BEST MODE FOR CARRYING OUT THE INVENTION

[0052] The multitask control device according to the embodiments of the present invention will be explained below with reference to the figures.

[0053] (First Embodiment)

[0054] FIG. 1 is a block diagram showing the overall structure of the multitask control device 1a according to the first embodiment of the present invention.

[0055] The multitask control device 1a causes a processor 10 to process a plurality of (3 in this figure) tasks (including a first task 101a< /italic>, a second task 102a and a third task 103a< /italic>) respectively for making a cell phone multifunctional, for example. The multitask control device 1a includes a task management unit 20a and a mode setting unit 30a, and is realized by functions incorporated in a real time OS which runs on the processor 10.

[0056] The first task 101a processed by the processor 10 is one program for realizing a first function (music recording (audio signal encoding), for instance), and has three operation modes (an operation mode A, an operation mode B and an operation mode C) depending on three levels of sound quality, high, medium and low. These levels are determined by algorithms of tools used for encoding audio signals (such as a psychoacoustic model, TNS (Temporal Noise Shaping), prediction, M/S (Middle/Side) stereo, AAC quantization/encoding) which vary depending on values of parameters determined by the mode setting unit 30a, according to the standard of ISO/IEC 13818-7 Advanced Audio Coding (hereinafter referred to as “AAC”). In the operation mode A, more load than the operation mode B or C is put on the processor 10 so as to improve the sound quality. More specifically, as a result that the algorithms questing for higher sound quality are used in the order of the operation modes A, B and C, more operational amount (a larger number of commands) is required in this order, and thus more load is put on the processor 10 so as to reproduce sound of higher quality in this order.

[0057] More specifically speaking, when the parameter value is “1”, the first task 101a executes the operation mode A for putting the load N11 of 200MIPS on the processor 10. In this operation mode A, the first task 101a generates compressed audio data for reproducing top quality sound which is almost same as the original sound. When the parameter value is “2”, the first task 101a executes the operation mode B for putting the load N12 of 50MIPS on the processor 10. In this operation mode B, the first task 101a generates compressed audio data for reproducing medium quality sound which is slightly inferior to the sound reproduced in the operation mode A. And when the parameter value is “3”, the first task 101a executes the operation mode C for putting the load N13 of 30MIPS on the processor 10. In this operation mode C, the first task 101a generates compressed audio data for reproducing low quality sound which is slightly inferior to the sound reproduced in the operation mode B.

[0058] The second task 102a is a program for realizing the second function (TV reception, that is, video decoding (Vdec) and audio decoding (Adec), for instance) by putting the load N2 of 150MIPS per unit time, for instance, on the processor 10.

[0059] The third task 103a is a program for realizing the third function (telephone call, for instance (speech and echo cancel)) by putting the load N3 of 100MIPS per unit time, for instance, on the processor 10.

[0060] The processor 10 is a processor having capacity of processing one of the first˜third tasks 101a< /italic>˜103a or a combination of two or more of them at a rate of 285MIPS=Nmax per unit time.

[0061] The task management unit 20a manages the startup and stop of each of the tasks 101a< /italic>˜103a. More specifically, the task management unit 20a includes status codes indicating task status, a task control block (TCB) including a header address of each task, a flag at the time of interrupting processing and a register save area, a dispatcher and a scheduler. When an event occurs, such that a button for recording music is operated, a button for watching TV is operated, or an off-hook button is operated when getting a call, or the event ends, the task management unit 20a specifies tasks to be started next or stopped, detects the status change of the tasks to be processed by the processor 10 based on the specification, and then notifies the mode setting unit 30a of the detected status change of the tasks, or starts up or stops respective tasks 101a< /italic>˜103a as scheduled using the TCB.

[0062] Upon receipt of the notification of the status change of the tasks, that is, the scheduled startup or stop of the tasks, from the task management unit 20a, if the first task 101a is included in the scheduled tasks, the mode setting unit 30a, having the operation mode setting table 31a, determines, with reference to the operation mode setting table 31a, the operation mode for the first task 101a depending on whether other tasks are to be executed concurrently with the first task 101a or not, and sets the parameter for the first task 101a based on the determined operation mode.

[0063] FIG. 2 is a diagram showing relation between load per unit time on a processor 10 and sound quality in the operation modes A˜C of the first task 101a as shown in FIG. 1.

[0064] AAC has a scalable structure so as to reproduce sound even if a part of a bit stream of an encoded audio signal is used, and in AAC, an algorithm questing for top quality can be adopted like a professional-grade encoder by combining a plurality of different algorithms, that is, all the tools, or an algorithm for lower quality than the professional-grade encoder, like an SD player, or an algorithm for still lower quality than the SD player, like an portable CD player, can be adopted by omitting a part of the tools (such as a prediction tool) or simplifying the tools.

[0065] For example, for high quality audio encoding, the processing called a psychoacoustic model imitating a masking phenomenon of human hearing is performed to improve the auditory sound quality (See AAC). However, an enormous number of commands are required for performing encoding processing for obtaining reproduced sound which is approximately same as the original sound, and thus a great deal of load is required for this encoding processing. On the contrary, if this processing is simplified, the sound quality is gradually lowered according to the simplification level, while the number of commands is sharply reduced, and thus the load required for this processing can be substantially reduced. In such a case, the program for performing encoding processing for obtaining reproduced sound which is approximately same as the original sound in full consideration of the psychoacoustic model is the operation mode A. The mode for performing the simplified processing of the first task is the operation mode B, and the mode for performing the still more simplified processing of the task is the operation mode C.

[0066] In addition, for high quality audio encoding, in order to suppress the compression distortion to the level as negligible for human hearing as possible, the encoding parameters are repeatedly changed or modified for compression (See AAC). This processing also requires a great deal of load. On the other hand, if this processing is simplified, the sound quality is gradually lowered according to the simplification level, while the load can be substantially reduced. In such a case, the mode for repeating change or modification of the encoding parameters many times is the operation mode A, and the modes for performing more simplified processing are the operation modes A and B.

[0067] More specifically, for the program of the operation mode A, the auditory characteristics are considered, such as variable minimum audible levels by frequency components (the so-called loudness characteristic), the masking characteristic, and insensitivity to absolute time deviation or phase lag. For example, the degree of raising levels in the lower and higher frequency bands is changed by changing the filter characteristic according to the audio signal levels, or the quantization noises are reduced by convoluting the audio signal along the line of the masking characteristic expressed in the Bark scale on the frequency axis (that is, the masking characteristic in which the masking range is narrowed in the lower frequency part and widened in the higher frequency part) according to the audio signal levels (amplitudes) and masking the lower level signal in the vicinity of the audio signal. Similarly, for the program of the operation mode B, the load per unit time is substantially reduced compared with that in the operation mode A by making the range of the masking characteristic same in both lower and higher frequency bands, for example. In addition, for the program of the operation mode C, the load per unit time is slightly reduced compared with that in the operation mode B by building up the levels in the lower and higher frequency bands using the same filter characteristic regardless of audio signal levels.

[0068] FIG. 3 is a diagram showing the detailed structure of the operation mode setting table 31a, and FIGS. 4A-4C are diagrams showing relations between the processing capacity of the processor and the operation modes A˜C for the first task 101a< /italic>. Specifically, FIG. 4A shows the case where only the first task 101a is started up, FIG. 4B shows the case where the first task 101a and the second task 102a are started up concurrently, and FIG. 4C shows the case where the first task 101a< /italic>, the second task 102a and the third task 103a are started up concurrently.

[0069] The operation mode setting table 31a in FIG. 3 shows the relation between the tasks which should be executed and the operation modes for the first task 101a to be executed. In this figure, “◯” indicates that the task is started up, “x” indicates that the task is stopped, and “−” indicates that no operation mode is set for the first task 101a< /italic>. The case where the first task 101a is not started up is also shown in this figure.

[0070] This operation mode setting table 31a shows that when receiving the notification from the task management unit 20a that only the first task 101a is started up, the mode setting unit 30a sets the operation mode A with the highest load (the load N11 per unit time is 200IMPS, for instance) for the first task 101a< /italic>. The reason why the operation mode A is set is as follows. If the processing capacity Nmax=285MIPS of the processor 10 is represented by blocks in FIG. 4A, all the capacity of the processor 10 can be assigned to the processing of the first task 101a< /italic>. In this case, any of the operation modes A˜C can be set because they are within the limit of the processing capacity of the processor 10, but the sound quality in the operation mode A is highest among them. Therefore, the operation mode A with the highest quality is set to be executed for the first task 101a< /italic>. In other words, the operation mode A is set to be executed with the highest quality within the limit that the processor 10 does not get out of capacity Also, this operation mode setting table 31a shows that when receiving the notification from the task management unit 20a that the second task 102a (150MIPS) and the first task 101a are started up concurrently, the mode setting unit 30a sets for the first task 101a the operation mode B (50MIPS) with the lower load than that of the operation mode A. The reason why the operation mode A is set is as follows. As shown in FIG. 4B, the load for processing the second task 102a accounts for N2=150MIPS among the processing capacity of the processor 10, and 135MIPS can be assigned to the processing of the first task 101a< /italic>. So, any of the operation modes B and C can be set within the limit that the processor 10 does not get out of capacity, but the operation mode B is superior to the mode C in sound quality. Therefore, the operation mode B is set to be executed for the first task 101a< /italic>. In other words, the operation mode B is set to be executed with the highest quality within the limit that the processor 10 does not get out of capacity.

[0071] Note that in this operation mode setting table 31a, when receiving the notification from the task management unit 20a that the third task 103a (100MIPS) and the first task 101a are started up concurrently, the mode setting unit 30a sets the operation mode B (50MIPS) for the first task 101a for the same reason as the case where the second task 102a (150MIPS) and the first task 101a are started up concurrently.

[0072] In addition, the operation mode setting table 31a shows that when receiving the notification from the task management unit 20a that all the first, second and third tasks 101a< /italic>, 102a and 103a are started up concurrently, the mode setting unit 30a sets the operation mode C (the load N13 per unit time is 30MIPS, for instance) for the first task 101a< /italic>. The operation mode C is set because only the mode C can be executed within the limit that the processor 10 does not get out of capacity. In other words, the operation mode C is set to be executed with the highest quality within the limit that the processor 10 does not get out of capacity.

[0073] FIG. 5 is a diagram showing the operation of the multitask control device 1a.

[0074] When detecting a change of task status such as a new task startup and a task stop due to an event occurrence (S11), the task management unit 20a specifies the tasks to be started up after the status change (S12). Then, the task management unit 20a notifies the mode setting unit 30a of the tasks to be executed next based on this specification.

[0075] Upon receipt of the notification, the mode setting unit 30a determines the operation mode for the first task 101a using the operation mode setting table 31a if the tasks include the first task 101a< /italic>, and sets the parameter corresponding to the determined operation mode for the first task 101a (S13). After setting the parameter for the first task 101a< /italic>, the task management unit 20a causes the processor 10 to execute the tasks by time-sharing (S14).

[0076] By repeating these processes (S11˜S 14), the multitask control device 1a concurrently processes the first-third tasks 101a< /italic>˜103a by time-sharing so as to cause the processor 10 to execute the first task 101a in the top quality operation mode within the limit that the processor 10 does not get out of capacity, while improving the operating efficiency of the processor 10.

[0077] FIG. 6 is a diagram showing relation between the task status transition and the transition of the load on the processor 10 in the case where only the first task 101a is started up.

[0078] Before the time T1 shown in FIG. 6, no task is started up. In this status, if the task management unit 20a starts up only the first task 101a at the time T1, the mode setting unit 30a sets the parameter “1” for the first task 101a to cause the processor 10 to execute it in the operation mode A. At that time, the load on the processor 10 is N11=200MIPS, and the top quality sound can be obtained. In other words, since this load is well within the limit of the capacity of the processor 10, all of which can be assigned to the processing of the first task 101a< /italic>, the mode setting unit 30a instructs the processor to execute the first task 101a in the operation mode A as a high quality encoding mode.

[0079] As described above, while the load of only 30MIPS is put on the processor 10 to reproduce low quality sound in the conventional art, the first task 101a is processed in the operation mode A for putting much more load of 200MIPS on the processor 10 in the first embodiment, and thus the operating efficiency of the processor 10 is drastically increased, which enables reproduction of top quality sound.

[0080] FIG. 7 is a diagram showing relation between the task status transition and the transition of the load on the processor 10 in the case where the first task 101a is further started up after the second task 102a has been started up.

[0081] If the task management unit 20a starts up only the second task 102a at the time T2 shown in FIG. 7, the load N2=150MIPS, which is required for the second task 102a< /italic>, is put on the processor 10. Next, if the task management unit 20a further starts up the first task 101a at the time T3, the processing amount which can be assigned to the first task 101a is (Nmax−N2)=135MIPS because the processing of the second task 102a requires the load of 150MIPS. The operation mode setting table 31a as shown in FIG. 3 shows that the operation mode B (50MIPS) is designated for the first task 101a< /italic>. According to this, the mode setting unit 30a determines the operation mode B and sets the parameter “2” for the first task 101a to cause the processor 10 to execute it in the operation mode B. In this case, the load N12 required for the operation mode B for the first task 101a is added to the processor 10. In other words, since the processor 10 here does not have so much spare capacity as that in the case of FIG. 7, the operation mode B with the highest quality within the limit that the processor 10 does not get out of capacity is set. Therefore, the operational load on the processor 10 is (N2+N12)=200MIPS.

[0082] Accordingly, while only the load of 30MIPS+150MIPS=180MIPS is put on the processor in the conventional art, as described above, much more load is put in the first embodiment to increase the operating efficiency of the processor 10, and thus the higher quality sound than ever before can be obtained.

[0083] FIG. 8 is a diagram showing relation between the task status transition and the transition of the load on the processor 10 in the case where the second and third tasks 102a and 103a are further started up after the first task 101a has been started up.

[0084] Before the time T4 shown in FIG. 8, no task is started up. In this status, if the task management unit 20a starts up the first task 101a at the time T4, the mode setting unit 30a sets the operation mode A for the first task 101a because the second and third tasks 102a and 102a have not been started up. At that time, the load on the processor 10 is N11=200MIPS. In other words, since this load is well within the capacity of the processor 10, the mode setting unit 30a instructs the first task 101a to encode in the operation mode A as a high quality encoding mode.

[0085] Next, it is assumed that the task management unit 20a further starts up the second task 102a at the time T5. In this case, the operation load N2=150MIPS which is required for the second task 102a is added to the processor 10. In this case, if the operation mode A for the first task 101a is maintained as it is, the excessive load of 350MIPS is put on the processor 10, which causes the processor 10 to go down. So, the mode setting unit 30a determines the operation mode B (50MIPS) for the first task 101a< /italic>, with reference to the operation mode setting table 31a. As a result, the total load of (N12+N2)=200MIPS is put on the processor 10. In other words, since the processor 10 here does not have so much spare capacity as assigning the operation mode A, the mode setting unit 30a instructs the first task 101a to encode in the operation mode B with the highest quality and the highest encoding speed within the limit that the processor 10 does not get out of capacity.

[0086] Next, it is assumed that the task management unit 20a starts up the third task 103a at the time T6. In this case, the load N3=100MIPS which is required for the third task 103a is added to the processor 10. If the operation mode B for the first task 101a is maintained as it is, the load on the processor 10 becomes 300MIPS, which causes the processor 10 to go out of capacity.

[0087] So, the mode setting unit 30a determines to set the operation mode C (30MIPS) for the first task 101a< /italic>, with reference to the operation mode setting table 31a. As a result, the total load of (N13+N2+N3)=280MIPS is put on the processor 10.

[0088] In other words, since the processor 10 does not have so much spare capacity as assigning the operation mode B, the mode setting unit 30a instructs the first task 101a to encode in the operation mode C with the highest quality and high encoding speed but with less load within the limit that the processor 10 does not go out of capacity.

[0089] Next, it is assumed that the task management unit 20a stops the third task 103a (telephone call) at the time T7. In this case, since only the first and second tasks 101a and 102a are running, the mode setting unit 30a sets the operation mode B for the first task 101a< /italic>, with reference to the operation mode setting table 31a. In other words, since the processor 10 has spare capacity in its operational load, the mode setting unit 30a instructs the first task 101a to encode in the operation mode B as a highest quality encoding mode within the limit that the processor 10 does not go out of capacity. As a result, the operational load on the processor 10 is (N12+N2)=200MIPS.

[0090] Next, it is assumed that the task management unit 20a stops the second task 102a at the time T8. In this case, since only the first task 101a is running, the mode setting unit 30a sets the operation mode A for the first task 101a< /italic>. In other words, since the processor 10 has spare capacity in its operational load, the mode setting unit 30a instructs the first task 101a to encode in the operation mode A as a highest quality encoding mode. As a result, the operational load on the processor 10 is N11=200MIPS.

[0091] As described above, according to the first embodiment of the present invention, operation modes for putting different load values per unit time, for high, medium and low quality sound, on the processor 10, are prepared in advance for the first task 101a< /italic>, and the mode setting unit 30a resets the operation mode for the first task 101a so as to put the highest load within the limit that the processor 10 does not go out of capacity whenever the status of the first and second tasks 101a and 102a in the task management unit 20a changes, that is, these tasks are respectively started up or stopped. More specifically, when a great deal of processing load is put on the processor 10 due to concurrent tasks, the processing load of the first task is reduced. Therefore, as long as the first task 101a is running, the load per unit time can be put on the processor 10 so that the load never exceeds the processing capacity Nmax of the processor 10. In other words, if the total load can be reduced and there is spare capacity to accept more load, a lot of processing resources can be assigned to the first task 101a to realize a high performance function. As a result, the operation mode with the highest load is set for the first task 101a within the limit of the capacity of the processor 10, which makes it possible to make full use of the processing capacity of the processor 10, and thus increase the operating efficiency of the processor 10.

[0092] In the first embodiment, the operation in the case where three tasks occur concurrently has been explained, but the present invention can be applied, based on the first embodiment, to the case where two tasks, or four or more tasks occur concurrently. Specifically, a plurality of operation modes, with different load values required for the processing per unit time, are switched for the first task, depending on the concurrence status of the tasks. The index for judging the switch of the operation modes for the first task is the total sum of the load of the running tasks other than the first task. If there is much spare capacity in the processor even with the total sum of the load being put on, the maximum load is given to the first task within that much spare capacity limit of the processor for high performance processing, and if there is not so much spare capacity in the processor with the total sum of the load being put on, the maximum load is given to the first task within such less capacity limit of the processor for high-speed processing.

[0093] In the first embodiment, the mode setting unit 30a determines the operation mode for the first task 101a using the operation mode setting table 31a, but the processing amount assignable to the first task 101a may be specified after the status change without using the operation mode setting table 31a so as to determine the operation mode for the first task for putting the highest load on the processor 10 within the limit that the processor 10 does not go out of capacity.

[0094] In the first embodiment, the total sum of the load of the running tasks other than the first task is used as an index for judging the switch of the operation modes for the first task, but it is possible to use the total sum of the operational amount required for the processing of the running tasks other than the first task, or the total number of commands included in the programs that form the running tasks other than the first task.

[0095] The number of the running tasks other than the first task may also be used. In this case, the operation mode may be determined and set in the manner as such the operation mode A for the number of other running tasks “0”, the operation mode B for “1” and the operation mode C for “2”. It is very easy to determine and set the operation mode.

[0096] In the first embodiment, three operation modes are available for the first task 101a< /italic>, but two, or four or more operation modes may be used.

[0097] In the first embodiment, a plurality of operation modes with different load values are available for only the first task 101a< /italic>, but they may be available for other tasks including the second task 102a< /italic>. In this case, as an index for judging the switch of the operation modes, the number of concurrent tasks after the status change may be used.

[0098] In the first embodiment, the first task 101a includes one program for changing operation modes according to parameters. However, a plurality of the first tasks may include programs in the same category corresponding to different functions so as to select for the processor one task corresponding to the highest function within the limit of the processing capacity of the processor from among these first tasks, when it is judged that one of the first tasks should be started up. More specifically, it is assumed that when there have already been three programs in the same category, a professional-use encoder (200MIPS) for a DVD device or the like, an encoder or an SD jukebox (50MIPS) for an SD player or the like, and an encoder or a CD ripper (30MIPS) for a portable CD player or the like, the first tasks are these three programs; the professional-use encoder (200MIPS), the SD jukebox (50MIPS) and the CD ripper (30MIPS). When it is judged that one of the first tasks should be started up, one task corresponding to the highest function within the limit of the processing capacity of the processor may be selected for the processor from among the professional-use encoder (200MIPS), the SD jukebox (50MIPS) and the CD ripper (30MIPS).

[0099] In this case, there is no need to create one program for changing operation modes according to parameters, existing software can be reused, and thus recycling of existing resources are promoted.

[0100] In addition, in the first embodiment, the present invention is applied to the case where the first task is audio signal encoding, but it can be applied to audio signal reproduction. For example, the present invention may be applied to a virtual surround function in the audio signal reproduction. The virtual surround function is realized by a digital filter with a plurality of taps. For that purpose, if the program is formed so that the numbers of taps (the numbers of FIR filter coefficients) are 8192 (2 to the 13th power) for the operation mode A, 128 (2 to the 7th power) for the operation mode B, and 32 (2 to the 5th power) for the operation mode C, for instance, the operation mode A is set to put more load when concurrent tasks are few in number, and thus the virtual surround effect for enlarging the acoustic field can be maximized. On the contrary, when concurrent tasks are large in number, if the operation modes B and C are set in this order, the virtual surround effect can be gradually degraded.

[0101] (Second Embodiment)

[0102] FIG. 9 is a block diagram showing the overall structure of a multitask control device 1b according to the second embodiment of the present invention. Note that the same numbers are assigned to the components corresponding to those of the multitask control device 1a in the first embodiment and the explanation thereof is omitted.

[0103] The multitask control device 1b has the approximately same structure as the multitask control device 1a in the first embodiment. For example, the multitask control device 1b, which causes the processor 10 to process a plurality of (four in this figure) tasks for making a cell phone multifunctional (a first task 101b< /italic>, a second task 102b< /italic>, a third task 103b and a fourth task 104b< /italic>), includes a task management unit 20b and a mode setting unit 30b, and is realized by functions incorporated in a real time OS which runs on the processor 10.

[0104] The multitask control device 1a in the first embodiment is configured so as to execute the first task 101a and other tasks 102a and 103a concurrently, and the operation mode for the first task 101a is set based on the operation mode setting table 31a in the mode setting unit 30a. On the contrary, the multitask control device 1b in the second embodiment is configured so as to disable or enable the concurrence of the first task 101b and other tasks (the second task 102b< /italic>, the third task 103b and the fourth task 104b< /italic>) based on the user configuration, and the operation mode for the first task 101b is determined based on the operation mode determination table 22 in the task management unit 20b. This is a notable difference from the first embodiment. Note that the mode for disabling the concurrence of the first task 101b and the second-fourth tasks 102b< /italic>˜104b is also hereinafter referred to as a “concurrence disabling mode”, and the mode for enabling the concurrence as a “concurrence enabling mode”.

[0105] The task management unit 20b includes a flag storage unit 21 for storing flags indicating the concurrence modes (the concurrence disabling mode and the concurrence enabling mode) set by a user, and an operation mode determination table 22 for determining the operation mode and the like for the first task 101b< /italic>. If the flag stored in the flag storage unit 21 indicates the concurrence disabling mode, when a request is made for starting up other tasks after the first task 101b is started up, the task management unit 20b disables the startup of the other tasks. Similarly, if the flag indicates the concurrence enabling mode, when a request is made for changing the status of the current task, such as starting up or stopping it, and the tasks to be started up include the first task 101b< /italic>, the task management unit 20b determines the operation mode for the first task 101b< /italic>, based on whether the first task 101b and other tasks occur concurrently or not, with reference to the operation mode determination table 22, and notifies the mode setting unit 30b of the determined operation mode. Or, when the processing amount of the running first task 101b could drop to less than a predetermined threshold if the requested other tasks are started up, the task management unit 20b disables the startup of requested other tasks.

[0106] The mode setting unit 30b sets for the first task 101b the parameter corresponding to the operation mode notified by the task management unit 20b so as to execute the operation mode according to the parameter.

[0107] FIG. 10 is a diagram showing an example of the structure of the operation mode determination table 22 as shown in FIG. 9.

[0108] As shown in this figure, the operation mode determination table 22 includes a record 221 for storing the processing capacity of the processor 10, records 222˜22 5 for storing the load values of the first task 101b< /italic>˜the fourth task 104b< /italic>, and a record 226 for storing the threshold indicating the processing amount required minimally for executing the first task 101b.

[0109] The first task 101b processed by the processor 10 is one program for executing the first function (music reproduction), which decodes audio signals and changes the number of taps of a digital filter used for virtual surround reproduction according to the parameter values set by the mode setting unit 30b. The operation mode for the first task 101b< /italic>, that is, the operation mode A, the operation mode B or operation mode C for reproducing sound with the level high, medium or low, is determined depending upon the number of filter taps. Specifically, the operation mode A is a program for performing virtual surround reproduction using a digital filter with 8192 taps after decoding, which puts the load of 250MIPS on the processor 10. The operation mode B is a program for performing virtual surround reproduction using a digital filter with 128 taps after decoding, which put the load of 200MIPS on the processor 10. The operation mode C is a program for performing virtual surround reproduction using a digital filter with 32 taps, which puts the load of 150MIPS on the processor 10.

[0110] The second task 102b is a program for decoding videos of digital satellite broadcasting and displaying them on a monitor (for TV view), which puts the load of 50MIPS on the processor 10. The third task 103b is a program for connecting to a desired server via the Internet to view (browse) the information uploaded on the server, which puts the load of 50MIPS on the processor 10. The fourth task 104b is a program for viewing video e-mails received via the Internet or the like, which puts the load of 50MIPS on the processor 10.

[0111] As a threshold, a value (140MIPS) which enables real-time decoding of audio data is set for reproducing sound continuously.

[0112] FIG. 11 is a diagram showing an example of the external structure of a cell phone in which the multitask control device 1b as shown in FIG. 9 is implemented.

[0113] As shown in this figure, a cell phone exit115 includes an antenna ex201 for transmitting and receiving radio waves between a cell site not shown in the figure and a satellite, an operation unit ex204 including a plurality of operation keys mounted on the body and operated by a user, a camera ex203 including a CCD camera or the like capable of shooting moving and still pictures, a display ex202 including a liquid crystal display or the like for displaying the pictures shot by the camera ex203, the TV videos and the like received via the antenna ex201, a voice input unit ex205 including a microphone for inputting voices, a voice output unit ex208 including a plurality of speakers for reproducing voices, a pin jack connector ex209 for connecting a headphone and an external speaker, a slot ex206 for inserting a recording medium ex207, and others.

[0114] Here, the recording medium ex207 is a plastic case such as an SD card containing a flash memory element, a type of EEPROM (Electrically Erasable and Programmable Read Only Memory) which is electrically rewritable and erasable nonvolatile memory, which holds the data of moving or still pictures, the data of received e-mails, the encoded audio data or decoded audio data, and others.

[0115] The task management unit 20b and the mode setting unit 30b included in the multitask control unit 1b are realized by a processor incorporated in the body of the cell phone ex115, a multitask OS executed by the processor, a ROM for storing application programs (such as software for an MPEG encoder/decoder, browser software and e-mail software), a memory for providing a work area for the running OS or application programs or for temporally memorizing flags indicating concurrence enabling/disabling set by the user and the operation mode determination table 22, and the like.

[0116] Next, the operation of the task management unit 20b and the mode setting unit 30b included in the multitask control device 1b will be explained below.

[0117] FIG. 12 is a flowchart showing the details of task startup and stop processing performed by the task management unit 20b and the mode setting unit 30b.

[0118] This task startup and stop processing is performed at predetermined time intervals.

[0119] In this processing, the task management unit 20b first judges whether a task startup request is received or not (S10). When the task startup request is received (Yes in S10), the task management unit 20b judges whether the task is a music reproduction task or not (S20). If it is the music reproduction task (Yes in S20), the task management unit 20b, in collaboration with the mode setting unit 30b, executes the music reproduction task startup control processing (S30) to end the task startup and stop processing. If the task is not the music reproduction task (No in S20), the task management unit 20b, in collaboration with the mode setting unit 30b, executes other task startup control processing (S40) to end the task startup and stop processing.

[0120] On the other hand, if the task startup request is not received (No in S10), the task management unit 20b judges whether a task stop request is received or not (S50). If the task stop request is received (Yes in S50), the task management unit 20b judges whether the task is the music reproduction task or not (S60). If it is the music reproduction task (Yes in S60), the task management unit 20b stops the music reproduction task (S70) to end the task startup and stop processing. If the task is not the music reproduction task (No in S60), the task management unit 20b, in collaboration with the mode setting unit 30b, executes other task stop control processing (S80) to end the task startup and stop processing.

[0121] If the task stop request is not received (No in S50), the task management unit 20b ends the task startup and stop processing.

[0122] FIG. 13 is a flowchart showing subroutines of the music reproduction task startup control processing (S30) as shown in FIG. 12.

[0123] In this music reproduction task startup control processing, the task management unit 20b first judges whether there is another running task or not (S301). If there is no other task running (Yes in S310), the task management unit 20b determines the operation mode “A” (high quality) for the music reproduction task (S302) to make the mode setting unit 30b select the operation mode A. Then, the task management unit 20b judges whether the concurrent disabling mode is set or not with reference to the flag storage unit 21 (S303). If the concurrence disabling mode is set (Yes in S303), the task management unit 20b starts up the music reproduction task (operation mode “A”) in the concurrence disabling mode (S304) to return to the main routine. If the concurrence disabling mode is not set (No in S303), that is, if the concurrence enabling mode is set, the task management unit 20b starts up the music reproduction task in the concurrence enabling mode (S305) to return to the main routine.

[0124] On the other hand, if there is the other running task (No in S301), the task management unit 20b calculates the processing amount assignable to the music reproduction task (assignable processing amount) by subtracting the processing amount of the other task from the processing capacity of the processor 10 (S306), and judges whether or not the calculated assignable processing amount is the threshold or more (S307). If it is the threshold or more (Yes in S307), the task management unit 20b determines the operation mode “B” or “C” (medium or low quality) for the music reproduction task based on the assignable processing amount (S308) to make the mode setting unit 30b select the operation mode B or C. How to determine the operation mode is as follows. If the assignable processing amount is the processing amount of the operation mode B or more, the task management unit 20b determines the operation mode B, and if the assignable processing amount is less than that of the operation mode B, it determines the operation mode C. After determining the operation mode, the task management unit 20b judges whether the concurrence disabling mode is set or not (S309). If the concurrence disabling mode is set (Yes in S309), the task management unit 20b starts up the music reproduction task (operation mode “B” or “C”) in the concurrence disabling mode (S310) to return to the main routine. And if the concurrence disabling mode is not set (No in S309), the task management unit 20b starts up the music reproduction task (operation mode “B” or “C”) in the concurrence enabling mode (S311) to return to the main routine.

[0125] On the other hand, if the assignable processing amount is not the threshold or more, that is, less than the threshold (No in S307), the task management unit 20b disables the startup of the music reproduction task (S312) to return to the main routine. That is why real-time decoding cannot be performed even if the music reproduction task is started up and thus the music is reproduced discontinuously.

[0126] FIG. 14 is a flowchart showing the subroutines of the other task startup control processing (S40) as shown in FIG. 12.

[0127] When the other task startup request is received, the task management unit 20b first judges whether the music reproduction task is running or not (S401). If the music reproduction task is running as a result of the judgment (Yes in S401), the task management unit 20b judges whether this music reproduction task is running in the concurrence disabling mode or not (S402). If it is running in the concurrence disabling mode (Yes in S402), it disables the startup of the requested other task (S403) to return to the main routine. This processing does not change the operation mode for the running music reproduction task. So, the music reproduction is continued until the end with constant sound quality.

[0128] On the other hand, if the music reproduction task is not running in the concurrence disabling mode, that is, it is running in the concurrence enabling mode (No in S402), the task management unit 20b calculates the processing amount which is assignable to the music reproduction task (assignable processing amount) when the requested other task is started up, with reference to the operation mode determination table 22 (S404), and judges whether the calculated assignable processing amount is less than the threshold or not (S405). If it is less than the threshold (Yes in S405), the task management unit 20b disables the startup of the requested other task to return to the main routine. This processing ensures that the processing amount assignable to the music reproduction task is always the threshold or more. Therefore, the music is reproduced continuously. On the other hand, if the assignable amount is not less than the threshold, that is, it is the threshold or more (No in S405), the task management unit 20b determines the operation mode “B” or “C” for the music reproduction task based on the assignable processing amount calculated in Step S404, and the mode setting unit 30b transitions the operation mode to “B or “C” (S407). Specifically, the operation mode is transitioned in the following manner. If the assignable processing amount is the processing amount of the operation mode B or more, it is transitioned to the operation mode B, and if the assignable processing amount is less than that of the operation mode B, it is transitioned to the operation mode C. In this case, the operation mode is transitioned to that of the lower load, and thus the sound quality is degraded. After the operation mode is transitioned, the task management unit 20b enables the startup of the requested other task (S408) to return to the main routine. As a result, the music reproduction task and the requested other task are executed by time-sharing.

[0129] On the other hand, if the music reproduction task is not started up (No in S401), the task management unit 20b enables the startup of the requested other task (S409) to return to the main routine. As a result, the requested other task is executed.

[0130] FIG. 15 is a flowchart showing the subroutines of the other task stop control processing (S80) as shown in FIG. 12.

[0131] When the other task stop request is received, the task management unit 20b first stops the other task (S801) and judges whether the music reproduction task is running or not (S802). If it is not running (No in S802), the procedure returns to the main routine.

[0132] If the music reproduction task is running (Yes in S802), the task management unit 20b judges whether the music reproduction task is running in the concurrence disabling mode or not (S803). If the task is running in the concurrence disabling mode (Yes in S803), the procedure returns to the main routine. In other words, the procedure returns to the main routine without allocating the spare processing capacity to the music reproduction task. Therefore, the operation mode for the running music reproduction task is not changed, and thus the music can be reproduced continuously without the change of the sound quality.

[0133] On the other hand, if the task is not running in the concurrence disabling mode (No in S803), that is, it is running in the concurrence enabling mode, the task management unit 20b calculates the processing amount assignable to the music reproduction task (assignable processing amount) (S804) and determines the operation mode “A” or “B” for the music reproduction task, and the mode setting unit 30b transitions the operation mode to “B” or “C” (S805), and then the procedure returns to the main routine. Specifically, the operation mode is transitioned in the following manner. If the assignable processing amount is the processing amount of the operation mode A or more, the operation mode is transitioned to “A”, and if it is less than that of the operation mode A, the operation mode is transitioned to “B”. In this case, since the operation mode is transitioned to that with heavier load, the sound quality is improved.

[0134] FIG. 16 is a diagram showing the transition of load depending upon the transition status of tasks in the concurrence enabling mode. Specifically, FIG. 16 shows the relation between the transition of task status and the transition of load on the processor 10 in the case where the music reproduction task is started up in the concurrence enabling mode and then other tasks are started up in sequence.

[0135] No task is started up before the time T10 as shown in FIG. 16. In this status, when the task management unit 20b starts up the music reproduction task at the time T10, no other task has not been started up. So, the mode setting unit 30b sets the operation mode A for the music reproduction task, and thus the load on the processor 10 is 250MIPS. In other words, since the processor 10 has spare capacity to accept more load, the parameter for reproducing music in the high quality virtual surround mode is set for the music reproduction task. When only the music reproduction task is executed like this, the operation mode “A”, the title of reproduced music, the singer, and others are displayed on the screen of the cell phone, as shown in FIG. 17A.

[0136] Next, it is assumed that the task management unit 20b starts up another task (TV view, for instance) at the time T11. In this case, the operational load, 50MIPS, required for the TV view is additionally put on the processor 10. If the operation mode A for the music reproduction task is maintained, the load on the processor 10 is 300MIPS, which is excessive and causes the processor 10 to