referring to fig7 of the drawings , there is shown a general arrangement of an automatic player piano embodying the present invention . fig7 is provided for focusing upon production of key touch information , and , for this reason , the other components omitted therefrom . the automatic player piano largely comprises a mechanical piano 21 and an automatic player system 22 , and the mechanical piano 21 has a keyboard with a plurality of typically 88 keys , a key action mechanism 23 coupled to the keys , a plurality of hammer assemblies linked with the key action mechanism 23 , a plurality of music wires capable of being struck with the hammer assemblies , respectively , and a damper mechanism 24 accompanied with a set of pedals 25 . the mechanical piano 21 is thus provided with multiple mechanical lines , however , fig7 shows only one mechanical line including the key 26 , the hammer assembly 27 and the music wire 28 in a modeled form . the automatic player system largely comprises a controller coupled at the input port thereof to a sensor unit 29 and at the output port thereof to a plurality of solenoid - operated actuators 30 , and the controller achieves functions as tracing means , sampling means , key velocity calculating means , final hammer velocity deciding means and key touch information producing means . the automatic player piano thus arranged is shifted between a recording mode of operation and a playback mode of operation . when a human player shifts the automatic player piano into the recording mode of operation and , then , begins to perform a music by successive keying - in operations , the keys are moved from undepressed states toward depressed states along respective traveling paths depending upon key touches , respectively . the forces exerted on the keys are transmitted through the key action mechanism 23 to the hammer assemblies , respectively . then , the hammer assemblies are driven for rotations toward the music wires , and sounds are mechanically produced upon respective strikes . the key motions from the undepressed states toward the depressed states are respectively detected by the sensor unit 29 and the tracing means produce pieces of a locis information representative of loci of the key motions . after formation of the pieces of the locus information , the sampling means access the pieces of the locus information to extract sections for uniform motions from the loci , respectively . in the sections , the keys respectively travel in uniform motions . when the sections are respectively extracted from the loci of the key motions , the key velocity calculating means decide respective key velocities in the sections , and the final hammer velocity deciding means estimate respective final velocities of the hammer assemblies on the basis of the key velocities . the final hammer velocities are thus estimated from the key velocities , respectively , then the key touch information producing means produce the pieces of the key touch information each representative of the the intensity of the sound produced upon striking the music wires with the hammer assemblies . these pieces of the key touch information are memorized in the controller for a latter usage . after memorizing the pieces of the key touch information into the controller , the automatic player piano is shifted into the playback mode of operation , then the controller retrieves the pieces of the key touch information in succession . the pieces of the key touch information are used for driving the solenoid - operated actuators 30 , and , accordingly , the keys are moved with respective powers tantamount to those transmitted to the key action mechanism 23 . then , the hammer assemblies are driven for rotations toward the music wires , and the sounds are reproduced with the intensities equivalent to those of the original sounds . turning to fig8 of the drawings , there is shown the mechanical arrangement of the automatic player piano illustrated in fig7 . the mechanical piano 21 is of the upright type , and the keyboard 31 including the key 26 is mounted on a key bed 32 . each of the keys is swingable with respect to a balance pin 33 , however , the key motion is restricted by a back rail cloth member 34 and a front rail member 35 . in this instance , the sensor unit 29 is provided on the key bed 32 between the front rail member 35 and the balance pin 33 , and the solenoid - operated actuators 30 are mounted on the key bed 32 between the balance pin 33 and the back rail cloth member 34 . though not clearly shown in fig8 the sensor unit 29 is provided with a plurality of photo couplers which are grouped by four and provided in association with the keys , respectively . each of the keys are accompanied with a shutter plate 36 projecting from the lower surface of the key , and the shutter plate 36 is downwardly moved with the key , so that optical paths of the photo couplers are successively interrupted by the shutter plate 36 , thereby detecting the locus of the key motion . every four photo couplers are provided in association with every key , and the photo couplers grouped by four are referred to as &# 34 ; photo coupler group &# 34 ; in the following description . in each of the photo coupler groups , the four photo couplers are called as first , second , third and fourth photo couplers from the key side to the key bed side . the other mechanical components are well known in the art , and , for this reason , no further description is incorporated in the sake of simplicity . on the upper front board of the mechanical piano is exposed the front panel of the controller 37 previously described with reference to fig7 . turning to fig9 of the drawings , the circuit arrangement of the controller 37 is illustrated and contains three micro - computer units 41 , 42 and 43 which are of the single chip type . the micro - computer unit 41 is provided for scanning the sensor unit 29 and periodically checks the sensor unit 29a to see whether or not any photo coupler detects the key motion . when the sensor unit 29a detects the key motion , the micro - computer unit 41 produces a piece of the key touch information as well as a piece of a note information representative of a note assigned to the key depressed by the human player . on the other hand , the micro - computer unit 43 is dedicated to a manipulating panel 44 , a midi unit 45 and a floppy disk driver unit 46 . on the manipulating panel 44 are provided various kinds of switches such as , for example , a power switch , a volume switch , a mode selecting switch and so on , then the micro - computer unit 43 periodically checks the manipulating panel 44 to see whether or not any switch is operated . the manipulating panel 44 is accompanied with a remote controller 47 , so that anyone can change the operation mode and the volume from a long distance . the floppy disk driver unit 46 is used for writing and reading out the pieces of the key touch information as well as the pieces of note information into and from a floppy disk 48 . if the pedals 25 are operated by the human player , pieces of a pedal actuating information is also memorized into the floppy disk 48 . the midi unit 45 is provided for a communication with another electronic musical instrument such as , for example , an autorhythmic system . however , the micro - computer unit 42 serves as a supervisor for the other computer units 41 and 43 and , accordingly , transfers the key touch information and the note information from the micro - computer unit 41 to the micro - computer unit 43 . the micro - computer unit 42 is further operative to check into the sensor unit 29b associated with the pedals 25 for producing the pieces of the pedal actuating information which is also transferred to the micro - computer unit 43 for the storage . when the pieces of the information are retrieved from the floppy disk 48 , the micro - computer unit 43 transfers the pieces of the information to the micro - computer unit 42 which in turn transfers them to a solenoid driver unit 49 . the solenoid driver unit 49 is responsive to the pieces of the information and selectively distributes electric power supplied from the power unit 50 , thereby causing the solenoid operated actuators 30a and 30b to be actuated . in order to produce the force tantamount to that originally transferred to the key action mechanism 23 , the solenoid driver unit 49 changes the duty ratio of the electric power depending upon the piece of the key touch information . turning to fig1 , description is made for a program sequence executed by the micro - computer units 41 to 43 of the controller 37 . when the power switch turns on , the controller 37 immediately executes an initialized subroutine program p1 . upon completion of the initialized subroutine program , the controller 37 proceeds to step p2 and checks to see whether or not the mode selecting switch is shifted to the recording mode of operation . if the answer to the step p2 is given in the positive , the controller 37 is branched to a recording subroutine program p3 which will be described hereinafter in detail . however , if the answer to the step p2 is given in the negative , the controller 37 further checks to see whether or not the automatic player piano is shifted into the playback mode of operation as by step p4 . if the controller 37 acknowledges the playback mode of operation , the answer to the step p4 is given in the positive , then the controller 37 is branched to a playback subroutine program p5 which is also described hereinafter in detail . however , when no operation mode is specified , the answer to the step p4 is given in the negative , then the controller 37 proceeds to step p6 . in the step p6 , the controller 37 checks to see whether or not any switches except for the mode selecting switch is operated . if the answer to the step p6 is given in the negative , the controller 37 returns to the step p2 and reiterates the loop consisting of the steps p2 , p4 and p6 until the answer to any one of the steps p2 , p4 and p6 is given in the positive . when any one of the switches except for the mode selecting switch is operated , the answer to the step p6 is given in the positive , then the controller 37 is branched to a subroutine program for the other switches p7 . whenever any one of the subroutine programs p3 , p5 and p7 are completed , the controller 37 proceeds to step p8 to see whether or not the power switch turns off . the answer to the step p8 is given in the negative in so far as the electric power is supplied from the source 50 , then the controller 37 returns to the step p2 and reiterates the loop consisting of the step p2 to p8 until the power switch turns off . as described above , when the mode selecting switch is shifted to the recording mode of operation , the answer to the step p2 is given in the positive , then the controller 37 is branched to the recording subroutine program p3 . the program sequence of the recording mode of operation is illustrated in fig1 a and 11b and starts with step p30 where an internal timer of the micro - computer unit 41 begins to count clock pulses . then , the micro - computer unit 41 writes value &# 34 ; 1 &# 34 ; into an index register i as by step p31 and , thereafter , checks to see whether or not the photo coupler group associated with the first key detects the key motion as by step p32 . prior to a first keying - in operation , no photo coupler group detects any key motion , so that the answer to the step p32 is given in the negative , then allowing the microcomputer unit 41 to proceed to step p33 . in the step p33 , the micro - computer unit 41 checks to see whether or not the index register i has been increased to value &# 34 ; 88 &# 34 ;. the index register i is provided for specifying the position of the key currently checked , so that the answer to the step p33 is given in the negative before all of the eighty - eight keys are checked . in this situation , the micro - computer unit 41 proceeds to step p34 to increment the index register i . upon completion of the step p34 , the micro - computer unit 41 returns to the step p32 to check to whether or not the photo coupler group specified by the index register i detects the key motion . the micro - computer unit 41 thus reiterates the loop consisting of the steps p31 to p34 until the answer to the decision step p32 is given in the positive . however , when all of the photo coupler groups are checked by the micro - computer unit 41 , the index register i maintains value &# 34 ; 88 &# 34 ;, then the answer to the decision step p33 is given in the positive . with the positive answer for the decision step p33 , the micro - computer unit 41 returns to the step p31 to rewrite value &# 34 ; 1 &# 34 ; into the index register i again and , then , reiterates the loop consisting of the steps p32 to p34 to find the key depressed by the player . when a performance of a music starts with a first keying - in operation followed by a series of keying - in operations , the answer to the decision step p32 is given in the positive under the index register i matched with the key position subjected to the first keying - in operation . then , the micro - computer unit 41 proceeds to step p35 and checks to see whether or not the key motion is detected by the first photo coupler . any key motion is firstly detected by the first photo coupler , so that the answer to the decision step p35 is given in the positive immediately after a fresh keying - in operation . if it is found that the key motion is detected by the first photo coupler , the micro - computer unit 41 proceeds to step p36 and checks to see whether or not a first register assigned the first photo coupler keeps value &# 34 ; 0 &# 34 ;. when the key is moved from the undepressed state toward the depressed state , the first register stores value &# 34 ; 0 &# 34 ;. then , it is found that the first register keeps value &# 34 ; 0 &# 34 ;, the answer to the decision step p36 is given in the positive , and the microcomputer unit 41 proceeds to step 37 and transfer the counting value of the internal timer to the first register . after the step p37 , the micro - computer unit 41 returns to the step p33 to continue the detecting operation . when the shutter plate 36 interrupts the optical path of the second photo coupler , the answer to the decision step p32 is given in the positive , however , the answer to the decision step p35 is given in the negative . then , the micro - computer unit 41 proceeds to step p38 and checks to see whether or not the key motion is detected by the second photo coupler . after the detection by the first photo coupler , the key motion is usually detected by the second photo coupler . then , it is found that the answer to the decision step p 38 is given in the positive . with the positive answer to the decision step p38 , the micro - computer proceeds to step p39 to see whether or not a second register assigned the second photo coupler keeps value &# 34 ; 0 &# 34 ;. on the way to the depressed state , the second register also keeps value &# 34 ; 0 &# 34 ;. then , it is found that the second register keeps value &# 34 ; 0 &# 34 ;, and the answer to the decision step p39 is given in the positive , then the micro - computer unit 41 transfers the counting value of the internal timer to the second register as by step p40 . after the completion of the step p40 , the micro - computer unit 41 returns to the step p33 so as to continue the detecting operation . with a lapse of time , the shutter plate 36 interrupts the optical path of the photo coupler again , so that the answer to the decision step p32 is given in the positive , however , the answers to the decision steps p35 and p38 are given in the negative . then , the micro - computer unit 41 proceeds to step p41 to see whether or not the key motion is detected by the third photo coupler . after the interruption of the optical path of the second photo coupler , the shutter plate 36 usually interrupts the third photo coupler . then , it is found that the answer to the decision step p41 is given in the positive , and the micro - computer unit 41 checks into a third register assigned to the third photo coupler to see whether or not value &# 34 ; 0 &# 34 ; is stored in the third register as by step p42 . since the third register keeps value &# 34 ; 0 &# 34 ; upon the depression of the key , it is found that the third register keeps value &# 34 ; 0 &# 34 ;, and the micro - computer unit 41 transfers the counting value of the internal timer to the third register as by step p43 , then returning to the step p33 . after a while , the shutter plate 36 interrupts the optical path of the photo coupler again , so that the answer to the decision step p32 is given in the positive , however , the answers to the decision steps p35 , p38 and p41 are given in the negative . then , the micro - computer unit 41 proceeds to step p44 to see whether or not the key motion is detected by the fourth photo coupler . after the interruption of the optical path of the third photo coupler , the shutter plate 36 usually interrupts the fourth photo coupler . then , it is found that the answer to the decision step p44 is given in the positive , and the micro - computer unit 41 checks into a fourth register assigned to the fourth photo coupler to see whether or not value &# 34 ; 0 &# 34 ; is stored in the fourth register as by step p45 . the fourth register has been reset to value &# 34 ; 0 &# 34 ;, so that it is found that the fourth register keeps value &# 34 ; 0 &# 34 ;, and the micro - computer unit 41 transfers the counting value of the internal timer to the fourth register as by step p46 , then returning to the step p33 . in this manner , the counting values are successively stored in the first to fourth registers when the key is fully depressed , however , if the key is partially depressed in the shallow touch , the key motion may not be detected by the fourth photo coupler . in any case , the registers store the respective counting values which are indicative of the locus of the key motion . for this reason , the tracing means are achieved by the steps p30 to p46 . when the key is released , the key is moved toward the undepressed state , and the shutter plate 36 interrupts the optical path of the photo coupler again . then , the answer to the decision step 32 is given in the positive , and any one of the decision steps p35 , p38 , p41 and p44 is given in the positive . then , the micro - computer unit 41 proceeds to step p47 and calculates time intervals t1 , t2 and t3 between the first and second photo couplers , between the second and third photo couplers and between the third and fourth photo couplers , respectively . after the calculation , the micro - computer 41 proceeds to step p48 and resets the first and second registers for the subsequent keying - in operation . the micro - computer unit 41 compares the time intervals t1 to t3 with an internal table ( not shown ) to decide the kind of the keying - in operation as by step p49 and , then , selects one of the time intervals depending upon the kind of the keying - in operation decided on the basis of the time intervals as by step p50 . the selected time interval stands for the section where the key moves in the uniform motion . then , the steps p47 to p50 as a whole achieve the function of the sampling means . when the time interval is selected , the micro - computer unit 41 decides the key velocity on the basis of the selected time interval as by step p51 . then , the key velocity calculating means are achieved by the step p51 . when the key velocity is decided , the micro - computer unit 41 estimates the final hammer velocity and , then , produces a piece of the key touch information as by step p52 . then , the final hammer velocity deciding means as well as the key touch information producing means are achieved by the step p52 . thus , the piece of the key touch information is produced by the micro - computer unit 41 , then the piece of the key touch information is transferred to the micro - computer unit 43 which in turn transfers the piece of the key touch information to the floppy disk driver unit 46 for storing into the floppy disk 48 as by step p53 . if the piece of the key touch information is thus memorized into the floppy disk 48 , the micro - computer unit 41 returns to the step p33 for the subsequent keying - in operation . in this way , the micro - computer unit 41 repeats the loop consisting of the steps p30 to p53 until the automatic player piano is escaped from the recording mode of operation . additionally , the detecting operation will be masked from the completion of the step p48 to the return to the undepressed state . in the program sequence described above , all of the time intervals are calculated in the step p47 , however , some kinds of the keying - in operation tends to be characterized by only one time interval . for this reason , the micro - computer unit 41 may calculate the time interval t1 after the step p40 and check to see if or not the time interval t1 features the keying - in operation . if the kind of the keying - in operation is decided from the time interval only , no calculation is carried out for the time intervals t2 and t3 . if not , the subsequent time interval is calculated . thus , the time intervals are sequentially calculated from one to another , the micro - computer unit 41 will be certainly decreased in the amount of job . as described in connection with the problem of the prior - art , some users request the piano manufacturer to remodel the mechanical piano into an automatic player piano . the component members are not standardized , however , the space between the keyboard and the key bed are substantially identical with one another . then , it is preferable to accommodate the sensor units and the actuators in the space in view of the standardization . turning to fig1 of the drawings , there is shown the arrangement of a part of an automatic player piano embodying the present invention . the automatic player piano partially illustrated in fig1 is similar in arrangement to the automatic player piano illustrated in fig8 except for a sensor unit 61 and solenoid - operated actuators 62 , so that description is focused upon the sensor unit 61 and the solenoid - operated actuators 62 , and the other component members are denoted by like reference numerals designating the corresponding component members of the automatic player piano illustrated in fig8 . as illustrated in detail in fig1 , the sensor unit 61 largely comprises an encoder plate 63 and two photo couplers 64 and 65 supported by a bracket member 66 . two small windows 67 and 68 are formed in the encoder plate 63 in such a manner that optical paths of the photo couplers 64 and 65 intermittingly pass the windows 67 and 68 , respectively , while the key 26 is moved toward the depressed state . in this instance , each of the windows is about 0 . 5 millimeter in height . since the windows 67 and 68 are slightly deviated from each other as seen from fig1 , the optical path of the photo coupler 65 firstly extends through the window 68 on the way to the depressed state , and , then , both of the optical paths are established through the windows 68 and 67 for the photo couplers 65 and 64 . if the key 26 is further moved , the optical path of the photo coupler 65 is blocked by the encoder plate 63 , but the optical path of the photo coupler 64 still extends through the window 67 . however , if the key 26 is further advanced , both of the optical paths are blocked by the encoder plate 63 . thus , the sensor unit 61 is capable of producing four bit patterns or a two bits of an encoded signal , which is summarized in the following table , with only two photo couplers . this results in reduction in the production cost . in the sensor unit 61 shown in fig1 , the photo couplers 64 and 65 are arranged in juxtaposition , but the windows are slightly deviated from each other . however , the photo couplers may be arranged in a deviated manner with the juxtaposed windows in another implementation . table______________________________________ optical path of optical path of photo coupler 64 photo coupler 65______________________________________first position blocked establishedsecond position established establishedthird position established blockedfourth position blocked blocked______________________________________ if the two bits of the encoded signal is supplied to the controller , the controller can trace the locus of the key motion on the basis of the four bit patterns . for this reason , the micro - computer unit 41 periodically checks to see whether or not the bit patter is varied for making decisions instead of the steps p35 , p38 , p41 and p44 . the solenoid - operated actuators 62 are supported by a bracket member and accompanied with lever members 69 , respectively . each of the lever members 69 is rotatably supported at an intermediate portion thereof by the bracket member and engaged at the rear end portion thereof with a plunger 70 . the plunger 70 passes through a solenoid , so that the plunger 70 is projectable from the bracket member . the lever member 69 is engaged at the front end portion thereof with the shutter plate 36 , and , for this reason , the key 26 is pulled down upon the projection of the plunger 70 . turning to fig1 of the drawings , there is shown an encoder plate 71 incorporated in a sensor unit which in turn is provided in an automatic player piano embodying the present invention . the encoder plate 71 cooperates with three photo couplers 72 , 73 and 74 which are accompanied with a signal processing circuit illustrated in fig1 . however , the other components are similar to those of the automatic player piano shown in fig8 so that the corresponding components are referred to with like reference numerals , however , no detailed description is made . the encoder plate 71 has a plurality of windows 75 to 81 arranged in three lines , all of the windows 75 to 81 are equal in width to one another . however , the windows in each line are different in height from the windows in another line . namely , the windows 75 to 78 are equal in height to one another but different from the other windows 79 to 81 . similarly , the window 79 is equal in height to the window 80 but different from another window . the windows in the respective lines intermittingly pass the optical paths of the photo couplers 72 , 73 and 74 , respectively , and the three photo couplers 72 to 74 are arranged in a juxtaposed manner , so that three bits of an encoder signal is produced by the photo couplers 72 to 74 when the key 26 is moved from the undepressed state toward the depressed state . this results in that the controller 37 can discriminate eight positions on the locus of the key motion from one another . the three bit encoder signal is supplied from the photo couplers 72 , 73 and 74 to the signal processing circuit , and the signal processing circuit largely comprises eight flip flop circuits 82 to 89 ( each of which is abbreviated as &# 34 ; ff &# 34 ; in fig1 ) and eight and gates 90 to 97 which are of the three input node type . the three input nodes of each and gate are selectively accompanied with an inverter circuit or inverter circuits ( which are indicated by small bubbles ), and , for this reason , the and gates 90 to 97 sequentially produces output signals . the output signals of the and gates 90 to 97 are respectively supplied to the set nodes of the flip flop circuits 82 to 89 , however , the reset nodes of the flip flop circuits 2 to 89 are supplied with the output signals of the adjacent and gates 91 to 90 , respectively . the flip flop circuits 82 to 89 thus arranged are sequentially shifted to the set states and , accordingly , produces an eight bit position signal . the bit string of the position signal is varied by advancement of the key 26 , so that the micro - computer unit 41 can trace the locus of the key motion with the variation of the bit string . turning to fig1 of the drawings , still another encoder plate 100 is illustrated . the encoder plate 100 is provided in association with two photo couplers 101 and 102 and , accordingly , formed with two lines of windows 103 to 110 . all of the windows 103 to 110 are identical in shape with one another and spaced at a regular interval , however , these windows are arranged in a staggered manner . the photo couplers 101 and 102 are respectively coupled to both pulse generators 111 and 112 as shown in fig1 , and the count pulses produced by the generators 111 and 112 are supplied to the count - up node and the count - down node of a counter circuit 113 , respectively . the signal processing circuit thus arranged is operative to increment or decrement the counting value which is indicative of discrete positions on the locus of the key motion . since the windows 103 to 106 are arranged in the staggered manner with respect to the windows 107 to 110 , the pulse generator 111 produces the clock pulses on the way to the depressed state , however , the pulse generator 112 keeps silent , so that the counter circuit 113 increments the counting value with time . on the other hand , when the key is released , the pulse generator 112 produces the clock pulses , however , the no clock pulse is supplied to the count - up node of the counter circuit 113 , then the counter circuit 113 decrements the value . thus , the counting value is incremented or decremented depending upon the direction of the key motion . then , the micro - computer unit 41 can trace the locus of the key motion with the output signal of the counter circuit 113 as will be understood from the waveforms in fig1 . although particular embodiment of the present invention have been shown and described , it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention . for example , the actomatic player system according to the present invention is applicable to a mechanical piano of the grand type .