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Патент USA US2126464

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Aug- 9, 1938.
Filed April 2, 1958
10 Sheets-Sheet 1
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Aug. ‘9, 1938.
Filed April 2, 1938
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Aug. 9, 1938.
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Aug. 9, 1938.
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Patented Aug. 9, 1938
Laurens Hammond, Chicago, Ill.
Application April 2, 1938, Serial No. 199,613
49 Claims. (Cl. 84-1)
My invention relates generally to electrical of the fundamental. If all the notes of a concert
musical instruments and more particularly. to type of grand piano, which is herein referred to
instruments of this type in which the tones are
as standard, are played with a blow of the same
produced by translating electrically originated
intensity, which I shall call medium intensity,
it will be found that the “brightness” of the notes 5
increases strikingly in playing down the scale.
By “brightness" is meant the amount of the
and controlled current pulsations into sound.
The main object of my invention is to produce
an electrical musical instrument playable from
a keyboard on which it is possible to produce
many novel and interesting effects, and at the
same time make it possible for a pianist~ade~
quately to interpret the great classical works of
piano literature.
A further object of my invention is to produce
an instrument upon which piano literature can
be played, of a tone quality very reminiscent of
piano, but with desirable characteristics not
heretofore found in the tones of pianos, but which
are found only in stringed instruments such as
the violin. For example, in the instrument "oi.
my invention there may be added to the piano
tone a frequency shifting system resulting in, a
characteristic of the tone resembling the vibrato
total energy contained in the harmonics as com
pared to the energy in the fundamental. For
example, the tones in a piano which I have meas 10V
ured, and the characteristics of which are graphed
in Figure 12, the highest A or note No. 85—(the
notes being numbered consecutively from 1 to 88,
the lowest note A, being note No. l)-has a total
energy which exceeds the energy of the funda 15
mental of that note by only 10%. That is to say,
about 90% of the energy is that of the funda
mental tone, whereas the harmonic energy is
only approximately 10%. In going down the
scale to note No. 37, which is A below middle 0, 20
it is found that thetotal energy on the harmonics
has risen to a value as great as that of the founda
of a violin.
I am of course aware that electrical pianos have
mental alone. Harmonic energy then increases
steadily and rapidly until for the lowest note of
25 been built in which the hammer action and string
the piano, A, note No. 1, the total energy 'has
have been retained, and by means of electrical
translation devices the characteristic motions of
the piano string have been translated into sound,
through the operation of electrical pick-ups, am
8% pliflers, and speakers. By this process it is pos
sible to build electrical instriunents on which
risen to a value which is six and one half times
as great as the energy of the fundamental fre
quency alone. So accustomed are we to this
standard piano music may be adequately played,
but in such systems it is not possible, for instance,
periodically to shift the frequency of the various
notes. In the instrument of my invention, Ii’ do
not employ hammers or strings, but seek by
purely electrical means to produce the same gen
eral acoustical e?ect as that of the string under
various intensities of hammer blow, with the
collateral ‘advantage of introducing other novel
effects which are hereinafter described.
In order fully to understand the operation of
the instrument of my invention, it is necessary
to review, in a general way, some of the more
45 obscure acoustic properties of the piano, so as
to show what'e?‘ects are necessary in the elec
trical system, in order that the music produced
will be su?iciently similar to that of the stand
ard piano adequately to interpret the e?ects in
50 tended by the composers of classical piano works.
In the ?rst place, piano notes are, on the aver
age, very complex. That‘is to say, a very large
tonal arrangement in the piano that we naturally
think of these tone qualities as being somewhat
the same throughout the range of the instru
ment, whereas, by physical measurement they
are entirely unlike in harmonic development.
If, instead of playing each note with the same
intensity or" blow, a single note is selected and.
played with intensities of blows ranging from
the softest to the hardest, it will be found that
the sound produced changes in two major re
spects. In the ?rst place, it will be found that
the total energy increases enormously with the g
, hard blow, the relationship being such that the
total energy is approximately in direct proportion
to the energy imparted to the key. The energy ‘
in an electric circuit is proportional to the square
of the current or voltage. The energy necessary
to move the key is proportional to-the square
of the velocity imparted to the key, so it follows
that in general, to simulate the touch response
of a standard piano in an electrical instrument,
it is necessary to produce an output signal of
which the voltage is directly proportional to the
velocity with which the key is struck. However,
with key blowsof increasing intensity on a stand
part of the energy of a single note, especially in
the lower register, is contained in the energy of ard piano, the audibility of the note increases .
55 the harmonic frequencies as compared to that - even more rapidly than its total energy increases,
because of the shift in the harmonic energy dis
tribution. This is the second major effect. If
portant part in determining the characteristics
of the piano tone. In the instrument of my
invention, the rate of decay may be controlled,
a harmonic analysis is made of a soft note and
then of a loud one, resulting from a change in
the strength of blow delivered to the key, it will
be found that the brightness of the note has in
and the constants of the circuit are so arranged
as to give, with at least one particular setting,
the same rate of decay as that of the standard
creased along with an increase in intensity. The
increase in brightness means that a larger pro
portion of energy is carried by the harmonics
with the increasing strength of blow, which adds
to the audibility, especially in the lower range,
where the higher harmonics fall in the region
piano. If these values are departed from, the
instrument may produce most interesting and
novel effects, but the similarity to the piano will
immediately be lost and many works of piano
literature cannot adequately be played if these
constants be appreciably changed.
of maximum sensitivity of the ear.
Thus, the skilled pianist has under his control,
15 not only the loudness, but the harmonic content
of the note, depending on the way in which he
strikes the key. In the electrical instrument
herein described, I have provided means whereby
the key, which operates switches in timed suc
20 cession, rather than actuating mechanical mech
We have seen therefore that a piano note con
sists of a complex tone quality which starts sud
denly and thereafter decays logarithmically to 15
zero. This is not the whole story. This may be
illustrated by the use of. an electric organ of the
type diclosed in my U. 8. Letters Patent No.
1,956,350 granted April 24, 1934. On this instru
anisms such as hammers, is nevertheless so ar
ment it is possible to set up a tone quality which
for any particular note in the middle range may
ranged as to be touch responsive in the same way
as is the piano key. I mean by that, that the
be quite similar to that of the standard piano.
By pressing the key and closing the swell pedal
acoustic energy output will very closely approxi
mate the square of the velocity with which the
key is struck, and that the brightness of the
note will increase with increasing key blow in
while so doing, it is possible to make the note come
tensity, but in a manner which can be regulated
on instantly and decay logarithmically to a lower
level. In this case, howeverhthe quality of the
note does not change as it decays. While the
note sounds generally like a piano note, there is
in the electrical instrument, whereas it cannot
be so regulated in the piano, except by taking out
the hammers and changing the dimensions and
properties of the felt covering the hammers, or
changing the shape of the relatively harder cores
of the hammers themselves.
Whereas, as stated above, the acoustic energy
a certain definite difference noticeable to the ear.
To simulate the piano, it is necessary that the
electrical apparatus be such that each note have
a different harmonic content depending on the
intensity with which it is sounding. This is simi
lar to the effect discussed previously in connec
tion with soft and hard key blows. Owing to
output of the piano is approximately proportional
the fact that the note, when loud, is brighter
'to the energy of the key blow, this relationship
than when it is soft, the audibility of the note as
it decays, decreases more rapidly at the start
than the straight logarithmic decay of a note of
does not hold true, for very light blows. In the
standard piano there is a threshold value of key
40 velocity below which there is no energy output.
This is due to the fact that the velocity with
which the key is struck must be sufllcient to
throw the hammer far enough to reach the string.
This limits the pianist in very soft passages be
constant harmonic content. As far as I am
aware, I am the first to propose an instrument
below which the acoustic energy falls to zero.
without hammer or strings in which this charac
teristic of the sound of a string struck by a ham
mer is obtained, and it is therefore a further
object of the invention to reproduce this effect.
A further characteristic of the standard piano
lies in the fact that the tone of a piano note,
especially in the lower register, is made up of a
fundamental and a long series of partials, which
partials‘ are not true harmonics of the funda
mental-in the sense that they are of frequencies
which are not exact integral multiples of. the
fundamental. This is due to the fact that where
as the string continues to generate sound over a
65 The operation of the key has been made such
considerable period of time, it is not continuously
that the lowest possible velocity produces a
threshold value of acoustic output. It therefore
excited, but receives all of its energy during the
blow of the hammer. If the string were an ideal
one--that is to say, having no stiffness of its
own, a uniformly distributed mass in a diameter
45 cause if he attempts to play more and more soft
ly he reaches a point where his technique no
longer permits him to play each note with equal
intensity, or at least with the intensity which he
desires and expects. This is one of the features
50 of the standard piano which I believe can be im
proved upon, and it is therefore a further object
of the invention to make the key operate in such
manner that there is no threshold of key velocity
follows that there is a small range of intensities
of key blows, all of them exceedingly soft, which
gives rise to the same energy output and therefore
greatly facilitates the pianist's technique in soft
A further important characteristic of the
standard piano is the way in which the sound
65 decays after the key is struck and held down.
Once the string is set in motion, its energy is
gradually transferred to the sounding board and
the acoustic output decreases in a logarithmic
fashion to approach the value of zero asymp
70 totically. That is, it would theoretically re
quire an infinite length of time for the string to
come to rest. The rate of decay of the sound is
strikingly different for the highest notes and
the lowest ones--the lower notes sounding much
75 longer. The exact rate of decay plays a very im
which could be completely neglected by com
parison with its length-—then it could be shown
that such a string would give rise to partials of
frequencies which were exact harmonics of the
fundamental. This is not the case in the stand
ard plane as may quickly be shown by watching
the pattern shown on a cathode ray oscilloscope
operated from a microphone picking up the sound
of a piano note. If all the partials of the tone
were exact harmonics, the shape of the pattern
would not change appreciably during the decay of 70
the note. Actual experiment will immediately
show small amplitude high frequency waves which
travel rapidly along on the wave produced by the
lower frequencies.
Pianos are usually tuned by a beat system, and II
‘_ '
3 '
whereas the methods of tuning differ in detail, all
a tremulant, which is usually defined as a period
of them tend to expand the musical scale in such ‘ ic change in intensity. The e?ect of the vibrato
a way that the highest notes-‘not the scale are is enhanced by applying it at different frequen
sharp and the lower notes are flat.
cies to different notes, such that when a chord
-' Let us assume therefore, that a piano has just ' is played no definite rhythmic beating is percep
been tuned in accordance with the best, practice, tible. It may be stated in a general way, that
and assume further, that double octaves are . when the listener becomes conscious of such
rhythm to the extent that he is aware of the beat
The second harmonic of the lowest M13) is very itself rather than of the effect' of warmth which
struck, for instance, A’s' Nos. 13, 25, 37 and 49.
close in frequency to the fundamental of the
M25) an octave higher. Similarly, the fourth
harmonic of the A03) is of a frequency in the
vicinity of the fundamental of the M37) two oc
taves higher, and in the vicinity of the second
15 harmonic of the M25) one octave higher. The
eighth harmonic of the~lowest M13) is close in
frequency to the fundamental of the M49) three
octaves up and close to the fourth harmonic of
the M25) one octave higher, and close to the
second harmonic of the A(37) two octaves higher,
etc. Now it is not possible to tune these four A's
so that there will be no beats between them. Not
only is it not possible to do so in a practical sense,
but it is theoretically impossible as well. With
‘out therefore considering the effects of the tame
pered musical scale which introduces beats in
chords where notes of other than unison pitches
it gives to the tone, the effect is then unsatis
factory to many listeners.
He is never able to
count the vibrato effects of the piano and has
therefore come to think of the piano as an instru
ment without vibrato, which is not the fact in a
technical sense. Very strong vibrato effects are 15
present in the standard piano, but they are of a
very complex nature. The beats between the dif
ferent frequencies do not last‘ long because the
notes decay, and there are a great many different
rates of beat going on at the same time. To 20
count them it is necessary to focus the attention
on one particular beat and to start to count.
Time does not permit much- in this process, be
cause before the rate is established clearly in the
mind, the effect is gone.
In the instrument of my invention, I have a
system for continuously shifting the frequencies
of ‘each note in a rhythmical manner. Thus, if
a single note is played, the effect may be counted,
if the attention is attracted to it, as is the case
‘ with the solo violin note. I provide a method,
notes of chords.
however, for putting a different vibrato rate on
Further, beats are introduced for another rea
son. Except for the very lowest notes of the the different notes so that when chords are played
standard piano, either two or three strings are there are many vibratos of different rates going
used for each note. The multiple strings cannot on at the same time. This produces an effect-too 35
be tuned exactly alike as a practical matter. or complex for the listener to analyze menta1ly—in
even if they were, they would not remain so for the sense that he is unable to pick up these com
very long after tuning. Chords played on a plex rhythms before they are gone. The shift
ing of the frequencies of different notes in a
\ freshly tuned piano give rise to groups of fre
40 quencies which are very close to one another but manner in which one is independent of another 40
produces a musical effect which is highly desir
are not the same. The amounts of their differ
able. The complex tone of a chord, as in the
ences are not of an order to suggest to the listen
standard piano, is then made up of many groups
er that the piano is out of tune, but these fre
quency differences play a very important part in of frequencies which are shifting in audibility, so
that some are growing more prominent while
45 characterizing the general effect and in giving
“life" to the tone. This is due to thefact that others are becoming less so. The specific means
when two frequencies which are close to one for accomplishing this result is not shown herein
another are temporarily in phase with one but is disclosed in detail and claimed in my co
are involved, it will be seen that beats will oc
cur between frequencies which are close to one
another and which are derived from the different
another they reinforce each other, whereas when
50 they are temporarily out of phase they tend to
cancel one another.
Thus, the acoustic spectrum
is continuously changing-certain frequencies
coming on while others are going off, etc. These
effects are generally similar to the effect of a
55 vibrato on a stringed instrument such as a violin.
In listening to the piano, one is ordinarily not
aware of the vibrato effect because of the fact
that a vibrato is associated in our minds with a
de?nite simple rhythmical change such as is
60 heard when a violinist rocks his ?nger on the
‘string. In that case, he changes the effective
length of the string in such a way as periodically
to raise and lower the pitch. This is a change in
,frequency which, together with the pattern of
65 the enclosure in which he is playing, produces at
the listener's ears a change both in frequency and
in intensity.
A simple vibrato on a melody instrument such
as the violin occurs at a periodicity which may
70 be counted by a listener. Depending on room
conditions and other factors, this may sound very
much like a tremulant to the listener, and there
is no very clear difference, under some circum
stances, between a vibrato, which is customarily
75 thought of as a periodic change in frequency, and
pending application Serial No. 199,612, filed April
2, 1938.
Other and more speci?c objects of my inven
tion will be apparent from the following descrip
tion, reference being had to the accompanying
drawings in which:
Figures 1 and '2 together constitute a wiring
diagram of a representative portion of the in
Figures 3 to 9a inclusive show wave forms, il
lustrating the operation of the distortion and
control tube.
Figure lii is a graph showing the effect of the
velocity of depression of the keys of the instru
Figure 11 is a wiring diagram showing the ad
justable potential supply means for the circuits 65
shown in Figures 1 and 2.
Figure 12 is a graph showing the relative energy
in the fundamental and the harmonics of a piano
Figure 13 is a transverse sectional view of the 70
keyboard and associated switches.
Figures 14 to 18 inclusive are sectional views of
the keyboard taken on the lines H-H, iii-l5,
i8-l8, l'I--I‘i and l8--i8 respectively, of Figure
Figure 19 is a chart showing the harmonic de
velopment of a number of notes.
Figure 19a is a wiring diagram showing the
setup used to obtain the‘readings charted in
Figure 19.
Figure 20 is a chart showing the effect of in
creased key velocity upon the harmonic develop
ment of the note.
Figure 20a is a diagram showing the setup
utilized in obtaining the readings charted in
Figure 20.
Figures 21 to 27 inclusive are various intensity
envelope diagrams showing a number of differ
ent ways in which the intensity of the note
sounded may be varied.
Figure 28 is a diagrammatic representation of
a modi?ed form of note control switch mecha
Figure 29 is a diagrammatic front elevational
view of the unitary bus contact bar; and
Figure 30 is a view similar to Fig. 29 showing
a sectionalized bus contact bar.
General plan of instrument
I believe that a better understanding of the
invention will be obtained if the general opera
tion of the instrument as a whole is first out
lined. As a complete instrument it includes thou
sands of parts, including at least 180 vacuum
30 tubes with their circuits, power supply systems,
amplifiers, etc. The whole instrument may be
divided into five general systems as follows: (1)
Generator system; (2) Control tube system; (3)
Touch responsive keyboard; (4) Touch control
system; and, (5) Output system.
Generator system
The generator system includes 88 alternating
.current generators which are continuously oper
ating to produce frequencies of a tempered musi
cal scale corresponding to the 88 notes of the
standard piano. The generator system per se
forms no part of the invention hereinafter de
scribed, being the subject of my aforesaid co
pending application, Serial No. 199,612, filed
April 2, 1938.
While any one of a number of suit-able high
audio-frequency generators known in the art
could be used, but for a numberof considerations
which will appear later, I prefer to employ 88
oscillating vacuum tubes. In one of the arrange
ments which I prefer, there are 12 vacuum tube
oscillators which I refer to as master oscillators,
and which oscillate at the frequencies of the 12
highest notes of the piano. In addition to this,
there are '76 tubes, only one of which is illustrated
in Figure l as tube 280. These tubes are special
3 element gas ?lled tubes and they operate as re
laxation oscillators. The grid of each such tube
60 receives a signal from a secondary winding 2",
of a transformer, the primary of which is in
the plate circuit of a similar tube one octave up.
Thus, the frequency at which each gas tube "re
laxes" is controlled, in cascade fashion, from a
frequency twice as great, produced by another
similar relaxation oscillator which is supplying
the signal for a note one octave higher. By
means which it is unnecessary here to describe,
the frequency of every gas tube oscillator for
70 similar notes in each octave, such as the A, for
instance, is thus derived from the frequency of
the master oscillator for the note A of the highest
for the entire instrument will shift accordingly.
The generator system also includes (but not
shown in this application) a selectively operable
system for periodically shifting the frequencies oi’
all the master oscillators, at different rates, so
that the control tubes 2“ hereinafter described
do not necessarily function at steady frequencies.
Control tube system
The control tube system includes 88 vacuum
tubes, one for each note, for which purpose I
prefer to use pentodes with sharp cutoff char
acteristics. Only one of these tubes is illustrated
as tube 294 in Figure 1, and its operation is de
scribed in detail elsewhere. Brie?y, this tube is
continuously receiving a signal from the corre
sponding relaxation oscillator for that note, this
signal being applied to the control grid "2. All
of these control tubes are connected to transmit
the signal from their plates P-IB, P--I0, etc.,
into a common output circuit described herein
after. They do not normally supply any signal,
however, because of the fact that the cathode of
each tube is connected to the common ground
oi’ the system by means of the condenser C4,
and unless the cathode receives direct current
through a resistance R1, it is of necessity cut off,
and no plate current is available to carry a sig
nal to the output circuit. This means that if
any direct current flows through the resistance
R1, such current will be immediately "chopped
up" into impulses‘ which pass into the output
system. as a result of the continuously varying
potential of the grid 292. The amplitude of these
impulses will depend wholly on the amount of
direct current supplied to the cathode from the
touch responsive keyboard.
Touch responsive keyboard
The keyboard includes 88 keys of conventional 40
piano dimensions. When struck, they move down
to stop against a felt cushion, and when released,
they are brought to rest in their upward motion
by a mechanical damper. Under each key are
several electrical switches which operate in suc
cession, the making and breaking of different
contacts occurring in regular succession in ac- ‘
cordance with the motion of the key, so that
the electrical events occurring in the circuits
associated with these switches take place in suc
cession with a rapidity which is wholly deter
mined by the velooity of the key on its down
ward stroke. One of the switches shown as
switch 262, 286 of Figure l, is adapted to be oper
ated upon by a spring catch 2'“ which is con 55
trolled by a foot pedal 210 corresponding to the
damper or sustaining ‘pedal of the standard
piano. The general purpose of they switches, oper
ated by the keys and sustaining pedal, together
with their associated circuits, is to supply to the
control tube system impulses of direct current
which are to be converted‘into signals by the
operation of the control tubes 294 Just described.
Thus, the direct current supplied from the key
board switches to the control tubes 284 may be
thought of as determining the envelopes of the
waves transmitted by the control tubes to the
output systems Thus, if the direct current sup
plied by the keyboard system be one which starts
suddenly, continues at the same rate for a period 70
of time and then suddenly stops, the signal trans
mitted from the control tube to the output sys
octave of the instrument. If this last frequency tem will resemble a sustained note, such as an
be shifted up and down through a smallrange, organ note, for example. If, on the other hand,
75 or even several notes, the pitch of every other A ‘the direct current supplied by the keyboard sys
8, 126,464
tem starts suddenly and then immediately begins
decreasing in a logarithmic fashion, the note
produced will resemble that of a percussion in
strument, such as a piano. Thus, the switches
operated by the keys merely control direct cur
rents and it would therefore be possible, if it
were desired, to place the keyboard system at a
distance from the rest of the apparatus, in the
same manner that organ consoles may be placed
10 remotely from the body of the instrument. Ex
cept for a common ground return, only one wire
per note would be necessary for this purpose.
Touch control system
Unlike any other musical instrument with
which I am familiar, the instrument of my in
vention contains means whereby the player may
instantly control the touch responsive behavior
of the keyboard upon which he is playing. This
20 touch control system includes a number of sep
arate controls described in detail elsewhere,
whereby the velocities of key motion may produce
different effects not only in the control of the
envelope of intensity, but also in the quality of
each note. This is accomplished in a general
way by shifting the direct current potential of
different points in circuits associated with the
key and sustaining pedal operated switches.
Output system
tension or "touch”.
Upward movement of the key is limited by a
thin ?exible metallic strip 2l8, the lower end of
which is suitably anchored by means of clamp
ing strips 220, and the upper end of which is se
cured to the forward end of the key bar 202.
Upon depression of the key, the strip 2|8 bows
forwardly as indicated by the dotted lines in
Figure 13.
Suitable means is provided to prevent rebound 15
of the key as it returns to its uppermost position.
This means comprises a snubber consisting of a
fabric ribbon 222, one end of which is clamped
to the center of the strip 218, and the other end
of which is shown as secured to the upper free 20
end of. a leaf spring 224. In actual practice the
latter may be secured to a suitable adjustable
clamping means 225 whereby its effective length
may be adjusted to maintain a slight initial bow
in the strip 2 IS. The ribbon 222 passes partially 25
around snubber rods 228, 228. As the key is
depressed to its lowermost position, and the strip
2l8 bows forwardly to its dotted line position,
the spring 224 will take up» the slack produced in
the ribbon 222 so that upon the return stroke of 30
the key, the ribbon will be under tension applied
This system includes circuits whereby the out
put of all the control tubes can be combined,
by the spring 224, and due to its frictional con
modi?ed, controlled as to intensity, ampli?ed,
tact with the snubber rods 226, 228 will dissipate
and transmitted to an electrical output system
energy at a rate increasing as the key approaches
the upper limit of its stroke and as the strip 2l8 35
resumes its normal position as shown in full lines
in Figure 13. As a result, the key will return to
its normal position without any rebound what
soever despite the fact that the tension of the
$5 for conversion into sound waves in a manner gen
erally similar to that employed in electric or
gans, but with such modi?cations as are described
in detail hereinafter.
Key action and switches
The keyboard employed in the instrument of
my invention will, in external appearances, re
semble that of the standard piano and extends
throughout the tonal range customary in stand
45 ard pianos, namely, throughout 88 notes com
mencing with Ar1 and extending to C7. For con
venience, the notes of the piano are herein desig
nated by‘ their respective numbers, 1 to 88.
While the key action per se does not form a
50 part of the subject matter claimed herein, a}
brief description thereof is believed to be neces
sary for an understanding of the invention as a
whole, and I have therefore illustrated in Figures
13 to 18 inclusive, 29 and 30, the more important
01 Ca parts of the key action, especially as it is related
to the subject matter of the invention claimed
For each .oflthje notes there is a key 200, pref
erably of a molded plastic composition of the
2 l6 by means of which the bar 2 ll may be moved
forwardly or rearwardly in guide slots 2 ll formed
in frame plates 208 'to vary the tension of the
springs 2|2 and thus adjust the keyboard as a
whole, or any portion thereof, for any desired key
general character disclosed and claimed in my
co-pending application, Serial No. 91,283 filed
July 18, 1936, which has matured into Patent
No. 2,117,002, granted May 10, 1938. The keys
200 are suitably secured to key bars 202. which
are pivoted upon a rod 204, the latter being suit
ably secured in a keyboard assembly and being
supported adjacent each group of keys by the
upwardly projecting portions 206 of key frame
plates 208.‘ There is riveted to each of the key
bars 202, adjacent its rearward end, an off-set
downwardly extending arm 2“), to the lower end
of which one end of a tension spring 2|2 is se
cured. The other end of the tension spring 2I2
is anchored to bar 2“ extending lengthwise of
75 the keyboard and provided with adjusting screws
spring 2&2 is sufficiently great relative to the 40
rotary moment of inertia of the key bar and
parts carried thereby, that the return stroke of
the key is very rapid.
Downward movement of the key is limited by
a felt pad 230, which is suitably mounted upon a 4.5
cross-bar 232 extending longitudinally of the key
board, which has notches 233 receiving portions
of the frame plates 20%, being held in engagement
with the latter by screws 234. Suitable means for
guiding the forward ends of the key bars 202, and
for holding the plates 208 in properly spaced rela 50
tion, are provided, but it is believed to be unnec
essary to describe these parts in detail in this
Each of thekeys is provided with a suitable 55
bearing bracket' 236 which is secured to the key
bar 202 and at its lower end is provided with a
pivoted touch-response switch arm 238. This \
arm pivots freely but is forced to move clock
wise (Fig. 13) by a tension spring 240, one end
of which is suitably anchored to a terminal part
24! ?xed to the key bar 202 (but insulated there
from), and the other end of which is secured
to the tail portion 242 of the switch arm 238.
When the key is in its normal upper position, the 65
switch arm 23!! makes contact with a bus contact
244, which may extend the full length of the key
board as shown in Fig. 29 or may, if desired,
be made in separate sections such as the sections
244a, 244b, and 244c shown in Fig. 30, each sec
tion for one or more keys. Adjacent the bus con
tact 2“ is a contact 246 individual to each key,
while forwardly of the. latter there is an insu
lating strip 248, and forwardly of the insulating
strip is a third- contact 250 individual to each 75
key. The upper edges of the contacts 244, 266,
266 and insulating strip 246, are at different ele
vations, so that upon depression of the key 266,
the contact arm 266 will successively make con
tact with the contact 246; break contact with
the contact 244; break contact with the contact
246; and make contact with the contact 266.
Upon the return stroke of the key, these contacts
are made and broken by the contact arm 266 in
10 the reverse order.
The "repeat" switch comprises a pivoted arm
262 carried by a bracket 264 secured to the key
bar 262. The repeat arm 262 does not pivot free
ly, but a certain amount of friction- is provided
15 so that the arm will not be moved by the forces
of gravity and momentum to which it may be
subiected. The am 262 projects through an
aperture 266 formed in an insulating strip 266,
and is adapted shortly prior to the completion
20 of the downward stroke of the key to make con
tact with a contact 266, the contact with this
contact 266 being broken Just prior to the com
pletion of the down stroke of the key and just
prior to the time rocker switch arm 236 makes
25 contact with contact 266. Upon the return stroke
of the key the switch arm 262 does not make
contact with the bus 266.
A sustaining switch arm 262 is suitably clamped
at 264' and is in the form of a leaf spring having
30 a contact making element 266 at its rear free
latter away from its complementary contact 266,
when later the key is released, as long as the
sustaining pedal is held depressed. Similarly, if
keys are operated while the sustaining pedal is
being held depressed, the sustaining switch arms
262 of such keys will be engaged by their re
spective resilient latches 214, and these switch
arms 262 will thereby be held away from their
associated contacts 266 until the sustaining pedal
is released and the resilient latches 214 thus
swung counterclockwise (Fig. 13) out of the path
of movement of the free ends of the sustaining
switch arms 262, whereupon these arms will re
sume their normal positions in contact with con
tacts 266.
From the above description it will be seen that
the depression of a key will result in switching
operations in the following sequence:
(0) Making contact between switch arm 266
and contact 246;
(b) Breaking contact between switch arm 236
and bus contact 244;
(c) Breaking contact between switch arm 266
and contact 246.
(11) Making contact between repeat switch
arm 262 and contact 266;
(e) Breaking contact between switch arm 262
and contact 266;
(f) Breaking contact
switch arm 262 and its complementary contact
end which is adapted to close a circuit by contact
(9) Making contact between switch arm 266
ing with a contact member 266. In normal posi
and contact 256; and upon the return stroke:
tion, as shown in Figure 13, the circuit is com
(h) Breaking contact between switch arm 226
pleted between the switch contact arm 262 and
35 contact member 266. Upon depression of the and contact 266;
(i) Closing contact between sustaining switch
key, an actuator 266 which is carried by the key
bar 262, is adapted to engage the contact am arm 262 and its complementary contact 266 (if
262 and move the rearward free end of the lat , the sustaining pedal is not depressed at the time
ter away from the contact 266, thereby opening the key is released) ;
(1) Making contact between switch arm 226
the circuit.
A shaft 266 is pivotally mounted in the frame and contact 246;
(Is) Making contact between switch arm 266
plates 266 (other suitable bearings may be pro
vidcd) and may extend the full length of the and bus contact 244;
(1) Breaking contact between switch arm 226
keyboard, or may be made in two or more sec
and contact 246.
45 tions. The shaft 266 is adapted to be moved
clockwise (Fig. 13) by means of a sustaining pedal
The functions and effects obtained by the se
quence of switching operations above set forth,
216 (illustrated schematically in Fig. 1), the sus
will be apparent from the description of the cir
taining pedal being connected by a suitable ten
sion member 21l with an arm 212 secured to the cuits and their operation which appears herein
after. The key action and associated switches,
50 shaft 269. If the shaft is made in two or more
sections, each section may be provided with a as distinguished from the combinations in which
separate sustaining pedal and linkage for actu
these elements are used, are disclosed in greater
ating that section of the shaft, duplicating the detail and are claimed in my copending appli
parts 269, 216, 2H and 212 shown in Fig. 1. For cation Serial No. 216,336, ?led June 28, 1938.
55 example, the instrument may be provided with
Oscillator generators
a shaft section 269 which extends throughout the
treble register of the keyboard and a similar
As previously stated, the oscillators for the
shaft section for the remaining keys of the key
various note frequencies may be of any of the
board, each section being controlled by a separate well-known types, but I prefer to use vacuum
tube oscillators for the twelve highest frequen
60 sustaining pedal. The sustaining pedals, how
ever, are preferably so positioned that the player cies and may use gas ?lled tube relaxation oscil
may easily with one foot selectively depress either lators for the remaining '16 notes. The gas filled
of the sustaining pedals, or both of them.
tube oscillators have their frequencies controlled
The shaft, or shaft sections, 269 have secured by means of the master oscillators, each master
oscillator controlling the frequency of the gas
65 thereto resilient latches 214 which, when the
shaft 269 is swung clockwise by the depression tube oscillators which are of the same note in
of the sustaining pedal, will press against the the octave, i. e., whose frequencies bear a sub
ends of the sustaining switch arms 262. If the multiple octave relationship to the frequency gen
latter are in their uppermost position, the pres
erated by the master oscillator. The gas tube
oscillators of a‘series are controlled in cascade
70 sure of the resilient latches 214 against the ends
thereof will not have any effect. If, however, in a manner such that the master oscillator con
a key is being held down while the sustaining - trols the frequency of the gas tube oscillator
pedal is depressed, the resilient latch 214 asso
which has a frequency one-half that of the
ciated therewith will swing above the free end master oscillator, and this gas tube oscillator
75 of the sustaining switch arm 262 and hold th‘i controls the frequency of the next gas tube oscil
denser C1. The junction M may be connected
to a direct current source of adjustable poten
lator of the series which has a frequency one
iourth of that of the master oscillator, and one
half the frequency of the preceding gas tube
tial through a resistance R14 upon closure of a
oscillator, etc.
switch 296 which isconnected, to terminal G.
This system or method of gen
erating oscillations of the frequencies of the
tones of the musical scale is disclosed in greater
detail, and is claimed in my copending applica
tion Serial No. 199,612, ?led April 2, 1938. An
other simpler and less costly arrangement is to
10 have the oscillations of the master oscillators
successively divided by two by means of the fre
quency divider circuit disclosed and claimed in
my‘ co-pending application Serial No. 199,614,
’ ?led April 2, 1938.
In Fig: 1, I have illustrated one such gas ?lled
tube relaxation oscillator as comprising a gas
?lled triode 280. The plate circuit of each oscil
lator includes a resistance 282 and an inductance
284 (which constitutes the primary of a trans
20 former, the secondary of which is connected in
the grid circuit of the gas tube oscillator gen
The junction M is connected to a terminal D
through a resistance Re, the terminal D being
connected to a direct current source of adjust
able potential.
The junction M is connected through a re
sistance Ra with a terminal L, which latter is 10
connected to the ?xed repeat switch contact 260.
The repeat switch‘arm 252 is connected by a
conductor 298 with a terminal F, which in turn
is connected to a direct current source oi’ ad
justable potential. A resistance Rm is connected 15
between terminal L and a terminal K, which is
the terminal on the sustaining switch contact
266. The resilient sustaining switcharm 262 is
connected to a terminal E, which in turn is‘ con
nected to adirect current source of adjustable 20
potential.v A resistance R0 is connected between
erating a frequency which is an octave lower the junction M and the terminal J on the con
than that of the oscillator illustrated). a source tact 250.
The contact 246 is connected through a resist
of adjustable direct current potential 286, a re
ance R13 to terminal B, which in turn is con 25
26 sistance R11 and a resistance R16. A condenser
C15 is connected between the source 266 and the , nected to a direct current source of adjustable
cathode of the tube 286. The grid of the tube
280 is connected to ground through a resistance
28‘! and an inductance 288, the latter constitut
ing the secondary of the transformer by which
the stabilizing signal from the oscillator of a
frequency twice that of the oscillator illustrated
is impressed upon the grid of the tube 280. The
resistance R17 is shunted by a condenser i314 and
may be shunted by an additional condenser C10
denser Cs. and connected to terminal C through
upon closure of a switch 2%. The signal from
this relaxation oscillator is taken from the point
P between the resistances Rm and, Ru and is
impressed upon the control grid 292 of the con
trol tube 294. The heaters for the cathodes of
tubes 2% and 2% are omitted from Fig. l for
the sake of clarity. A description of the opera
tion of this oscillator will appear hereinafter.
nected to a direct current source of adjustable
Control tube and circuits
The functions of the control tube are four
fold: (a) It serves as a distorter tube to change
the harmonic content of the note fed to the
output circuit from that oi the signal which is
applied to its grid; (b) it serves to control the
envelope of the energy output, beginning-at cut
off, when the note is not sounding, to maximum
intensity. and progressively to lower. intensities.
finally to cutoff again; (0) it serves as a
1 means for changing the harmonic content oi’ the
note in a manner which results in increasing the
brightness with increasing output intensity:
(d) lastly. the tube operates in such a way that
it makes possible a wide variety oi‘ tone qualities
supplied to the output circuit by a change in the
value of a condenser connecting its control grid
to ground. A change in the value of this con
The bus contact 244 is connected to
a terminal A, which is connected to a direct cur
rent source of adjustable potential. The touch
response switch arm 236 is connected to a ter 30
minal H, the latter being» connected through a
resistance R12 to a terminal 300, and the latter
terminal is connected to ground through a con
a high resistance Ru, the terminal 0 being con
The screen grid 3% of the control tube 294 is
connected to a source of direct current of adjust
able potential, while the suppressor grid QM may 40
be grounded. The plate 79-49 of the tube 94
is preferably connected to a wire 306 in parallel
with the plates ‘EL-5d, P-Ell and P--§2 which
are the plates of the control tubes 294 for the ad
jacent notes Nos. 50, 51 and 52.
Operation of control tube and circuits
I will first describe the operation or the tube
‘2% with r'espect to its control of the energy out
put. When playing the instrument as a piano 50
this energy output is of course changing all the
time from the beginning of, the note when it
rises suddenly, until the end, when the tube is
again cut oil‘ by release of the key or release of
the sustaining pedal, in the event the latter has 555
been used. The tube I prefer is a pentode having
sharp grid cutoff characteristics. A tube oi.’ the
type 666, for instance, is satisfactory. The con
trol grid 293 receives a continuous signal from
a generator, which in this case is illustrated as 60
a continuously oscillating vacuum tube. One of
the systems which I prefer, and which is herein
partly shown and described, is the three-elec
trode gas ?lled tube 280 previously mentioned,
but which'might be some other form of oscilla 65
content of the output signal in a manner which tor or generator.
The screen 302 of the control tube 294, like the
is entirely different from the effect a condenser
screen of the control tube for every other note,
would have in a linear circuit.
The cathode of the control tube 294, of which is supplied with a steady potential of 25 volts.
there is one for each note of the instrument. is. The suppressor 304 is connected to ground and 70
denser changes the sharpness of the positive
peak of the signal impressed on the grid. and
this change produces a change in the harmonic
connected to a terminal N which is connected to
' the ground through a blocking condenser C4.
high resistance R1‘ is connected between a junc
tion M and the terminal N. The junction M is
connected to ground through a blocking con
the plate is continuously supplied with voltage,
(at point 3i2, Fig. 2) in this case some 200 volts,
from an output circuit which receives the signal
from all the control tubes. Since the cathode is
connected by means of the condenser C4 to 75
ground‘, and by means of the high resistance R1
to ,the touch responsive key circuit, no direct
current can flow to the cathode of the control‘
tube unless it does so by way of the high resist
ance R1. Thus, if the control tube 294 is not
supplied with direct current from the key circuit,
it cannot continue to function to transmit the
' signal (which is continuously put on its grid 292)
to the output circuit. when direct current stops
10 ?owing in R1 the potential of the cathode will
shift in the positive direction such that the con
trol grid becomes more negative with respect
thereto, until the tube is cut off. In this condi
tion there is no output signal despite the fact that
15 the grid continues to receive an input signal, and
the plate P—-49 is shielded from the capacltative
effect of the grid by the screen I02 and suppressor
I04. The key circuit, which is described else
where herein, controlslthe direct current com
20 ponent of the pulsating current in the cathode
circuit. The impedance of the tube 204 from
cathode to ground for direct current is extremely
small by comparison with the impedance of the
resistance R1. This is due to the fact that if the
25 potential of the point N with respect to ground
should change by a very small amount in the
negative direction, an enormous plate current
would immediately now through the tube.
The control tube may be thought of as nor
80 mally cut off. As soon as cathode current is sup
plied from the key circuit, the direct current potential of the cathode will shift slightly such that
the next most positive excursion of grid poten
tial will be just su?lcient to allow an impulse of
35 current to pass through the tube during a frac
tion of the cycle of grid voltage. During the re
malnder of the cycle the tube will be cut off. and
the potential of the cathode will again be sinking
in the negative direction for another impulse of
40 current on the succeeding positive grid swing.
To understand the operation, it may be con
venient to think of the tube as a device whereby
whatever direct current is supplied by the resist~
ance R1 to the cathode, will be immediately
45 "chopped off" by the tube and delivered in im~
pulses to the output circuit, the frequency of these
impulses being of course determined by the fre
quency of the signal which is continuously sup
plied to its grid. If the current supplied by the
50 resistance R1 be double, for instance, then the
cathode current will be doubled and so will the
average value of plate current. The control tube
operates to amplify the signal supplied to its
grid and to transmit a signal to the output cir
55 cuit, but it does not act as a linear amplifier.
The harmonic content of the output differs from
that of the input, and the tube may therefore be
thought of as .a distortion device.
It is the par
ticular type of distortion which is here of inter
60 est and which may be explained as follows:
In the circuit illustrated in Figure l the signal
applied to the grid 202 of the control tube 294
is derived from a generator having an output
which is not a sine wave.
I may use a sine wave,
65 however, and in the preferred form of construc
tion do use a sine wave for the highest octave
of notes. If a sine wave is supplied to the grid,
the tube functions for only a portion of the cycle
during the period of most positive grid excursion,
70 so that the quality of the output is dependent on
the shape of only that portion of the input sig
nal wave which is su?iciently, positive to operate
the tube.
To illustrate this point, there is shown in Fig
75 ures 3 to 6 inclusive a number of different input
waves which would result in the same output sig
nal, illustrated in Figure 9a. The harmonic con
tent of these various input signals would be en
tirely different, and yet the output would be the
same because the shapes of the upper parts of
the waves above the dot-dash cut-off lines, are
similar in each case. The distortion introduced
by the tube is therefore of a peculiar nature. A
sine wave input will produce an output wave
which may be rich in harmonics.
If the output wave, such as that shown in Fig
ure 9a, is connected to a harmonic analyzer (using
a circuit such as shown in Figure 19a) and the
amount of energy of each harmonic is measured
separately, it will be found that the amplitude 15
of each harmonic bears a simple relationship to
the amplitude of all other harmonics. This re
lationship is a simple geometric one in which if
the amplitude of the fundamental is unity and
the amplitude of the second harmonic is, for ex
ample, .8, then the amplitude of the third har
monic will be .8><.8 or .64, and the amplitude of
the fourth harmonic will be .8><.64 or .512, etc.
If the amplitude of each harmonic be plotted on
semi-logarithmic paper, a series of points such
as that shown in Figure 19 will be obtained, and
it will be found that these points lie on a straight
line with a high degree of accuracy. If the
amplitude of the input wave be changed, a dif
ferent series of harmonics will be obtained, but
in this case the points will again fall in a straight
line with a difference in the slope of the line.
The harmonics present from what appears to
be an in?nite series extending upward in fre
quency in accordance with this formula as far as
the measuring apparatus will permit. In meas
urements which I have made, the ratio of energy
between the 42nd and 43rd harmonics, for ex
ample, will be found to be just the same as that
between the second and the third harmonic. The
complete harmonic analysis of tone is therefore
determined by simply specifying the difference
in intensity measured in decibels between any
harmonic and the next succeeding one. This is
of course the same as specifying the slopes of
the lines in Figure 19, and I therefore refer to this
slope in terms of decibel decrease per harmonic
number as, for instance, 2.05 db/H for note No.
25. The curves of Figure 12 are plotted with a
value oi’ db/H as abscissas and various energies
as ordinates.
For a wave having a fundamental
of a given ampltiude it will be found that the
total energy increases very rapidly as the har
monic content increases, corresponding to armlyses in which the db/H factor decreases. This
factor is therefore a convenient measure of the
brightness of the tone.
Except for the highest octave of notes, each of
the control tubes 294 obtains its signal from a re
laxation oscillator shown as comprising the gas
?lled triode 280 in Figure i. In the cathode cir
cult of this tube are two resistors R16 of 300,000
ohms, for example, and the resistance R11 of
10,000 ohms. A pulsating direct current flows
through these resistors and the wave of this cur~ 65
rent is of a saw-tooth nature illustrated as the
curve shown in Figure '7. The voltage developed
across the resistance Rn would be of the same
shape were it not for the eiTect of the condenser
On, which shuts it. If this condenser be of 70
proper value, the voltage across the mesh of
the resistance R17 and the condenser C“ will
be of wave form illustrated in the curve of Fig
ure 8.
It will be noted that the curve of Figure 7 is 75
2,126,464 ~
of saw-tooth form, one leg of which is almost
the values of the condensers C14 and C4 for each
vertical, and there is a sharp corner to the wave
note are arrived at by “voicing" the instrument
in such a manner as most closely to approximate
the tone quality for a similar blow on a certain
concert grand piano of well known make. The
on the most positive and negative points. The
excursion of voltage in the positive direction does
not produce a vertical line because of the im
pedance in the plate circuit of the tube which
does not permit of an instantaneous change in
charge of the condenser C15. The slope of this
portion of the curve may be controlled by chang
10 ing the constants in the plate circuit, The curve
of Figure 8 shows the effect of the condenser C14
which has the effect of rounding off the most
positive portion of the wave. The shape of the
most negative part of the wave is substantially
15 unaffected.
Comparison of this curve with the
curves of Figures 3, 4, 5 and 6 will show that
only the most positive part of the wave is of im
portance in determining the signal put out by
the control tube. Thus, a saw-tooth wave may
20 be employed in the same manner as a sine wave,
and the harmonic content of the output of the
control tube will have the same general char
acteristics as those described for the sine wave
curves show the enormous change in the har
monic content for the same note in different
octaves. Similar curves plotted for different in
tensities of blows show the same general arrange
ment, except, of course, that all notes are more
harmonically developed with increasing strength
or blow.
Operation of touch responsive key circuits
As previously described the touch responsive
key circuit is connected as shown in Figure 1, in
which, when the key is in its upper position of
rest, the rocker arm 238 electrically connects
the point H with the terminal A. As the .key
goes down, the rocker arm connects H to B 20
through R1: and opens the connection between
'H and A. After a period of time, depending upon
the velocity with which the key is moving, the
contact from H to B (through resistance R13) is
in turn broken and when the key is all the way 25
An additional condenser Cm may be shunted
across the condenser Cm by means of the switch , down, contact is made between H and J.
290. In a complete instrument this switch is a
When the key has been depressed a certain dis
gang switch, operating to ground one end of a tance the damper switch 262-266 connecting E
similar condenser provided for each note for i to K is likewise opened. When the key is allowed
to return upward this switch will again be closed
which a relaxation oscillator is used as the gen
erator. With the added capacity, Cm, the curve unless it is held open by the action of the sus
representing the voltage at the point P (with
‘respect to ground) now changes to a shape more
like that of curve of Figure 9. In this wave the
35 positive loop is rounded off evenv more.
Figure 20 shows the result of the analysis of
the harmonic content of the signal put out by
the control tube of note No. 25 of a complete in
strument. The circuit. used and the constants
40 used are shown in the diagram, Figure 20a.
Five different series of points were obtained which
may be joined by substantially straight lines, rep
resenting the voltages of each separate harmonic
entering into the composition of the wave. The
45 five diiferentlines arise from ‘five different volt
ages of the battery B1. These curves show how
an increase in this voltage produces an increase
in the energy of the output signal, and more par
ticularly, how the harmonic content of the re
sulting note changes with an increase in intensity.
50 The slope of each line is denoted by the “bright
ness factor” db/H previously described, and it
will be noted that the “decibel decrease per har
monic number" changes from a value 2.75 for
the softest note to 1.41 for the loudest, and that
55 this change is systematic. Further experiments
have shown that'the harmonic content of the
output may be controlled in two major respects
by changing the values- OfQm and C4 respective
ly. For any given battery voltage the slope of
269 through a small angle in a clockwise direc
tion. In that event, the damper switch will re
main open regardless of whether the key is al 35
lowed to rise or not.
The repeat arm 252 whose mechanical opera
tion has been described, connects the terminal L
with the terminal F for a brief period during only
the down stroke of the key. The circuit from L 40
to-F is open both at the top and at the bottom of
the key stroke and the parts are so dimensioned
that contact is made on the down stroke at about
the same time that the rocker arm 238 is passing
over the insulating strip 248, so that at that time, 45
point H is not connected to either A or J.
The terminals A, B, C, D, E, F, and G are, for
purposes of illustration, each connected by‘means
of a battery to ground.‘ - The switch 296 is herein
assumed to be open. These batteries are merely 50
to indicate that each of these points is to be held
at a ?xed potential relative to ground.
The volt- '
age on each one of these points may be adjusted
in accordance with the requirements of the play
er. In actual practice, such batteries would not 55
be used, but the ?xed potentials would be secured
from a single source of direct current supply by
shunting this supply with a tapped resistor in\a ,
manner well known in the design of power supply
systems for radio sets and the like and as de 60
the line, or db/I-I factor may be changed by
changing the value of the condenser C14.‘ Once
this slope is established for a certain battery volt
age, the rate of change of slope with increasing
and decreasing battery voltage may be largely
controlled by the proper selection of the value of
the condenser C4. The desirability of the bright
scribed hereinafter with respect to Figure 11. By
means of various multiple switches, hereinafter
ness change to simulate piano-like effects was
circuits we will assume that the performer wants
to obtain a normal touch responsive action which
described previously.
taining pedal 210 which operates to turn the shaft
In Figure 19 are plotted the harmonic analyses
for a plurality of different A~’s of the complete in-,
strument. The same battery voltage was used
in each case, and was of a potential correspond
ing to the starting potential of a blow con
75 sidered to be of medium strength,
In this case,
described in detail, the performer may change .
the potential -of each one 01' these points inde
pendently in order to obtain a different mode of 65
operation for the circuit.
In order to- explain the operation of the key
will closely approximate that of the standard 70
piano. In that case, he will ‘make such adjust
ments that the potential at the various points
with respect to ground is as follows: A at minus
450 volts, B, C and F at ‘ground, D and E at plus 6
volts. The points A, B, C, D, E, F, and G are 75
common for all notes of the keyboard, (or, if de
sired, for only certain registers thereof) but the
values of the resistors or condensers, or both, may
change' from note to note. Assuming that the
diagram applies to note No. 25, for instance, then
good values for the various elements are as fol
lows: C1 and Ca, .3 mfd; R1 and R11, 4 megohms;
R12 and R13, 6000 ohms; R0 for this note may have
‘a value of zero and consequently be omitted, (but
10 for higher notes Re may be considerable); Ra.
10,000 ohms; Rm, 600,000 ohms; and ‘Re 20
I prefer to make the condensers C1 and C1 of
the same capacity, and it will be noted that one
end of each is connected to ground. The charges
on these condensers vary during the playing of
the note, the general plan being that a charge on
C1; is carried and delivered to C1. A simple me
chanical analogy may beused to illustrate the
20 operation as follows: A runner has a bucket with
which he is carrying water from a lake to ?li a
tank. The bucket has a hole in it. .On his way
between the lake and the tank, the water is con
tinually running ‘out of the bucket. He always
25 starts with a full bucket but when he reaches the
tank, he may have much or little, or no water at
all, depending on how fast he ran.
If he goes
slowly enough he will obviously arrive with no,
water. In this analogy, the condenser Cu is the
30 bucket. Condenser C1 is the tank. The hole in
the bucket is the resistance path from B through
start of the note. These voltages are not ob
tained, of course, until, after a very short period
of time when the two condensers Ca and C1 have
had their ‘charges equalized 'through the path
from M through Re to J to H and through Rm.
It will be noticed that if the velocity of the key
is near zero, the voltage of point M will likewise
be close to zero. As the velocity increases to 1,
voltage begins to rise slowly, the curve being
shown as the curve A-IO.
The characteristics of the control tube and the
constants of the circuit are such that the tube is
cut oi! when the cathode is at a voltage of plus 6
volts with respect to ground. Accordingly, if the
cathode is more positive than plus 6 volts, there is
no direct current path from the point N through
the resistance R1 to ground. Likewise, the point
N always remains at a potential which is exceed
ingly close to plus 6 volts with respect to ground,
because as soon as any current flows through R1
tending to make the point N grow more nega
tive, the current through the tube will increase
very sharply. For operation of the key circuit
it may therefore be assumed that the point N
is always at a fixed potential of plus 6 volts with '
respect to ground.
It will be seen that when the point M is at a
voltage of zero with respect to ground, there is
therefore 6 volts difference between the point M
and the point N, and there is therefore a small 30
current ?owing through the resistor R1. This is
the resistance Ru, the rocker arm to H, the re ,the cathode current of the tube and there is,
sistance R11 to the bucket condenser Ca. The therefore, a signal,.albeit a small one, with M at
lake is the source A, the potential of which is 450 the potential of zero.
35 volts more negative than ground.
From the curve in Figure 10, it may therefore ‘
With the key up before the down-stroke of the be seen that if the velocity of the key is anything
key, the potential across Cu is that of the source
A, and the condenser therefore has a high nega
tive charge. The potential across C1 is the same
40 as that of the points D and E which were both
assumed to be at the same voltage, namely, plus
6, with respect to ground. No current is ?owing
in the resistance R1 because the control tube 284
is cut off.
Assume that the key is struck very violently.
In that case, connection through the rocker arm
238 from H to B will not last long, and despite the
fact that B has been assumed to be at ground
potential, time will not permit a great discharge
50 of the "bucket" condenser C; because of the re
sistances R1: and R1: in the circuit.
As a result,
when the connection H to J is established through
the rocker arm 238, the charge on the condenser
Ca will be something less than 450 volts, say for
55 example 250 volts, depending of course on how
~ hard the key was struck. As soon as the rocker
less than 1, the intensity of the signal will be sub
stantially constant and may be represented by
the voltage between the cutoff voltage of the tube
and the voltage of the point M. A straight line 40
F-J? drawn from plus 6 volts and zero velocity
toward the point B—-l0 will therefore represent
a theoretical value which the voltage of point M
should have if the intensity of the signal, ex
pressed in volts, was exactly proportional to the
key velocity.
This relationship will be exactly
true at three points where the straight line inter
sects the curve 11-", namely, at C, D and E.
The agreement, generally speaking, is very close
except for very low velocities, which gives this 50
system a great ‘advantage over the touch respon
siveness of the standard piano, for reasons previ
ously described.
The constants of the circuit, especially the
values of the resistors R1: and R12 must be my 55
chosen that the hardest blow which a good pianist
can deliver to the key will not carry the voltage
arm 238 establishes contact between H and J, the
two charges on C1 and Ca will begin to equalize. . of the point M a great deal beyond the point
This takes only a very short time because of the E—l0, where the curve A—|0 begins to depart
60 low resistance of Ru and Re, and as a result, the
more and more rapidly from the straight line.
potential at the point M will suddenly shift from If this precaution is not observed, the instrument
its previous value of a low potential with respect might be used to advantage by one who was ac
to ground to a potential about halfway towards customed to it, but there would be a range of
lninus 250 volts, or say minus 125 volts.
hard blows which would all result in intensities
Had the key been pressed very slowly ‘the po
which differed only slightly from one another.
tential delivered’ by Ca would be substantially
Assume that the key has been struck and held
nothing. That is to say, its charge would be down so that there is a connection from H to J,
substantially lost and when the connection H to J and the point M has reached a negative potential‘
was established the condensers C: and C1 would with respect to ground, of 125 volts, There are
70 neither of them have any substantial charge. In now three paths whereby the charge on the con 70
that event, the potential at the junction point M densers C1 and Ca‘ may leak off. One path is
would be left substantially unchanged.‘
through theresistance R1 to the point N, the
In Figure 10, a curve has been plotted in which potential of which is plus 6 volts. Another path is
the abscissa is the velocity of key motion. -The through the resistance R11 to the point C, the
75 ordinates are the voltages of the point M at the ' potential of which is zero,‘ and the third is through ’
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