ELECTRONIC STRINGED MUSICAL INSTRUMENT

Abstract: Trigger strings (12) of a guitar-like electronic musical instrument have transverse coils (10, 40) adjacent them which are cyclically supplied with h.f. a.c. current from a source (24). The coils provide signals dependent upon the absolute positions of the strings, and not just their velocities, to provide a reliable indication of when triggering of a note is desired by plucking of the string.

Description: ELECTRONIC STRINGED MUSICAL INSTRUMENT BACKGROUND OF THE INVENTION This Invention relates to electronic musical instruments, and in particular with guitar-like instruments with improved means for detecting plucking of the strings. The invention is particularly suitable for use with the SYNTHAXE (Registered Trade Mark) instrument described in our International Patent Application Publication No. 084/04619 (United States Patent Application No. 691,486), to which reference should first be made for a detailed description of the background to this invention. That publication describes a guitar-like electronic musical instrument for use with a synthesizer. The instrument has a neck carrying six pitch strings which the player depresses onto conductive frets to determine the selected note, and a body carrying six trigger strings, separate from the pitch strings, which can be plucked or strummed to initiate or trigger the desired notes. The neck also includes coils embedded in the finger board which are designed to detect sideways movement of the pitch strings, a feature known as "string bending" often used by guitarists to vary slightly the quality and pitch of the note produced. The trigger or pluck detecting system described in publication 084/04619 (see Figure 26) detects triggers (plucks) by means of attaching each trigger (plucking) string to a magnet within a spring loaded barrel. As the string is tensioned, a mechanical counter- load is established within the spring, due to the increasing compression built up within the spring. When the string is released the magnet moves within the barrel, and a Hall effect device at the end of the barrel senses the movement of the magnet. Further analogue signal conditioning is applied to the raw signal thus generated to determine the velocity and the direction of movement (sense) of the magnet when it is released during a plucking action. Trigger signals (valid plucks) are defined according to the velocity of the magnet within the barrel. When a defined velocity threshold is exceeded in the correct direction or sense, a valid trigger is deemed to have been made. While this system has represented a considerable advance on the systems previously proposed, we have appreciated that in certain circumstances it can produce less than optimum results, for a variety of reasons. The first of these is that the perceived sensitivity of the system reduces with an increase in string tension. The slacker the string, the more magnet movement due to a given pluck, and therefore the larger the signal that is detected. Thus the detection system works best with relatively slack strings. Unfortunately, because guitarists like to use relatively high tensions, they find problems adjusting to the slackness of the strings. When the strings are tensioned up, the system becomes relatively insensitive, and plucks considered valid by the player may not be detected. We believe that the mass of the magnet holder is the main contributor to insensitivity, but that the friction between the magnet holder and the barrel is also a significant problem. Both factors act detrimentally in reducing the velocity of the magnet movement and therefore the sensitivity of the system, particularly at higher frequencies. Guitars can be played either by finger picking, or by using a plastic, bone or metal pick or plectrum. Plucks from a pick tend to produce a higher frequency spectral response than finger picking. This is because when the strings are picked by the finger, the finger itself has a considerable mechanical damping effect on the string; whereas a pick or plectrum does not have the inherent damping characteristic of a flesh covered finger. Therefore, the string trigger design of our earlier publication was slightly unbalanced In being less insensitive to finger picking than to using a pick or plectrum. Thus the sensitivity of the response fell off particularly when playing with plastic picks or plectrums. Furthermore, the lack of balance presented another assimilation problem to the guitar player. The above problems manifested themselves in lost triggers, and therefore "missing notes". This loss of sensitivity could be recovered by electronic means but then "crosstalk", mechanical vibrational coupling between strings, became significant and unwanted or spurious triggers were being generated on adjacent channels. Another problem can arise because, when plucking a guitar, the plectrum or finger often flies off a string at the moment of plucking, and mechanically overshoots, or flies into the adjacent string, with a considerable amount of energy. In doing so, the adjacent string is displaced from its position of rest, but although the adjacent string does not get properly plucked, there is a tendency for the string to spring back to its rest position due to the extra tension imparted to the adjacent string by the overshooting plectrum or finger. Sometimes this spring-back happens over a sufficient mechanical range and at above the required velocity threshold so as to produce an unwanted trigger signal, and therefore an unwanted note. If the synthesiser being addressed was one which did not have a velocity related loudness, then any trigger however large or small would produce the same intensity of result. This could be annoying in the extreme. The system is also susceptible to external impacts on the instrument covers which could cause spurious notes to sound. Finally, the complexity of the trigger transducing scheme has reached such a level that manufacturing costs are unduly high in order to achieve acceptable reliability and performance. SUMMARY OF THE INVENTION The present invention is defined in the appended claims to which reference should now be made. We have appreciated that the problems could be eased if the displacement signal, such as from the Hall effect device, was also used to produce constant data relating to the absolute position of the string, as well as the velocity of the movement of the string. If this data were available and properly processed, a valid trigger could be defined not just in terms of string movement velocity in a particular sense, but a further condition could be defined so that the string would have to return to its nominal mechanical zero point within a defined period of time. The principle object of the present invention is to remove peripheral mechanical attachments to the string and reduce the mass of the moving system to the minimum, which is the bare string alone. This permits the perceived relative sensitivity to different methods of plucking the string to be equalised, the string tension to be increased, and the energy imparted to the whole mechanical system to be less, hence reducing crosstalk to almost immeasurable levels and no susceptibility to extraneous knocks to the instrument covers. A further object is to faithfully determine string displacement and hence whilst a displacement exists to use subsequent electronic logic to inhibit the generation of trigger events unless the string was deliberately damped, thus satisfying the requirement to prevent unwanted triggers when a string was accidentally knocked. Another object is to reduce the complexity of the mechanical aspects of the design to reduce accordingly manufacturing costs. BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which:- Figure 1 is a broken away perspective view of the mechanical part of a trigger detection system embodying the invention; Figure 2 is a view of one of the coils; Figure 3 is a block circuit diagram of the circuity connected to the coil outputs; Figure 4 is a waveform diagram showing the coil output when there is a single simple trigger; Figure 5 is a similar diagram when there are three triggers in close succession; Figure 6 is a similar diagram of a single trigger when the string is manually damped; Figure 7 is a block diagram of the relevant part of the instrument circuitry based on Figure 24 of our earlier application; Figure 8 is a block diagram of processor A in the circuit of Figure 7; Figure 9 is a block diagram of processor B in the circuit of Figure 7; Figure 10 is a view similar to Figure 1 of a modification of the trigger detection system; Figure 11 shows the displacement transfer characteristic obtained from the coil in the modification of Figure 10; and Figure 12 illustrates a typical waveform obtained from the coil of Figure 10 with the strings switched on cyclically each two cycles of the energising current. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The example of the invention to be described comprises a string displacement transducing system that permits precise location measurement of an electrically conductive string that may form part of the playing system of an electronic stringed musical instrument. The improved trigger system illustrated is shown in Figure 1 as comprising a line of six coils 10 each beneath a respective one of six triggerable strings 12. One of the coils 10 is shown in more detail in Figure 2 and comprises a rectangular section silicon iron core 14 on which is mounted a bobbin 16 carrying a winding 18. The coil is positioned symmetrically under the string 10 with the longer axis of the rectangular core section parallel to the string. Thus the coils are in a plane adjacent and parallel to the plane containing the strings. The coils 10 are mounted on a printed circuit board 20 and located in an appropriate housing 22 in the body of the guitar-like instrument. The circuit board also includes suitable amplification circuitry. The trigger strings 12 are sequentially energised with an A.C. current in a similar manner to the string drive system used on the neck (pitch) strings in our earlier patent application. This A.C. string drive current induces a signal in the string trigger coil 10. Movements of the string in both the X and Y planes (i.e. laterally and at right angles to the fingerboard respectively) produce changes in the amplitude of the signal at the output of the coil. When the trigger string is plucked, complex vibrations produce variations in the amplitude of the coil output signal, but when the string is at rest a steady output signal is generated by the coil. This steady state is used as a reference. More particularly, a high frequency current of small amplitude is passed sequentially through each string, in the case of conventional guitars and guitar-style synthesiser controllers, there being six such strings as shown. The small pick-up coil is sited beneath each string relatively close to the bridge for the string set such that no impediment to free movement of string and/or player exists. The magnetic field emanating from the string is received in the coil and by a suitable amplification and phase-referenced detector method a bipolar signal that relates to the string position is obtained. This signal has been found to give very accurate positional information of the string in the plane of movement that is normal to the axis of the coil and Is moderately accurate in determining displacement along the axis of the coil. This characteristic along with several other advantages that this system has lends itself to application as a trigger or trigger and pitch determining method. The output of the phase-referenced detector may in principle be utilised to produce the sound of the vibration of the string by further amplification, or may be processed further to determine If a trigger event has occured or if the string vibration has been subsequently damped by touching. Thus in the example shown in Figure 1 the system includes a string current driver circuit 24 which applies current to the strings in a cyclical sequence through an electrical contact 26 associated with the physical string attachment point 28. As indicated in Figure 3, the six coils are in this example connected to a multiplexer 30 which operates in synchronism with the string driver circuitry 24. The selected coil output Is applied to an analogue band-pass filter 32, and through a sample-and-hold circuit 34 which acts as a synchronous detector, to an analogue-to- digital converter 36. From there the signal is applied to processing circuitry of the type described in our earlier application (processor 2) which is adapted to undertake the necessary additional processing functions. It should be noted that the string driver circuit 24 can be constructed in a similar manner to the string drive circuitry used to apply current to the pitch strings for string bend detection purposes in our earlier application (Figure 9 thereof). The system relies on the fact that when a current is passed through a thin straight conductor a circular, co-axial magnetic field exists around that conductor. If that current is oscillatory in nature, then the changing field will induce an emf into any adjacent conductor and a coil with many turns will particularly respond by generating a voltage at the output terminals. If the coil is placed with its axis normal to the string conductor, then a zero output is observed but as the conductor becomes displaced from that axis, then the output voltage will again increase but this time with a phase reversal. Thus a detector sampling the output waveform with a phase-sensitive ability will see bipolar changes of voltage to indicate the string distance away and which side it is situate. The energising current is produced from a constant current source to exclude variation in performance caused by production spread in electronic components and the use of different string types. The use of a high frequency to energise the coils allows the subsequent electronic filtering to considerably reduce the effects of external field pick-up, such as that generated by power lines, and the resulting performance is rugged. One embodiment of the invention uses six small coils of approximately 1500 turns each of fine wire wound over a rectangular cross-section former fitted with the small flat, centrally placed soft iron core. This enables the six coils to be small enough for siting side by side beneath the trigger string set and sufficiently far removed to be fully encapsulated along with the small printed circuit board providing head-amplifica ion. The physical dimensions and shape of the coil coupled with the distance it is placed away from the energising string interelate closely and in a determined manner to assure maximum rate of change of output voltage as the string traverses its operating region coupled with good linearity. It is desirable at this stage to provide common-mode rejection to signals induced in the coils. The voltage on the strings is not zero due to the passage of the energising current through the resistance of the multiplexing system and this voltage capacitively couples into the coil and, if not rejected, would constitute an interfering signal. The energising current typically operates at approximately 50 kHz, being sufficiently high (e.g. above 20 kHz) to enable it to carry modulating information up to several kilohertz and be subjected to conventional filtering without loss of that information, and not so high as to create risk of either generating emissions or being susceptible to externally generated emissions. Its amplitude is typically of 10 to 20 mA. A microprocessor can be utilised to generate the high frequency waveform which is converted to triangular form which when communicated through loose coupling approximates to a square waveform. The microprocessor can then operate the current switching scheme for each string in turn and the appropriate switching of each pick-up coil to the main receiver amplifier and coordinate the activity of the sample and hold circuit followed by the analogue-to- digital conversion in a perfectly related manner. Thus a complete digitised description of the movement and lateral behaviour of each string is acquired. The refresh rate for this data is approximately 2kHz. Software, described below, performs the necessary processes to that data to satisfy the performance objectives of the instrument. The processing will be illustrated by considering the waveforms shown in Figures 4 to 6. Figure 4 shows the output of one of the coils 10 when a simple trigger takes place. When the string is at rest, the trace is typically a straight line. Figure 4 shows a trace when the string is plucked. Note that in the moments while the string is being deflected, just prior to the moment of "pluck" when the displaced string is let go, the trace clearly shows the relative position of the string even before the string starts vibrating. This data is not available either on the string trigger system of our earlier application, or on a conventional electric guitar pickup. Figure 5 shows the coil output for a series of three triggers with the string being plucked in both senses, i.e. the third trigger is in the opposite sense. Here again the displacement effect is clearly seen. The system must also be able to produce reliable trigger information when the trigger string is mechanically damped by the heel of the player's hand. This technique produces special damped effects on a guitar, and the Synthaxe system can also mimic this effect. However, trigger detection must not be affected in this mode. Figure 6 shows a trace when a trigger occurs during a period of manual damping. The offset trace before and after the trigger can be clearly seen. The system is microprocessor controlled and thus valid trigger definitions can be as subtle and sophisticated as the software engineer wishes. One possible sort of definition might be as follows. A trigger signal is defined as valid when the amplitude of the signal produced by the oscillations of the trigger string exceeds a predefined trigger threshold, and whose mean amplitude coincides with the steady state amplitude produced by a stationary string, plus or minus an operating window or tolerance. The sensitivity of each string may now be set in software by altering the trigger threshold value. By comparing the mean level of the string oscillations with the steady state, spurious triggers produced by overshooting fingers or picks are ignored. This is because, although an overshoot may produce a burst of oscillations, the string is mechanically offset by the overshooting finger or pick under these conditions, and the mean level of these oscillations will not coincide with the signal level of the steady state of the string. So that an oscillating string does not continue to generate a series of trigger signals during a burst of plucking activity, the system preferably does not re-arm itself to generate another trigger until the oscillations have fallen below a predefined re-arming threshold. This threshold is also adjustable as a value in software. The system is Inherently more sensitive and less frequency selective than the previous method. This is due to the fact that when the string is plucked, there is no mass to consider other than the mass of the string itself, and there are no frictional problems due to magnet barrels and housings etc. Also variations in string tension do not significantly alter the performance of the system, and the high tensions preferred by guitar players produce good, reliable results. Nevertheless, it may be possible to design the detection system to be even quicker in operation than that of our earlier application. The processing hardware and its operation can be based on that of our earlier application. Figures 7 to 9 of the present application illustrate the alterations required to implement the present invention. Figure 7 corresponds to Figure 24 of our earlier application to which reference is made for a description thereof, and shows that the analogue processor 3 is now divided into a processor A shown in Figure 8 and a processor B shown in Figure 9. The left hand touch sensing function of our earlier application can, in fact, be transferred into processor 1 and is not shown in Figures 8 and 9. Processor B outputs an inhibit command to processor 2 as shown for reasons discussed below. For compatibility with existing circuitry analogue signals from the transducers are converted to digital form in processor block 3 and then reconverted back to analogue form for onward passage to processor 2, processors A and B thus operating digitally. Figure 8 shows the steps of getting the "new string position' from a digital-to-analogue converter (DAC). The velocity is calculated by taking the difference between the new string position and the old string position. The peak, velocity is held in a peak hold circuit and this is emptied when the sign of the velocity changes (indicating the end of a velocity half-cycle). This continues cyclically with the peak velocity values being sent to Figure 9. The logical steps set out in Figure 8 are thus described by the following loop: Velocity = new string position - old string position IF velocity's sign has changed THEN send peak velocity to Figure 9 peak velocity = ABS (velocity) ELSE IF peak velocity is less than ABS (velocity) THEN peak velocity = ABS (velocity) Repeat loop. ABS means absolute value (modulus). In Figure 9 the operations illustrated are as defined in the following eighteen lines. The capital letters G to M are various constants used in the system. 1) wait for 6 peak velocities from processor A 2) take highest peak velocity of those 6 and call it string velocity 3) look if string velocity is more than twice as high as the last calculated string velocity 4) if this is not true then go back to line 1 PRETRIGGER STATE : 5) record string velocity by calling it "on velocity" from now on 6) wait until processor A has sent H more peak velocities 7) look if the string is touched 8) if the string is touched then proceed at line 13, otherwise just go on STRING UNTOUCHED : 9) get "on velocity", divide it by I 10) if the result is less than J then make it equal to J 11) compare the result to the last peak velocity sent by processor A 12) if it is smaller, then start again at line 1, else go on at line 16 STRING TOUCHED : 13) get "on velocity" divide it by K 14) compare the result to the last peak velocity sent by processor A 15) if it is smaller, then start again at line 1, else just go on TRIGGER SENSED : 16) signal to processor 2 that a trigger has occurred and send "on velocity" divided by L to it 17) wait for M milliseconds to prevent "double triggers" 18) start again at line 1. The processor block 3 has seven mode lines which are placed under control of processor 2. There are two modes per channel, inspect "on-velocity" and observe "off-velocity". When "on- velocity" exceeds a given threshold, "note-on" events are initiated by processor 2 which then flips the mode line for that channel and peruses the "off-velocity" signal now being sent through instead. "Note-off" events are initiated by processor 2 when the separately acquired after-level signal reduces ostensibly to zero, implying that a trigger event is concluded. "Off-velocity" data is part of the "Note-off" signal protocol under MIDI (Musical Instrument Digital Interface standard) just as "on-velocity" is part of the "note-on" protocol. When employed in the instrument of our earlier application the inclusion of the present invention means that there are now two high frequency asynchronous string energising and multiplexing systems, one for the neck pitch and string bend determination, and the other associated with the trigger strings to generate triggering information. There is thus a possibility of mutual interference, particularly from the neck scanning system into the trigger pick-up coils. To obviate these Interference problems, processor 2 receives an inhibit signal from processor block 3 which is relayed to processor 1 and only allows processor 1 to carry out any neck switching operations when processor A is not acquiring a voltage from the trigger string coils, e.g. it is in the middle of a conversion from the sample/hold circuit when the desired voltage is already acquired and is then relatively immune to interference. In the embodiment described the end result is the comprehensive control of the note triggering of a musical synthesiser. However, related systems can utilise the same transducing system to simultaneously acquire from the six strings of a conventionally strung and tuned guitar-style instrument by determining displacement behaviour the following:- i) fretted pitch ii) pitch bend iii) trigger event initiated iv) strigger event concluded by natural decay v) trigger event concluded by artificial damping Another related system could utilise the same transducer method to extract pure audio from the guitar string. Such a design has the great advantage of allowing individual outputs from each string to be available with excellent crosstalk separation and noise performance. In this design, the system is followed by simple parallel analogue processing to minimise cost and decrease response delays. Figure 10 illustrates a modification of the arrangement of Figure 1 in which the six coils 10 under each trigger string 12 are all replaced by a single larger coil 40 extending across the width of all six strings. As well as needing less amplifiers, etc., this has the advantage of closer matching in performance between different strings, and also the ability to mount the coil 40 a substantial distance away from the strings. This makes it easier to house unobtrusively in the body of the instrument. The chief disadvantage is a certain susceptibility over smaller coils to extraneous electrical noise pick-up, particularly from the microprocessor systems in the instrument. A lesser disadvantage is the reduced output per string now available, which implies the need for extra amplification and/or offsets to optimise the dynamic range. Current returns for each energised string can be suitably placed to provide automatic offsets. Figure 11 illustrates the displacement transfer characteristic for such an arrangement, and Figure 12 shows typical waveforms obtained, assuming that the system cyclically switches between the strings so that each string receives two cycles of energisinig current from the string driver circuit 24. The sampled waveform is also shown.

Claims: CLAIMS 1. An electronic musical instrument, comprising a plurality of strings to be plucked to signify the initiation of an Intended note, transducer means responsive to string movement to provide an electrical signal, and circuit means connected to the transducer means and responsive to the plucking of a string to provide an electrical output signal indicative of the initiation of a desired note, the transducer means comprising coil means in a plane adjacent and parallel to the plane of the strings, and means for applying a high frequency current to the strings to be sensed by the coil means. 2. An instrument according to claim 1, in which the strings are fixedly secured at each end. 3. An instrument according to claim 1, in which the circuit means comprises bandpass filter means and sample-and-hold means. 4. An instrument according to claim 1, in which there is one coil for each string. 5. An instrument according to claim 1, in which one coil spans several strings. 6. An instrument according to claim 1, in which the current applying means applies current to the individual strings cyclically. 7. An electronic musical instrument, comprising a plurality of strings to be plucked to signify the initiation of an intended note, transducer means responsive to string movement to provide an electrical signal, and circuit means connected to the transducer means and responsive to the plucking of a string to provide an electrical output signal indicative of the initiation of a desired note, the circuit means being responsive to the amplitude of lateral displacement of the string to provide the output signal indicative of note initiation. 8. An instrument according to claim 7, in which the circuit means determines whether the mean amplitude of the lateral string displacement substantially coincides with a steady state amplitude produced by the string when stationary. 9. An instrument according to claim 7, in which the circuit means determines whether the string returns to its position when stationary within a predetermined time period. 10. An instrument according to claim 7, in which the transducer means comprising coil means in a plane adjacent and parallel to the plane of the strings, and means for applying a high frequency current to the strings to be sensed by the coil means.

Inventors: AITKEN WILLIAM ALEXANDER, DIXON MICHAEL STEPHEN, SEDIVY ANTHONY JERRY, AITKEN, WILLIAM, ALEXANDER, DIXON, MICHAEL, STEPHEN, SEDIVY, ANTHONY, JERRY
Applicants: SYNTHAXE LTD, SYNTHAXE LIMITED
Application year: 1987
Main classification: G10H3
IPC codes: G10H3