Direction of bowing detection method and apparatus

An apparatus for producing a signal indicative of the direction of bowing of a musical instrument by analyzing the signal from a transducer monitoring the displacements of a bowed vibrating element in the plane of bowing. The indicative signal is stored for the entire duration of a played note or musical passage.

TECHNICAL FIELD 
This invention relates to tone processing systems and more specifically to 
the electronic detection of specific characteristics of tones produced by 
transducers of bowed musical instruments. 
BACKGROUND ART 
It is customary when writing sheet music for bowed musical instruments to 
specify the direction of bowing of each note or passage to ensure 
consistency of interpretation and correct articulation of the musical 
phrases thus annotated. 
Devices capable of writing sheet music automatically in response to a tone 
signal from a musical instrument are known in the prior art. When a 
musical passage played on a bowed instrument is recorded onto sheet music 
using a device capable of such operation automatically, it is desirable to 
obtain a partition containing the bowing information along with the rest 
of the musical notation. This has not been the case in the past and bowing 
information was either lost or written in manually after the fact with the 
inherent possibility of error from the lack of simultaneity of the events. 
The mechanics of bowing and the related motional behavior of bowed 
vibrating elements in musical instruments are known in the prior art. 
When a bow is drawn across a vibrating element to produce a musical tone, 
the gripping surface of the bow displaces the vibrating element in the 
direction of bowing at a relatively low speed until a limit is reached 
where the friction between the contacting surfaces becomes insufficient to 
displace the vibrating element further in that direction. 
If the bow continues to move in the direction of bowing, the vibrating 
element slips under the bow in the opposite direction at a relatively high 
speed until a point is reached where the elasticity limits of the 
vibrating element force it to stop, and the forces of friction between the 
surfaces are once again sufficient to pull the vibrating element in the 
direction of bowing and start another similar cycle of vibration for as 
long as the bow is drawn across the vibrating element with sufficient 
speed and contact pressure to maintain the tone. 
A transducer responsive to the above-mentioned displacements of a vibrating 
element in the plane of bowing produces a tone signal, the waveform of 
which resembles a sawtooth. The most vertical slope of each cycle of this 
waveform corresponds to the sudden slipping of the vibrating element under 
the bow. 
This characteristic of each cycle of the tone signal appears consistently 
upon bowing and is independent of which note is being played. 
It is therefore an object of the present invention to detect the polar 
direction of this rapid voltage change which is directly related to the 
direction of bowing, irrespectively of the frequency of the played note. 
It is a further object of the present invention to produce an indication of 
the detection in the form of a signal, the magnitude of which is 
independent of the amplitude or frequency of the signal from the 
transducer. 
It is a still further object of the present invention to provide an 
indication of the direction of bowing for the entire duration of the note 
or passage being played, including the period during which the bow leaves 
the vibrating element in a state of free vibration at the end of a bow 
stroke. 
SUMMARY OF THE INVENTION 
According to the invention, a transducer monitors the displacements of a 
bowed vibrating element such as a violin string under tension. The 
transducer must be positioned with respect to the instrument so that the 
waveform of the resulting tone reverses when the direction of bowing is 
reversed. 
U.S. Pat. No. 3,453,920 issued to Scherer exemplifies a suitable transducer 
for this application in a stringed instrument. In such a transducer, the 
piezoelectric elements are connected so as to produce minimum sensitivity 
in the vertical plane. A microphone positioned so as to have its diaphragm 
very near the top of an acoustic instrument preferrably near the bridge, 
or a magnetic pickup near a ferrous vibrating element can also be used in 
this application. As long as the waveshape of the tone produced by the 
transducer, as seen on an oscilloscope, becomes recognizably inverted when 
the direction of bowing is reversed, there is a detectable portion of the 
signal which is indicative of the direction of bowing and the transducer 
is suitably monitoring the vibrating element for the purpose of this 
invention. 
The preferred transducer mentioned above produces a waveform which closely 
approximates a sawtooth and which reverses symmetrically when the 
direction of bowing is reversed, even when playing in the highest 
positions capable of producing a musical tone. In most applications such 
accuracy is not required but the transducer should preferrably have good 
high frequency response to accurately monitor the rapid motions of the 
vibrating element throughout its entire playable range. 
It is preferable that the resonances in the transducer be low in amplitude 
and high in frequency to avoid creating spurious frequencies in the 
transducer signal, the waveform of which can be assumed to be a sawtooth, 
for ease of understanding of the present invention. 
The signal from the transducer is applied to a slope detector in order to 
produce a signal in which the specific characteristics of the direction of 
bowing will be significantly enhanced and possibly become dominant, thus 
simplifying the detection process. A first function of the slope detector 
is to respond to the rates of voltage change occurring in each cycle of 
the transducer signal and convert each one into a value, either in the 
form of a voltage or in other forms such as a number,a frequency, etc. . A 
second function of the slope detector is to produce a reference value 
corresponding to a fixed portion of the cycle being analyzed. 
It is desirable to store the information obtained from the slope detector 
for the entire duration of the cycle being analyzed and further until new 
information supersedes the preceding one. In practice, a value can be 
assumed valid for more than one cycle and new information generally takes 
more than one cycle to develop. If the slope detector produces more than 
one value during a cycle of the transducer signal, the required 
arithmetical operations are preformed to obtain a single significant value 
which is not directly frequency related. The significant value obtained is 
preferrably independent of the frequency of the played note. 
The significant value is then compared to the reference value from the 
slope detector to find the sign or polarity of the difference between the 
two compared values. This sign or polarity is indicative of the direction 
of bowing and can be expressed as an indication of magnitude such as a 
voltage, a number, a frequency, etc. according to the requirements of the 
application requiring the information. 
In practice, it is desirable to store this indication of the direction of 
bowing to avoid erroneous indications when in bow leaves a string 
vibrating, for example at the end of a bow stroke. 
A data latch would typically be used if the indication is in the form of a 
binary voltage. The indicative signal is stored shortly after the 
beginning of a note and held until the end of a note. 
It may also be desirable to exhibit a neutral state when no note is being 
played. For example, a gate or switch responding to the presence of signal 
from the transducer can be used to disconnect the indicative signal from 
the rest of the system whenever the amplitude of the trans ducer signal 
falls below a predetermined value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1 of the drawings, a transducer 11 responsive to the 
displacements of a bowed vibrating element in the plane of bowing produces 
a signal A illustrated in FIG. 2A, the waveform of which resembles a 
sawtooth. 
In spite of the presence of a small amount of friction ripple in one 
portion of each cycle of the waveform of FIG. 2A, it can be seen that two 
major voltage changes of opposite polarity occur in each cycle of signal 
A. These voltage changes occur at different rates, the highest of which 
corresponds to the slipping of the vibrating element under the bow. This 
is confirmed by the absence of any friction ripple in that portion of each 
cycle of the transducer signal A. 
Signal A is applied to a slope detector 12 consisting of a high-pass filter 
13 and a gain stage 14. The slope detector 12 produces a signal B 
illustrated in FIG. 2B, which exhibits peaks of opposite polarity, the 
magnitude of which substantially corresponds to the rates of voltage 
change occurring in transducer signal A. 
In a first portion of the circuit operating in the amplitude domain, signal 
B is applied to a positive peak follower 15 to produce a positive voltage 
D, and is also applied to a negative peak follower 16 to produce a 
negative voltage E. Voltages D and E are stored values substantially 
corresponding to the rates of voltage change occurring in the transducer 
signal A. Signal B is also applied to an averager 17 to produce a 
reference voltage H by which the magnitude of the peaks can be evaluated. 
Voltages D and E are of unequal magnitude with respect to voltage H since 
they are effectively equal to the peak voltages appearing in signal B. The 
polarity of the largest peak of each cycle of signal B is indicative of 
the direction of slipping of the vibrating element under the bow and 
consequently will change according to the direction of bowing. 
Voltages D and E are applied respectively to the inputs of a voltage summer 
18 to produce a voltage F which is effectively the difference between 
voltages D and E in the direction of the largest peak in each cycle of 
signal B. 
The magnitude of voltage F depends on the amplitude of signal B, and 
consequently, on the amplitude of transducer signal A. It is desirable to 
produce an indication of fixed magnitude, irrespectively of the amplitude 
of signal A. 
For this purpose, voltage F is smoothed by an averager 19 to produce a 
ripple free voltage G, the magnitude of which is independent of the 
frequency of the played note. Voltages G and H are applied respectively to 
the inputs of a voltage comparator 20 to produce a bi-state signal J, the 
magnitude of which is independent of the amplitude or frequency of the 
played note, and the state of which is indicative of the direction of 
bowing. 
In a second portion of the circuit operating in the time domain, signal B 
and voltage H are applied respectively to the inputs of a voltage 
comparator 21 to produce a square wave C illustrated in FIG. 2C, the pulse 
width of which coincides with the duration of the peaks in signal B. 
Square wave C exhibits two states of unequal duration which are 
effectively two time values substantially corresponding to the rates of 
voltage change in the transducer signal A. 
The two time values contained in each cycle of square wave C are converted 
into a single significant voltage value by applying square wave C to an 
averager 22 which effectively sums the positive and negative portions of 
each cycle of the square wave C and produces a significant voltage K, the 
value of which changes about a fixed level according to the direction of 
bowing. 
Voltage K is applied to a first input of a voltage comparator 23. The 
second input of comparator 23 is referenced to a fixed voltage L which is 
equal to the fixed level around which the significant voltage K changes 
according to the direction of bowing. Comparator 23 produces a bi-state 
indication of the direction of bowing, voltage M. Voltages J and M can be 
used jointly or separately if desired. 
For simplicity and ease of understanding of the portion of the present 
invention illustrated in FIG. 3, voltage J only will be used as the signal 
indicative of the direction of bowing. 
Referring now to FIG. 3, the transducer signal A is applied to a signal 
detector 24 comprising a rectifier 25, an averager 26 and a comparator 27, 
the output of which goes positive when the magnitude of voltage P which 
corresponds to the amplitude of the transducer signal A, becomes larger 
than a reference voltage R. Comparator 27 produces voltage S which is 
applied to a delay network 28 which delays the transition from LO to HI 
while the transition from HI to LO is not significantly delayed. The delay 
provided by delay network 28 ensures that data latch 30 and switch 31 will 
be activated only after signal J has become stable and exhibits the 
correct indication. Delay network 28 produces a voltage T which is also 
applied to a delay network 29 which provides a small amount of delay when 
voltage T goes negative to ensure that the switch 31 opens before the data 
latch 30 is reset. Delay network 29 produces a voltage U which activates 
the data latch 30. The data latch 30 produces a voltage V which is applied 
to the switch 31. Switch 31 produces a tri-state voltage W, the extreme 
states of which are indicative of the direction of bowing. The third state 
of voltage W is indicative of the absence of a played note. 
APPLICABILITY 
To use the present invention in a music writing device, the transducer 
signal A is fed to the instrument input of the music writing device so 
that the pitch and amplitude characteristics of the played note may be 
converted into musical notation while voltage V or W is fed to a control 
input which governs the synchronous notation of the direction of bowing. 
The present invention may otherwise be implemented in a computer controlled 
music writing device in the form of logic instructions, thus minimizing 
the hardware costs in the device and allowing the possibility of updating 
older devices without any hardware modifications. 
While a specific embodiment of the present invention was described and 
illustrated, it is clear that the use of similar functions in another form 
for the purpose intended here does not depart from the true spirit and 
scope of this invention as described in the appended claim.