GB2465154A

GB2465154A - A stringed musical instrument, played by tapping

Info

Publication number: GB2465154A
Application number: GB0820244A
Authority: GB
Inventors: Anthony Richard Hardie-Bick
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-11-05
Filing date: 2008-11-05
Publication date: 2010-05-12
Also published as: GB0820244D0

Abstract

A musical instrument 101 played by tapping is disclosed, comprising: a fretboard 102; frets 103; strings 104-113; transducers 115, 119 for converting string oscillations into signals representative of the oscillations; and a signal processing circuit (401). String oscillation is initiated by tapping a string against a fret 103 on the fretboard 102. The signal processing circuit (401) detects an amplitude (406) of string oscillation, defines a filter characteristic (407) such as resonance in response to the detected amplitude, and obtains a natural dependence of the harmonic content (409) of string oscillation upon the intensity with which the string 104-113 is set in motion. This is achieved by filtering (408) the signals in accordance with the defined filter characteristic, which may be defined in a look-up table and calculated using a z-transform. Signal processing may be performed independently (401, 402) for signals from strings 104-113 played by the left and right hands.

Description

INTELLECTUAL

. .... PROPERTY OFFICE Application No. GB0820244.2 RTM Date:7 March 2009 The following terms are registered trademarks and should be read as such wherever they occur in this document: Chapman Stick Intellectual Property Office is an operating name of the Patent Office www.ipo.gov.uk

Patent Specification

Supporting an Application under the Patents Act 1977 Title: Musical Instrument

Field of the Invention

The present invention relates to stringed musical instruments and in particular musical instruments in which strings are set in motion by tapping strings against frets along a fretboard, and in which resulting string oscillations are converted into electrical signals.

Introduction to the Invention

The range of expression of a musical instrument is dependent upon the musical language it provides. To be useful as a means of expression, this language must be self-consistent, so this is not the same as providing the musician with an unlimited variety of sounds. The electric guitar provides considerable opportunity for sound processing using effects. The variety of effects available is almost unlimited, from a technical perspective.

However, the simple application of an effect to the sound of an instrument generates a discontinuity in its musical language, and a more integrated approach may result from the exploration of different playing styles. As a result, musicians constantly explore the varieties of expression possible from playing techniques within a particular instrument, so that newly discovered forms of expression are connected with a family of existing sound characteristics.

On a guitar, it is possible to play notes by tapping strings at frets in order to set them in motion, rather than by plucking. This is particularly applicable to amplified guitars, because the resulting notes tend to be quieter than usual. Without the need to pluck strings to obtain a sound, both hands are free to play at the frets. This freedom to play notes by tapping with fingers of both hands has resulted in the development of instruments intended primarily for playing by tapping. Such instruments provide more strings than an ordinary guitar, greatly extending the pitch range. The best known instrument of this kind is the Chapman StickTM.

The Stick, and other instruments played by tapping, open up a new musical space that can be very exciting to explore. However, the range of tones that may be obtained by tapping is somewhat limited. As a result, these instruments are usually played in a way that generates musical contrast primarily through harmonic progression, rather than through variations in the sound of the instrument itself.

Summary of the Invention

It is an aim of the present invention to provide an improved musical instrument played by tapping, comprising a fretboard, strings adjacent to said fretboard arranged to be set in motion by tapping said strings against frets on said fretboard; transducer means for converting oscillations of said strings into signals representative of said oscillation; and signal processing means arranged to perform the steps of detecting an amplitude of string oscillation in said signals, defining a filter characteristic in response to said detected amplitude, and obtaining a natural dependence of the harmonic content of string oscillation upon the intensity with which a string is set in motion by filtering said signals in response to said defined filter characteristic. Preferably said steps for processing signals are performed independently for strings played by the left and right hands.

Brief Description of the Drawings

Figure 1 shows a musical instrument designed for playing by tapping, including a pickup module containing signal processing circuitry; Figure 2 details the process of tapping used to play the instrument shown in Figure 1; Figure 3 shows frequency spectra resulting from the process of tapping detailed in Figure 2; Figure 4 shows a block diagram of signal processing circuitry contained in the pickup module shown in Figure 1, including melody and bass signal processing channels and an output circuit; Figure 5 shows a schematic diagram of a signal processing channel of the type shown in Figure 4; Figure 6 shows an algorithm for digitally implementing a signal processing channel of the type shown in Figure 4; and Figure 7 shows a schematic diagram of the output circuit shown in Figure 4.

Detailed Description of the Preferred Embodiments

Various techniques for playing guitar are known. Variations in timbre can be achieved by plucking a string at different places along its length, using a finger, fingernail or pick. This provides significant variation in the harmonic structure of the notes played. Additionally, a guitar string may be set in motion by tapping the string against a fret, without the need to pluck the string at all. This makes it possible to play the fretboard with both hands. An ordinary guitar can be played in this way, but the fact that both hands are then free to play the fretboard means that a greater number of strings can be usefully provided, giving a greater range of notes than a traditional guitar. Several other advantages relating to the playing position, height of the strings above the frets, and so on, are also evident on instruments that have been created specifically for playing by tapping with both hands.

An instrument designed for playing by tapping is shown in Figure 1 The instrument 101 is a Chapman Stick and includes additional electronic circuitry. The Chapman Stick is manufactured by Stick Enterprises Incorporated, 6011 Woodlake Avenue, Woodland Hills, CA 91367-3238, USA. The instrument 101 comprises a fretboard 102 with frets 103, ten strings 104 to 113, and a pickup module 114 containing the additional electronic circuitry. The pickup module 114 includes a melody pickup 115 with melody channel controls 116, 117, 118, a bass pickup 119 with bass channel controls 120, 121, 122, an output switch 123 and an output socket 124. The output socket 124 is used to connect the instrument to an amplifier.

The melody pickup 115 acts as a transducer for a first set of five strings 104 to 108. The bass pickup 119 acts as a transducer for a second set of five strings 109 to 113. The musician's right hand 125 rests with fingers over the melody strings 104 to 108. The left hand 126 rests with fingers over the bass strings 109 to 113. Fingers of either hand may play at any position; flexibility in this respect is an essential part of the playing technique on this instrument.

The process of tapping is illustrated in Figures 2a and 2b. In Figure 2a, a string 111 is shown at rest. The pickup 119 contains a magnet and a coil. The string 111 is static, and so no variations in the magnetic field occur, and no electrical signals are generated. A note is about to be played by a finger at position 201 just above the string 111 and just behind a fret 103 on the fretboard 102. In Figure 2b, the finger has moved downwards to a position 202, thereby setting the string 111 in motion, and resulting in oscillations of the string 111 at a frequency determined by the length of its freely oscillating portion and also its tension.

Variations in the magnetic field of the pickup 119 result in electrical signals representative of the string oscillations. The technique of playing the string is best described as tapping, as it involves a light touch just behind the fret of the desired note. Louder notes are played by tapping the string harder, or with greater intensity. This increases the downward velocity of the string as it hits the fret 103, which is then translated into string oscillations of a greater amplitude. The term tapping describes the process whereby the string is caused to impact against a chosen fret by downward movement of a finger just behind the fret. A second method by which a string can be set in motion is known as a pull-off, in which a fingertip pulls away from a fretted note, effectively plucking the string at that fret and causing it to sound at a lower frequency determined by a fret which is held by another finger of the same or other hand. Tapping is the primary method by which new or ascending notes are played and pull-offs are used to play descending notes on the same string. In this way, upward or downward melodic movement across one or several strings is achieved, although the terms tapping and tap-style are generally used to describe the combination of both techniques.

Both tapping and pull-offs suffer from a lack of variation in timbre. String oscillation, as shown in Figure 2b, varies in amplitude according to the intensity with which the string is set in motion by tapping 202. However, the harmonics present in string oscillation vary their amplitude in the same relationship with tapping intensity as does the amplitude of the fundamental frequency of the note. As a result, the brightness of tone of a note remains the same, regardless of tapping intensity. The volume varies in an expressive way, but the tone remains unchanged.

The pull-off also suffers from this limitation. A musician typically has very little choice, during a sequence of notes, as to which fret the pull-off is made at, and therefore must accept the particular tone that arises from a pull-off made at that fret. Very little change in tone occurs in response to intensity, although some subtle variation does occur, because pull-offs are made with the soft part of the fingertip, which compresses in varying degrees according to pressure.

In a guitar, the tone of a note can be varied by the location along the string where the string is plucked. Each location generates a different balance of harmonics in the tone. In addition, the type of pick which is used, or fingertip or other implement, facilitates considerable variation in overall tone brightness. All of these possibilities are open to the player of a tap-style instrument, of course, but then tapping with both hands is prevented, and the instrument will start to sound much like a guitar. Tap-style playing opens up a completely new vocabulary in terms of the phrases, chords and progressions which are playable. The dynamic range, in terms of volume, is comparable to that of a grand piano, and the tone, too, is unique, although fixed in character.

The instrument most associated with variation in tone in response to touch is the piano. Variation in the brightness of a piano note results from the variable compressibility of the felt which covers the hammer that hits a string. This felt is less dense on the outside.

When a note is played softly, only the outer layer of felt is compressed. When a note is played loudly, the outer layer is fully compressed against the dense inner layer, so the string is hit by a harder object, resulting in greater amplitude of higher harmonics. The density of the felt used in piano hammers is carefully regulated in a process known as voicing.

The differences between tapped strings and piano strings at high and low impacts are summarised in Figure 3. Graph 301 shows the frequency profile of a string tapped at high intensity, recorded 200mS after the start of the note. Along the x axis are frequencies ranging from the lowest at the origin to the highest audible frequency. The harmonic content varies from a peak at the fundamental to lower amplitudes for the higher harmonics. The graph is normalised with respect to the amplitude of the fundamental, so that variations in relative strengths of the harmonics may be seen more easily. Graph 302 shows a frequency profile of a string tapped at a low intensity. The graph is normalised, so the fundamental frequency of oscillation is shown at the same amplitude as graph 301. The relative strengths of the harmonics are unchanged. Graphs 301 and 302 would look almost the same for pull-offs; only slight changes in relative harmonic amplitudes would be present. The same graphs for a piano string are shown in graphs 303 and 304. Graph 303 looks very similar to graph 301.

However, graph 304 is clearly different from graph 302. The soft outer felt of the piano hammer imparts a much lower rate of acceleration to the piano string than the inner dense felt, with the result that the fundamental is far louder than the accompanying harmonics.

The importance of such variation in tone arises from the fact that the human ear is sensitive not only to volume but also to timbre. When a sound changes only in its amplitude, a reduced amount of differentiating information is sent to the auditory processing centres of the brain. By contrast, when a sound has a timbre that varies in correspondence with its amplitude, becoming brighter with increasing volume, the effect is psychologically very different. This perceptual aspect arises from the evolution of those parts of the brain which appreciate music also being associated with those which interpret the emotional content of speech or human-made vocal sound. Words and vocal expressions are naturally emphasised by increasing brightness of tone as well as volume, because bright sounds carry more characterising information and are therefore more easily distinguished at a distance.

A parallel exists between the structure of the piano hammer and the human finger.

Both have outer layers which compress easily and inner layers which are more dense. For this reason one may take the response of the piano as a model for the desired response of a string instrument played by tapping. From the point of view of the musician, a variation in tone that conforms to tactile sensations that occur when a string is tapped would be considered natural as well as musically useful.

A block diagram of the circuitry contained in the pickup module 114 shown in Figure 1 is shown in Figure 4. The melody transducer 115 supplies electrical signals representative of oscillations of the melody strings 104 to 108 to a melody channel signal processing circuit 401. The bass transducer 119 supplies electrical signals representative of oscillations of the bass strings 109 to 113 to a bass channel signal processing circuit 402. Both signal processing circuits 401 and 402 operate independently. An output circuit 403 supplies processed melody and bass signals to the output socket 124 shown in Figure 1. A power supply 404 supplies power from a nine volt battery to the signal processing circuits 401 and 402. The unprocessed response of the transducers 115 and 119 is illustrated in graphical form at 405, which is the same response as that shown in graphs 301 and 302 in Figure 3.

The melody channel signal processing circuit 401 includes a circuit 406 for detecting amplitudes of string oscillation. The amplitude of string oscillation is supplied to a circuit 407 for defining a filter characteristic. A signal representing the defined filter characteristic is supplied to a filter circuit 408. A natural dependence of the harmonic content of string oscillation upon the intensity with which a string is set in motion by tapping is obtained by filtering signals from the transducer 115 in accordance with the filter characteristic defined by circuit 407. The bass channel signal processing circuit 402 includes amplitude detection 410, a circuit for defining a filter characteristic 411 and a filter 412, and these operate in the same way, except for the filter 412. The filter in the bass channel has a slightly higher degree of resonance.

The modified pickup response that results from signal processing 401 or 402 is shown in graphical form at 409. This is similar to that shown in graphs 303 and 304 in Figure 3, which show the response of a piano.

The provision of two separate signal processing circuits 401 and 402 makes possible the clear musical articulation of two separate parts, which may most naturally be played by the two hands on the respective groups of strings 104 to 108 and 109 to 113. Although separate channels could be provided for each individual string, the amount of circuitry required for this is prohibitive, particularly if high sound quality is required. Conversely, the use of a single channel is also possible. However, the natural use of two hands for tapping, and the two separate channels already provided by the Chapman Stick, form the basis of a relatively simple apparatus in which the range of musical expression is greatly increased using the arrangement that has been described.

The melody channel signal processing circuit 401 shown in Figure 4 is detailed in Figure 5. In the amplitude detection circuit 406, oscillation signals received from the melody pickup 115 are supplied to a sensitivity control 118 shown in Figure 1. A high pass filter is formed by a capacitor 501 and a resistor 502. This high pass filter equalises sensitivity across strings of different gauges by filtering signals below three hundred hertz using a six decibel-per-octave characteristic. The high pass filtered signal is amplified by a transistor 503. This amplified signal is rectified by a circuit based around an operational amplifier 504, which has the effect of giving all signal peaks the same polarity. This is considered as forming the process of amplitude detection 406 shown in Figure 4.

Definition of the filter characteristic 407 is performed in the following way. An operational amplifier 505 adds the detected amplitude to a bias signal 506. This is supplied to a transistor 507 which charges up a capacitor 508 to the voltage given by the sum of the detected amplitude and bias signals. The capacitor 508 discharges slowly through a second transistor 509, giving a filter characterising signal 510. Two vactrols 511 and 512 are used to set the filter characteristic. A vactrol is an optical device containing a light-emitting diode (LED) and a light-dependent resistor (LDR) coupled together. In this circuit, the light-emitting diodes of the vactrols 511 and 512 are supplied with the filter characterising signal 510. This sets the resistances of the LDRs in the vactrols which define the cutoff frequency of the filter 408. A light-emitting diode has an exponential relationship between its optical output and the voltage applied to it. Furthermore, the resistance of an LDR varies in a non-linear way with the light incident upon it. The translation of the filter characterising signal 510 into a filter characteristic is therefore an extremely complex process. Series resistances 513 and 514 are used to partially linearise this translation. It will be appreciated that the specific component values shown result from listening experiments and provide a natural relationship between intensity of tapping and brightness of the resulting tones.

In the filter circuit 408, signals from the pickup 115 are supplied to a volume control 116 shown in Figure 1. The volume control 116 is used to set the overall volume of the melody channel. Filter pre-amplification is performed by an operational amplifier 515. The filter comprises two low-pass sections. The first of these is formed by a capacitor 516 and the resistance resulting from the combination of the resistor 514 and the LDR of the vactrol 512.

A transistor 517 supplies signals to the second filter section, which is formed by a capacitor 518 and the resistance resulting from the combination of the resistor 513 and the LDR of the vactrol 511. The filter output is provided by an operational amplifier 519. Feedback is supplied from the output of the operational amplifier 519 to the capacitor 516 in the first filter section. This provides the filter with a degree of resonance. The two filter sections provide a 12dB-per-octave low pass resonant filter characteristic. The resonance causes harmonics around the cutoff frequency of the filter to be boosted. Therefore the filter not only removes frequencies above a cutoff frequency defined in response to tapping intensity, it also imparts a degree of amplification to harmonics around the cutoff frequency. This is an aesthetic consideration, and adds character to the sound, giving a variation in tone colour as well as brightness. The combination of tone colour and brightness is considered as being defined by the harmonic content of a note.

The overall brightness of the filter is set by a brightness control 117 which is also shown in Figure 1. The brightness control 117 is part of a bias circuit which includes a variable voltage reference 520. The resulting reference voltage signal 521 varies between 3.75 and 4.1 volts, according to the setting of the brightness control 117. This is used as a bias voltage for the amplitude detection circuit 406, giving a filter characterising signal 510 with a range of 2.85 to 3.1 volts. The filter characterising signal 510 supplies a voltage just high enough to cause the LEDs in the vactrols 511 and 512 to generate light. Detected signal amplitudes increase the light emitted, increasing the cutoff frequency of the filter 408 for a short time. The sensitivity control 117 increases the degree to which signal amplitudes increase the cutoff frequency of the filter 408.

The circuit shown in Figure 5 is also used for bass channel signal processing 402.

There is a slight difference between the two circuits. In the bass channel, a resistor 522 and a capacitor 523 provide slightly increased gain in the output operational amplifier 519, giving the bass channel filter 408 a slightly higher degree of resonance than the melody channel filter 412. Resistor 522 and capacitor 523 are not included in the melody channel circuit.

Diodes are provided across the base-emitter junctions of two of the transistors 503 and 507 in order to prevent emitter-base breakdown from discharge of the electrolytic capacitors in their emitter circuits.

The signal processing circuit shown in Figure 5 requires a power supply of nine volts and operates at a quiescent current of one milliamp. The melody and bass channels together consume two milliamps, making this circuit suitable for battery operation.

In an alternative embodiment the analogue signal processing circuit shown in Figure 5 is replaced by a digital signal processor (DSP) such as the D5P56367 available from Freescale Semiconductor Incorporated, 6501 William Cannon Drive West, Austin, Texas 78735, U.S.A. Suitable analogue-to-digital and digital-to-analogue converters are widely available, including the ALl 101 G and ALl 201 G manufactured by Wavefront Semiconductor, Scenic View Drive, Cumberland, RI 02864, U.S.A. Application notes detailing connections between the DSP and converters are available from the manufacturers.

An algorithmic equivalent to the analogue circuit shown in Figure 5 is shown in Figure 6. Those familiar with digital signal processing techniques will be able to convert this algorithm into assembly language for the DSP56367 or other DSP, either manually or using an automated signal processing design system.

Amplitude detection 406 includes a high pass filter 601 having a -3dB characteristic at 300Hz. An absolute (ABS) function inverts the polarity of negative samples. Definition of the filter characteristic 407 includes an integrator based around an adder 603. The integrator's feedback loop is modified to include a maximum (MAX) function 604 which gives as its output the maximum of its two inputs. As a result, high signals pull the output of the integrator up to their value, which then decays during subsequent samples according to a time constant defined by the feedback factor K1. The time constant should be equal to one second. The output of this integrator is supplied to a multiplier 605 which multiplies by a value between zero and one which is derived from the setting of the sensitivity control 118 shown in Figure 1. This is added to a brightness value provided by a multiplier 606, which multiplies by a value derived from the setting of the brightness control 117 shown in Figure 1. The resulting samples are supplied to a low pass filter 607, which has a time constant of two milliseconds.

This low pass filter is necessary to avoid the sudden transitions which can occur at the start of a new note, which can result in an audible click. The low pass filtered samples are supplied as addresses to a filter coefficient look-up table 608, which looks up three values, A, B and R which define the filter's characteristic.

The filter 408 includes two integrating low pass stages based around adders 609 and 610. These integrators have time constants defined by the values A and B respectively, which are supplied from the look-up-table 608. A feedback path 611 provides the filter with a degree of resonance. A differentiator based around adder 612 supplies a multiplier 613 which defines the resonance using the value R from the look-up table 608. In this filter, R varies in compensation for the values of A and B, and therefore varies with cutoff frequency, even though the degree of resonance of the filter 408 may be fixed. The channel's volume control is implemented in the digital domain by multiplier 614, which multiplies samples by a value derived from the setting of the volume control 116 shown in Figure 1. In an alternative embodiment, channel volume can be defined in the analogue domain, thereby making best use of the range of the digital-to-analogue converters.

Coefficients in the look-up table 608 may be obtained with reference to the z-transform of the filter, which is: Hzj z-Az--Bz--Cz+AB+C where A and B are the integration coefficients of the filter. C is an overall feedback factor which is a conflation of 1-B and R. The resonance coefficient R in the look-up table 608 is obtained from the values of B and C by:

C

B

A frequency-to-coefficient look-up table may be defined initially using these equations.

The initial look-up table may then be gradually distorted using a normalised logarithm or other exponential function until listening tests confirm the required natural relationship between tapping intensity and harmonic content of string oscillation. Linear interpolation between coefficients may be used to ensure smooth transitions without the need for a large look-up-table. The algorithm shown in Figure 6 is repeated for the bass channel.

The output circuit 403 shown in Figure 4 is detailed in Figure 7. The output 701 from the melody signal processing channel 401 is supplied to one terminal of the output switch 123, which is also shown in Figure 1. The output 702 from the bass signal processing channel 402 is supplied to a second terminal of the output switch 123. The negative terminal of the battery in the power supply 404 is also supplied to a terminal of the output switch 123.

Using the configuration shown, there are two connection options. Both outputs may be combined and connected to a monophonic quarter inch jack plug via the output socket 124.

In this case, power is supplied to the signal processing circuits 401, 402 only when a monophonic jack plug is connected. In the other switch position, separate bass and melody channel connections are made to a stereophonic quarter inch jack plug, and power is always applied when the switch is in this position.

Claims

Claims 1. A musical instrument played by tapping, comprising a fretboard, strings adjacent to said fretboard arranged to be set in motion by tapping said strings against frets on said fretboard; transducer means for converting oscillations of said strings into signals representative of said oscillations; and signal processing means arranged to perform the steps of: detecting an amplitude of string oscillation in said signals; defining a filter characteristic in response to said detected amplitude; and obtaining a natural dependence of the harmonic content of string oscillation upon the intensity with which a string is set in motion by filtering said signals in response to said defined filter characteristic.
2. A musical instrument according to claim 1, wherein said filter characteristic has higher relative amplitudes of low frequencies with respect to amplitudes of high frequencies.
3. A musical instrument according to claim 2, wherein said filter characteristic includes resonance.
4. A musical instrument according to claim 1, including light-emitting means and light-dependent resistance means arranged to set the cutoff frequency of filtering.
5. A musical instrument according to claim 1, wherein said step of detecting an amplitude includes compensating for different string gauges by processing said signals with a high pass filter.
6. A musical instrument according to claim 1, including a sensitivity control for varying said detected amplitude of string oscillation independently of the instrument's volume.
7. A musical instrument according to claim 1, including a brightness control for varying the brightness of said string oscillations.
8. A musical instrument according to claim 1, wherein said filter characteristic is defined in a look-up-table that has been calculated with reference to the z transform of a digital filter.
9. A musical instrument according to claim 1, including digital signal processing means configured to filter said signals by a process having the z-transform of: H[z] 2 z-Az--Bz--Cz+AB+C
10. A musical instrument according to any of claims 1 to 9, including signal processing means configured to perform said processing steps separately for signals from sets of strings associated with left and right hands respectively.
11. A method of generating musical tones by processing signals derived from the oscillation of a string set in motion by tapping said string against a fret on a fretboard adjacent to said string; characterised by the steps of: detecting an amplitude of string oscillation in said signals; defining a filter characteristic in response to said detected amplitude; and obtaining a natural dependence of the harmonic content of string oscillation upon the intensity with which it is set in motion by filtering said signals in response to said defined filter characteristic.
12. A method of generating musical tones according to claim 11, wherein said filter characteristic has higher relative amplitudes of low frequencies with respect to amplitudes of high frequencies.
13. A method of generating musical tones according to claim 12, wherein said filter characteristic includes resonance.
14. A method of generating musical tones according to claim 11, including obtaining said harmonic content of string oscillation by optically modifying a resistance.
15. A method of generating musical tones according to claim 11, wherein said step of detecting an amplitude includes compensating for different string gauges by processing said signals with a high pass filter.
16. A method of generating musical tones according to claim 11, including adjusting a volume control for said string oscillations without varying their detected amplitude.
17. A method of generating musical tones according to claim 11, including adjusting a brightness control to set the brightness of string oscillations.
18. A method of generating musical tones according to claim 11, including accessing a look-up-table that has been calculated with reference to the z transform of a digital filter.
19. A method of generating musical tones according to claim 11, including filtering said signals by a process having the z-transform of: H1zj z2-Az---Bz--Cz�AB+C
20. A method of generating musical tones according to any of claims 11 to 19, wherein said steps for processing signals are performed independently for strings played by the left and right hands.