Abstract:
A performance control apparatus that prevents erroneous key depressions from disturbing musical performance and allow an inexperience player to play at ease. A performance operator is adapted to generate performance operation information in response to performance operations by a user, the performance operation information including information indicative of performing timing in automatic performance. A storage device is adapted to store data of a music piece comprising sequence data of note information for individual musical tones. A performance control device is adapted to, each time the performance operation information is generated, calculate tempo of automatic performance on the basis of the difference in generation time between the present performance operation information and the previous performance operation information, and to read out the data of the music piece from the storage device with the tempo; wherein the performance control device is adapted to exclude currently the present performance operation information from calculation of the tempo if the difference in generation time is less than a predetermined threshold.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a performance control apparatus that sequences data of a music piece for a predetermined duration according to operation by a player, as well as a program for the performance control apparatus. 
   2. Description of the Related Art 
   Conventionally, there have been known electronic musical instruments that generate musical tones in response to operation by a player. Such electronic musical instruments are modeled on, for example, pianos and generally carry out performance operations in a manner similar to pianos that are acoustic musical instruments. These electronic musical instruments require skill to perform and much time to learn. 
   An electronic musical instrument (electronic piano) detects the keying velocity of a player and generates musical tones in accordance with the keying velocity. The electronic piano is equipped with sensors, one for each key, for detecting the keying velocity. The sensors measure the on/off time of multiple contacts, or use elastically deforming members for contacts and utilize the behavior of the members to detect the keying velocity. However, the use of contacts in the sensors causes chattering (repetitive on and off behavior). To prevent the chattering, an apparatus according to Prior Art 1 has been proposed that ignores on/off switching that occurs in a short period of time (see for example Japanese Patent Laid-Open No. 2002-244662). 
   On the other hand, electronic musical instruments are used by a wide variety of users at all levels from beginners to skilled players. Skilled players want electronic musical instruments capable of providing a wide range of nuance in accordance with performance operations like acoustic musical instruments. In contrast, beginners want electronic musical instruments that allow them to play by simple operations. 
   In order to meet these demands, an apparatus according to Prior Art 2 has been proposed that automatically plays musical tones for a given time period (for example ½ bar) when a player performs a simple operation (swing by hand) (see, for example, Japanese Patent Laid-Open No. 2000-276141). Japanese Patent Laid-Open No. 2000-276141 describes a musical instrument consisting of multiple slave units and a single master unit. Such an electronic musical instrument generates musical tones in accordance with a player&#39;s performance operation. That is, when a player performs a performance operation using a performance operator, information such as the velocity of the performance operation by the player is sent from a slave unit to the master unit, where musical tone data for the musical part assigned to the slave unit is read and a timbre and other characteristics of the musical tone are determined on the basis of the velocity of the player&#39;s performance operation. 
   There has been proposed an apparatus according to Prior Art 3 that sets an upper limit on the velocity of performance operations and, if an operation is performed at a velocity exceeding the predetermined threshold, the operation is assumed to be treated at the upper limit velocity (see, for example, Japanese Patent No. 3720004). The threshold can be varied to change the level of response to performance operations. Thus, the level of difficulty of controlling musical characteristics (stability or musical expression ability) can be adjusted according to player&#39;s proficiency level. 
   As stated above, there has been demand for musical instruments that can be played even by inexperienced players with ease in recent years. It is conceivable that slave units of an electronic musical instrument such as the apparatus according to Prior Art 2 are used as electronic pianos. 
   However, a beginner can perform wrong operations (accidentally hit neighboring keys at approximately the same time) on an electronic piano that is a slave unit. The apparatus according to Prior Art 1 prevents key chattering but not erroneous performance operations. Furthermore, the keyboard of the apparatus has a complex contact structure and therefore a complex algorithm. 
   An electronic musical instrument such as the apparatus according to Prior Art 3 treats performance operations performed at a velocity exceeding a predetermined threshold as operations performed at an upper limit velocity to reduce variations in tempo. However, the apparatus does not prevent erroneous performance operations. If keys are depressed at approximately the same time, the tempo of performance significantly changes, causing irregularities in performance. 
   SUMMARY OF THE INVENTION 
   The present invention provides a performance control apparatus and a program therefor that prevent erroneous key depressions from disturbing musical performance and allow an inexperience player to play at ease. 
   In a first aspect of the present invention, there is provided a performance control apparatus comprising: a performance operator adapted to generate performance operation information in response to performance operations by a user, the performance operation information including information indicative of performing timing in automatic performance; a storage device adapted to store data of a music piece comprising sequence data of note information for individual musical tones; and a performance control device adapted to, each time the performance operation information is generated, calculate tempo of automatic performance on the basis of the difference in generation time between the present performance operation information and the previous performance operation information, and to read out the data of the music piece from the storage device with the tempo; wherein the performance control device is adapted to exclude currently the present performance operation information from calculation of the tempo if the difference in generation time is less than a predetermined threshold. 
   According to the present invention, the difference in generation time between musical performance operations is detected and, if the difference in generation time is less than a threshold, it is determined that the operations are successive key depressions performed accidentally and the performance operations are ignored and determination of characteristics such as tempo of the music tones is omitted. Thus, erroneous operations do not cause irregularities in musical performance and therefore an inexperienced player can enjoy playing at ease. 
   According to the present invention, when a player uses a performance operator to play a performance operation (for example, a key depression), an operation signal including information indicating timing of performance is generated. The performance timing is indicated at regular intervals such as every single beat, two beats, or ½ beat by a direction by a facilitator, for example, who guides the performance. The performance control apparatus determines parameters such as the volume and quality of a musical tone on the basis of the operation signal and musical piece data (for example, MIDI data). When an operation signal is generated by a performance operation, the difference in generation time between the present operation signal and the generation of the previous operation signal is calculated. If the calculated time difference is greater than or equal to a predetermined threshold, tempo of the musical tones and the volume and intensity of each tone are determined on the basis of the time difference. If the calculated difference in generation time is less than the threshold, it is determined that successive key depressions have been accidentally performed and determination of characteristics such as sound volume and intensity is omitted. 
   The performance control device can be adapted to update the threshold on the basis of the difference in generation time. 
   According to the present invention, the threshold is updated, even during performance, on the basis of the difference in generation time after the time when the previous operation signal has been generated. 
   The performance control device can be adapted to count the present performance operation information as performance operation information generated by an erroneous operation if the difference in generation time is less than the threshold and to record information including the number of pieces of performance operation information generated by erroneous operations in the storage device. 
   According to the present invention, the number of erroneous operations identified by differences in generation time less than thresholds is counted and recorded as a log. A facilitator can check the log to see the number of erroneous operations and thereby know the level of proficiency of each player, for example. In addition to the number of erroneous operations, other information such as the times at which the erroneous operations occurred, the keys depressed (note numbers), key depression velocities, and the title of the music piece played may be recorded. 
   The performance control device can be adapted to determine the threshold or the basis of information including the number of pieces of performance operation information generated by erroneous operations recorded in the storage device. 
   According to the present invention, specifically the threshold is determined on the basis of the number of erroneous operations recorded as a log. For example, if many erroneous operations occurred, a larger threshold is set to prevent change of tempo due erroneous operations, thereby preventing irregularities in performance. 
   The performance operator has a plurality of keys adapted to generate performance operation information in response to performance operations by a user, the performance operation information having different note numbers for different keys, and the performance control device can be adapted to exclude the present performance operation information from calculation of the tempo if the difference in generation time is less than a predetermined threshold and the key corresponding to the present performance operation information and the key corresponding to the previous performance operation information are adjacent to each other. 
   According to the present invention, the operation element has multiple keys. When a player depresses one of the keys, a note number associated with the key is included in the operation signal generated. When an operation signal is generated by a performance operation, the difference in generation time between the present operation signal and the previous operation signal is calculated. If the calculated difference in generation time is greater than or equal to a predetermined threshold, the tempo of musical tones is determined on the basis of the difference in generation time and other parameters such as the volume and quality of the musical tones are determined on the basis of the difference in generation time. If the difference in generation time is less than the threshold, the key corresponding to the current operation signal is compared with the key corresponding to the previous operation signal. If they are not adjacent to each other, the key depressions are not considered as an erroneous operation and tempo of the musical tones and parameters such as the volume and intensity of each musical tone are determined on the basis of the difference in generation time. Since a key adjacent to an intended key is likely to be accidentally depressed, determination as to whether a key depression is erroneous can be restricted to keys adjacent to the previously depressed key. 
   The performance operator can be adapted to, in every performance operation by a user, generate a note-on message for the performance operation information at the start of the performance operation and generate a note-off message for the performance operation information at the end of the performance operation, and the musical performance control device can be adapted to exclude the present performance operation information from calculation of the tempo if the difference in generation time is less than a predetermined threshold and no note-off message is generated for the previous performance operation information 
   According to the present invention, when a player depresses a key, a note-on message is generated; when the player releases that key, a note-off message is generated. When an operation signal is generated in response to a performance operation, the difference in generation time generation between the present operation signal and the previous signal is calculated. If the calculated time difference is greater than or equal to a predetermined threshold, tempo of the musical tones and parameters such as the volume and quality of each musical tone is determined on the basis of the difference in generation time. If the difference in generation time is less than the threshold, determination is made as to whether a note-off message for the previous performance operation has been generated. If the note-off message has not been generated, it is determined that the operations are successive erroneous key depressions and determination of the parameters such as the volume and quality of the musical tones is omitted. A key adjacent to an intended key is likely to be accidentally depressed at approximately the same time as the intended key is depressed. Therefore, determination as to whether or not a key depression is an erroneous operation can be restricted a case where a note-off message of the previous key depression has not been received. 
   In a second aspect of the present invention, there is provided a program for causing a musical performance control apparatus, comprising a performance operator adapted to generate performance operation information in response to performance operations by a user, the performance operation information including information indicative of performing timing in automatic performance, and a storage device adapted to store data of a music piece comprising sequence data of note information for individual musical tones, to execute: a performance control module of, each time the performance operation information is generated, calculate tempo of automatic performance on the basis of the difference in generation time between the present performance operation information and the previous performance operation information, and reading out the data of the music piece data from the storage device with the tempo, wherein the performance control module comprising excluding the present performance operation information from calculation of the tempo if the difference in generation time is less than a predetermined threshold. 
   The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing the construction of an ensemble system including a controller as a musical performance control apparatus according to an embodiment of the present invention. 
       FIG. 2  is a block diagram showing the construction of the controller shown in  FIG. 1 . 
       FIG. 3  is a block diagram showing the construction of a performance terminal shown in  FIG. 1 . 
       FIG. 4  is a diagram showing the relationship among musical piece data, a player&#39;s key depression velocity, and a specified sound volume value used when sound generation instructing data is determined by the controller. 
       FIG. 5  is a flowchart of a procedure for determining sound generation instructing data performed by the controller. 
       FIGS. 6A and 6B  are diagrams showing the relationship between data of a music piece, a player&#39;s key depression velocity, and a specified sound volume value in variations of the example shown in  FIG. 4 .  FIG. 6A  shows an example in which information indicating a pitch (note number) sent from a performance terminal  2  is used to detect an erroneous operation and  FIG. 6B  shows an example in which a note-off message sent from a performance terminal  2  is used to detect an erroneous operation. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present invention will be described with reference to the accompanying drawings. 
     FIG. 1  is a block diagram showing an ensemble system including a controller  1  which is a performance control apparatus according to an embodiment of the present invention. The ensemble system  100  includes a controller  1  and a plurality of (six in  FIG. 1 ) performance terminals ( 2 A- 2 F) connected to the controller  1  through a MIDI interface box  3 . In this embodiment, the interposition of the MIDI interface box  3  allows the performance terminals  2  to be connected to the controller  1  through separate MIDI channels. The MIDI interface box  3  is connected to the controller  1  through a USB. 
   In the ensemble system  100  according to the embodiment, the controller  1  controls the performance terminals  2  so as to automatically play different musical parts, thereby playing in ensemble. A musical part is a tune, for example, constituting an ensemble. Examples of musical parts include one or more melody parts, rhythm parts, and multiple accompanying parts played by different instruments. 
   In the ensemble system  100 , each of the performance terminals  2  does not perform full automatic performance but a player of each of the performance terminals  2  indicates a sound volume, intensity, timing, and tempo by performance operation for each piece of data for each of the musical parts in a predetermine length of time (for example, sectional data such as ½ bar). The ensemble system  100  performs an ensemble at appropriate playing timing when each player performs a performance operation at particular operation timing. 
   The operation timing may be common to the performance terminals  2  or may be indicated by a performance operation performed by a facilitator (for example, the player of performance terminal  2 A) acting as a guide, or may be indicated by a direction using a hand by the facilitator to the players. If the players play in accordance with the operation timing indicated, appropriate ensemble is performed. 
   Each of the performance terminals  2  is implemented by an electronic keyboard instrument such as an electronic piano. The performance terminal  2  accepts a performance operation (for example a depression of one of the keys on the keyboard). The performance terminals  2  have the capability of communicating with the controller  1  and send an operation signal indicating operation information (for example, a note-on message in MIDI data) to the controller  1 . The operation information includes information indicating a pitch. The controller  1  in the present embodiment uses operation information as information indicating timing of a performance operation by ignoring (filtering out) information indicating a pitch. Therefore, depression of any key with the same force causes the same operation signal to the controller  1 . Thus, a player unfamiliar with playing keyboard instruments can play simply by pressing any one of the keys. 
   The controller  1  may be implemented by a personal computer, for example, and software installed in the personal computer controls musical performance on the performance terminals  2 . In particular, musical data consisting of multiple musical parts is stored in the controller  1 . The controller  1  allocates a musical part (or parts) to each of the performance terminals  2  before starting an ensemble. 
   The controller  1  has the capability of communicating with the performance terminals  2 . When the controller  1  receives an operation signal indicating a performance operation from a performance terminal  2 , the controller  1  determines, on the basis of the operation signal, tempo and timing of the musical part allocated to the performance terminal  2  that output the operation signal. The controller  1  then sequences a predetermined time length of musical piece data for the allocated musical parts with the determined tempo and timing and sends the data to the performance terminals  2  as sound generation instruction data. The sound generation instruction data includes timing of sound generation, the length of sound, sound volume, timbre, effects, pitch variations, (pitch bends), and tempo. 
   The performance terminals  2  plays automatic performance of different musical parts in accordance with sound generation instruction data by using a built-in sound generator. Thus, the performance terminals  2  play the musical parts allocated by the controller  1  with the intensity indicated by the players through performance operations and, as a result, an ensemble is performed. The performance terminals  2  are not limited to electronic pianos. The performance terminals  2  may be other electronic instruments such as electronic guitars. Of course, the appearance of the performance terminal is not limited to a natural musical instrument. It may be a terminal equipped with simple operating elements such as buttons. 
   Each of the performance terminals  2  does not need to have a built-in sound generator. A separate sound generator may be connected to the controller  1 . In this case, a single sound generator or as many sound generators as the number of the performance terminals  2  may be connected to the controller  1 . If as many sound generators as the number of the performance terminals  2  are connected, the controller  1  may associate the sound generators with the performance terminals  2  and allocate musical parts of musical piece data to them. 
   Constructions of the controller  1  and the performance terminal  2  will be described below in detail. 
     FIG. 2  is a block diagram showing the construction of the controller  1  shown in  FIG. 1 . As shown, the controller  1  includes a communication section  11 , a control section  12 , a hard disk drive  13 , a RAM  14 , a user operation console  15 , and a display  16 . Connected to the control section  12  are the communication sections  11 , the hard disk drive  13 , the RAM  14 , the user operation console  15 , and the display  16 . 
   The communication section  11  communicates with performance terminals  2  and has a USB interface. Connected to the USB interface is a MIDI interface box  3 . The communication section  11  communicates with the six performance terminals  2  through the MIDI interface box  3  and MIDI cables. The HDD  13  stores operating programs with which the controller  1  operates and musical piece data consisting of multiple musical parts. 
   The control section  12  reads an operating program stored in the HDD  13  and loads it in the RAM  14 , which is a work memory, and executes processing of musical part allocating section  50 , a sequencing section  51 , and a sound generation instructing section  52 . The musical part allocating section  50  allocates musical parts of musical piece data to performance terminals  2 . The sequencing section  51  determines tempo and timing based on operation signals received from the performance terminals  2  and sequences (determines parameters such as the sound volume and timbre of) each musical part of the musical piece data using the determined tempo and timing. The sound generation instructing section  52  sends parameters such as the volume of sound and timbre determined at the sequencing section  51  to the performance terminals  2  as sound generation instruction data. 
   The user operation console  15  is used by a player (mainly a facilitator) for issuing instructions to the ensemble system  100  to operate. The facilitator operates the user operation console  15  to specify musical piece data to play and allocate musical parts to the performance terminals  2 . The display  16  is a monitor. The facilitator and players look at the display  16  while playing. The display  16  displays information such as performance timing for playing in ensemble. 
   The control section  12  determines the tempo for sound generation instruction data on the basis of the difference in time between a performance operation and the next performance operation. That is, the control section  12  determines the tempo on the basis of the input time difference between note-on messages in operation signals it has received from the performance terminals  2 . 
   It should be noted that the moving averages of multiple performance operations (the last several performance operations) may be calculated and a time-weight may be assigned to them. The heaviest weigh is assigned to the last performance operation and increasingly lighter weights are assigned to older performance operations. By determining tempo in this way, the tempo can be naturally changed in accordance with the flow of a music piece without a sudden change of tempo even if there is a significant irregular change in the time intervals between performance operations. 
     FIG. 3  is a block diagram showing the construction of the performance terminal  2  shown in  FIG. 1 . As shown, the performance terminal  2  includes a communication section  21 , a control section  22 , a keyboard  23 , which is a performance operator, a sound generator  24 , and a loudspeaker  25 . The communication section  21 , the keyboard  23 , and the sound generator  24  are connected to the control section  22 . The loudspeaker  25  is connected to the sound generator  24 . 
   The communication section  21  is a MIDI interface which communicates with the controller  1  through a MIDI cable. The control section centrally controls the performance terminal  2 . 
   The keyboard  23  has  61  or  88  keys, for example, and is capable of playing 5 to 7 octaves. In the ensemble system  100 , however, the keys are not differentiated but instead note-on/note-off messages and data indicating how hard the keys are depressed (key depression velocity) are used. In particular, each key has a built-in sensor that senses the on/off operations and a built-in sensor that senses key depression intensity. The keyboard  23  provides an operation signal responsive to the fashion in which keys are operated (such as which key has been pressed and how hard) to the control section  22 . The control section  22  sends note-on and note-off messages to the controller  1  through the communication section  21  on the basis of an operation signal input to it. 
   The sound generator  24  generates a musical sound waveform in accordance with the control (namely the sound generation instruction data) of the control section  22  and outputs it as a sound signal to the loudspeaker  25 . The loudspeaker  25  reproduces the sound signal input from the sound generator  24  and outputs musical tones. While the sound generator  24  and the loudspeaker  25  are contained in each of the performance terminals  2  in this embodiment, the present invention is not so limited. For example, a sound generator and a loudspeaker may be connected to the controller  1  so that musical tones are output from a location different from the locations of the performance terminals  2 . In this case, as many external sound generators as the number of the performance terminals  2  or a single sound generator may be connected to the controller  1 . 
   In the present embodiment, the control section  22  sends a note-on/note-off message to the controller  1  when a key of the keyboard  23  is depressed and a musical tone is generated in response to an instruction from the controller  1  (local off) instead of the note message from the keyboard  23 . However, the performance terminal  2  can also be used as a conventional electronic musical instrument, of course, in addition to functioning as described above. When a key of the keyboard  23  is depressed, the control section  22  can instruct the sound generator  24  to generate a musical tone in accordance with that note message (local on). Switching between the local on and local off may be made by a user through use of the user operation console  15  of the controller  1  or a terminal operation console (not shown) on the performance terminal  2 . Furthermore, some of the keys may be set to local-off mode and the others to local-on mode. 
   The control section  12  of a conventional controller  1  has determined tempo on the basis of the time difference between note-on message receptions. However, beginners intending to depress one of the keys of a keyboard  23  have often accidentally depressed an adjacent key as well. In such a case, more than one note-on message is transmitted in a short time, considerably changing the tempo. According to the present embodiment, a threshold for the time difference between note-on message receptions is set and continuous key depressions performed in a time less than the threshold are ignored to prevent fluctuations in tempo due to erroneous performance operations. Thus, inexperienced player can enjoy playing at ease. 
   Operation for determining sound generation instruction data according to the present embodiment will be described below.  FIG. 4  is a diagram showing the relationship among musical piece data, key depressions by a player, and the time differences between note-on message receptions when sound generation instruction data is determined by the controller  1 . The horizontal axis in  FIG. 4  represents the flow of time. When the player depresses a key of the keyboard  23  of a performance terminal  2 , a note-on message is sent to the controller  1 , sound generation instruction data for a predetermined length (for example, 1 beat) is determined, and a musical tone is generated. 
   The control section  12  receives the note-on message and calculates the time difference Δt 2  between the reception of the previous note-on message (the timing of key depression  1 ) and the reception of the current note-on message (at key depression  2 ). The time difference Δt 2  is compared with a predetermined threshold Δt 5  (which will be described later). If the time difference Δt 2  between the key depressions is greater than or equal to the predetermined threshold Δ 5   t , the current key depression is considered as a correct performance operation and timing and tempo are determined. The tempo may be determined on the basis of the time difference Δt 2  or may be average value of the previous time difference Δt 1  and the current time difference Δt 2 . Alternatively, it may be determined on the basis of the average of the past time differences. As described above, the heaviest weight may be assigned to the latest time difference and increasingly lighter weights may be assigned to time differences between older performance operations. 
   Then, musical piece data for 1 beat is read with the determined timing and tempo and sound generation instruction data is determined. The determined sound generation instruction data is sent to the performance terminal  2 . The control section  12  updates the threshold on the basis of the time difference Δt 2 . The updated threshold Δt 6  will be used when the next note-on message is input. For example, Δt 6 =Δt 2 /2. That is, the threshold Δt 5  compared with the time difference Δt 2  at key depression  2  is represented as Δt 5 =Δt 1 /2, which has been updated when key depression  1  is performed. The method for updating the threshold is not limited to the example that is based on the latest key depression time difference. The threshold may be determined on the basis of the average value of the past key depression time differences. Furthermore, a fixed threshold may be used for performance of a music piece. The fixed value may be allowed to be manually changed by a facilitator. 
   When a note-on message is input in response to an erroneous key depression  1  (erroneous key depression made when key depression  2  was performed) in  FIG. 4 , the time difference Δt 4  between the reception of the previous note-on message (the timing of key depression  2 ) and the reception of the current note-on message (at key depression  1 ) is calculated. The time difference Δt 4  is compared with threshold Δt 6 . If the time difference Δt 4  is less than the threshold Δt 6 , the current key depression is considered as an erroneous operation and the current note-on message is ignored. Therefore, for this note-on message, determination of tempo and timing is omitted and sound generation instruction data is not determined. Of course, the threshold is not updated. 
   When a note-on message is input in response to the next key depression  3 , the time difference Δt 3  between key depressions  2  and  3  is calculated. The time difference Δt 3  is compared with the threshold Δt 6 . If the time difference Δt 3  is greater than or equal to the threshold Δt 6 , the current key depression is considered as a correct performance operation and timing and tempo are determined. Consequently, sound generation instruction data is determined from the key depression  3 . The threshold is updated based on the time difference Δt 3 . The threshold Δt 7  to be used when the next note-on message is input is updated as Δ 7 =Δt 3 /2. 
   The operation performed by the control section  12  for determining sound generation instruction data will be described with reference to a flowchart.  FIG. 5  is a flowchart illustrating a procedure performed by the controller  1  for determining sound generation instruction data. This operation is triggered by input of a note-on message from a performance terminal  2 . First, the time difference between the input of this note-on message and the input of the previous note-on message is calculated (step S 11 ). It should be noted that when the first note-on message is input at the beginning of performance, normally there is no previous note-on message input. In the present embodiment, the time difference from a previous note-on message when the first note-on message is input at the beginning of performance is determined as follows. 
   When players depresses keys in response to a cue by a facilitator after allocation of musical parts to the performance terminals  2  for playing in ensemble, musical piece data is not read, musical tones are not generated (or only rhythm sound “tum-tum” is generated), and only note-on messages for determining tempo are input for the first several performance timings (for example, four key depressions). In this case, determination of sound generation instruction data is omitted (or determination is made that rhythm sound is to be generated) at the step of determining sound generation instruction data (step S 15 ), which will be described later. It is not until the fifth performance timings that musical piece data is read, sound generation instruction data is determined, and performance is started. It should be noted that time difference calculation at step S 11  is not performed for the first one of the note-on messages used for determining the tempo because there is no previous performance timing. 
   Then, the control section  12  determines whether the time difference calculated at step S 11  is greater than or equal to a predetermined threshold (step S 12 ). The threshold may be a value updated at the previous performance timing (processing at step S 17 , which will be described later) or may be a fixed value for performance of one music piece. If the time difference is greater than or equal to the threshold, the current key depression is considered as a correct performance operation and steps S 13  to S 17  are performed. If the time difference is less than the threshold, the current key depression is considered as an erroneous operation and the process will terminates. As mentioned above, there is no previous performance timing for the first note-on message input after allocation of musical parts, therefore it is assumed at this decision step that the current key depression is a correct performance operation and steps S 13  to S 17  are performed. 
   Then, the control section  12  calculates the moving averages of time differences between note-on message inputs (step S 13 ). As described earlier, weighted moving averages may be calculated by assigning the heaviest weight to the latest performance operation and increasingly lighter weights to older performance operations. Then, tempo and timing for a predetermined time length (for example, 1 beat) are determined on the basis of the calculated moving averages (step S 14 ). Musical piece data is read for the predetermined time length with the determined timing and tempo and sound generation instruction data is determined, including the length of musical tone to be generated, sound volume, timbre, effect, pitch changes, and tempo (step S 15 ). The determined sound generation instruction data is sent to the performance terminals  2  (step S 16 ). In the case of a note-on message for the operation for determining tempo described above, determination of sound generation instruction data is omitted (or data for generating a rhythm sound is determined). In this case, the process of step S 14  for determining tempo is not performed, of course. 
   Finally, the threshold is updated on the basis of the calculated moving average (step S 17 ). The threshold may be updated with a half the time equal to the moving average as described above. For the first note-on message input after allocation of musical parts, there is no moving average calculated and therefore the threshold is not updated. Alternatively, the threshold may be updated to a predetermined value. If the threshold is fixed for performance of a music piece, the threshold is not updated. An initial threshold value may be preset on the basis of tempo data contained in musical piece data. Alternatively, a facilitator may manually set an initial threshold value. In this case, it may be assumed that there was a virtual previous key depression a predetermined amount of time (fore example an amount of time equal to twice a threshold) before the detection of the first key depression. This allows an erroneous key depression to be detected even it is the first key depression. Thus, players can enjoy playing without concern for erroneous performance operation from the beginning. 
   Since a threshold is set for the time difference between inputs of note-on messages and, if the time difference between inputs of note-on messages is less than the threshold (NO in step S 12 ), steps S 13  to S 17  are skipped (ignored) as described above, erroneous performance operations will not disturb tempo and therefore even an inexperienced player can enjoy playing at ease. 
   The following variations of the present embodiment are possible.  FIGS. 6A and 6B  are diagrams showing variations of the relationship among musical piece data, player&#39;s key depressions, and the time difference between receptions of note-on messages shown in  FIG. 4 .  FIG. 6A  shows a diagram illustrating an example in which information indicating a pitch (note number) sent from a performance terminal  2  is used to detect an erroneous operation. The same elements as those shown in  FIG. 4  will be labeled the same reference symbols (Δt 1 -Δt 7 ) and the description of which will be omitted. 
   When a player depresses a key of the keyboard  23  of a performance terminal  2 , a note-on message is sent to the controller  1 . The note-on message includes information indicating a note number. For example, note-on messages of key depressions  1  and  2  include information indicating note number  68 . 
   A controlling section  12  receives the note-on message and calculates the time difference Δt 2  between the reception of the previous note-on message (the timing of key depression  1 ) and the reception of the current note-on message (the timing of key depression  2 ). The time difference Δt 2  is compared with a predetermined threshold Δt 5 . If the time difference Δt 2  is greater than or equal to the threshold Δt 5 , the current key depression is considered as a correct performance operation and timing and tempo are determined. 
   Then musical piece data for 1 beat is read with the determined timing and tempo and sound generation instruction data is determined. The determined sound generation instruction data is sent to the performance terminal  2 . The control section  12  updates the threshold on the basis of the time difference Δt 2 . The updated threshold Δt 6  will be used when the next note-on message is input. 
   When a note-on message caused by an erroneous key depression  1  is input (an accidental key depression made when key depression  2  was performed), the time difference Δt 4  between reception of the previous note-on message (the timing of key depression  2 ) and the reception of the current note-on message (the timing of erroneous key depression  1 ) is calculated as in the example described above. The time difference Δt 4  is compared with the threshold Δt 6 . If the time difference Δt 4  is less than the threshold Δt 6 , the note number included in the current note-on message (of erroneous key depression  1 ) is compared with the note number included in the previous note-on message (of key depression  2 ). If the note number included in the current note-on message (of the erroneous key depression  1 ) is a consecutive note number,  69 , (or  67 ) immediately succeeding or the preceding note number,  68 , of the previous key depression  2 , the current key depression is considered as an erroneous operation and the current note-on message is ignored. 
   When a note-on message caused by the next key depression  3  is input, the time difference Δt 3  between key depressions  2  and  3  is calculated and is compared with the threshold Δt 6 . If the time difference Δt 3  is greater than or equal to the threshold Δt 6 , it is determined that this key depression is a correct performance operation and timing and tempo are determined. As a result, sound generation instruction data is determined based on key depression  3 . Also, the threshold is updated based on the time difference Δt 3 . The updated threshold Δt 7  to be used when the next note-on message is input is Δt 7 =Δt 3 /2. 
   When subsequently a note-on message caused by key depression  4  is input, the time difference Δt 8  between key depression  3  and key depression  4  is calculated and is compared with the threshold Δt 7 . If the time difference Δt 8  is less than the threshold Δt 7 , the note number contained in the current note-on message (of key depression  4 ) is compared with the note number contained in the previous note-on message (of key depression  3 ). If the note number ( 38  in  FIG. 6A ) contained in the current note-on message (of key depression  4 ) is not a consecutive note number before or after the note number  68  of the previous key depression  3 , the current key depression is considered as a correct performance operation and timing and tempo are determined. As a result, sound generation instruction data is determined based on key depression  4 . 
   In this way, an erroneous operation may be detected on the basis of whether note numbers are consecutive numbers, in addition to the time difference between inputs of note-on messages. If a key is mistakenly depressed by an erroneous operation, the key is likely to be a key adjacent to an intended key. Therefore, determination as to whether an operation is an erroneous operation can be restricted to keys adjacent to the previous key depressed. This can ensure an accurate determination as to whether a key depression is an erroneous one. 
     FIG. 6B  is a diagram illustrating an example in which a note-off message sent from a performance terminal  2  is used to detect an erroneous operation. The same elements as those shown in  FIG. 6A  will be labeled the same reference symbols (Δt 1 -Δt 8 ) and the description of which will be omitted. 
   When a player depresses a key of the keyboard  23  of a performance terminal  2 , a note-on message is sent to the controller  1 ; when the player releases the depressed key, a note-off message is sent to the controller  1 . 
   A control section  12  receives the note-on message and calculates the time difference Δt 2  between the reception of the previous note-on message (the timing of key depression  1 ) and the reception of the current note-on message (the timing of key depression  2 ). The time difference Δt 2  is compared with a predetermined threshold Δt 5 . If the time difference Δt 2  is greater than or equal to the predetermined threshold Δt 5 , the current key depression is considered as a correct performance operation and timing and tempo are determined. 
   Then musical piece data for 1 beat is read with the determined timing and tempo and sound generation instruction data is determined. The determined sound generation instruction data is sent to the performance terminal  2 . The control section  12  updates the threshold on the basis of the time difference Δt 2 . The updated threshold Δt 6  will be used when the next note-on message is input. 
   When subsequently a note-on message caused by an erroneous key depression  1  (an accidental key depression made when key depression  2  was performed) is input, the time difference Δt 4  between the reception of the previous note-on message (the timing of key depression  2 ) and the reception of the current note-on message (the timing of erroneous key depression  1 ) is calculated as mentioned above. The time difference Δt 4  is compared with the threshold Δt 6 . If the time difference Δt 4  is less than the threshold Δt 6 , determination is made as to whether a note-off message of the previous key depression  2  has been received. If the note-off message of the previous key depression  2  has not been received, the current key depression is considered as an erroneous operation and the current note-on message is ignored. 
   When a note-on message caused by the next key depression  3  is input, the time difference Δt 3  between key depression  2  and key depression  3  is calculated and is compared with the threshold Δt 6 . If the time difference Δt 3  is greater than or equal to the threshold Δt 6 , this key depression is considered as a correct performance operation and timing and tempo are determined. As a result, sound generation instruction data is determined based on key depression  3 . The threshold is updated on the basis of the time difference Δt 3 . The updated threshold to be used when the next note-on message is input is Δt 7 =Δt 3 /2. 
   When a note-on message caused by the next key depression  4  is input, the time difference Δt 8  between key depression  3  and key depression  4  is calculated and is compared with the threshold Δt 7 . If the time difference Δt 8  is less than the threshold Δt 7 , determination is made as to whether a note-off message of the previous key depression  3  has been received. If the note-off message of the previous key depression  3  has been received, the current key depression is considered as a correct performance operation and timing and tempo are determined. As a result, sound generation instruction data is determined based on key depression  4 . 
   In this way, an erroneous operation may be detected on the basis of whether a note-off message caused by the previous key depression has been input. A key adjacent to an intended key is likely to be depressed at approximately the same time as the intended key is depressed. Therefore, determination as to whether or not a key depression is an erroneous operation may be restricted to a case where a note-off message of the previous key depression has not been received. This can ensure more accurate determination as to whether a key depression is an erroneous key depression. 
   Determination as to whether or not a key depression is an erroneous operation may be made on the basis of a logic of key depression and release (namely a sequence of a depression and release of a key) in addition to the time difference between operations, the difference between note numbers, and whether a note-off message has been received. For example, if a key is depressed and then multiple keys are depressed before the key is released, it may be determined that the depressions of the multiple keys are erroneous depressions. 
   Furthermore, information indicating the intensity of a key depression (velocity) contained in an operation signal sent from a performance terminal  2  may be used to detect an erroneous operation. If the time difference between note-on message inputs is less than a threshold, the velocity of the previous key depression may be compared with the velocity of the current key depression and, if the velocity of the current key depression is approximately equal to the velocity of the previous key depression (if the difference between the velocity values is within a predetermined range), it may be determined that the current key depression is an erroneous operation. 
   The control section  12  of the controller  1  may count the number of erroneous key depressions performed on each of the performance terminals  2  and may records the count as a log on a HDD  13  after one music piece has been played. A facilitator can check the log to see the level of proficiency in each player. The control section  12  may determine a threshold on the basis of the number of erroneous key depressions recorded on the log. The control section  12  may set a greater threshold for a performance terminal  2  on which many erroneous key depressions have been made (such as a performance terminal  2  played by a beginner), thereby preventing erroneous operations from changing tempo and disturbing performance. On the other hand, the control section  12  may set a less threshold for a performance terminal  2  on which fewer erroneous key depression have been made (such as a performance terminal  2  played by a skilled player) to allow the player to play music with drastically varying tempo. 
   The ensemble system according to the present embodiment can also provide the following rendering by taking into account the gate time between a note-on and a note-off in determining tempo. For example, when a particular key is pressed and released quickly, the control section  12  (sequencing section  51 ) of the controller  1  may provide a short tone for the beat whereas when a key is pressed and released slowly, the control section  12  may provide a tone with a long tone for the beat. In this way, a musical rendering in which sounds are disconnected crisply (staccato) without significantly changing tempo can be implemented on a performance terminal  2  or a musical rendering in which a tone is sustained for a long time without significantly changing tempo (tenute). 
   Some keys of a keyboard  23  may be enabled to play staccato or tenute and the others not. The controller  1  may change the length of sounds while maintaining a constant tempo only when a note-on message or a note-off message is input from a particular key (for example, E 3 ). 
   It is to be understood that the object of the present invention may also be accomplished by supplying a computer, for example, the controller  1  with a storage medium in which a program code of software which realizes the functions of the above described embodiment is stored, and causing a computer (or CPU or MPU) of the system or apparatus to read out and execute the program code stored in the storage medium. 
   In this case, the program code itself read from the storage medium realizes the functions of any of the embodiments described above, and hence the program code and the storage medium in which the program code is stored constitute the present invention. 
   Examples of the storage medium for supplying the program code include a floppy (registered trademark) disk, a hard disk, a magnetic-optical disk, a CD-ROM, a CD-R, a CD-RW, DVD-ROM, a DVD-RAM, a DVD−RW, a DVD+RW, a magnetic tape, a nonvolatile memory card, and a ROM. Alternatively, the program may be downloaded via a network. 
   Further, it is to be understood that the functions of the above described embodiment may be accomplished not only by executing a program code read out by a computer, but also by causing an OS (operating system) or the like which operates on the computer to perform a part or all of the actual operations based on instructions of the program code. 
   Further, it is to be understood that the functions of the above described embodiment may be accomplished by writing a program code read out from the storage medium into a memory provided on an expansion board inserted into a computer or in an expansion unit connected to the computer and then causing a CPU or the like provided in the expansion board or the expansion unit to perform a part or all of the actual operations based on instructions of the program code.