 |
Description  |
|
|
BACKGROUND OF THE INVENTION
Numerous devices are presently available for the purpose of tuning musical
insturments. Some devices known in the art are mechanical, some are
electronic, and other operate on electromechanical principles. A typical
electromechanical device known in the art includes a tunable sub-audio
oscillator which drives a synchronous electronic motor, which in turn
rotates a binary-divided strobe disk. Positioned behind the disk is a
glow-discharge tube which flickers in response to the output of a musical
instrument. Visual observation of the resulting light pattern provides an
indication of the frequency. Numerous difficulties are inherent in this
device as well as in other mechanical and electromechanical tuning
devices. For example, moving parts are included which are subject to
degradation with age and dislocation due to external mechanical shocks.
Most devices are heavy and otherwise non-portable. A particular limitation
of mechanical and electromechanical devices is the restricted range of
frequencies over which operation is effective. Typically, operation is not
possible below 25 hertz or above 10K hertz.
A few electronic tuning devices are also known in the art. Typically, such
devices are null-reading devices and therefore indicate when a signal
frequency is precisely equal to a preset frequency, but do not provide any
indication of the amount or direction of tuning error when the signal
frequency is not precisely at the preset frequency. The electronic devices
heretofore known typically utilize analog electronic circuitry and involve
frequency-to-voltage conversions to perform the measurement and provide a
display. The accuracy of these analog instruments is therefore limited,
even when a highly accurate frequency standard is employed.
It would, therefore, be desirable to have available a digital electronic
tuning device whose accuracy was concomitant with that obtainable with
digital electronics. Preferably, the device should be portable and easy to
read. It should indicate when an incoming signal frequency is equal to a
preset frequency, and also provide an indication of the magnitude and
direction of the tuning error when the signal frequency is not precisely
equal to the reference frequency. When the signal under test comprises a
complex waveform, the device should give some indication of the harmonic
content of that waveform.
SUMMARY OF THE INVENTION
In accordance with the illustrated preferred embodiments, the present
invention provides a digital electronic tuning device (hereinafter
referred to as a "digituner") particularly well suited for the tuning of
musical instruments, but also generally useful whenever the frequencies of
any two signals are to be compared. In a preferred embodiment, the device
consists of a sequence of lights preferably arranged in a circle. With
presently available digital components, eight lights provide a convenient
display. Each light is activated by the output of an associated AND gate.
According to the invention, one input of each AND gate is the incoming
signal whose frequency is to be determined, while the other input is an
enabling pulse which activates the AND gates sequentially at a preselected
frequency. A simplified description of the operation of the invention is
possible if it be assumed that the incoming signal has a frequency which
is precisely one-eighth the frequency of the enabling pulses (i.e., the
period is eight times the period of the enabling pulses), but has only a
12.5% (one-eighth) duty cycle. Thus, the incoming signal pulse will always
be coincident with the enabling pulse for only one of the eight AND gates,
and the corresponding LED will blink at the frequency of the enabling
pulse. When the frequency is in the audio range or higher, the blinking
will occur at a rate above the flicker fusion frequency of the human eye,
and the visual appearance will be of one particular light of eight glowing
continuously. However, if the incoming signal is not precisely one-eighth
the enabling pulse frequency, but is slightly higher, it will
progressively coincide with earlier enabling pulses, thereby causing
adjacent lights to appear to glow in a moving sequence. In a circular
display, the visual appearance will be of a whirling or spinning glow. The
apparent direction of spin reverses if the incoming signal is of a lower
frequency. When the duty cycle of the incoming signal is greater than
12.5% (e.g., 50% as in a sine wave), then not one light but several lights
will glow and appear stationary when the incoming frequency is precisely
one-eighth the enabling pulse frequency. When there is a frequency
mismatch, the entire pattern of glowing lights will appear to spin, as
described above. A similar pattern of glowing and/or spinning lights
appears when the waveform is complex.
In accordance with another preferred embodiment of the invention,
additional circuitry is included so that the output of each AND gate may
drive successive poles of a stepping motor. The stepping motor may then in
turn drive a tuning wrench connected to a stringed instrument, thereby
effecting the automatic tuning of the instrument.
In accordance with yet another preferred embodiment of the invention, the
fundamental frequency of the enabling pulses may be driven by a chromatic
scale generator to provide tuning to any note of the chromatic scale.
In accordance with still another preferred embodiment of the invention, a
number of individual displays may be employed, each display corresponding
to one particular note of the chromatic scale.
It is apparent from the above description that in addition to tuning
musical instruments, the invention may be of general applicability for the
comparison of any two electronic frequencies. Furthermore, if a magnetic
or photoelectric pickup device is employed to supply the input from a
desired external signal, the invention may be fruitfully employed as a
tachometer.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a preferred embodiment of the present
digital electronic tuning device.
FIG. 2 shows a clock waveform and an input signal under test having a
frequency equal to a preset frequency. A pattern appearing on the display
elements is shown.
FIG. 3 shows a clock-enabling sequence and an input signal whose frequency
differs from the preset frequency. An indication of a resulting
time-varying pattern appearing on the display elements is shown.
FIG. 4 illustrates an embodiment of the invention using a chromatic
generator to provide tuning to notes of the chromatic scale.
FIG. 5 illustrates am embodiment of the invention in which tuning to
different notes of the chromatic scale is displayed on a plurality of
display arrangements.
FIG. 6 shows an embodiment of the invention in which the digituner drives a
stepping motor to provide automatic tuning of certain instruments.
DETAILED DESCRIPTION OF THE INVENTION
In FIG. 1 there are schematically illustrated a number of display elements
labeled 11. Display elements 11 are preferably light-emitting diodes
[LED's], but other suitable display devices such as liquid crystals or
neon bulbs may be employed. In the illustrated embodiment, the display
elements are arranged in a circle to provide a particular visual
appearance suitable for many purposes. However, the display elements may
also be arranged in other configurations such as linear or rectangular
arrangements if desired.
Each display element in FIG. 1 is activated by the output of an associated
digital logic AND gate, several of which are labeled 13 in the figure.
Parallel connection from a pickup device 15 which provides a frequency
under test is made to one input of each of the logic AND gates 13. The
input device 15 may be any suitable pickup device such as, e.g., a
microphone if the desired input signal is from a musical instrument. Other
transducers such as inductive pickups or photocells may also be suitable
for different applications. The input signal from pickup device 15 is
transmitted through a signal processing unit 17 to suitably process the
signal before application to the sequence of AND gates 13. Signal
processing any be desired, e.g., to modify the input waveform to achieve
amplitude and impedance match with the AND gates. For example, a signal
derived from a high impedance microphone at 0.001 volt would preferably be
signal processed to yield a 5 volt signal at 50 ohms for compatibility
with conventional digital logic circuitry. It may also be desirable to
shape or filter the input waveform to produce a particular appearance in
the output display. For example, signal processing unit 17 may include a
comparator or saturation amplifier to produce a square wave from a sine
wave; or a low pass filter may be employed to eliminate higher harmonics
which are not of interest. Signal processor 17 may also include automatic
gain control [AGC] to maintain a level amplitude for consistency of the
visual appearance of the output display. AGC is useful, for example, to
restore the amplitude generated by a vibrating string as it decays with
time.
The second input of each AND gate 13 is driven from one output of a driver
unit 19. The basic function of driver 19 is to supply an enabling pulse at
a fundamental frequency to each of the AND gates 13 in sequence. Thus, a
suitable element known in the art for driver 19 is an N-bit shift register
containing a single circulating bit. Presently, eight bit shift registers
are commonly available in the present embodiment and would be used in
conjunction with eight display elements. For illustrative purposes, a
device having eight elements will be discussed hereinafter. It should be
apparent, however, that with appropriate digital circuitry, the device may
include any desired number of display elements. Driver 19 may also
comprise, e.g., a one-out-of-eight counter which produces pulses
sequentially at each output when driven at a fundamental clock frequency.
The input to driver 19 is supplied by a clock 21 which may be a
crystal-controlled oscillator or other suitable ocillator. Alternately,
the frequency may be derived from the line frequency powering the device
or from a broadcast standard or from any other suitable clock standard. If
desired, another external input may replace the clock if it is desired to
use the device to directly compare two frequencies. A particular clock
driver will be described below in connection with particular embodiments
of the invention.
Understanding of the operation of the device will be facilitated by
reference to FIGS. 2, 3 and 4. In FIG. 2 there is illustrated a waveform
23 which is the basic clock frequency signal from clock 21. For purposes
of illustration, this signal is taken to be at a frequency 8f, where f is
a prescribed frequency to which tuning is desired. In response to this
clock pulse, driver unit 19 (of FIG. 1) produces an enabling pulse which
activates the AND gates in sequence at the periodic frequency f. As
discussed above, this may be accomplished by using a circulating shift
register containing one bit which is sequentially presented to each AND
gate. A second signal 25 is illustrated which represents the output of
signal processor 17 being derived from, e.g., a musical instrument by
means of pickup 15. In FIG. 2 signal 25 is shown as a square wave of
frequency and having a 50% duty cycle. FIG. 2 illustrates a condition in
which the frequency of the instrument is precisely in tune with the
preselected frequency set on the digituner. A representation of the
display elements 11 of FIG. 1 is also shown, in which circular elements
containing an "x" indicate an "on" state of the display element, while
those not containing an "x" represent an "off" state. As indicated, the
"in tune" condition here described will produce a pattern of four lights
on and four lights off. More precisely, it can be seen from FIGS. 1 and 2
that input signal 25 will present a logic "high" level to one input of
four AND gates coincident with the periodic "high " level produced by the
enabling pulse 23 at the other input. The four display elements associated
with these gates will therefore flicker at the frequency of the enabling
pulses; the remaining four display elements will be "off". If the flicker
frequency is in the audio range or higher, it will be above the flicker
fusion frequency of the human eye, and the visual appearance will be of
four lights "on" and four lights "off" in a stationary pattern. Thus, in
the circular display of FIG. 1, a stationary pattern indicates that the
frequency under test is precisely equal to the preselected frequency f of
the digituner.
FIG. 3 illustrates a condition when the detected signal is at a frequency
other than the frequency f. As an example, suppose it is desired to tune
to a frequency of 50 Hz. Then the basic clock frequency should be 400 Hz,
which will insure that driver 19 produces enabling pulses at 50 Hz. The
first line of FIG. 3 indicates which of the eight display elements is
enabled at any particular time. A waveform 27 is illustrated at a
frequency other than f, here taken as 53.33 Hz, with a duty cycle of 50%.
By comparing the "high" or "low" state of the input waveform with the
"enabled" or "not enabled" state of such display element, it may be seen
that at any particular time certain display elements will be activated.
FIG. 3 also indicates a persistent glow of each display element resulting
from the response of the human eye; the persistence is shown as being
about 20 msec here. It may be seen that as time advances (to the right in
the figure), the pattern of flowing display elements appears to shift
upward.
If the display elements were arranged in a circle as in FIG. 1, the
appearance of the display would consist of a pattern of lights appearing
to shift to the right around the circle. This pattern spinning to the
right indicates that the frequency of the instrument under test is higher
than the preset frequency of the tuning device. If the instrument
frequency were below the frequency of the tuning device, the pattern of
lights would appear to rotate to the left. The speed of rotation of the
pattern indicates the magnitude of the deviation form the desired
frequency. The device, therefore, provides an indication of both the
magnitude and direction of the deviation from a desired frequency. As
described above, a stable pattern clearly indicates when the desired
frequency is obtained.
If harmonics of the fundamental frequency are present in the test signal,
the display elements will indicate an additional pattern superimposed on
the fundamental pattern. As with the fundamental, this pattern will be
stationary if the harmonic is precisely in tune, but will rotate if the
harmonic is out of tune. A skilled observer may therefore obtain
information as to the harmonic content of a signal and to any harmonic
mistuning.
In FIG. 4 there is schematically illustrated a particular arrangement of
electronic elements to serve as clock 21. Although in general a clock may
be used which allows tuning to any preselected frequency, for the tuning
of musical instruments, it is desirable to provide tuning to the notes of
the chromatic scale. A basic clock 29 is here used to generate a
fundamental frequency. Clock 29 drives an octave divider 31 which produces
outputs at multiples of the fundamental clock frequency from clock 29. A
switch 33 is used to select an appropriate output from octave divider 31
to serve as an input to a chromatic generator 35. Chromatic generators are
known in the art and comprise circuitry which generates the twelve notes
of a chromatic scale from a given fundamental frequency. Here, a switch 37
is used to select a particular note to be used as the input to driver 19.
Tuning of an instrument to particular notes of the chromatic scale is thus
provided. For this musical purpose it is especially useful to provide that
switch 37 take the form of twelve off-on switches whose physical form is
that of the notes of one octave of a piano keyboard.
FIG. 5 illustrates an embodiment of the device in which each output of
chromatic generator 35 is directed to a different visual display unit. In
this embodiment of the invention, the presence in the test signal of a
component at a particular frequency of the chromatic scale will be
indicated by a stationary pattern on a corresponding display.
In FIG. 6 there is illustrated a driver unit 19 shown as a one-out-of-four
counter. As before, one input of each gate is supplied by driver 19 while
the other inputs of the AND gate are supplied by a signal whose frequency
is to be determined. In this embodiment of the invention, however, the
outputs of AND gates 13 are directed to four motor driver units 39. These
may be any conventional units which serve to process the output signal
from AND gates 13 to provide outputs which are suitable for driving a
stepping motor, here labeled 41. Other electromechanical devices which
respond to the order of a sequence of input pulses are also suitable,
e.g., linear motors known in the art. Stepping motor 41 will, therefore,
rotate in response to the outputs of AND gates 13 directly, much as the
pattern of lights of display elements 11 rotated. The rotation will be in
one direction when the instrument frequency is above the desired frequency
and in the other direction when it is below the desired frequency. If the
stepping motor is connected to a wrench or "tuning hammer", the tuning
hammer will rotate also, and may provide automatic tuning of an instrument
such as a piano. If desired, the output of AND gates 13 may also be
connected in parallel to a visual display unit as described above to
provide visual observation of the tuning process.
* * * * *
|
|
 |
|
 |
Description |
|
