low frequency range components in the same range as that of the low frequency range signal SLFE are extracted. The resulting signals are supplied to the adding circuit  86  and added to the low frequency range signal SLFE at a specified level or ratio. 
     Thus, the level of components in a low frequency range included in the audio signals SLF to SRB of a digital audio signal S 5 . 1  is also prevented from being lowered by converting circuits  3 F and  3 B. 
     It should be noted that in the embodiment described with reference to FIG. 2, a low-pass filter is provided for each of the audio signals SLF to SRB outputted from the decoder circuit  10 ; however, it is also possible to configure the reproducing apparatus in such a manner that only at least one audio signal among these audio signals SLF to SRB is each provided with a low-pass filter, so that a component in a low frequency range extracted by the low-pass filter is supplied to the adding circuit  86  or the adding circuits  4 L and  4 R. 
     Moreover, in the configuration described above, it is possible to form the converting circuits  3 F and  3 B by using a DSP. It is also possible to include adding circuits  21 ,  22 ,  4 L, and  4 R in the DSP. 
     In addition, when a low frequency range signal SLFE from the decoder circuit  10  is supplied to the adding circuits  4 L and  4 R, a delay equal to the delay time occurring in the converting circuits  3 F and  3 B can be provided for the low frequency range signal SLFE by using a delay circuit. Furthermore, in supplying a low frequency range signal SLFE to the adding circuits  4 L and  4 R, if the level of the low frequency range signal SLFE is increased, for example, by providing a level adjusting circuit, a low frequency range can be boosted independently. 
     It is of course possible to form the delay circuit and the level adjusting circuit by using a DSP. 
     Moreover, connection between the reproducing apparatus  300  and the headphones  7  can be made wireless. In this case, for example, the reproducing apparatus  300  may convert audio signals SL and SR outputted from D/A converter circuits  5 L and  5 R into FM signals, and then transmit the FM signals by using infrared rays as carrier. The headphones  7  may obtain the FM signals by receiving the infrared rays, and demodulate the original audio signals SL and SR from the FM signals. 
     In addition, the above description has dealt with a case where the multichannel audio signal is a 5.1-channel digital audio signal employed in the AC-3 system; however, the present invention is applicable in cases where at least one signal of the multichannel audio signal is an audio signal representing a component in a low frequency range. The present invention is also applicable in cases where there is provided at least one sound source signal in which a component in a low frequency range is included. In this case, it is necessary to provide a low-pass filter in order to extract the component in a low frequency range from the sound source signal. Moreover, the present invention does not specifically limit the method for transmitting the single-channel or the multichannel audio signal, nor the medium for its transmission. Thus, DSB may be used, for example. 
     The following is a list of abbreviations used herein. 
     D/A: Digital to Analog 
     DSB: Digital Sound Broadcasting 
     DSP: Digital Signal Processor 
     DVD: Digital Versatile Disc 
     FM: Frequency Modulation 
     According to the present invention, a reproduction sound field similar to a single-channel or a multichannel stereo reproduction sound field obtained by speaker reproduction can be reproduced by means of headphones. In this case, low frequency sound obtained from an audio signal representing a component in a low frequency range or from a component in a low frequency range included in an audio signal can be outputted at its original level. Besides, it is not necessary to add a special circuit to achieve this effect.