Multiplexed multiple intensity reproducing piano

A reproducing piano is provided which is capable of reproducing the notes of a chord or a sequence of multiple intensity notes within several groups of common intensity. This is accomplished by sorting the notes into a plurality of groups and then assigning intensity levels to the various groups so that notes within a group will be played at the same intensity. A limited number of solenoid driver circuits are multiplexed among the solenoids according to the grouping of notes, thereby providing faithful reproduction of the music, but at a lower cost than by individual control of each key.

BACKGROUND OF THE INVENTION 
The present invention relates to a reproducing piano and more particularly 
to a reproducing piano that sorts the notes to be reproduced according to 
intensity and subsequently reproduces the notes at or near the original 
recorded intensity. 
It is known to record performances of a piano on magnetic tape, for 
example, and reproduce the performance by playing the tape and causing the 
keys to be actuated mechanically. During the record mode, the piano is 
played by a musician, and sensors detect the timing and velocity with 
which the keys are depressed or the hammers are moved, and this 
information is stored digitally in a permanent memory, such as a magnetic 
tape. During playback, the digital information is retrieved from the tape 
and converted to control signals that energize actuators to play the keys 
in the same pattern and with the same dynamics as during the original 
performance. 
In U.S. Pat. No. 4,307,648, which patent is incorporated herein by 
reference, there is disclosed a method and apparatus for measuring the 
dynamics of a piano performance wherein a shutter is provided for each 
hammer shank of the piano, as well as a separate optical switch assembly 
and counter that is responsive to the trigger signals produced as the 
shutter eclipses the light beam, the counter is responsive to an 
initiating signal from the optical switch assembly to start the counter 
and to an end of count signal from the optical switch to terminate the 
count, the total count defining the count increment. The total count 
registered comprises a digital signal constituting an inverse function of 
the near terminal hammer velocity, that is, the velocity of the hammer 
just before it strikes the string. Digital information corresponding to 
the count is stored on magnetic tape for recall during playback and 
reproduction of the original performance. 
A microprocessor retrieves the data from the magnetic tape and produces a 
digital drive value corresponding to the particular key velocity required. 
A digital-to-analog converter converts the digital drive value to an 
analog voltage, and a feedback servomechanism comprising a plurality of 
operational amplifiers and a sense coil is connected to a solenoid and 
energizes the solenoid with a current that produces a constant velocity. 
The velocity is maintained constant by means of the auxiliary sense coil 
within which a permanent magnet connected to the solenoid plunger moves; 
the coil is connected to the input of the first operational amplifier. 
This circuit arrangement causes the solenoid to operate as a linear motor 
with constant velocity, thereby ensuring that transit times and key 
velocity can be maintained within very close tolerances so that the 
playback performance is an accurate reproduction of the original 
performance. 
This linear key velocity technique and system for playback of musical 
performances yields an extremely accurate reproduction; however, at a 
considerable expense. Each of the 88 keys has its own operational 
amplifier servomechanism comprising: a digital-to-analog converter, three 
operational amplifiers and an input from a triangular wave generator as 
shown in FIG. 2 of U.S. Pat. No. 4,593,592, which is hereby incorporated 
by reference. For all the keys on a piano, then, this extremely accurate 
reproduction system requires a total of 264 operational amplifiers and 88 
digital-to-analog converters in order to drive the 88 solenoid actuators 
for the 88 key analog to digital converters. 
Considering that the maximum number of notes that can be played by a 
performer is twelve, most of the 88 individual circuits of the key 
velocity reproduction system are not being used at any one time. 
Accordingly, it is desirable to provide a reproducing piano which 
multiplexes a number of digital-to-analog converters and a number of 
driving circuits in order to reduce the expense and the complexity of 88 
individual circuits. 
A type of multiplexing reproducing piano is known from U.S. Pat. No. 
4,135,428. This known reproducing piano has two multiplexed pulse-width 
modulators for driving all of the solenoid actuators, one of which is 
assigned to the bass half of the keyboard and the other of which is 
assigned to the treble half of the keyboard. With this arrangement, notes 
that are played on the same half of the keyboard and substantially 
concurrently, such as the notes of a chord, are all reproduced at the same 
intensity, because all the treble solenoid actuators are driven from one 
pulse-width modulator drive signal output and all the bass solenoid 
actuators are driven from the other pulse-width-modulator drive signal 
output. This system then reproduces two concurrently struck notes on the 
same half of the keyboard with equal intensity, and similarly it 
reproduces all of the notes in most chords with equal intensity. When all 
of the notes are played with equal intensity, the reproduction has a 
certain "mechanical" quality to it, which is both noticeable and 
objectionable. Some of this "mechanical" quality may be overcome by time 
shifting the notes using known techniques, but this requires an extensive 
amount of processing of the recorded performance in order to produce a 
recording tape for playback. 
Accordingly, it is desirable to provide a reproducing piano which has a 
number of possible solenoid actuator drive signals to which the keys may 
be multiplexed, thereby reproducing multiple sound intensities for notes 
which are struck substantially concurrently. Moreover, it is desirable to 
provide a "non-mechanical"-sounding reproducing piano which does not 
require extensive monetary investments in the editing and processing 
required between the recording stage and the reproduction stage of the 
musical performance. 
SUMMARY OF THE INVENTION 
The disadvantages of expense and complexity, on the one hand, and 
objectionable quality of the sound of the reproduction on the other hand 
are overcome by the present invention, in one form thereof, by providing a 
plurality of independent drive signals which can be varied digitally to 
provide three or more sound intensity levels at which any one note can be 
reproduced substantially concurrently with any other note. Each of the 88 
keys has a solenoid actuator and a decoder which selects from the two or 
more independent solenoid drive signals the corresponding one to drive the 
solenoid actuator during the time segment reproduced. 
Applicant's invention reproduces a piano performance more faithfully and 
with a more natural sound than the two drive signal system described 
above, by grouping the notes that are performed substantially concurrently 
within a specific time segment into a plurality of individual sound 
intensity groups. Thus, if the original performance had a prominent note 
played concurrently with other notes, the prominent note would be 
reproduced at one sound intensity and the remainder of the notes grouped 
into one or more sound intensities upon reproduction. The solenoid 
actuator of the prominent note will be decoded to the prominent sound 
intensity drive signal and the remainder of the notes will be averaged 
within their groups and each group will have the solenoid actuators for 
its keys decoded to a corresponding sound intensity drive signal. In this 
manner, only a small number of operational amplifiers and 
digital-to-analog converters are required to reproduce a piano 
performance, with a natural sound and negligible pre-reproduction 
processing. 
In a system constructed in accordance with this invention which employs 
three drive signals for actuating the keys and causing the hammers to 
strike the piano strings, one of the three drive signals would provide a 
"loud" group sound intensity and a second drive signal would provide a 
"soft" group sound intensity. The third drive signal would be typically 
reserved for the "hold" intensity, which is a level that energizes the 
solenoid actuator just enough to hold the key depressed, thereby 
reproducing the effect of the performer holding down the keys during the 
performance. In unusual cases, when many notes are played or the hold 
intensity is not required, the third drive signal may be used to provide a 
third sound intensity, in which case the notes are sorted into three 
groups, providing an even more faithful reproduction. Conceptionally, the 
three drive signal system has a fourth sound intensity drive signal, the 
null intensity, which is always zero. An open circuit connection input to 
each solenoid actuator decoder provides the null intensity function. All 
notes which are not played or are "released" after being played with a 
"loud", " soft", or "hold" drive signal, are assigned to the null sound 
intensity. 
The digital data concerning the notes to be reproduced, and the sound 
intensity of each of the notes may be recorded on a permanent medium 
using, for example, a method and apparatus as described in U.S. Pat. No. 
4,593,592. This data is subsequently supplied to a programmable 
microprocessor. The program of this computer divides the continuous 
digital data stream into various time segments, which are not necessarily 
equal, and then sorts the notes which are to be reproduced within each 
time segment. As mentioned above, the notes typically are sorted into a 
"loud" group, a "soft" group, a "hold" group and a "null" group. The notes 
in the "loud" group are assigned the sound intensity level of the loudest 
note within the loud group. The sound intensity of the "soft" group is 
determined by a statistical average of the sound intensities of every 
"soft" note reproduced within the time segment. The "loud" sound intensity 
level and the "soft" sound intensity level are converted to digital 
numbers which control the drive signal duty cycle and thereby control the 
strength with which the solenoid actuator strikes the key. The digital 
information for the hold group and the null group are predetermined and 
are simply assigned as required. Once assigned, the sound intensity drive 
signals usually remain constant within a time segment, and transitions 
from "loud" to "hold", "soft" to "hold", "loud" to "release", "soft" to 
"release", or "hold" to "release" are accomplished by having the decoder 
select the next required drive signal for the note. 
The programmable microprocessor operates so rapidly relative to the playing 
of the keys that it is possible for the sorting and the sound intensity 
assigning to be done in real time as the digital tape of a previously 
recorded performance is supplied at the input to the microprocessor. 
Alternatively, the sorting and sound intensity assignments can be 
accomplished as an intermediate step in which case an output tape can be 
produced which then does nothing but set the sound intensities to be 
produced in each time segment and instruct the decoder of each key as to 
which sound intensity drive signal to select in each time segment in a 
real-time reproduction. 
Briefly stated, the invention, in one aspect thereof, provides a 
reproducing piano for reproducing a sequence of notes from data 
representing a musical performance. This data, for example, may be 
recorded on a recording medium such as a disk or magnetic tape or it may 
be in the form of a data stream from a source such as a Midi synthesizer. 
The reproducing piano includes a plurality of keys, each corresponding to 
a respective note of the sequence of notes, and a plurality of solenoid 
actuators, each connected to a respective key of said plurality of keys. 
The entire sequence of notes is divided into segments which are 
sequentially reproduced, it is determined which notes are to be reproduced 
within each of the segments, and notes are sorted, within each segment 
according to their sound intensity in the performance, into a plurality of 
groups. The system assigns a representative sound intensity level to each 
of the groups of sorted notes, and each solenoid actuator reproduces each 
note at its respective representative sound intensity within its 
respective time segment. For purposes of this application, the term "key" 
refers to the mechanical key-action assembly that causes a hammer to 
strike a string when the key-action assembly is actuated by a performer. 
It is not limited to the actual playing key itself and may comprise other 
portions of the action. 
The invention, in another aspect thereof, also provides a method of 
reproducing a sequence of notes on a reproducing piano from a stream of 
data representing a musical performance. The method includes the the steps 
of: playing back the sequence of notes from the recorded performance, 
dividing said sequence of notes into a plurality of consecutive segments, 
determining which notes are to be reproduced in each of the consecutive 
segments, sorting the notes to be reproduced in each of the consecutive 
segments according to sound intensity into a number of groups, assigning a 
representative sound intensity to each of the groups, and driving each key 
corresponding to each of the notes to be reproduced to produce the 
representative sound intensity of each respective group at the respective 
position within the segment. 
It is an object of this invention to provide a method and an apparatus for 
reproducing a sequence of notes with a natural sound which is less 
expensive than presently available. 
It is a further object of this invention to provide a method and apparatus 
for reproducing a sequence of notes which can reproduce a performance with 
a natural sound that does not require intermediate processing and editing.

DETAILED DESCRIPTION OF THE INVENTION 
The invention will be described in connection with musical data in a time 
division multiplexed format and in connection with expression achieved by 
modulation of the width of the pulses applied to drive the solenoids. The 
invention, however, is not restricted to use with such data format or such 
means for achieving expression. 
Referring now to FIG. 1, one embodiment of the invention comprises a 
digital playback unit 12 which has stored digitally, in a reproduceable 
medium such as magnetic tape, the inverse hammer velocities for a sequence 
of notes on a piano especially equipped for such purposes such as the one 
discussed in U.S. Pat. No. 4,593,592, in which a digital time record of 
every note that is played and the intensity of every note played is 
produced. A magnetic tape interface 14 receives the data from the digital 
playback unit 12 and converts the signals into a form which can be read 
into the microprocessor 16 without changing its information content. The 
digital information is read in through input port 18, thereby supplying to 
the microprocessor the time relationship of the notes to be reproduced, 
the notes to be played, and the intensity in the form of inverse hammer 
velocity that the note had during the original performance. The 
microprocessor 16 divides the continuous record of notes played and sound 
intensities into individual time segments. This information is then stored 
in random access memory 20 and further processed by the programs stored in 
the programmable read only memory 22. The sorting and reproduction 
programs delay the start of the reproduction slightly and then the 
playback of the notes of each segment is kept in the proper sequence with 
respect to the notes of the other time segments. 
In a preferred embodiment of the invention, the microprocessor 16 performs 
the note sorting and sequencing according to the program shown in FIG. 8. 
The first portion of the program collects, calculates and stores in RAM 20 
the time information for each note in the time segment, also known as the 
time window. Once the initiation times and the duration times of all the 
notes are established, the program examines these times to see if more 
than three notes ever overlap within the time segment. If three or fewer 
notes overlap in the time segment, then the system has sufficient 
circuitry to reproduce these notes without further processing. Therefore, 
the hammer velocities that determine the sound intensity of the notes are 
assigned directly and the sorting and sequencing program is completed for 
this segment. If there are more than three notes being played at any one 
time, then some approximations must be made because the simplified 
circuitry cannot necessarily reproduce every note in the time segment with 
a one-to-one correspondence to the level. In such a case, the data stored 
in RAM 20 is examined to find the note with the highest hammer velocity 
and this note is labeled as "loud". The remainder of the notes in the 
segment are examined to see if they are within a small percentage, such as 
20%, of the loudest note and if so, these notes are also labeled "loud". 
At the end of this examination, the "loud" group of notes is collected in 
RAM 20. All of these notes will be reproduced at the same sound intensity 
as that produced by the hammer velocity of the loudest note of the group. 
After the "loud" group has been processed, the remaining notes are 
gathered into a "soft" group and an average sound intensity level of the 
notes in the soft group is calculated. A hammer velocity corresponding to 
this average sound intensity is then assigned to each of the notes of the 
"soft" group. This completes the note sequencing and sorting program for 
this time segment and the results are stored in RAM 20. 
Next, the microprocessor 16 executes one of the two note reproducing 
programs shown in FIGS. 8 and 9. The program shown in FIG. 9 is executed 
when there are very few notes played at the same time and each of the 
notes can be reproduced at its original sound intensity. The 
microprocessor 16 in such a case looks up each corresponding solenoid 
drive from the EEPROM 24 and installs each digital solenoid drive level in 
a respective digital-to-analog converter 26. Data from timing and control 
circuit 28 is connected by bus 29 to shift register 31 and the shift 
command is connected thereto over line 33. The time division multiplexed 
data is shifted through shift register 31 and then latched into a 
plurality of one-of-four decoders 30 by latch 35, the latter being 
connected to timing and control circuit 28 over latch line 37. Shift 
register 31 has 88 stages corresponding to the 88 keys of the piano and 
there are 88 one-of-four decoders 30, also assigned respectively to the 
various keys. Key actuation data will be shifted into stages of shift 
register 31 corresponding to keys to be depressed within a given time 
window, such as the window shown in FIG. 5 for the A#, G and D# keys. A 
two-place binary number within each of the shift register stages controls 
one-of-four decoder 30 to activate one of its four outputs corresponding 
to a "loud" intensity, a "soft" intensity, a "hold" intensity or a null 
condition, in which case the unconnected "NC" output line would be 
activated. 
After the solenoid drive level has been set, the microprocessor 16 and the 
timing and control circuit 28 determine the time for each solenoid to be 
energized, at which time the decoder 30 selects the desired solenoid drive 
circuit to drive each solenoid. Each of the solenoid drive circuits 32 
comprises a pulse-width modulator 39 having one input connected to a 
corresponding digital-to-analog converter 26 and a further input connected 
to power supply 41. Pulse-width modulators 39 produce on their outputs 43 
pulse streams wherein the widths of the individual pulses vary depending 
on the analog value from digital to analog converter 26, which corresponds 
to the intensity of the note to be played. AND gates 45 have one of their 
inputs connected to respective outputs of decoder 30 and the other inputs 
are connected to the outputs of pulse-width modulators 39 of the loud, 
soft and hold values. The outputs of AND gates 45 are combined by OR gate 
47 and control transistor 34 to drive solenoid 36. There are 88 such 
solenoids 36 connected to the respective keys as illustrated in FIG. 2. 
If, in the original performance, after the striking of either a "loud" note 
or a "soft" note, one or more of the notes were held by holding the key 
depressed after striking the note, then the microprocessor 16 and the 
timing and control circuits 28 will reassign that key to the "hold" level 
solenoid drive circuit 32 for as long as the key was held in the original 
performance. 
FIG. 2 shows a representative key 38 and conventional action 46 of the 
reproducing piano which can either be played in the normal manner by 
depressing key 38 or be operated as a reproducing piano. When operated as 
a reproducing piano, solenoid 36 drives pushrod 40 which then drives the 
end 49 of key 38 upwardly. If the solenoid 36 is driven with a 
sufficiently large current, the pushrod 40 drives the key 38 with 
sufficient velocity to cause hammer mechanism 46 to throw hammer 48 
against piano string 44 thereby sounding the note of the string 44. If 
after the hammer 48 strikes the string 44, driving the solenoid 36 at the 
"hold" level will cause the pushrod 40 to hold the key 38 depressed and 
the damper mechanism 42 elevated from the piano string 44, allowing the 
string 44 to vibrate freely. The velocity with which hammer 48 strikes 
string 44 is determined by the effective value of the pulse-width 
modulated drive current in solenoid 36 for driving actuator 40. 
The output of digital-to-analog converters 26, as shown in FIG. 4, controls 
the duty cycle of the rectangular pulse output of solenoid drive circuits 
32 by pulse-width modulation. It is well known that increasing the duty 
cycle of a rectangular wave signal increases the effective current of that 
signal. A known pulse-width modulator circuit 39 which can be used for 
solenoid drive circuit 32 is shown in FIG. 7. The output of 
digital-to-analog converter 26 is essentially a DC voltage which changes 
the switching point of the final operational amplifier shown in FIG. 7 
from a symmetrical position with regard to the triangular wave input to 
the non-symmetrical switching position and will thereby have a rectangular 
output signal of greater than 50% duty cycle. 
The output of DAC 26 is connected through input resistor 50 to a summing 
node 52 connected to the inverting input of operational amplifier 54. The 
output 56 of operational amplifier 54 is connected to one input of 
comparator 64. The other input 66 to comparator 64 is connected to 
triangle wave generator 68, which is fed by a reference voltage through 
square-root circuit 70. Comparator 64 produces on output 72 a pulse-width 
modulated signal wherein the average voltage is proportional to the input 
on line 74. This technique of providing a pulse-width modulated signal to 
drive a solenoid is well known in the field of servo circuit design. As 
the pulse width increases, the solenoid is driven with higher average 
current, thereby increasing the force and velocity with which key 38 is 
driven and increasing the intensity with which hammer 48 strikes string 44 
(FIG. 2). 
FIG. 3 illustrates the solenoid decoding and driving circuitry of FIG. 1 in 
somewhat greater detail. 
Four solenoid drive signals, three from solenoid drive circuits 32 and a 
fourth which is a null circuit or an open circuit are connected to 88 
one-of-four decoders 30. Of the four signals connected to the decoder, 
only one will be selected to drive solenoid drive transistor 34. The 
selection is made by digital logic acting upon the two inputs S.sub.0 and 
S.sub.1 of each decoder. Each selector input has two possible binary 
states, either logic 1 or logic 0, and together S.sub.0 and S.sub.1 can 
select among the four different inputs. The selector inputs are provided 
by the microprocessor 16 and the timing control circuit 28. Together they 
constantly update the data in 22 eight-bit latching shift registers 31, 
35. In the preferred embodiment, each of the 88 decoders must select one 
of the four solenoid drive signals including the null signal. If a note is 
to be reproduced within a time segment, at the note initiation time which 
is stored in RAM 20, the respective shift and latch register 31, 35 will 
select one of the outputs of one of the three solenoid drive circuits 32 
for the length of time corresponding to the determined hammer velocity and 
transit time set forth in the EEPROM lookup table 24, and after that time 
either the "hold" or the "release" solenoid drive signals are selected 
according to the note information stored in RAM 20. The output of each 
decoder drives its respective solenoid drive transistor 34, which is 
connected in series with solenoid 36 across the power supply 41. As shown 
in FIG. 6, as the solenoid drive transistor 34 is driven by the 
pulse-width modulated signal, the effective solenoid current switched by 
the transistor 34 from the power supply 41 into the coil 62 of solenoid 36 
will determine with what force actuator 40 will drive the key 38 and the 
ultimate velocity with which hammer 48 will strike the string 44. Diode 54 
allows the flux in the solenoid to die out rapidly by short circuiting the 
solenoid when the solenoid is no longer driven by transistor 34. 
After all the notes of the segment have been reproduced by means of the 
mircoprocessor programs and the circuitry described above, each subsequent 
segment is reproduced using the same microprocessor programs and the 
circuitry until all segments, and of course the entire sequence of notes, 
have been reproduced. 
Although the invention has been described as having a preferred design of 
four solenoid drive signals including the null signal, it will be 
appreciated by those skilled in the art that by using other, larger 
decoders and by changing the microprocessor program the notes can be 
sorted into a larger number of groups. For example, the notes could be 
grouped into eight different intensities, including a "hold" condition and 
a "release" condition. In this case, one-of-eight decoders would be 
utilized and the pulse-width modulation circuitry would be replicated 
seven times. 
In the illustrated embodiment, the data representing the musical 
performance is recorded on a reproduceable medium such as a magnetic tape. 
It will be understood, however, that this data may be in the form of a 
data stream from, for example, a Midi synthesizer. 
While this invention has been described as having a preferred design, it 
will be understood it is capable of further modification. This application 
is, therefore, intended to cover any variations, uses or adaptations of 
the invention following the general principles thereof and including such 
departures from the present disclosure as come within known or customary 
practice in the art to which the invention pertains and fall within the 
limits of the appended claims.