Abstract:
An audio reproducing apparatus includes: a decoding module configured to perform a decoding process on a digital broadcast signal demodulated by a demodulating module and generate an audio signal; a generating module configured to generate suspension audio data based on the audio signal of a first given time period, the audio signal being output from the decoding module; a switching module configured to switch a source of the audio signal to be output from the audio output module between the decoding module and a storage module; and a control module configured to control the switching module to switch the source to the storage module for causing an audio output module to output selected suspension audio data to be faded-out when a suspension of the decoding by the decoding module is occurred, the selected suspension audio data being the suspension audio data stored immediately before the suspension.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     The present disclosure relates to the subject matters contained in Japanese Patent Application No. 2008-333140 filed on Dec. 26, 2008, which are incorporated herein by reference in its entirety. 
     FIELD 
     The present invention relates to an audio reproducing technology and to an audio reproducing apparatus for suitably reproducing digital audio data. 
     BACKGROUND 
     In recent years, various apparatus which are equipped with a tuner unit for receiving digital broadcasts such as digital TV broadcasts and digital radio broadcasts, have been introduced into the market. Portable apparatus such as notebook personal computers and cell phones have come to be equipped with such a tuner unit to allow users to receive digital broadcasts via wireless transmission path even while a user is out of the door and while the user is moving. 
     Although capable of receiving digital broadcasts during movement, these apparatus sometimes suffer from a loss of data in a wireless transmission path. This is because broadcast waves are interrupted by an obstacle such as a building, a tree, a bridge, or a tunnel as the apparatus passes the shadow of the obstacle during the movement. 
     When errors occur in video data or audio data due to a loss of data, picture disorder or a sound interruption occurs in the apparatus. An uncomfortable feeling of the viewer due to picture disorder among these problems can be relieved relatively easily and effectively by, for example, maintaining a still image. 
     In contrast, it is difficult to reduce an uncomfortable feeling due to a sound interruption by simple processing such as mere muting. 
     In connection with the above, an audio decoding device is known which can suppress degradation in auditory feeling by reducing a sound interruption feeling caused by muting. An example of such device is disclosed in JP-A-7-336311. This device can reduce an uncomfortable feeling due to a sound interruption by replacing immediately preceding coding parameters with coding parameters of a past silent frame when code errors are detected in consecutive frames. 
     In conventional techniques as typified the one disclosed in the publication, JP-A-7-336311, attention is focused on how to compensate for, that is, whether to repair or to replace, errors in audio data. These replacement (interpolation) techniques are effective in the case where errors in audio data last only a very short time. However, when errors in audio data last long time because the apparatus receiving a broadcast enters a tunnel, managing to compensate for a silent portion may produce an unnatural sound contrary to the intention. As such, at present, the replacement techniques for compensating for a silent portion are not necessarily used effectively. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A general configuration that implements the various feature of the invention will be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention. 
         FIG. 1  is a perspective view, in a state that a display unit is opened, of a notebook personal computer (computer) which is an embodiment of the audio reproducing apparatus according to the present invention. 
         FIG. 2  is a block diagram showing an example system configuration of the computer according to the embodiment. 
         FIG. 3  is a functional block diagram of a TV tuner unit of the computer according to the embodiment. 
         FIG. 4  is a functional block diagram showing the configuration of an MPEG decoder of the computer according to the embodiment. 
         FIG. 5  is a functional block diagram showing the configuration of an audio decoder of the MPEG decoder. 
         FIG. 6  is a flowchart of an audio signal output process (for suspension) which is performed by the computer according to the embodiment. 
         FIGS. 7A and 7B  are graphs showing how the audio volume varies when audio output is suspended. 
         FIG. 8  is a flowchart of an audio signal output process (for restart) which is performed by the computer according to the embodiment. 
         FIGS. 9A and 9B  are graphs showing how the audio volume varies when audio output is restarted. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     An embodiment of the audio reproducing apparatus according to the present invention will be hereinafter described with reference to the accompanying drawings. 
     In the embodiment, the audio reproducing apparatus according to the invention is implemented as a battery-powered, portable notebook personal computer. 
       FIG. 1  is a perspective view, in a state that a display unit  12  is opened, of a notebook personal computer (hereinafter simply called “PC”)  1  which is an embodiment of the audio reproducing apparatus according to the invention. 
     The main body of the PC  1  is provided with a base unit  11  and the display unit  12 . The display unit  12  has a display device, such as an LCD (liquid crystal display) panel  13 . The display screen of the LCD panel  13  is disposed approximately at the center of the display unit  102 . 
     The display unit  12  is connected to and supported by a rear end portion of the base unit  11 . The display unit  12  as a second body is attached so as to be rotatable between an open position where it exposes the top face  20  of the base unit  11  and a closed position where it covers the top face  20  of the base unit  11 . The base unit  11  has a thin, box-shaped body, and a keyboard  14 , a power button  15  for powering on or off the PC  1 , and a touch pad  16  are arranged on its top face  20 . 
     Two speakers  17   a  and  17   b  are arranged on a rear end portion of the top face  20  of the base unit  11  so as to be directed upward. The speakers  17   a  and  17   b  function as an L-channel speaker and an R-channel speaker, respectively. 
     A side surface  23  of the base unit  11  is provided with a headphone jack  24 . When the plug (not shown) of a headphone is inserted into the headphone jack  24 , audio can be output from the headphone as from the speakers  17   a  and  17   b.    
       FIG. 2  is a block diagram showing a system configuration of the PC  1  according to the embodiment. 
     The PC  1  according to the embodiment is mainly equipped with a CPU  30 , a host hub  31 , a main memory  32 , a graphics controller  33 , the LCD panel  13 , an I/O hub  34 , a hard disk drive (HDD)  35 , a sound controller  36 , the speakers  17   a  and  17   b , a DVD drive  37 , a BIOS-ROM  38 , an LPC bus  39 , an embedded controller/keyboard controller (EC/KBC)  45 , the keyboard  14 , the touch pad  16 , a power controller  50 , a PCI bus  47 , and a TV tuner unit  48 . 
     The CPU  30  is a processor which controls the operation of the PC  1 . The CPU  201  runs an operating system and various programs including utility programs that are loaded onto the main memory  32  from the HDD  35 . 
     The host hub  31  is abridge device which connects a local bus of the CPU  30  and the I/O hub  34 . The host hub  31  incorporates a memory controller for controlling the main memory  32 . The graphics controller  33  which controls the LCD panel  13  is connected to the host hub  31 . 
     The I/O hub  34  functions as an I/O controller which controls various I/O devices connected to the LPC bus  39  and the PCI bus  47 . The I/O hub  34  incorporates an IDE for controlling the HDD  35  and the DVD drive  37 . The I/O hub  34  controls access to the BIOS-ROM  38  and the sound controller  36  for controlling the speakers  17   a  and  17   b . The BIOS-ROM  38  is a flash ROM for storing a system BIOS in an electrically rewritable manner. 
     The EC/KBC  45  is a one-chip microcomputer in which an embedded controller (EC) for communicating with the power controller  50  which controls the power supply to the individual sections of the PC  1  and a keyboard controller for controlling the keyboard  14  and the touch pad  16  are integrated together. The PC  1  can be powered by either of a power supplied from a battery (not shown) and a power supplied from an external power source (not shown). The power controller  50  communicates, to the EC/KBC  45 , power information such as the presence/absence of power supply from the external power source and the residual energy of the battery. 
     The TV tuner unit  48  receives digital TV broadcast data, performs demodulation and decoding on the received data, and outputs resulting data to the PCI bus  47 . Among digital TV broadcast data that are output to the PCI bus  47  from the TV tuner unit  48 , video data is supplied to the LCD panel  13  via the graphics controller  33 . Audio data is supplied to the speakers  17   a  and  17   b  via the sound controller  36 . While the digital TV broadcast data are being recorded, the video data and the audio data are output to the HDD  35  or the DVD drive  37 . 
       FIG. 3  is a functional block diagram of the TV tuner unit  48 . 
     In the embodiment, the TV tuner unit  48  functions as a receiver module for receiving a digital broadcast signal including an audio signal and a demodulating module the digital broadcast signal received by the receiver module. 
     A signal that is transmitted over a wireless transmission path and input via an antenna  61  is converted into an IF signal by a tuner  62 , which is output at a given level from an AGC (automatic gain control) circuit which is incorporated in the tuner  62 . A demodulator  63  converts a digital broadcast signal that is input in an analog signal format into a signal having a digital signal format and performs error detection and correction. The demodulator  63  outputs ordinary digital data that can be expressed by 0s and 1s. 
     The digital data that is output from the demodulator  63  is supplied to an MPEG de coder  65  as a TS (transport stream) signal. 
       FIG. 4  is a functional block diagram showing the configuration of the MPEG decoder  65  of the PC  1  according to the embodiment. 
     In the embodiment, the MPEG decoder  65  functions as a decoding module for decoding a digital broadcast signal demodulated by the demodulating module and thereby outputting an audio signal. 
     The TS signal produced by the demodulator  63  is separated into respective streams as a video TS, an audio TS, and a system TS by a demultiplexing module (switch  66 ). Data of the TSs thus separated from each other are accumulated in respective TS buffers  71 - 73 . The accumulated data are output from the TS buffers  71 - 73  in the same order as the input order, converted into ESs (elementary streams), and accumulated in respective STD (system target decoder) buffers  74 - 76 . 
     System information is extracted by a system decoder  79  from data that is readout from the STD buffer  76 , and pieces of information that are necessary for video decoding and audio decoding are sent to a video decoder  77  and an audio decoder  78 , respectively. If necessary, the system decoder  79  controls the video decoder  77 , the audio decoder  78 , and the switch  66  based on an instruction received through the keyboard  14 , for example. 
     The video decoder  77  receives a video ES from the STD buffer  74  and decodes it into video data, that is, a video signal. The audio decoder  78  receives an audio ES from the STD buffer  75  and decodes it into audio data, that is, an audio signal. 
     In general, video data and audio data that are used in digital broadcast are compressed. Therefore, input video data and audio data cannot be output-transferred consecutively to the LCD panel  13  and the speakers  17   a  and  17   b . Instead, data that amount to a given size are collected and then the collected data are expanded and output. 
     In the case of video data, data portions may actually be output to the LCD panel  13  in different order than are received and output to the video decoder  77 . Therefore, the times necessary for video data and audio data to become ready for output to the LCD panel  13  and the speakers  17   a  and  17   b  may be different from each other. 
     In view of the above, to synchronize an output video signal and audio signal, time information called a PTS (presentation time stamp) is added to each video data and each audio data. By virtue of the addition of PTSs, video data and audio data can be output when a clock (not shown) of the MPEG decoder  65  comes to correspond to a PTS. 
     Next, a description will be made of the configuration of the audio decoder  78  of the MPEG decoder  65 . The audio decoder  78  according to the embodiment is equipped with an audio data generating module  80  which generates suspension audio data and restart audio data and outputs them when necessary. 
       FIG. 5  is a functional block diagram showing the configuration of the audio decoder  78  of the MPEG decoder  65 . 
     An audio signal that is output from an audio decoder  78  is supplied to the speakers  17   a  and  17   b  and output for the user of the PC  1 . At the same time, the audio signal is supplied to the audio data generating module  80 . 
     The audio data generating module  80  is equipped with an A/D converter (ADC)  81 , a data generating module (FFT)  82 , a buffer  83 , a D/A converter (DAC)  84 , and a volume controller  85 . 
     The A/D converter  81  converts an audio signal of a given time period that is output from the audio decoder  78  into an audio signal having a digital signal format. 
     The data generating module  82  performs a frequency analysis on the audio signal of the given time period by FFT (fast Fourier transform). The data generating module  82  generates the suspension audio data or the restart audio data having frequency components obtained by the frequency analysis. In the embodiment, the data generating module  82  functions as a generating module for generating the suspension audio data and the restart audio data. 
     The buffer  83  temporarily stores the suspension audio data and the restart audio data generated by the data generating module  82 . In the embodiment, the buffer  83  functions as a storing module for storing the suspension audio data and the restart audio data. 
     The D/A converter  84  converts the suspension audio data and the restart audio data stored in the buffer  83  into an audio signal having an analog signal format. 
     The volume controller  85  adjusts the volume of the audio signal that is output from the D/A converter  84 . The audio signal that is output from the volume controller  85  is supplied to the speakers  17   a  and  17   b.    
     The speakers  17   a  and  17   b  are provided with a switch  86 . The switch  86  has a function of switching the supply source of the audio signal to be output from the speakers  17   a  and  17   b  between the audio decoder  78  and the audio data generating module  80 . In the embodiment, the switch  86  functions as a switching module. 
     In the embodiment, the audio decoder  78  having the audio data generating module  80  functions as a control module. 
     Next, an audio signal output process which is performed by the PC  1  according to the embodiment will be described. First, a description will be made of a process that is performed in a case that normal audio data cannot be obtained due to, for example, a degradation of the receiving situation of a digital TV broadcast signal received by the tuner  62  of the PC  1 . 
       FIG. 6  is a flowchart of an audio signal output process (for suspension) which is performed by the PC  1  according to the embodiment. 
     For example, this audio signal output process is performed when the PC  1  receives a digital TV broadcast signal and outputs a video signal and an audio signal to the LCD panel  13  and the speakers  17   a  and  17   b  or a headphone (not shown) that is connected to the PC  1  via the headphone jack  24 . 
     At step S 1 , it is determined whether audio data is normal. If it is determined that audio data is normal, at step S 2  the audio decoder  78  decodes the audio data and produces an audio signal. 
     At step S 3 , the audio decoder  78  switches the switch  86  to the audio decoder side so that the audio signal that is output from the audio decoder  78  can directly be output from the speakers  17   a  and  17   b , for example. 
     At step S 4 , the audio decoder  78  monitors PTSs that are attached to the audio data and determines whether the current time that is based on the clock of the MPEG decoder  65  coincides with or has passed a PTS time. If determined that the current time coincides with or has passed a PTS time, at step S 5  the audio decoder  78  causes the speakers  17   a  and  17   b  to output the audio signal. On the other hand, if determined that the current time has not reached a PTS time, the audio decoder  78  stands by until the current time reaches a PTS time. 
     At step S 6 , the audio signal that is output from the audio decoder  78  is also output to the audio data generating module  80 . The audio data generating module  80  generates and stores suspension audio data. More specifically, when an audio signal of a given time period has been supplied to the audio data generating module  80 , the A/D converter  81  converts the audio signal of the given time period into an audio signal having a digital signal format. 
     The data generating module  82  performs a frequency analysis on the audio signal of the given time period by FFT. Furthermore, the data generating module  82  generates suspension audio data having frequency components obtained by the frequency analysis and stores it in the buffer  83  (temporary storage). 
     On the other hand, if determined at step S 1  that the audio data is not normal and the supply of audio data to be decoded is interrupted, at step S 7  the audio decoder  78  determines whether all of an audio signal that has been decoded by the occurrence of interruption has been output from the audio decoder  78 . 
     An event of interruption of supply of audio data occurs when data is lost (errors occur) in a wireless transmission path because, for example, broadcast waves are interrupted by an obstacle such as a building, a tree, a bridge, or a tunnel as the PC  1  passes the shadow of the obstacle. 
     If determined that not all of the audio signal that has been decoded by the occurrence of interruption has been output from the audio decoder  78 , the audio decoder  78  continues the audio signal output until all of the audio signal that has been decoded by the occurrence of interruption is output from the audio decoder  78 . 
     On the other hand, if determined that all of the audio signal that has been decoded by the occurrence of interruption has been output, at step S 8  the audio decoder  78  switches the switch  86  to the audio data generating module side (storage module side). Since the switch  86  has thus been switched, at step S 9  the audio data stored in the buffer  83  is output to the speakers  17   a  and  17   b  via the D/A converter  84  and the volume controller  85 . The D/A converter  84  converts that portion of the suspension audio signal stored in the buffer  83  which was stored immediately before the interruption of the decoding into audio data having an analog signal format. The audio data is output from the speakers  17   a  and  17   b  after its volume is adjusted by the volume controller  85 . The volume controller  85  adjusts the volume so that the suspension audio data fades out in a given time period and the audio output is stopped after a smooth decrease in volume. 
       FIGS. 7A and 7B  are graphs showing how the audio volume varies when audio output is suspended. The vertical axis represents the volume of audio that is output from the speakers  17   a  and  17   b  and the horizontal axis represents time. 
     When decoding is suspended and audio signal output from the audio decoder  78  is suspended at time t 0 , as shown in  FIG. 7A , the volume decreases abruptly to a silent state because no waveform is generated if no measure is taken. It is highly likely that this abrupt decrease in volume causes the user of the computer auditory discomfort. 
     In contrast, in the PC  1  according to the embodiment which is equipped with the audio data generating module  80 , even if the audio signal output from the audio decoder  78  is suspended at time t 0 , as shown in  FIG. 7B , suspension audio data that was stored in the buffer  83  immediately before the suspension of decoding can be output so as to fade out. 
     Therefore, the volume does not vary abruptly and the degree of auditory discomfort that the user suffers due to the variation in the receiving situation can be lowered. 
     Next, a description will be made of an audio signal output process that is performed when normal audio data comes to be produced because of improvement in the receiving situation after audio output was suspended due to disappearance of normal audio data. 
       FIG. 8  is a flowchart of an audio signal output process (for restart) which is performed by the PC  1  according to the embodiment. 
     At step S 11 , it is determined whether audio data is normal. If determined that audio data is normal, at step S 12 , the audio decoder  78  acquires a PTS from the system decoder  79  and acquires a difference time between the current time and the PTS time. 
     At step S 13 , the audio decoder  78  determines whether the difference time is longer than a fade-in output enabling time. The fade-in output enabling time is a predetermined time. More specifically, it is a time that will be taken for restart audio data to be generated based on an audio signal of a given time period that is output immediately after a restart of decoding processing of decoding normal audio data and to be output from the speakers  17   a  and  17   b  so as to fade in. If determined that the difference time is shorter than the fade-in output enabling time, the process moves to step S 18 . In this case, the fade-in output processing, which is one of main features of the PC  1  according to the embodiment, is not performed when audio signal output is restarted. This is because a sufficient time for the processing cannot be secured. 
     On the other hand, if determined that the difference time is longer than the fade-in output enabling time, at step S 14  the audio decoder  78  turns off the switch  86  so that no audio signal is supplied to the speakers  17   a  and  17   b.    
     At step S 15 , an audio signal is output from the audio decoder  78  to the audio data generating module  80 . This audio signal is an audio signal of a given time period starting from the head of an audio signal that is output after the restart of decoding. When the audio signal of the given time period is supplied to the audio data generating module  80 , the A/D converter  81  converts the audio signal of the given time period into an audio signal having a digital signal format. The data generating module  82  performs a frequency analysis on the audio signal of the given time period by FFT. Furthermore, the data generating module  82  generates restart audio data having frequency components obtained by the frequency analysis and outputs the restart audio data to the buffer  83 . 
     At step A 16 , the audio decoder  78  switches the switch  86  to the audio data generating module side (storage module side). Then, at step S 17 , the restart audio data stored in the buffer  83  is supplied to the speakers  17   a  and  17   b  via the D/A converter  84  and the volume controller  85 . The volume controller  85  increases the volume so that the restart audio data fades in within a given time period and the audio level increases smoothly. The audio level is increased to its normal level. 
     At step S 18 , the audio decoder  78  monitors PTSs that are attached to the audio data and determines whether the current time that is based on the clock of the MPEG decoder  65  coincides with or has passed a PTS time. If determined that the current time has not reached a PTS time, the audio decoder  78  stands by until the current time reaches a PTS time. 
     On the other hand, if determined that the current time coincides with or has passed a PTS time, at step S 19  the audio decoder  78  switches the switch  86  to the audio decoder side. At step S 20 , the audio signal is output from the speakers  17   a  and  17   b.    
     At step S 17 , when the restart audio data is output while fading in, the audio volume reaches the normal level of the audio data at a time point when the current time reaches a PTS time. On the other hand, if the process has jumped to step S 18  from step S 13  in which case no restart audio data is being output, the audio signal starts to be output at a time point when the current time reaches a PTS time. 
     At step S 20 , the audio signal is output normally from the audio decoder  78 . Then, the process returns to the audio signal output process for suspension and the audio signal output is continued until an instruction to stop the audio output is received. 
       FIGS. 9A and 9B  are graphs showing how the audio volume varies when audio output is restarted. The vertical axis represents the volume of audio that is output from the speakers  17   a  and  17   b  and the horizontal axis represents time. 
     When decoding is restarted and audio output is started at time t 0 , as shown in  FIG. 9A , the volume increases abruptly to its normal level if no measure is taken. It is highly likely that this abrupt increase in volume causes the user of the computer auditory discomfort. 
     In contrast, in the PC  1  according to the embodiment which is equipped with the audio data generating module  80 , even if audio output should be restarted at time t 0 , as shown in  FIG. 9B , a head (decoding restart portion) audio signal can be output in advance (i.e., from a given time period before time t 0 ) so as to fade in because restart audio data is stored in the buffer  83 . 
     Therefore, the volume does not vary abruptly and the degree of auditory discomfort that the user suffers due to the variation in the receiving situation can be lowered. 
     According to the PC  1 , even if audio signal output from the audio decoder  78  is suspended due to a variation in the receiving situation, audio that is continuous with audio that was output immediately before the suspension can be output. Furthermore, even if audio signal output from the audio decoder  78  is suspended, the PC  1  can stop the audio output after a smooth decrease in volume by decreasing the volume gradually using suspension audio data stored in the buffer  83 . 
     When audio output should be restarted, audio can be output from before an original output time by using the buffer  83 . The PC  1  can restart audio output smoothly by increasing the volume gradually using restart audio data that is generated from a head audio signal. 
     As such, even if audio output is suspended by errors in audio data, the PC  1  according to the embodiment can suspend and restart audio output without causing the user auditory discomfort. 
     In the embodiment, suspension or restart audio data is generated from an audio signal that is output from the audio decoder  78 , and is stored in the buffer  83 . An alternatively configuration is possible in which the line for directly supplying an audio signal from the audio decoder  78  to the speakers  17   a  and  17   b  is omitted and all portions of an audio signal including a portion for suspension or restart pass through the audio data generating module  80  (buffer  83 ). 
     The invention can be applied to not only apparatus which receive a digital TV broadcast signal and outputs a video signal and an audio signal but also apparatus which reproduce only a digital audio signal. Furthermore, the invention can be applied to not only apparatus which use a wireless transmission path but also apparatus which use a wired transmission line. 
     The invention can be applied to not only notebook personal computers (the case of the embodiment) but also various audio reproducing apparatus such as desktop computers, word processors, cell phones, audio apparatus, and communication apparatus. 
     Each process (series of steps) described in the embodiment can be performed either by software or by hardware. 
     Although in the embodiment the steps of each process are performed in time-series order as shown in the flowchart, the invention is not limited to such a case. Each process may include steps that are performed parallel or individually. 
     Although the embodiments according to the present invention have been described above, the present invention is not limited to the above-mentioned embodiments but can be variously modified. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.