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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains generally to the field of electronic musical
instruments and is more particularly directed to a novel sound pick-up for
string instruments and associated electronic circuits for encoding the
pick-up output in M.I.D.I. compatible serial digital format for driving a
sound synthesizer or other sound processing system equipped with a
standard M.I.D.I. interface.
2. State of the Prior Art
Sound or music synthesizers have come into widespread use and are available
from a growing number of manufacturers. While the various models currently
offered differ in terms of capability flexibility and complexity many are
equipped with a M.I.D.I. (Musical Instrument Digital Interface) interface,
a unified data communication standard for electronic musical intruments
which enables systems of different manufacture to be used together. For
example two or more synthesizers of different make may be simultaneously
controlled or played from a single keyboard.
At present, the keyboard remains the most commonly used input device to
MIDI sound synthesizers. However, many performers and musicians are more
proficient with or have a preference for non-keyboard musical instruments,
particularly among the great numbers of guitar players. A need therefore
exists for MIDI interfaceable string instruments and more precisely, for
sound pick-up systems for musical string instruments having a MIDI
compatible output.
U.S. Pat. No. 4,606,255 issued Aug. 19, 1986 to Hayashi et al. and other
patents cited therein, particularly U.S. Pat. No. 4,357,852 issued Nov. 9,
1982 to Suenaga, are directed to guitars for use with a sound synthesizer.
These guitars derive an input signal to the synthesizer by detecting the
fundamental frequency of a vibrating guitar string. This requires that the
guitar pick-up be connected to circuits capable of accurately
discriminating between the fundamental and the multiple harmonic
frequencies simultaneously generated when any of the guitar strings is
played. In practice however, it happens under certain circumstances that
the system erroneously identifies a harmonic frequency as the fundamental,
and thus causes the sound synthesizer to generate an output note other
than the one actually played by the performer. In addition, the currently
available MIDI guitar sound pick-up systems are excessively costly and not
widely affordable.
A continuing need therefore exists for more reliable and lower cost MIDI
interfaceable pick-up systems for musical string instruments.
SUMMARY OF THE INVENTION
This invention advances the state of the art by providing a sound pick-up
system based on a novel method of sensing a musical note played on a
string of a musical instrument. A test signal is applied by a transmitter
to one fixed end of the string and propagates along the string towards the
opposite end of the string. Upon reaching the opposite end or an arbitrary
intermediate point of the string which has been clamped against vibration,
a return signal or echo results from reflection of the test signal which
return signal is detected by a receiver. A selected characteristic of the
propagated and return signals are compared by a string length detection
circuit which derives a first output representative of the physical length
of the string between the propagation end and the reflection point of the
signal, which first output is therefore also indicative of the fundamental
frequency of vibration of the string and thus of the musical note
obtainable by playing the string. A level sensing circuit monitors the
string for acoustic vibration induced by a player touching the string to
elicit a musical note and upon sensing such vibration a second electrical
output is derived indicative of the intensity of such sensed vibration and
thus representative of the level of the musical note played. The playing
of a note on the string may be sensed either in a normal mode by detection
of a note level output exceeding a preset level or in a fret trigger mode
upon sensing a change in the string length irrespective of any note level
output.
The first and second electrical outputs thus derived may be used in their
original form for driving other systems or may be encoded in any suitable
format for input to a sound synthesizer or other sound processing system.
In particular, the first and second outputs of the sound pickup may be
operated on by a suitably programmed microprocessor and encoded for system
output in M.I.D.I compatible serial digital format for input to a standard
M.I.D.I. synthesizer.
The pick-up system of this invention may also be configured for detecting a
relative phase shift in the return signal which has been found to occur
due to bending of the string by the player of the instrument while holding
a particular note, and deriving a third electrical output related to such
phase shift and thus indicative of the degree of string bending. This
third output may also be encoded in a manner similar to that of the first
and second outputs for input to a sound synthesizer or other system.
The test signal propagated along the string may be an acoustic wave of a
frequency which preferably lies outside the normal tonal range of the
instrument, as for example, an ultrasonic frequency. The test signal is
generated by a transmitter circuit and applied to the string by means of a
suitable transducer driven by the transmitter and coupled to the string.
The return signal is detected by a receiver circuit which may
advantageously share the same transducer with the transmitter circuit. In
the case of an ultrasonic test signal, the transducer may be a
piezo-electric transducer mechanically coupled to the string.
In a presently preferred form of the invention, the test signal is
intermittently applied to the string and the string length or first output
is derived from a measurement of the time lapse between propagation of the
test signal and detection of the return echo. In alternate forms of the
invention however, the test signal may be propagated continuously and the
first electrical output derived from the phase relationship between the
return echo and propagated test signals. String length information may be
derived in still other ways, as for example, by periodically applying a
sweep frequency signal to the string, the signal being of either
acoustical or electrical nature, receiving a return sweep signal reflected
from the opposite end of the string, and then operating on the
relationship between the transmitted and returned signals at various
discrete frequencies in a manner known in the art to derive the first
electrical output. Furthermore, the acoustic test signal may be propagated
by mechanically coupling a suitable transducer to the string or
alternatively by inductive coupling of an electromagnetic transducer.
In general, the present invention broadly contemplates that string length
information is to be derived from information provided by a transmitted
and a reflected signal propagated along the string of the musical
instrument
The string length output will normally be updated at a string length
sampling rate much greater than the rate at which a player is capable of
producing different notes on the instrument string, as for example, by
touching the string to different frets on a guitar. The string is also
monitored for acoustic vibration at a frequency within its normal tonal
range. In the normal mode a "note on" output signal is triggered to the
MIDI if such string vibration is detected above a preset level. In an
alternative fret trigger mode MIDI "note on" output is triggered upon
sensing a change in string length irrespective of other vibration of the
string. MIDI "note off" output is triggered in either mode by a change in
the string length output of the pick-up.
In a system configured for sensing relative phase shift of the return
signal indicative of string bending, as will normally be desirable, the
microprocessor programmed for encoding the first, second and third pick-up
outputs to MIDI format is set to sample the three outputs at a cyclic
rate, each cycle producing a MIDI output frame. In any given frame, only
two pick-up output conditions can hold true: a new note has been played,
in which case a "note off" command is issued terminating the previously
played note followed by a "note on" command initiating a new note, or no
new note is sensed by the pick-up, in which case only a MIDI string bend
command is issued to the synthesizer without turning off the note
currently being played, if any.
These and other features and advantages of this invention will be better
understood from the following detailed description of the preferred
embodiment considered with reference to the accompanying drawings,
describing and illustrating a MIDI compatible sound pick-up system for
guitar.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a top plan view of a typical guitar equipped with a M.I.D.I.
interfaceable sound pick-up system of this invention;
FIG. 2 is a side view of the guitar of FIG. 1;
FIG. 3 is a perspective view partly broken away of the piezo-electric
transducer pick-up unit mounted to the guitar of FIG. 1;
FIG. 4 is a top plan view of a segment of the pick-up unit of FIG. 3
comprising a single piezo-electric transducer element;
FIG. 5 is a section taken in elevation along line 5--5 in FIG. 4;
FIG. 6 is a longitudinal section taken in elevation of the pick-up unit
taken along line 6--6 in FIG. 3;
FIG. 7 shows the top of the printed circuit bridge strip of the pick-up
unit of FIG. 3
FIG. 8 shows the underside of the printed circuit bridge strip of the
pick-up unit of FIG. 3;
FIG. 9 is an enlarged fragmentary cross-section in elevation showing the
stacked elements making up the pick-up bridge;
FIG. 10 is a basic circuit block diagram of a single string transmitter
receiver channel providing string length and note level outputs for
further processing or encoding;
FIG. 11 is a more detailed block diagram of the system with M.I.D.I. output
according to this invention, showing a single string channel only for
clarity;
FIG. 12 illustrates the waveforms at various stages of the echo receiver
front end section for deriving the string length output;
FIG. 13 is a circuit diagram of the receiver front end section of FIG. 12;
FIG. 14 is a flowchart showing a typical main sequence of operations
executed by the microprocessor in the system of FIG. 11 for converting the
string length and level outputs to M.I.D.I. formated system output;
FIG. 15 is a flowchart of an auxilliary microprocessor step sequence for
converting the string length output of the receiver to a guitar fret
number;
FIG. 16 is a flowchart of an auxilliary microprocessor step sequence for
generating the M.I.D.I. formated serial digital system output;
FIG. 17 is the "note on" step sequence for the FIG. 16 sequence;
FIG. 18 is the microprocessor step sequence for deriving string bend data
in the system of FIG. 11;
FIG. 19 is a flowchart of a tuning sequence for calibrating the system of
FIG. 11 to a particular string instrument;
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a typical guitar 10 comprising a guitar body 12 from which
extends a guitar neck 14 terminating in a head 16. A string bridge 18 of
conventional construction is affixed to the guitar body, and six guitar
strings 20 are strung between the bridge 18 and individual tuning pegs 22
mounted to the head 16 for stretching the strings to a desired degree of
tautness, thus tuning the strings to acoustically vibrate and give off a
desired scale of musical notes according to the physical characteristics
of each string, particularly the string length. The tuning pegs and head
assembly is constructed in such a manner that all strings 20 are bent over
and clamped against a nut bar 24, but spaced above a series of frets 25 as
best appreciated by reference to FIG. 2. The strings 20 have an effective
acoustic length measured between the bridge anchor points 18a of each
string and the nut bar 24. The six strings 20 thus lie in a common plane
generally parallel to and spaced from an guitar body upper surface 26 as
shown in FIG. 2.
The guitar 10 is equipped with a novel sound pick-up unit 30 which includes
a transducer bridge assembly mounted to the guitar body 12 underneath the
six strings 26 adjacently to the bridge assembly 18.
The construction of the transducer bridge 30 is shown in FIGS. 3-9 where
the bridge assembly 30 is seen to have a bridge bracket 32 supported at
two opposite ends 38 in spaced relationship to the guitar surface 26 by
means of spacer blocks 34, and secured in place by means of two screws 36
threaded through spacers 34 and the bridge bracket 32 into the guitar
body. The bridge bracket 32 may conveniently be a strip of sheet metal
bent upwardly at each end 38 in the manner illustrated. The intermediate,
dropped portion 42 of the bridge bracket extending between the vertical
sections 44 holds a transducer assembly which includes an electrical
insulator strip 46 covering the upper surface of the bridge section 42,
and a printed circuit bridge strip 48 of the double-clad type having top
and bottom surfaces 50, 51 respectively. Resting on the upper surface of
the printed circuit strip 48 are six elongated printed circuit strips 52
laid transversely to the bridge strip 48 as best seen in FIG. 5. Each
strip 52 is also double-clad circuit board having upper and lower
conductive surfaces, the lower conductive surfaces of the strip 52 being
in electrical contact with individual, electrically mutually insulated
conductive pads etched 54 on the upper surface 51 of bridge strip 48. A
transducer support arm 56 lies flat on and in electrical contact with the
upper conductive surface of each strip 52, and has a short forked end
section 58 bent upwardly at an angle relative to the printed circuit
strips 52 and the plane of the guitar surface 26. A piezo-electric
transducer element 60 is supported on the end 58 of the arm 56. A suitable
commercially available piezo-electric transducer element is Panasonic
#EFR-TAB 45K1. The transducer 60 consists of a metallic disk 62 on which
is permanently mounted a ceramic wafer 64. The steel disk 62 is spot
welded to the underside of the forked end 58 of the transducer support and
is therefore in electrical contact with the upper conductive surface 53a
of the printed circuit strip 52. The ceramic wafer 64 is connected by an
electrical lead 66 to the conductive underside 53b of the support strip
52. The transducer bridge assembly is completed by an electrically
conductive clamping strip 68 which extends transversely to the six strips
52 and parallel to the bridge strip 48, and is secured to the bridge
bracket 32 by means of seven clamping screws 70 each of which extends
through aligned bores in the elements 68, 48, 46 and is threaded into
bracket 32 so as to securely hold together the stacked strip elements in
positive electrical contact.
A transducer pick-up arm 72 has one end spot welded to the steel disk 62 of
each transducer element 60, while the opposite free end 74 of the
transducer pick-up arm is forked for receiving between two prongs an
overlying guitar string 20 and make light but positive mechanical spring
contact with the string at a point adjacent to the string bridge end 18a.
Both the transducer support arm 56 and the pick-up arm 72 may be
fabricated of 0.010 inch thick spring steel and arranged so that the
pick-up arm end 74 is urged upwardly against the string.
A plated-through hole 55 connects each contact pad 54 with a corresponding
conductor run 57 etched on the underside of printed circuit board strip
48. Six such runs 57 terminate at one edge of the strip from where a
connection is made through a multiconductor cable to a connector (not
shown) conveniently mounted on the guitar body. A ground connection is
made to the bridge clamp 68 or other convenient point which is
electrically common to all steel discs of the six transducer elements 60.
Turning now to the block diagram in FIG. 10, each string transducer element
60 is electrically connected to the input of a high impedance current
buffer amplifier stage 80 through an analog switch 82. A transducer drive
circuit such as an oscillator 84, generates a waveform of ultrasonic
frequency such as the 40 Khz waveform illustrated in FIG. 12 which is
applied as the test signal to each guitar string 20 through the
transducers 60. The test signal in the system being here described is
generated by a system microprocessor. The switch 82 is toggled by a switch
controller 86, which in the present system is also included in the system
microprocessor, for alternately switching the transducer 60 between the
output of drive oscillator 84 and the input of buffer amplifier 80. In an
initial condition, the switch 82 is connected for applying the output of
the oscillator 84 to the transducer 60 which causes an ultrasonic test
signal to be mechanically applied to a guitar string 20 at the bridge end
18a of the string. The applied test signal acoustically propagates along
the string 20 towards the head end of the string which for acoustic
purposes is at the contact point with the nut bar 34, assuming the string
is open i.e. is not being pressed down against the guitar neck at any fret
25 or other point intermediate to the open string ends. The acoustic test
signal wave is reflected at the nut bar 24 and returns along the string 20
back towards the bridge end 18a where the signal is sensed and converted
from mechanical to electrical form by transducer 60.
The switch 82 is toggled between the output of oscillator 84 and the input
buffer stage 80 at a fast enough rate such that the return or echo signal
sensed by transducer 60 is fed to the input of buffer stage 80 which in
turn provides an input signal to two receiver sections operating in
parallel, namely a string length detector circuit 88 which derives a first
output indicative of the current acoustic length of the guitar string 20
associated with the particular string transducer 60, and a note level
detector 90 which derives a second output indicative of acoustic vibration
of the guitar string at frequencies other than the ultrasonic frequency
output of the transmitter oscillator 84 within the instrument's normal
tonal range and in particular acoustic vibration such as produced by a
player plucking or strumming the string 20.
The FIG. 10 diagram shows a single channel transmitter/receiver system with
a single transducer 60. In a complete six string guitar system such as
here described, a separate transducer 60 is coupled to each of the six
guitar strings 20 as described in connection with FIGS. 1-9 and each
transducer element is driven by a suitable transmitter or oscillator
circuit, preferably one which drives all transducers simultaneously in
parallel. Likewise, six mutually independent receiver channels such as
shown in FIG. 10 and operating in parallel are provided, each receiver
channel being associated with one of the string transducers 60, and each
receiver channel delivering separate first and second output signals.
Turning now to FIG. 11, a complete single channel system is shown in more
detailed block diagram form and in particular showing the block components
of each receiver section 88, 90. The processing of the echo signal to
derive string length information is illustrated in FIG. 12. The string
length detector section 88 comprises a rectifier stage 92 which receives
the 40 kilohertz echo signal output A of buffer stage 80 shown in FIG. 10
and rectifies the waveform A to a rectified waveform B. The rectified
output is passed through low pass filter stage 94 which removes the high
frequency components of the rectified 40 kilohertz signal and produces an
output waveform C which is substantially the envelope of the rectified
high frequency signal B. The low pass filter output then is fed to a
differentiator stage 96 which operates to produce a differentiated output
waveform D wherein the zero voltage crossing point d corresponds to the
peak point c in the envelope waveform C. The differentiated output D is
then fed to peak detector 98 the output of which is represented by
waveform E. The peak level e of the waveform E represents the magnitude of
the peak c, i.e. the peak voltage of the return signal A. The receiver
front end section 88 is completed by a comparator circuit which includes
first, second and third comparators 102, 104 and 106 respectively.
Comparator 102 receives as comparative inputs the outputs of peak detector
98 and differentiator 96 to derive a first comparator output C1 whenever
the peak height c exceeds the level e of a previous peak C held by peak
detector 98, and whenever this condition occurs, an output pulse is
derived by comparator 102 at output C1. This comparison of consecutive
peak heights by receiver circuit 88 is a noise discrimination function
based on the premise that while various random or non-random noise
transients and waveforms may be picked by the transducer 60 and processed
by the receiver front end circuit 88, none of the noise waveforms will
exceed the peak level c of the test signal echo waveform A.
Comparator 104 references the output of differentiator 96 against system
ground and derives an output pulse at comparator output C2 upon detecting
the zero crossing d of the differentiated waveform D. Output C2 therefore
provides a precise indication in time of the occurrence of the echo signal
peak c. Finally the third comparator 106 references the test signal echo
40 kilohertz signal A to system ground and derives at output C3 a clock
pulse at each rising phase edge of the echo waveform A. The three
comparator output C1, C2, C3 are inputs to a J-K flip-flop 108. The
flip-flop resets at every high peak output C1, is then set by the C2
output indicative of an actual peak occurrence and is triggered by the
immediately following phase edge output C3 into the clock input of the
flip-flop. When all three conditions required for flip-flop output have
been meet by the comparator signals C1, C2, C3, a string length output Q
is derived by the flip-flop 108 and is connected as a reset input to a
binary counter 110. Counter 110 is continuously clocked from a
microprocessor 120 and the binary count output of the counter 110 is reset
to zero by every output pulse of flip-flop 108. Therefore, as noise peaks
are detected by the receiver circuit 88, the counter is reset by each
consecutive noise peak of increasing magnitude until a true echo return
signal is detected which will also cause the count 110 to be reset to
zero, but no noise peaks subsequent to the true echo return within a given
system time frame will again cause the counter to be reset because as has
been explained the peak detector output will hold the echo return value as
the highest peak value and will not be overridden by any subsequent noise
peaks. The true count of the binary counter therefore begins upon being
reset by the echo peak c. In a six string instrument six receiver sections
88 are provided each driven by its own buffer 80 and driving a separate
binary counter. The binary outputs of the six counters 110 is such a
system are read into microprocessor 12: through the processors data bus
130. A circuit diagram for the string length receiver section 88 is shown
in FIG. 13.
Simultaneously with the string length signal processing by receiver section
88, a note amplitude output is derived by parallel receiver section 90
which as shown in FIG. 11 comprises a two kilohertz low-pass filter stage
122 which receives its input from the buffer 80 output and operates to
remove the high frequency components of any string vibrations sensed by
transducer 60, and in particular removing the ultrasonic test signal
waveform A processed by receiver section 88. The output of low pass filter
122 therefore contains waveforms representative of acoustic vibrations of
guitar string 20 within the audible frequency range such as produced by
playing the string to produce a musical note. The filtered note signal is
fed to the input of a second peak detector 144, the output of which holds
the peak value of the note signal detected during the particular system
time frame. This peak value output is fed from each of six receiver (only
one receiver channel being shown for clarity in FIG. 11) through a string
select demultiplexer 126 to the input of an analog-to-digital converter
128 where the analog value of the peak detector output of each receiver
channel is converted to digital form suitable for input to the
microprocessor 120 through the microprocessor data bus 130. The output of
A/D converter 128 is a note amplitude value representative of the loudness
of the musical note played on the guitar string 20 during the particular
system time frame.
The microprocessor 120 operates, among other functions, as a timer to clock
a system time frame which preferably is approximately 4 milliseconds in
length. During each system time frame a test signal burst is applied to
the instrument strings, its echo detected by the receivers, and the string
length and note level outputs derived. In addition, during a particular
time frame or at least before completion of the subsequent time frame, the
microprocessor operates on the derived first and second receiver outputs
to convert the information to MIDI compatible digital serial output. The
system time frames are repeated at a rate sufficiently high so that
substantially continuous monitoring of the acoustic state of the
instrument strings is achieved. At the end of each time frame the peak
detectors 98 and 124 and the binary counter 110 are cleared by the
processor 120 and the transmit/receive cycle repeats itself.
The processor 120 controls the operation of the transmitter/receiver system
and also effects the data conversion and enconding, all under software
control. The sequence of operations performed by the processor 120 will
now be described with reference to the flow charts of FIGS. 14 through 19.
PROCESSOR MAIN SEQUENCE
The steps in this sequence which is executed once for each system time
frame are numbered in parentheses with reference to the numerals in FIG.
14. The microprocessor 120 operates through an output port to toggle the
switch 82 which selects the transducer connection between the test signal
source 84 in FIG. 10 and the echo receiver circuit. In an initial
condition of the system, the microprocessor toggles switch 82 to disengage
(200) the receivers and thus transmit (202) a test signal from transmitter
84 through transducer 60 which is propagated along the instrument strings
20. The microprocessor 120 then clears (204) the peak detector 98 and
substantially simultaneously starts (206) the four millisecond frame
count, immediately thereafter, toggling switch 82 to engage (208) the
receiver system i.e. connecting the receivers to the respective
transducers. During the time interval while the receivers are waiting for
the return echo signal, the processor 120 retrieves (210) certain data,
particularly "fret numbers"s, "delta"s and note "level" values, previously
stored in memory during one the auxilliary sequences described below for
conversion to the desired M.I.D.I. output format and stores these values
in the processor's registers. In step 212 the processor performs
conversion of the data of the first and second outputs collected during
the previous time frame to digital values on scales compatible with
command formats understandable by the synthesizer to be driven by the
guitar and the output data is encoded in MIDI compatible format and sent
out the M.I.D.I. output. The processor then waits (214) to the end of the
current four millisecond time frame at which time the processor 120 stops
(216) the binary counters 110. The outputs of the binary counters now show
a value representative of the time elapsed between detection of the echo
return signal and the end of the system time frame. By now subtracting the
counter output values, the processor 120 reads (218) the counter output
values and subtracts these from the time frame length to derive the actual
"time" lapse between transmission of the test signal at the beginning of
the time frame and detection of the echo signal during the time frame.
This "time" value is stored for subsequent retrieval. The processor then
reads and stores (220) the note "amplitude" values at the outputs of the
A/D converter 128 for each of the six strings.
The microprocessor 120 also performs a number of subroutines described
below in flow chart form, which are auxiliary to the aforedescribed main
sequence.
TIME-TO-FRET NUMBER CONVERSION SEQUENCE
This sequence which is executed once for each string of the instrument,
i.e. for each receiver channel of the system, during each system time
frame is illustrated in FIG. 15. As a first step the processor retrieves
(222) a previously stored "delay" value which is derived and stored during
an instrument or tuning or system calibration sequence which is described
below. The "delay" value is then substracted (224) from the echo time
value stored at step 218 in FIG. 14. The remnant of this subtraction is
then divided (226) by a "zero fret time" value, also previously derived
and stored in the aforementioned tuning routine the resulting dividend
being stored as a "ratio" value. The "ratio" value is then scaled (228) to
a number between 0 through 255 according to a preset scale factor which
may be user selected. The scaled "ratio" is then used (230) by the
processor 120 as a pointer in a stored "fret number" look-up table to find
the "fret number" corresponding to the particular echo "time" value. The
"fret number" thus obtained is then stored (232) for use in other
microprocessor sequences, ending the time to fret number sequence after
which the microprocessor may proceed (234) to a string bend value
sequence.
STRING BEND VALUES DERIVATION SEQUENCE
Before describing this microprocessor sequence illustrated in FIG. 16,
reference is again made to the circuit diagrams of FIGS. 11 12 and 13.
It has been discovered by these applicants, quite surprisingly, that the
phase of the echo signal shifts relative to the envelope of the echo
signal as a function of string bending by a player while holding a
particular musical note. This phenomenon is relied upon in the system of
this invention to monitor string bending and derive a string bend output
convertible to M.I.D.I compatible data format. This relative phase shift
is illustrated in FIG. 13 where the waveform shifts from its zero bend
condition at A1 to a bent string condition at A2 relative to the echo
signal envelope shown in dotted lining i.e. such that no change in the
envelope peak return time occurs. In other words, the relative phase shift
is a change in the time difference between the envelope peak c and the
next subsequent rising edge of the echo signal A. This "delta" is seen in
FIG. 13 as the change along the horizontal time axis between points c and
p in waveform A1, and between points c and p' in shifted waveform A2.
Point c has not shifted relative to system frame start time; only point p
has shifted so that the output of flip-flop 108 is now triggered at point
p'. This relative phase shift information is inherently contained in the
time value output of binary counter 110 because as earlier described, the
count of counter 110 is triggered at the phase edge immediately subsequent
to the zero crossing d indicative of the envelope peak. The time value
output therefore combines the fret number information with the relative
phase shift information. The contribution of the relative phase shift
value to the counter output however, is relatively small compared to the
numerical magnitude contributed by the fret number information and it is
therefore possible to construct a "fret number" look-up table which will
accurately permit location of the "fret number" within a "time" value
range for each fret of the guitar the range for each number being
sufficient to include the maximum possible deviation in time "value"
attributable to string bending.
The string bend information itself is isolated from the time value output
of binary counter 110 in a string bend processor sequence illustrated in
FIG. 16 and which is executed separately for each string of the
instrument. This sequence involves the steps of retrieving the counter
"time" value (236) previously stored at step 218, performing (238) a
modulus operation on the same which may be arbitrarily a modulus 25,
storing (240) the remainder value, which is a value between 0 and 24, as
the "current" relative phase value. The previously stored relative phase
value called here "delta" is then retrieved (242) by the processor and
subtracted (244) from the "current" relative phase value. The result of
this subtraction is tested for its sign and if the result is negative, the
value 25 is added to it (246), and if positive nothing is added, and the
result of this addition or no addition is stored (248) as the "bend up"
value. The next step (250) is to subtract "delta" from the "current"
relative phase value, and then again test the result for its sign: if the
sign is negative, 25 is again added to it, if positive no addition is made
(252). The number resulting from the addition or no addition is then
stored (254) as the "bend down" value. The stored "bend up"and "bend down"
values are then compared (256) for magnitude. If "bend up" is smaller than
"bend down" then the new value of "delta" is the previous "delta" plus
"bend up" (258); if instead "bend down" is smaller than "bend up" then the
new value for "delta" is equal to the previous "delta" minus "bend down"
(260). The final step in this sequence is the updating (262) of the stored
"delta" value by entering the newly computed value into the approp | | |
