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BACKGROUND OF THE INVENTION
The present invention relates to a note analyzer which automatically
determines the musical note of a musical tone and displays this note as an
aid in the tuning of musical instruments and the like.
Note analyzers of various types are known to the art. Generally, these note
analyzers operate to provide a visual indication of the musical note which
corresponds to the tone being analyzed. For example, one approach of the
prior art utilizes a spinning disc which is strobed at a rate which varies
in accordance with the frequency of the tone being analyzed. This approach
relies on the user to vary the rotation rate of the disc manually until
the strobe rate matches the rotation rate of indicia on the disc. This
approach suffers from the disadvantage that it requires considerable user
intervention and is therefore not automatic in operation. Furthermore, it
may be difficult for a user to find the proper adjustment for the rate of
rotation of the disc if the approximate note of the tone is not known in
advance.
SUMMARY OF THE INVENTION
The present invention is directed to an improved, fully automatic note
analyzer which is capable of accurately indicating the note of an input
musical tone for a wide range of musical tones.
According to a first aspect of this invention, an analyzer is provided with
means for receiving an input signal indicative of a musical tone and means
for automatically generating and updating a measured signal which is
indicative of a parameter, such as the period for example, of the input
signal. In addition, means are provided for automatically generating a
target signal as a function of the measured signal. This target signal is
indicative of the expected value of the parameter, and is used to evaluate
later measured values of the measured signal. That is, later measured
values of the measured signal are stored only if they match the target
signal. Stored measured signals are then used to modify the target signal.
Preferably, the measured signal is classified as matching the target
signal whenever the measured signal is substantially equal to selected
binary factors of the target signal, where the term "binary factor"
denotes any number which can be expressed as 2.sup.N, where N is an
integer.
This first aspect of the invention provides improved rejection of spurious
signals, since the target value is used to screen out measured signals
which do not fall within the expected range. Thus, the first aspect of the
invention contributes to increased accuracy and reliability. In addition,
by considering selected binary factors of the target signal as matches,
use is made of certain harmonic and subharmonic signals which can be
interpreted unambiguously, thereby improving the utilization of the
measured signal. In effect, the note analyzer of this invention can be
constructed to recognize certain harmonics and subharmonics, and to treat
them as such rather than simply ignoring them as erroneous signals.
According to a second aspect of the invention, a note analyzer is provided
with means for generating a sequence of measured signals, each of which is
indicative of a parameter, such as period, which is related to the tone of
an input signal at a respective time. In addition, means are provided for
automatically averaging a plurality of measured signals, including the
most recent measured signal, to generate an averaged measured signal. This
averaged measured signal is then used to generate a note signal which is
indicative of the corresponding musical note, which is then displayed.
This second aspect of the invention provides increased stability to the
note analyzer. Because an averaged value of the measured signal is used
rather than an instantaneous value, the displayed note signal changes more
slowly than otherwise would be the case. This second aspect of the
invention cooperates advantageously with the first aspect discussed above,
in that both aspects contribute to stable operation of the note analyzer
and to increased rejection of erroneous measurements.
According to a third aspect of the invention, a note analyzer is provided
with means for generating a plurality of note signals, each of which is
indicative of the musical note corresponding to an input signal at a given
time. In addition, means are provided for generating and storing a
correction signal indicative of the deviation of a first note signal from
a stored reference value. This correction value is then combined with a
later note signal to generate a transposed note signal indicative of the
musical tone of the later note signal with reference to the first note
signal.
This third aspect of the invention is especially useful in connection with
the tuning of musical instruments, where one instrument is to be tuned to
the pitch of another instrument. For example, it is often desirable to
tune a violin or guitar such that the violin or guitar is in tune with a
piano. In this case, the piano can be used to generate the input signal
which corresponds to the first note signal, and the violin or guitar can
be used to generate the input signal corresponding to the later note
signal. In this way, the transposed note signal of the violin or guitar is
determined in the relative pitch of the piano, rather than in absolute
pitch.
The invention itself, together with further objects and attendant
advantages, will best be understood by reference to the following detailed
description taken in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the presently preferred embodiment of the
automatic note analyzer of the present invention.
FIG. 2 is a flowchart of the initialization sequence and the main control
sequence of the embodiment of FIG. 1.
FIG. 3 is a flowchart of the input signal interrupt routine of the
embodiment of FIG. 1.
FIG. 4 is a flowchart of the timer interrupt routine of the embodiment of
FIG. 1.
FIG. 5 is a block diagram of a preferred embodiment of the circuit 30.
FIG. 6 is a detailed circuit diagram of the circuit 30.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, FIG. 1 presents a block diagram of the
presently preferred embodiment of this invention. The embodiment of FIG. 1
includes a microphone 20 which generates an electrical signal which is
applied to a filter and squaring circuit 30. This filter and squaring
circuit 30 does not form a part of the present invention as such, and will
not, therefore, be described in detail here. However, a detailed
description of the circuit 30 is provided in a co-pending U.S. patent
application Ser. No. 180,626, filed Aug. 25, 1980, assigned to the
assignee of the present invention. This co-pending application is entitled
"Filter Circuit" and names Daniel G. Prysby as the inventor. Reference
should be made to this application for a detailed understanding of the
filter and squaring circuit 30. Here, it is enough to state that the
circuit 30 processes the electrical signal received from the microphone 20
to generate an input signal on the line 32. This filter circuit 30 serves
to filter out undesired high frequency harmonics, and to provide a digital
input signal on the line 32 substantially corresponding in period to the
fundamental period of the electrical signal generated by the microphone
20. As will be explained below, the input signal on line 32 may on
occasion have a period equal to a subharmonic or a harmonic of the
fundamental period rather than to the fundamental period itself. However,
the object of the filter circuit is to produce an input signal on line 32
which as nearly as possible corresponds in period to the period of the
fundamental tone of the microphone signal.
FIGS. 5 and 6 present details of the circuit 30. FIG. 5 provides a block
diagram of the presently preferred embodiment of the filter circuit 30. As
shown in FIG. 5, this filter circuit 30 includes a preamplifier 210 which
provides automatic gain control. This preamplifier 210 accepts an input
signal which is preferably the electrical signal generated by the
microphone 20. The preamplifier 210 acts to amplify this input signal and
to bring the amplified input signal within a preferred amplitude range.
The output of the preamplifier 210 is supplied via line 230 to a variable
frequency filter 240. The construction of this variable frequency filter
240 is shown in detail in FIG. 6. Here it is enough to state that the
filter 240 accepts the amplified input signal on line 230 and produces a
filtered signal on line 260. In this preferred embodiment, the filter 240
is a low-pass filter having a variable rolloff frequency. The
instantaneous value of the rolloff frequency is determined by a control
signal which is applied to the filter 240 by means of the control signal
line 340. By varying the rolloff frequency of the filter 240, this filter
can be adjusted to selectively attenuate high frequency components of the
amplified input signal which have a frequency above the variable rolloff
frequency.
The filtered signal generated by the filter 240 is applied to an output
logic circuit 270 which shapes the filtered signal to form a two-state
digital signal suitable for input to a microprocessor. In addition, the
filtered signal on line 260 is applied as an input to an integrator 280
which generates an intermediate signal on line 302. This intermediate
signal is related to the difference between the peak amplitude of the
filtered signal on line 260 and the threshold signal generated on line 292
by the circuit 290. This intermediate signal on line 302 is applied to the
non-inverting input of a voltage comparator 320. A sawtooth signal
generator 310 is provided which provides a sawtooth signal on line 312 to
the inverting input of the voltage comparator 320.
The voltage comparator 320 acts to generate the control signal on line 340
which is applied to the variable frequency filter 240 to determine the
filter characteristics of the filter 240. This control signal 340 is a
two-state digital signal which alternates between two voltages. The
control signal on line 340 is held in the low voltage state by the voltage
comparator 320 when the amplitude of the sawtooth signal on line 312
exceeds the amplitude of the intermediate signal on line 302. Conversely,
the control signal on line 340 is held in the high voltage state when the
intermediate signal on line 302 is greater than the sawtooth signal on
line 312. Because the sawtooth signal provides a progressive ramp, the
percentage of the total time that the control signal is in the high
voltage state (that is, the duty cycle of the control signal 340) is
determined by the amplitude of the intermediate signal on line 302.
FIG. 6 presents a detailed circuit diagram of the presently preferred
embodiment of the circuit 30, in which the reference numerals of FIG. 5
have been applied to facilitate understanding.
The input signal on line 32 is applied as an interrupt to the interface
unit 40, which is in turn coupled to a CPU 60. In addition, a manually
operated switch 50 is provided to control a second signal which is applied
to the interface unit 40 via line 52. As will be explained below, this
manually operated switch 50 is used to select the mode of operation of the
automatic note analyzer of FIG. 1.
Standard integrated circuit components are included in the embodiment of
FIG. 1 to form a microcomputer system. This system includes the CPU 60, a
programmable read-only memory circuit 70, random access memory circuits
80,90, as well as an address decoder 100. An interface circuit 110 is
provided which controls two display driver circuits 120,122. These driver
circuits 120,122 provided multiplexed control over the display circuit
130. In addition, a one megahertz oscillator 140 is coupled to the CPU to
provide a time base for period measurement operations to be described
below.
In FIG. 1 the number of parallel conductors included in each of the buses
is indicated by a numeral adjacent a slash mark. For example, FIG. 1
indicates that eight parallel conductors are routed between the PROM 70
and the data bus, while four parallel conductors are routed between each
of the RAM's and the data bus. Table 1 indicates the integrated circuits
used to implement the preferred embodiment of FIG. 1.
TABLE 1
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Reference No.
Manufacturer Circuit Type No.
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40 Rockwell 6532
60 Rockwell 6502
70 Motorola 2716
80 Motorola 2114
90 Motorola 2114
100 Texas Instruments
74LS138
110 Rockwell 6522
120 Dionics DI514
122 Dionics DI514
130 Futaba 5LY01
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In this preferred embodiment the PROM 70 stores two kilo-bytes, each byte
eight bits in length, and the RAM's 80,90 each store one kilo-byte, each
byte four bits in length.
The circuit of FIG. 1 is a computer based circuit which receives an
interrupt input signal via line 32 and a switch signal via line 52 and
processes these signals to generate a digital output on display 130. As
will be explained below, this system operates to analyze the musical note
corresponding to the input signal, and to display this note digitally on
the display 130. In this preferred embodiment the precision of the
measured note is indicated in cents, or hundredths of a tone. Thus the
display 130 indicates the nearest note (in semitones) corresponding to the
input signal as well as a number between -50 and +50 which indicates the
number of cents by which the input signal deviates from the displayed
note.
Turning now to FIGS. 2-4, the computer program which processes the input
signals on lines 32 and 52 will now be described. A complete listing of
the computer program flowcharted in FIGS. 2-4 can be found in U.S. patent
application File Ser. No. 180,860 on file with the U.S. Patent and
Trademark Office.
Turning now to FIG. 2, the program of this preferred embodiment begins with
an initialization routine. In this initialization routine various hardward
parameters are initialized and software variables are set to initial
values. Then a set of data counts is measured and stored. As used herein,
the term "data count" will be used to denote a time measurement
corresponding to the number of oscillator pulses counted within a
measurement period extending over P consecutive cycles of the input signal
on line 32.
The CPU 60 includes a counter which is decremented by the oscillator 140 at
the rate of one million decrements per second. In this embodiment a data
count corresponds to the number of times this counter is decremented
within the measurement period. The measurement period is determined by the
input signals applied via line 32 to the interface unit 40. In the
simplest case, which corresponds to the situation immediately following
power up, the variable P is equal to 1 and the measuring period extends
between two adjacent interrupts on line 32. As explained previously, the
circuit 30 serves to provide an interrupt signal substantially once per
cycle of the fundamental period of the electrical signal generated by the
microphone 20. Thus, each data count will, during the first execution of
the initialization sequence, correspond to a number indicative of the
fundamental period of the input signal. During the initialization
sequence, a number of sequential data counts equal to N+5 are measured and
stored. During the first execution of the initialization routine N equals
1 and thus thirteen consecutive data counts are stored.
These data counts are than analyzed to determine whether a consistent
pattern can be found. In particular, the program attempts to find N
separate data counts which are equal to one another, with a tolerance of
about 3%. If N consistent data counts cannot be found, then the
initialization procedure is reexecuted, and another set of data counts is
measured and examined for consistency. Once N consistent data counts are
found, then the variable K is set equal to the sum of these N data counts.
Thus, K is a variable which corresponds to the sum of a selected number of
N consistent data counts. Expressed algebraically,
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Data Count = P .times. C.sub.1 ;
(Eq. 1)
K = N .times. Data Count; and
(Eq. 2)
K = N .times. P .times. C.sub.1 ;
(Eq. 3)
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where C.sub.1 equals the period of the input signal, that is the time delay
between adjacent interrupts on line 32; P equals the number of inputs
ignal cycles included in the data count measurement period, and N equals
the number of measurement periods which are summed to determine K. Of
course, C.sub.1 is not actually a constant value and therefore the
equalities of Eq. 1, 2 and 3 are only approximate. Thus, the variable K is
a function of three variables: (1) the time delay between adjacent
interrupts on line 32, (2) the duration of the measurement period of a
single data count in terms of the number of interrupts which occur between
the starting and the stopping of the counter, and (3) the number of data
counts which are summed to determine K. In this preferred embodiment, P
can take on any of the following values: 1, 2, 4, 8, 16 or 32. When P
equals 32, the measurement period is stopped on the 32nd interrupt
following the start of the counter. In this preferred embodiment, N can
take on any one of the following values: 1, 2, 4, 8, 16. Thus, when N
equals 16, 16 separate data counts are summed to form the variable K. The
variables N and P are used to control the data collection process as a
function of the measured period of the interrupt signals on line 32.
Once an initial value for the variable K has been determined, K is then
normalized to place it within a desired range. In this preferred
embodiment, the desired range is between 0135CD (hexadecimal) and 009AE6
(hexadecimal). The variable K is normalized to fall within the range
described above by multiplying or dividing it (as necessary) by factors of
2 until it falls within the desired range. Furthermore, at least one of
the variables N and P is adjusted each time the variable K is normalized
in order to adjust the measurement procedure to the prevailing measured
period.
Thus, for example, if the value of K is lower than the minimum acceptable
value, K is doubled and then either N or P is doubled as well. Doubling
either N or P will cause the next calculated K to be approximately double
its previous value, thereby reducing the need for further normalization of
K. If K is still below the minimum acceptable value after being doubled,
it is then doubled again, and once again either the variable N or the
variable P is doubled as well. On the other hand, if K is greater than the
maximum allowed value, then the value of K is divided by two, and one of
the variables N or P is divided by two as well. In this way the variable K
is normalized to fall within the desired range, and the variables N and P
are adjusted to increase or decrease K as appropriate in the next pass
through the program.
The normalized value of K is then averaged with previously calculated
values of K to smooth out fluctuations. In this preferred embodiment the
current value of K is summed with the fifteen immediately preceding values
of K and the sum is divided by sixteen to generate K. K is then used in
all further processing.
The first step in this further processing is to set the variable T equal to
the K divided by N. T is a measure of the expected data count for a
measurement period lasting over P cycles of the input signal. The variable
T is used as a target signal to define a window which is used to screen
incoming data to ascertain whether that incoming data is consistent with
previously measured values of the data count, and thereby to screen out
erroneous measurements.
In addition, K is used to determine the musical note corresponding to the
input signal on line 32. In particular, K is compared with a look-up table
which lists values for K at the halfway points between adjacent semitones.
In this way the semitone closest to K is determined. In addition, the
difference between K and the table entry for the nearest semitone is
determined as the fractional deviation of K from the nearest semitone in
cents. The entire look-up table is arranged in terms of a predetermined
reference pitch, such as International Concert Pitch in which A above
middle C corresponds to 440 Hz.
This preferred embodiment can operate in one of two modes. In the first
mode (the absolute mode) the semitone and cents determined in the previous
step are then loaded into a display buffer for display. In this mode the
period of the input signal on line 32 is analyzed in terms of absolute
pitch based on International Concert Pitch. In the second mode of
operation (the transposer mode) the semitone and cents value obtained from
the look-up table are modified by a variable amount. As will be explained
below, transposer correction values for both the semitone and for the
cents value are stored when the analyzer is operating in the transposer
mode. When in the transposer mode these correction values are added to the
semitone value and to the cents value, respectively. This effectively
transposes the note measurement to the pitch corresponding to the
correction value. In the transposer mode, it is these transposed values of
the semitone and the cents value which are loaded into a display buffer
for later display.
Once the display buffer has been loaded, the program then waits for a new
data count to be entered into the data buffer. The entry of new data
counts into the data buffer will be explained below in connection with
FIG. 3. Once a new data count has been entered in the data buffer, the
program then determines a new value for the variable K by summing N
separate data counts. Then the program branches back to normalize K, to
determine the averaged value K and the variable T as explained above. Thus
the program continues to loop, updating the note and cents display with
each pass through the main loop of the program.
Turning now to FIG. 3, the interrupt service routine for interrupts on line
32 is there flowcharted. In this routine the Start Timer Flag is first
checked. This flag indicates whether the timer is to be started on the
next interrupt. If the flag is set, the computer simply starts the timer
(that is, initializes the timer and causes it to be decremented at a rate
of one megahertz) and returns from the service routine.
If the timer flag is not set, the program reads and stores the
instantaneous value in the timer and then checks to see whether enough
values of the timer have been read. The number of timer values to be read
varies as a function of the variable P such that if P is equal to 1 or 2
exactly one reading of the timer is sufficient, if P equals 4 or 8 then
two consecutive readings of the timer are required, and if P is 16 or 32
then three readings of the timer are required. In the following discussion
C.sub.1 will be used to denote the elapsed time count between the start of
the timer and the next interrupt, C.sub.2 will be used to denote the
elapsed timer count between the start of the timer and the second
interrupt, and C.sub.3 will be used to denote the elapsed timer count
between the start of the counter and the third interrupt. The Start Timer
Flag is then set to begin collection of another set of data with the next
interrupt.
A number of tests are then performed on C.sub.1 (and on C.sub.2 and C.sub.3
as well if they have been measured) to determine whether or not any of
these variables falls within the expected range. In the flowchart of FIG.
3 these comparisons are indicated for the variable C. It should be
understood that each of the three comparisons of FIG. 3 related to the
variable C is in fact performed for each of the variables C.sub.1,
C.sub.2, and C.sub.3 which has been measured. The first of these tests is
to determine whether C is equal to T/P.+-.6%. It will be recalled that T
is the anticipated data count for measurement period extending over P
interrupts. Thus, if C is equal to T/P, then C falls within the
anticipated window. In this case a running sum is incremented by an amount
equal to the value of C.
If C is not equal to T/P it is then compared with 2T/P. If C is
substantially equal to 2T/P then C is approximately twice the anticipated
amount. This corresponds to detection of the subharmonic having a period
twice that of the expected period. The computer is programmed to recognize
this subharmonic and to increase the running sum by an amount equal to
C/2.
Finally, C is compared with 4T/P. If C is found to be approximately equal
to 4T/P then the sum is incremented by amount equal to C/4. This
corresponds to detection of the subharmonic having a peiod four times that
of the expected period and recognition by the program that the variable C,
though not equal to the anticipated fundamental tone, is in fact equal to
the period of a subharmonic. In this case the value of C is divided by
four in order that it may be properly treated as a subharmonic before it
is added to the running sum.
When C.sub.2 and C.sub.3 are compared with T/P, 2T/P and 4T/P the program
is effectively searching to determine whether C.sub.2 or C.sub.3
corresponds to a harmonic of the expected period. For example, if C.sub.2
is approximately equal to T/P, then the program interprets C.sub.2 as a
first harmonic of the anticipated fundamental and augments the sum by an
amount equal to C.sub.1. In this way both harmonics and subharmonics which
have a period differing from that of the fundamental by a binary factor
are recognized as such and incorporated in the period measurement. Such
harmonics and subharmonics can be interpreted unambiguously, because they
differ from the fundamental by one or more octaves, and therefore
correspond to the same note. Other harmonics and subharmonics are not so
unambiguous and are therefore rejected. For example, a harmonic with a
period 1/3 that of the fundamental could in fact correspond to a note
different from that of the fundamental.
If, however, no match is obtained with either T/P, 2T/P or 4T/P, then the
program increments a count of measurements outside the anticipated window.
When an excessive number of consecutive failures to match the measured
elapsed counter value with the anticipated window are sensed, then the
program re-initializes, effectively starting over in an attempt to
determine an appropriate value of T.
Assuming that the program finds a match between T and C.sub.1, C.sub.2 or
C.sub.3, as explained above, then the variable PCOUNT is decremented and
checked. If PCOUNT is equal to zero from decrementing, the sum is then
compared with T, the anticipated value of the sum. Generally, the sum will
be equal to T.+-.3% and in this case the sum will be entered in the data
buffer as a new data count for processing in the main control loop as
described above. The running sum and the variable PCOUNT are then
initialized for the next pass through the interrupt service routine of
FIG. 3, and control is returned to the main loop.
FIG. 4 shows a second interrupt service routine which services a timer
interrupt timed to interrupt the program every two milliseconds. This
timer interrupt is serviced by first multiplexing the display in order to
update the display to conform to the display buffer. In this preferred
embodiment the note value and the cents value are displayed as characters.
The note value is one of the 12 semitones and the cents value is an
integer between -50 and +50. In alternate embodiments it may be preferable
to display the cents value in an analog or pseudo-analog form. For
example, a 100 element row of discrete digital indicators (such as LED's
or LCD's) could be used with one half of the row used to indicate negative
cents and t | | |
