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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a circuit which generates reference frequency
signals to be used in the tuning of various musical instruments, and more
particularly to a reference frequency signal generator for a tuning
apparatus capable of generating reference frequency signals which are
based on various tuning characteristics.
2. Prior Art
Heretofore, in a musical instrument tuning apparatus, a reference frequency
signal generator utilizing an analog circuit arrangement has been known in
which the circuit constants of inductances (L), capacitances (C),
resistances (R), etc. are varied so as to obtain various reference
frequencies. Although this known generator has the advantage that the
variations of the reference frequency can be made continuous, it has such
disadvantages that the stability and precision of the reference frequency
are low, to make it practically impossible to tune a musical instrument as
accurately as within +1 cent, so that if it is desired to generate the
reference frequencies in accordance with the changes of the pitch or
different tuning curves, the circuit constants L, C, R, etc. need to be
changed by referring to a correction value table or the like each time,
which is troublesome in procedure and requires much time as well as much
labor, and that since the range of the reference frequencies which can be
generated is comparatively narrow, it is often required to tune a specific
tone first with the tuning apparatus and thereafter to tune a desired tone
with reference to the tuned specific tone so that the efficiency of the
tuning job is lowered and the tuning precision as a whole is also lowered.
SUMMARY OF THE INVENTION
Accordingly, the primary object of this invention is to provide a novel
reference frequency signal generator for a tuning apparatus which is free
from the above disadvantages.
More particularly, an object of the invention is to provide a reference
frequency generator capable of selectively generating reference frequency
signals of various tuning characteristics with simple operation and
sufficient accuracy.
According to one embodiment of this invention, there is provided a
reference frequency generator circuit wherein frequency change ratio
(frequency division ratio or frequency multiplication ratio) data
corresponding to necessary reference frequencies, also including data
corrected in accordance with various pitches or tuning curves, are stored
in a memory device in advance, a variable frequency oscillator circuit
being controlled in compliance with the frequency change ratio data read
out from the memory device, so as to obtain a desired reference frequency
signal.
According to another embodiment of this invention, there is provided a
reference frequency generator circuit comprising a first memory device
which stores therein frequency change ratio (frequency division ratio or
multiplication ratio) data serving as references, a second memory device
which stores therein data required for corrections that are to be applied
to the frequency change ratio data serving as references in accordance
with changes of pitches or tuning curves, and an arithmetic circuit which
operates the data read out from the first and second memory devices and
forms frequency change ratio data corresponding to a necessary reference
frequency, a variable frequency oscillator circuit being controlled in
accordance with the output data from the arithmetic circuit so as to
obtain a desired reference frequency signal.
The above-mentioned features and objects of the present invention will
become more apparent with reference to the following description taken in
conjunction with the accompanying drawings, wherein like reference
numerals denote like elements.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graphical diagram showing examples of tuning curves available
in a tuning apparatus according to an embodiment of this invention;
FIG. 2 is a block diagram showing the tuning apparatus according to an
embodiment of this invention;
FIG. 3 is a circuit diagram showing a reference frequency signal generator
used in the apparatus of FIG. 2;
FIG. 4 is a circuit diagram showing another embodiment of a reference
frequency signal generator used in the apparatus of FIG. 2; and
FIG. 5 is a circuit diagram showing another variable frequency oscillator
circuit usable in the circuit of FIGS. 3 or 4.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to FIG. 1, there are shown various tuning characteristics
covering a range of seven octaves, which are available with a tuning
apparatus according to an embodiment of this invention. The abscissa
represents the frequency based on an equal temperament scale, while the
ordinate represents the extent of deviation of a frequency to be given by
the tuning, in terms of a cent value. Numeral 1 designates a flat tuning
curve in accordance with the equal temperament, numeral 2 a tuning curve
similar to that used in tuning of a piano in which tuning notes are
lowered in the lower octaves and raised in upper octaves relative to the
equal temperament, symbol 2A or 2B a tuning curve where the curve 2 is
shifted upwards or downwards by setting pitches to be high (for example,
A.sub.4 =444 Hz) or low (for example, A.sub.4 =436 Hz), respectively, and
numeral 3 another tuning curve in which the deviations is upper and lower
octaves are more moderate than in curve 2.
FIG. 2 shows a tuning apparatus according to an embodiment of this
invention which is capable of demonstrating the various tuning
characteristics as described above. Numeral 10 designates an
acoustical-electrical transducer such as microphone which picks up a tone
of a musical instrument to be tuned and converts it into a corresponding
electric signal. This signal is amplified by an amplifier 12 which also
includes a filter (not shown) for removing high frequency noise etc. from
the output signal of the acoustical-electrical transducer 10. The filtered
and amplified signal is introduced into a fundamental wave extracting
circuit 14 which extracts the fundamental wave signal from the output
signal of the amplifier 12. A fundamental wave extracting circuit such as
is disclosed in the U.S. patent application No. 915,758, now U.S. Pat. No.
4,198,606, filed by the same assignee, may be used as the circuit 14.
Numeral 16 indicates a frequency assignment operation circuit which
includes a first assignment portion 16a for specifying a note in a musical
scale or octave comprising twelve notes, a second assignment portion 16b
for specifying a pitch deviation, a third assignment portion 16c for
specifying an octave, and a fourth assignment portion 16d for specifying a
tuning curve, and which delivers to a reference frequency specifying
signal FS consisting of a note specifying signal NT, a pitch deviation
specifying signal PT, an octave specifying signal OC and a tuning curve
specifying signal TC in response to the operations of switches such as key
switches incorporated in the operations of switches such as key switches
incorporated in the operation circuit 16. The reference frequency signal
generator generates a signal f.sub.o of a reference frequency determined
by the reference frequency specifying signal FS, the details of which will
be described later with reference to FIG. 3.
The fundamental wave signal f delivered from the fundamental wave
extracting circuit 14 and the reference frequency signal f.sub.o delivered
from the reference frequency signal generator 13 are compared by a
comparator 20, which supplies a display device 22 or an automatic tuning
system 24 with a comparison output corresponding to the difference between
the comparison inputs f and f.sub.o. The display device 22 displays
digitally the deviation of the frequency f to be tuned with respect to the
reference frequency f.sub.o in terms of, for example, a cent value, while
the automatic tuning system 24 automatically drives or adjusts tuning
parts in the musical instrument such as tuning pins in a piano, so as to
minimize the deviation of the frequency-to-be-tuned f from the reference
f.sub.o.
Thus, according to the tuning apparatus of FIG. 2, by appropriately
operating the key switches in the frequency assignment operation circuit
16, it is possible to generate the reference frequency f.sub.o suited to
the desired tuning characteristic for each note in each octave, to detect
the deviation between this reference frequency f.sub.o and this
frequency-to-be-tuned f, and to use the deviation signal for the display
or the automatic tuning.
Now, the details of the reference frequency signal generator will be
described with reference to FIG. 3. A variable frequency oscillator
circuit 30 comprises a stable fixed oscillator 32 such as a quartz
oscillator, and a variable frequency divider 34 constructed of a
programmable counter which divides a frequency signal f.sub.s generated
from the oscillator 32, by a frequency division ratio indicated by
frequency division ratio data DS as a control input thereof. A frequency
signal f.sub.T delivered from the variable frequency divider 34 is
appropriately subjected to a frequency division in octave by an octaval
frequency divider 36. A gate circuit 38 derives frequency signals from the
respective frequency division stages of the frequency divider 36 in
accordance with the octave specifying signal OC supplied from the
frequency assignment operation circuit 16 in FIG. 2. It operates so that,
regarding the top octave, the frequency signal f.sub.T may be derived as
it is, while regarding lower octaves, the frequency signals (1/2, 1/4,
1/8, ect. of f.sub.T) from the corresponding frequency division stages may
be respectively derived. The output signal f.sub.o of the gate circuit 38
is the reference frequency signal, and is supplied to the comparator
circuit 20 in FIG. 2.
On the other hand, the circuitry for forming the frequency division ratio
date DS to be supplied to the variable frequency divider 34 is provided
with a decoder 40 and a read only memory (ROM) 42. The decoder 40 receives
from the frequency assignment operation circuit 16 in FIG. 2 with the
frequency specifying signal FS including the note specifying signal TC,
and it is adapted to produce a data readout address signal for the ROM 42
in accordance with the combination of the note, the pitch deviation, the
octave and the tuning curve.
The ROM 42 stores the frequency division ratio data which is necessary for
obtaining the reference frequencies corresponding to the twelve notes C,
C# . . . and B of one octave according to the equal temperament. These
note frequencies of the equal temperament are specified by the note
specifying signals NT. The ROM 42 also stores for the respective notes the
frequency division ratio data modified in accordance with the alterations
of the pitches and tuning curves. Each time the output signal of the
decoder 40 assigns a specified readout address, the frequency division
ratio data in the address is read out. That is, the frequency division
ratio data corresponding to the respective notes of the top octave are
read out from the ROM 42 in correspondence with the pitch deviation or the
tuning curve assigned by the frequency specifying signal FS, while
regarding the notes having other frequencies than those according to the
equal temperatment, the frequency division ratio data for equal
temperament top octave to corrections in accordance with the specified
pitch deviation or tuning curve are stored in the ROM 42 for the
respective notes, whereby the frequency division ratio data DS is formed
by the data read out.
Here, assuming that the value indicated by the frequency division ratio
data DS from the ROM 42 has increased from N to (N+1), the ratio of
(N+11)/N determine the degree of adjacency between the immediately
adjacent frequencies concerning the specified notes in the circuit of FIG.
3. More specifically, in order to obtain frequencies at intervals of x
cents, the following is the required condition in view of the definition
of the cent value:
##EQU1##
Letting f.sub.m denote the desired maximum frequency and f.sub.s the
oscillation source frequency, f.sub.m .ltoreq.f.sub.s /N holds. Therefore,
supposing x to be 1 cent and f.sub.m to be about 4 kHz corresponding to
the note B.sub.7, N>1730 and f.sub.s >6.9 MHz hold. Accordingly, the
quartz oscillation frequency of the OSC 32 is set at f.sub.s >6.9 MHz. as
the variable frequency divider circuit 34 a 12-bit programmable counter
bits in necessary because in spite of N>1730 the frequency ratio between
B.sub.7 and the lowest note C.sub.7 within the same octave as that of
B.sub.7 is nearly double. Further, when it is intended to tune the 88 keys
of a piano with n.sub.1 sorts of tuning curves and n.sub.2 sorts of
pitches, data of 12 bits amount to 88.times.n.sub.1 .times.n.sub.2 words,
and the ROM 42 is required to have such a memory capacity as mentioned
above.
Next, the details of another reference frequency signal generator circuit
18' will be described with reference to FIG. 4 wherein numerals 30, 32,
34, 36 and 38 denote the same elements as in FIG. 3. The circuit 18'
differs from one shown in FIG. 3 in that the circuitry for forming the
frequency division ratio data DS to be supplied to the variable frequency
divider circuit 34 comprises read only memories (ROMs) 44, 48 and 52,
decoders 56 and 50 and full adders 54 and 56. The circuitry is supplied
from the frequency assignment operation circuit 16 in FIG. 2 with the note
specifying signal NT, the pitch deviation specifying signal PT, the octave
specifying signal OC and the tuning curve specifying signal TC.
The ROM 44 stores the frequency division ratio data which are necessary for
obtaining the reference frequencies of the top octave according to the
equal temperament which are specified by the note signals NT, and each
time the note signal NT specifies a note, the frequency division data
corresponding to that note is read out. Although, in this example, the
note signal NT is not encoded, the note signal NT may well be encoded, and
that case, it may be applied to the ROM 44 through a suitable decoder.
The decoder 46 serves to form an address signal for ROM 48 on the basis of
the note signal NT and the pitch deviation signal PT, and it is adapted to
assign a data readout address of the ROM 48 in accordance with the
combination of the note and the pitch. The pitch deviation may be
different for every note in an octave. The ROM 48 stores modification data
for the frequency division ratio data of the ROM 44 for the respective
notes in an octave in correspondence with pitches to be specified by the
pitch deviation signals PT, and in response to the address signal from the
decoder 46, the modification data on the assigned pitch is read out for
every note in an octave.
The decoder 50 serves to form an address signal for the ROM 52 on the basis
of the note signal NT, the octave signal OC and the tuning curve signal
TC, and it is adapted to assign a data readout address of the ROM 52 in
accordance with the combination of the note, the octave and the tuning
curve. Here, the tuning curve signal is a kind of pitch deviation
information to plot the tuning curves other than the equal temperament,
which curves have a non-linear relationship relative to the equal
temperament as shown in FIG. 1 so that the octave signal OC is also
required to define the pitch of a note according to a certain tuning
curve. The ROM 52 store modification data for the frequency division ratio
data of the ROM 44 for the respective notes of each octave in
correspondence with tuning curves to be assigned by the tuning curve
signals TC, and in response to the address signal from the decoder 50, the
modification data on the assignment tuning curve is read out for every
note of each octave.
The modification data respectively read out from the ROM's 48 and 52 are
added to each other by the full adder 54, and sum data from the full adder
54 is supplied to the full adder 56 as one addition input thereof. As the
other addition input of the full adder 56, the frequency division data
corresponding to the equal temperament read out from the ROM 44 is
supplied, and the full adder 56 forms the frequency division ratio data DS
by totalizing both the addition inputs. Since the frequency division ratio
data DS is formed by adding the modification data on the pitch or the
tuning curve to the frequency division ratio data of the top octave, it
indicates a quantity in which the frequency division ratio corresponding
to each note of the top octave has been modified in the light of the pitch
or the tuning curve. In case where it is desired to obtain reference
frequencies corresponding to the equal temperament characteristics
specified by the data stored in the ROM 44 (as indicated by symbol 1 in
FIG. 1), the output data of the ROM 44 need not be subjected to any
correction.
In accordance with the circuit 18' as mentioned above, the frequency
division ratio data corresponding to the 12 tones of the top octave are
stored in the ROM 44, while the modification data for the frequency
division ratio data are stored in the ROM's 48 and 52, and the data from
the ROM's 44, 48 and 52 are digitally operated, thereby to form the
frequency division ratio data DS corresponding to the assigned notes, and
hence, the memory capacities of the ROM's 44. 48 and 52 may be very small,
which makes it possible to put the circuit of FIG. 4 into a one-chip IC
except quartz oscillator and the like.
More specifically, the ROM 44 may be of such a memory capacity that data of
12 bits for the respective 12 notes can be stored. Since the ROM 48 is
provided in order to modify the pitch of each note specified by the ROM
44, the ROM 48 need not store data on the pitch specified by the ROM 44
and its memory capacity may be smaller to that extent. In order to
simplify the circuit, the pitch specified by the ROM 44 may be treated in
the ROM 48 as a maximum or minimum. Thus, the signs of the data stored in
ROM 48 can be unified into either plus or minus. By way of example, in
case where the pitch is changed in 6 stages (n.sub.2 =6) from 440 to 445
Hz in about 20 cents, and hence, 6 bits suffice as the number of bits of
the data. Accordingly, the memory capacity of the ROM 48 in this case may
be 6 bits.times.5 (the number of stages of pitch adjustment).times.12
(notes). Further, the ROM 52 stores the pitch deviations from the equal
temperament characteristics (symbol 1 in FIG. 1) as the modification data
and need not store the data corresponding to the equal temperament itself
and hence, its memory capacity may be smaller to that extent. In the
example of the piano, deviations of approximately .+-.30 cents with
respect to the equal temperament need to be produced. Therefore, about 7
bits are necessary as the number of data bits, and 1 bit of them is used
for expressing the sign. Accordingly, the memory capacity of the ROM 52 in
this case may be 7 bits.times.83(keys).times.m (corresponding to n.sub.1
-1).
Now, the full adder 54 functions to operate the data from the ROM's 48 and
52 and therefore suffices with 6 bits. While various methods may be
considered as the method of operating the data of the ROM's 44, 48 and 52,
it is advantageous from the viewpoint of reducing the number of bits that
the data of the ROM's 48 and 52 are operated in advance as in this
example. In addition, in order to make the operations possible with only
the adder without using an adder/subtractor, it is more preferable that
minus data stored in ROM's 48 and 52 are converted into ones expressed by
complements in advance. Since the full adder 56 functions to add the data
of 12 bits from the ROM 44 and the data of 6 bits from the full adder 54,
an adder of 12 bits suffices for the adder 56. On account of the
difference of the numbers of bits of the full adders 54 and 56, upper bits
of one input of the full adder 56 are in excess. However, it is possible
to dispense with the subtraction function of the full adder 56 otherwise
required by appropriately controlling the full adder 56 depending upon the
signs of the data within the ROM's 48 and 52 or the state of the full
adder 54.
By way of example, in case where the circuit of FIG. 4 was embodied so as
to generate reference frequency signals at intervals of .+-.1 cent in
order to tune the 88 keys of a piano with 3 sorts of tuning curves and 6
sorts of pitches, the memory capacity could be reduced as much as about
90% in comparison with that in the case of storing the frequency division
ratio data in one ROM, and also the decoders could be miniaturized.
Therefore, the circuit of FIG. 4 could be integrated in an IC (LSI) within
a single semiconductor chip except that the quartz oscillator and some
components for the tuning thereof were externally mounted, and the tuning
apparatus of FIG. 2 was made small in size and light in weight to the
extend that it was in the palm of a hand as a whole.
FIG. 5 shows another variable frequency oscillator circuit 60 which is
usable in the circuit of FIGS. 3 or 4. Numeral 62 designates a stable
fixed oscillator (OSC) such as quartz oscillator, numeral 64 is a phase
detector (PD) one input terminal of which is supplied with a frequency
signal f.sub.os from the OSC 62, numeral 66 a low-pass filter (LPF) which
removes a ripple component from an output signal of the PD 64 and provides
a d.c. output, numeral 68 a voltage-controlled variable frequency
oscillator (VCO) which has its oscillation frequency controlled by the
output signal of the LPF 66 and oscillates at a frequency being K times
higher than the output frequency f.sub.os of the OSC 62, and numeral 70 a
frequency divider which is constructed of a programmable counter adapted
to divide the frequency of the frequency signal from the VCO 68 by K, the
frequency division output of the frequency divider 70 being fed to the
other input terminal of the PD 64. That is the circuit of FIG. 5 operates
as a frequency multiplier circuit employing a PLL (phase locked loop), and
the frequency multiplication output K.multidot.f.sub.os which is stable is
provided from the output terminal of the VCO 68. In using this circuit in
the circuit of FIG. 3 or 4, data DS' indicative of the frequency
multiplication ratio K are applied as control inputs of the frequency
divider 70. The multiplication ratio data DS' can be formed by the circuit
of FIGS. 3 and 4, and to this end, the data of the ROMs 42, 44, 48 and 52
are redetermined concerning the multiplication ratios of K.
As described above in detail, according to the reference frequency signal
generator for a tuning apparatus of this invention, the excellent
functional effects as listed below are achieved:
(1) In spite of the single circuit, many functions are performed. That is,
reference frequencies corresponding to any desired notes of any desired
octaves are obtained in accordance with the equal temperament
characteristic, various pitches or various tuning curves. This makes it
possible to sharply reduce time and labor required for the tuning.
(2) Since the reference frequency signal generator comprises a stable fixed
oscillator and digital circuitry in combination, the stability and
precision of its operation is high, so that for example, a tuning on the
extend of +1 cent having heretofore been impossible can be stabily carried
out.
(3) The frequency change ratio (frequency division ratio or multiplication
ratio) data are formed by combining a plurality of ROM's and arithmetic
circuitry, so that in comparison with a case of storing them in and
reading them out from a single ROM, the memory capacity may be much
smaller, also peripheral circuitry becoming simpler to permit a sharp
miniaturization of a reference frequency signal generator, which is very
advantageous for constructing the entire tuning apparatus to be compact
and light in weight.
(4) By appropriately subjecting to octave frequency divisions frequency
signals formed for the respective notes of the top octave, reference
frequencies corresponding to the respective notes of the lower respective
octaves are obtained. As compared with a case where reference frequencies
are directly obtained in correspondence with the respective notes of all
the octaves, the circuit arrangement is greatly simplified, which is very
advantageous for rendering the tuning apparatus small in size and light in
weight.
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