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Description  |
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BACKGROUND OF FIELD OF INVENTION
This invention relates to aids to musical instrument tuning, particularly
the tuning of keyboard instruments in the equal-tempered scale.
There are two broad categories of apparatus for tuning musical instruments:
reference pitch generators and pitch comparators. A reference pitch
generator is a device that produces sound of the correct pitch, such as a
set of tuning forks or an electronically controlled frequency generator. A
pitch generator, on the other hand is a device that usually provides a
visual indication of the pitch of the note that is sounded. (The terms
"pitch" and "frequency" are used interchangeably to denote that feature of
the sound that is being tuned.)
Using a single tuning fork as a reference pitch generator has been
traditionally the most common method of profession piano tuning. After
several notes are tuned for a zero-beat with the tuning fork, the
remaining notes are then tuned in a special sequence, employing a method
know as interval tuning. This method requires considerable skill. It is
also susceptible to the effects of cumulative error, which require
frequent consistency checks and possible retracting of steps.
Using a large set of tuning forks (e.g., all the notes in one octave)
reduces the cumulative error, but it still relies on very precise
zero-beats being detected by ear. The same could be said of any reference
pitch system, such as an electronically controlled frequency generator.
However, a reference pitch generator is useful when tuning a note that is
very far from the corrected pitch, such as when new strings are being
installed on a piano.
Pitch comparators, on the other hand, can offer a much more accurate
evaluation of the pitch of a sound. The best pitch comparators are
actually phase comparators. They compare the phase of the sounded note
against the phase of an internally generated signal of precise pitch. The
rate of change of the phase difference between these two signals is a
precise measure of the error in pitch.
The visual means employed to show the phase difference is usually a moving
pattern. It is hard to see which way the pattern is moving if it is moving
very fast, so phase-sensitive pitch comparators are only useful when the
sounded note is too far off from the correct pitch.
In a pitch comparator, the internally generated reference frequency is not
usually heard by the person using the device. It is only an electronic
signal whose phase is compared against the phase of the sounded note. For
example, in the motorized strobe tuners, the reference pitch, or
frequency, exists as the rotation rate of the indicator. The sounded note,
detected by a microphone, is used to modulate (strobe) a light which
shines on the rotating indicator. The relative phase of these two signals
is seen as the position of the visible strobe pattern. The rate of
movement of the strobe pattern indicates the difference between the two
frequencies. The goal is to tune the musical instrument until the visible
strobe pattern is nearly stationary.
The motorized indicator in the motorized strobe tuner is bulky, hard to
control, and prone to mechanical failure. (A solid state alternative is
desirable.) One such device (described in U.S. Pat. No. 4,014,242, issued
Mar. 29, 1977 for an "Apparatus for Use in the Tuning of Musical
Instruments") uses quadrature reference signals and synchronous
demodulation to display the phase comparison in a circle of LEDs. The
brightness of each LED represents the degree to which the sounded note is
in phase with that particular reference signal. The quadrature signals and
their inverses provide a set of four reference signal that are supposed to
cover all possible phase conditions. But since there are only four
reference phases, poor phase resolution does not permit displaying sounded
notes that are harmonically related to the reference note. So this device
employs narrow bandpass filters that, together with the reference
frequency octave selection, must be adjusted when different octaves are
being sounded.
The frequency of the reference signal is the only source of error in a
phase-sensitive pitch comparator. In order to achieve the highest
accuracy, it is desirable to use precise digital frequency synthesis
techniques to generate the reference signal. These techniques usually lock
the reference signal to a quartz crystal oscillator using rational number
frequency ratios. Unfortunately, the frequencies that comprise
equal-tempered tuning are based on irrational number ratios that can only
be approximated by rational numbers. To attain excellent accuracy, the
whole numbers used in frequency synthesis must be very large. This means
that either a very high frequency quartz frequency must be used, or else a
very complicated series of whole-number multiplication and division
circuits must be applied to the quartz frequency.
Usual frequency synthesis systems rely on fixed division ratios to achieve
control of the generated frequency. In such systems, the period of the
controlled frequency must be a whole-number multiple of the period of the
high-frequency quartz reference signal. This limitation can be overcome by
dynamically varying the division ratio so as to achieve a long-term
average relationship between the quartz reference and the synthesized
frequency. In musical instrument tuning, the average period of the
reference signal is much more significant than the instantaneous period of
each reference pulse.
Even after very good approximations to the perfect ratios are implemented,
there is still the problem of offset adjustment. Usual frequency synthesis
techniques do not adapt well to continuous adjustment of the ratios
involved. The methods are better suited to "hopping" from one frequency to
another. Therefore, most pitch comparators utilizing digital frequency
synthesis, nevertheless use analog frequency synthesis to implement pitch
scale offsetting. This is sometimes implemented by replacing the quartz
oscillator by a variable frequency oscillator. A more accurate method is
to use hetrodyning technology to mix a quartz signal with a (low
frequency) variable signal. The resultant frequency sum retains much of
the quartz signal's accuracy. However, the greater the offset range, the
greater the potential frequency error.
BRIEF SUMMARY OF THE INVENTION
Based on the considerations of the prior art, the following are objects of
my invention. Both an audible reference pitch and a visual pitch
comparison are provided. Harmonically-related notes are displayed without
re-adjustment of the device. Pitch comparison is displayed by a
phase-sensitive pattern entirely using solid-state technology. All
reference frequencies and frequency offsets are digitally synthesized
using no variable frequency oscillator.
My invention provides these features through the use of a microprocessor to
control the synthesis of a signal that is typically 32 times the frequency
of the selected note. This signal is used together with a shift-register
display circuit to show a dynamic picture of the sounded note with
temporal resolution of 1/32 of a cycle of the selected note in an array of
32 indicator lights. Special considerations are made to accommodate a wide
dynamic range in the sound being detected and a complex sound signal
(composed of several frequency components).
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is block diagram of the tuning aid showing the major function
components.
FIG. 2 is a schematic of the analog signal processing portion of the sound
detection system.
FIG. 3 is a schematic of the shift register and display portion of the
tuning aid.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As shown in FIGS. 1, 2, and 3 my tuning aid 21 comprises an audio input
circuit 10, a comparison and display circuit 17, and a control circuit 18.
The sound of a musical instrument in sensed by a microphone 11. After being
pre-amplified by circuit 13, the microphone signal 23 is processed by an
active filter stage 13 that is switchable between flat response and
low-pass response, depending on the state of a control signal 24 from the
microprocessor 19. The low-pass filter is enabled by the microprocessor
when the selected reference note has a frequency below a certain
threshold. This threshold is set so as to suppress excessive harmonics
when tuning very low notes. This has the effect of clarifying the pattern
observed in the array of LEDs 28-1, 28-2, 28-3, 28-4, contained in the
circuit display circuit 16.
After being filtered, the microphone signal is digitized by a comparator 14
whose threshold tracks the average value of the signal. This tracking
action is accomplished by the charge on the capacitor 27. This charge
compensates for DC offsets in previous stages, and causes the pattern in
the LEDs to be comprised of about half of the LEDs on and half of them
off. The non-filtered leg of the comparator inputs is offset by a
high-value resistor 25. This offset has the effect of biasing the
threshold so that all LEDs are off when there is very little sound. This
action further enhances the clarity of the visual pattern, and saves on
battery power as well.
The output 26 of the comparator is a digital signal which is used as the
input to a shift register 15. The shift register is clocked by a reference
signal 29 generated by the microprocessor. After enough clocks have been
sent to the shift register to shift a new data to every stage (32 in this
embodiment), the microprocessor sends a load strobe 30 to cause the
instantaneous state of all shift register stages to be transferred to the
parallel output latches. The shift register and latch functions are
implemented in an integrated logic circuit, available commercially as a
type "74HC595" 31-1, 31-2, 31-3, 31-4.
These output latches together with the LEDs they control comprise the
display latches and display LEDs circuit 16 shown in FIG. 1. The effect of
issuing a number of serial-shift clocks followed by a single load strobe
is to capture a pattern which represents the instantaneous phase
relationship between the digitized microphone signal and the load strobe.
The resolution of this phase comparison is determined by the number of
shift clocks per load strobe, which is the same as the number of stages in
the shift register (32 in this embodiment). If the frequency of the
microphone signal and the frequency of the load strobe are near enough to
a unison or a harmonic relationship, then the dynamic pattern in the LEDs
can be visually observed. The direction and rate of movement of this
pattern then gives the comparison between the reference and the input
frequencies.
The microprocessor control circuit 18 implements the user interface and the
reference frequency generation. The user interface is accomplished through
a piano-like keyboard with various other control switches and status LEDs
20, and a speaker 22. The piano-like keyboard serves multiple functions in
this embodiment. Its primary function is to select a musical note. The
keys also have digits associated with them and they can be used for
numeric entry of offsets and frequencies. Some of the keys have special
functions associated with them which are used in conjunction with a
function control switch. The special functions include direct entry of
frequency or frequency offset, requesting a readout of current frequency
or offset, and system reset. Status LEDs associated with each key show
which note is currently selected. There are also status LEDs showing which
octave is currently selected. In addition to its use as an audible
reference tone generator, the speaker is used during user interaction to
acknowledge keypresses and otherwise provide feedback to the user.
To facilitate sequencing through a chromatic scale, two control panel
keyswitches and an external foot pedal switch 21 cause the next note in
chromatic sequence to be selected. This permits hands-off operation, when
it would be inconvenient to press a keyswitch for every note selection.
As a keyboard selectable option, the speaker 22 is driven by a signal which
in synchronous with the load signal 30 to provide an audible reference
tone. When the speaker is thus enabled, the visual display is not
generally used because the device is being used as a reference pitch
generator rather than a pitch comparator.
The means by which this embodiment synthesizes the clock and load
frequencies and the optional speaker signal is software timing loops. The
microprocessor uses the known execution time of each instruction to
measure out periods of time to the nearest microprocessor cycle for clock,
load, and speaker signals. To further increase the resolution of the
frequency synthesis, the cycle counts are dynamically varied so that the
average period of the synthesized signal is not restricted to being a
multiple of the microprocessor cycle period. This provides resolution well
beyond what is needed for musical instrument tuning, and in fact well
beyond what is normally achievable in terms of the accuracy of the quartz
crystal oscillator which provides the microprocessor timing.
For any given selected note, the microprocessor software starts with a
look-up table of equal-tempered frequencies for twelve notes in one
octave. These frequencies are stored in floating point format with a
resolution of 32 bits of mantissa. The frequency from the table is them
modified by the selected octave and the selected offset (if any) to arrive
at the actual desired frequency.
When the timing aid is not servicing the user interface, it is running the
frequency synthesis loop. This loop contains a software delay controlled
by a loop counter. The value of that loop counter is calculated in order
to make the frequency of the synthesis software loop close to the desired
frequency. Within the synthesis software loop is a branch that takes one
more cycle of time if it goes one way than if it goes the other. A
"fine-tuning" parameter controls the average number of time the branch is
taken or not taken. This fine-tuning parameter is calculated to make the
average synthesis software loop time as close as possible to the desired
period. This results in a slight amount of phase jitter in the synthesized
signal, which has negligible effect on the visual pattern. At the higher
frequencies, however, the jitter is barely audible in the reference tone
in the speaker.
In one of its operating modes, the tuning aid varies the two parameters
that determine the software synthesis timing according to the state of
keyswitches on the control panel. This allows the user to gradually
"slide" the synthesized frequency in order to match an external musical
note. Means are then provided using the status LEDs to display to the
operator the exact offset from standard tuning that the previous "slide"
represents. The user may also enter an explicit offset numerically through
the keyboard. For non-tempered scales or engineering applications, the
user may also enter the desired frequency in Hertz directly, bypassing the
note-table look-up.
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