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DESCRIPTION JP2001177900

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DESCRIPTION JP2001177900
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
audio signal recording apparatus suitable to be applied to a small apparatus such as a camera
integrated type video tape recorder such as a digital video camera.
[0002]
2. Description of the Related Art In general, in an audio signal recording apparatus of a camera
integrated type video tape recorder such as a digital video camera, a built-in microphone is
generally an omnidirectional microphone which is less susceptible to the influence of cabinet
shape and wind noise. There is.
[0003]
In order to obtain a sense of stereo from the audio signal from this nondirectional microphone,
the audio signals from two or more nondirectional microphones are supplied to a stereo
processing circuit to be electrically processed to make directional characteristics There is a need.
[0004]
In this camera integrated video tape recorder such as this digital video camera, a plurality of, for
example, two nondirectional microphones are mounted relatively close to each other, and
arranged side by side on the upper surface or front surface of this camera integrated video tape
recorder, for example There is.
10-04-2019
1
[0005]
Here, in general, the microphone gap d between a plurality of microphones disposed close to
each other and the level difference and the phase difference of the audio signal output from each
microphone will be described with reference to FIG.
[0006]
FIG. 7A shows the case where two microphones 1 and 2 are arranged side by side at intervals d,
and a sine wave of amplitude a is incident on the two microphones 1 and 2 from the direction of
the sound source A. Assuming that the signal 1 can be obtained from the output of the
microphone 2, the delayed signal corresponds to the phase φ1 from the point c intersecting the
perpendicular drawn from the microphone 1 to the sound source incident direction from the
microphone 2 Signal 2 is obtained.
[0007]
Here, if the distance d between the microphones 1 and 2 is sufficiently smaller than the distance
to the sound source A, the respective amplitudes a of the signals 1 and 2 are equal, and there is a
phase difference of only the phase φ1.
The signal 1 and the signal 2 are expressed by the following equations.
Signal 1 = acos ωt Signal 2 = acos (ωt-φ1)
[0008]
FIG. 7B shows the case where a sine wave of amplitude b is incident from the direction of the
sound source B which is an extension of the two microphones 1 and 2 in the arrangement of the
microphones 1 and 2 as in FIG. 7A. The two output signals 1 and 2 differ depending on the
frequency of the incident sine wave.
[0009]
Normally, sound is known to propagate in air as a compressional wave, but the sine wave of
sound source B has a maximum value of + (or a minimum value of-) dense, and a minimum value
of-(or + of Assuming that the maximum value is sparse, if the interval d is larger than a half
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period of the wavelength λ of the sine wave (λ / 2 <d), in other words, the frequency of the
incident sine wave is higher than the frequency with the interval d as a half period. If it is high,
the sine wave reaching the microphone 2 will be in the shadow of the microphone 1 and the
amplitude of the signal 2 will be smaller than that of the signal 1 because the level drops and
reaches the microphone 2.
[0010]
However, if the interval d is smaller than a half cycle of the sine wave (λ / 2> d), in other words,
the sine reaching the microphone 2 if the frequency of the incident sine wave is lower than the
frequency with the interval d as a half cycle Since the waves are hardly influenced by the
microphone 1, the amplitudes of the signal 1 and the signal 2 hardly change.
[0011]
In this case, it can be easily imagined considering the sense when human's both ears are fitted to
the microphone 1 and the microphone 2, that is, in the case of relatively low frequency (several
hundreds Hz), no sense of direction, high frequency In the case of (several kHz), the sense of
direction can be relatively easily obtained because high frequencies can cause shadows of the
head.
[0012]
As an example, in FIG. 7, when the distance d is 30 mm, the frequency F with the distance d as a
half cycle is as follows.
Frequency F = sonic speed in air / 2d = 340/2 × 0.030.035.7 kHz
[0013]
Therefore, if the incident sine wave in FIG. 7B is limited to the frequency F or less, the amplitudes
of the signal 1 and the signal 2 hardly change, and are shifted by the phase difference φ2
corresponding to the delay d.
That is, it is expressed by the following equation.
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Signal 1 = b cos ωt signal 2 = b cos (ωt−φ 2)
[0014]
FIG. 7C shows an example in which three nondirectional microphones 1, 2 and 3 are used, and
the microphone 3 is disposed at the apex of an equilateral triangle whose base is the microphone
1 and the microphone 2 at the interval d. .
Here, considering the case where a sine wave of amplitude a is incident from the direction of the
sound source A, if the input frequency is limited to the frequency F determined from the
microphone spacing or less as described above, each microphone spacing d The effect of is only
the phase difference due to the delay, and the amplitude a hardly changes.
When the relative spacing between the microphones 1, 2 and 3 is different, the frequency F is
determined from the maximum spacing.
[0015]
In this case, signal 1, signal 2 and signal 3 which are outputs of the microphones 1, 2 and 3 are
expressed by the following equations.
Signal 1 = acos ωt signal 2 = acos (ωt-φ1) signal 3 = acos (ωt-φ2)
[0016]
In addition, the above-mentioned relationship is considered to be naturally established even
when the arrangement of nondirectional microphones other than the example described above
and when four or more microphones are used.
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4
The following description is given on the premise of the above.
[0017]
Next, referring to FIG. 8, an example of an audio signal recording device of a conventional digital
video camera will be described. In FIG. 8, 10L and 10R are separated by a distance d, for example
30 mm. The omnidirectional left and right audio signals obtained by the microphones 10L and
10R are supplied to the automatic gain control circuit 12 through the amplifier circuits 11L and
11R, respectively.
[0018]
The automatic gain control circuit 12 controls signal levels in order to optimize the dynamic
range of the left and right audio signals from the microphones 10L and 10R for signal processing
in the subsequent stage, and the level controlled left and right audio signals are analog signals
Are supplied to the stereo processing circuit 14 through the A-D conversion circuits 13L and 13R
which convert the digital signal into a digital signal.
[0019]
In this stereo processing circuit 14, the digitized left and right audio signals are processed
through the matrix circuit and equalizing circuit to calculate directional stereo signals L and R
from the nondirectional left and right audio signals. Get processed.
[0020]
The stereo signals L and R obtained at the output side of the stereo processing circuit 14 are
recorded on a recording medium through the recording signal processing circuit.
In this case, when the recording medium is reproduced, a stereo feeling can be obtained.
[0021]
However, when there are variations in the levels of the left and right audio signals input to the
stereo processing circuit 14, the level difference or the level between the stereo signals L and R
obtained at the output side is obtained. There is a problem that a phase difference occurs and
stereo sound field localization is disturbed.
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[0022]
Generally, variations in level of the left and right audio signals are variations in manufacturing
characteristics of the microphones 10L and 10R, variations in characteristics of the amplifier
circuits 11L and 11R configured with operational amplifiers, etc., and the automatic gain control
circuits 12 and A- It occurs due to variations in the characteristics of the D conversion circuits
13L and 13R, and all of them are integrated.
[0023]
For example, looking at the amount of variation generated in FIG. 8, the characteristic variation
of the nondirectional microphones 10L and 10R is about ± 2 to ± 4 dB, the characteristic
variation of the amplifier circuits 11L and 11R is ± 0.5 dB, and automatic The characteristic
variation of the gain control circuit 12 is ± 1 dB, and the characteristic variation of the AD
conversion circuits 13L and 13R is about ± 0.5 dB.
[0024]
Therefore, in total, the variation of ± 4 to ± 6 dB may be integrated up to the maximum before
this stage of the stereo processing circuit 14 and ± 1 dB which is a level difference generally not
noticed by human hearing. There is a risk of exceeding it.
[0025]
In order to suppress variations in the level of the left and right audio signals of the stereo
processing circuit 14, selection (pairing) of the microphones 10L and 10R, characteristics of the
amplification circuits 11L and 11R, analog components such as capacitors and resistors Care
must be taken in the selection of components, which increases the cost of parts and the number
of parts to increase the accuracy, and the ICs (semiconductor integrated circuits) that constitute
the automatic gain control circuit 12 and the AD conversion circuits 13L and 13R. There was
also a problem that it would be expensive.
[0026]
In view of the foregoing, it is an object of the present invention to eliminate the level difference
between the levels of a plurality of audio signals supplied to the stereo processing circuit
irrespective of the non-directional microphone and the characteristic variations of various
circuits. I assume.
[0027]
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6
According to the present invention, an audio signal recording apparatus supplies first and second
audio signals from first and second nondirectional microphones to a stereo processing circuit,
respectively, and performs this stereo processing. In the audio signal recording apparatus for
recording a stereo signal obtained at the output side of the processing circuit, level control
means for controlling the levels of the first and second audio signals are provided and the first
and second audio signal recording apparatuses are provided. A level detection means is provided
for detecting the level of the low frequency component of the audio signal, and a level control
signal corresponding to the level difference between the first and second audio signals obtained
by the level detection means is obtained. The level control means is controlled to eliminate the
level difference between the first and second audio signals.
[0028]
According to the present invention, there is provided level control means for controlling the level
of the first or second audio signal, and the level control signal corresponding to the level
difference between the low frequency components of the first and second audio signals Since the
control means is controlled to eliminate the level difference between the first and second audio
signals, it is related to the variation of the characteristics of the omnidirectional microphone in
the previous stage of the stereo processing circuit, various circuits, etc. Therefore, the difference
in level between the first and second audio signals can be eliminated, and a stereo signal can be
obtained without disturbance in stereo sound field localization.
[0029]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the audio signal
recording apparatus according to the present invention will be described below with reference to
FIGS.
In FIG. 1, parts corresponding to FIG. 8 are given the same reference numerals.
This FIG. 1 example is also an example in which the present invention is applied to an audio
signal recording apparatus of a digital video camera.
[0030]
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In the example shown in FIG. 1, two nondirectional microphones 10L and 10R are built in the left
and right sides of the digital camera at a distance d, for example, 30 mm.
The nondirectional left and right audio signals obtained by the microphones 10L and 10R are
supplied to the automatic gain control circuit 12 through the amplifier circuits 11L and 11R
formed of operational amplifiers, respectively.
[0031]
The automatic gain control circuit 12 controls the signal level in order to optimize the dynamic
range of the left and right audio signals from the microphones 10L and 10R for signal processing
in the subsequent stage.
[0032]
In this example, the left audio signal obtained at the output side of the automatic gain control
circuit 12 is supplied to the stereo processing circuit 14 through the AD conversion circuit 13L
for converting an analog signal into a digital signal, and The right audio signal obtained at the
output side of the gain control circuit 12 is made stereo via a series circuit of a level control
circuit 20 including, for example, a voltage control amplifier (VCA) and an A-D conversion circuit
13R converting an analog signal to a digital signal. The data is supplied to the arithmetic
processing circuit 14.
[0033]
Further, in this example, the left audio signal obtained at the output side of the automatic gain
control circuit 12 and the right audio signal obtained at the output side of the level control
circuit 20 are supplied to the level control signal forming circuit 21 respectively.
[0034]
The level control signal formation circuit 21 detects the level of low frequency components of
the left and right audio signals, for example, lower frequency components of 5.7 kHz or less, and
a level control signal corresponding to the level difference of the left and right audio signals. The
level control signal is supplied to the level control circuit 20 to control the level of the right audio
signal obtained on the output side of the level control circuit 20 so that there is no difference
between the levels of the left and right audio signals.
[0035]
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8
In this case, since the distance between the nondirectional microphones 10L and 10R is 30 mm
because the low frequency component of 5.7 kHz or less is set, the distance d is smaller than half
of the wavelength λ (d <λ / 2) The level components of the left and right audio signals obtained
at the output sides of the microphones 10L and 10R are detected to detect the level components
of the frequency components.
[0036]
An example of this level control signal formation circuit 21 is shown in FIG.
In FIG. 2, reference numeral 30a denotes a left audio signal input terminal to which the left audio
signal obtained at the output side of the automatic gain control circuit 12 is supplied, and 30b
denotes a right audio signal obtained at the output side of the level control circuit 20. The right
audio signal input terminal supplied is shown.
[0037]
The left and right audio signals obtained at the left and right audio signal input terminals 30a
and 30b are supplied to the low pass filters 31a and 31b, respectively.
The low-pass filters 31a and 31b pass audio signals having a frequency F or less at which the
distance d between the nondirectional microphones 10L and 10R corresponds to a half cycle λ /
2.
[0038]
In this case, at the output side of the nondirectional microphones 10L and 10R, audio signals of
levels not affected by the shadows of the microphones are obtained.
[0039]
The left and right audio signals, for example, as shown in FIGS. 3A and 3C obtained at the output
side of the low pass filters 31a and 31b are leveled by unifying the codes as shown in FIGS. 3B
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and 3D via absolute value conversion circuits 32a and 32b. The signal is supplied to peak
detection circuits 33a and 33b that constitute a detection circuit.
The peak detection circuits 33a and 33b obtain peak values as shown by broken lines in FIGS. 3B
and 3D, and the peak values obtained at the output sides of the peak detection circuits 33a and
33b are supplied to the subtraction circuit 34. It does like that.
[0040]
In FIGS. 3A and 3C, when the right audio signal has a delay Td due to the interval d between the
microphones 10L and 10R with respect to the left audio signal, the absolute value processing
makes this become as shown by the solid line in FIGS. When peak detection processing is further
performed, it becomes as shown by a broken line.
Now, at t = T2, when this level is compared, a level difference after absolute value generation
occurs, but almost no level difference occurs in the broken line after peak detection.
In this example, the delay component Td hardly affects the level detection by handling a signal
having a long cycle with respect to the delay component Td, that is, a low frequency, and further
increasing the hold time at peak detection.
[0041]
In the subtraction circuit 34, in this example, detection of the level difference component is
performed at audio sampling frequencies such as 32 kHz, 44.1 kHz or 48 kHz.
The positive sign, zero and negative sign signals obtained at the output side of the subtraction
circuit 34 are supplied to the sign detection circuit 35.
[0042]
The code detection circuit 35 detects the positive code, zero or negative code in synchronization
with the audio sampling frequency, for example, 32 kHz, 44.1 kHz or 48 kHz, and supplies the
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detection signal to the control signal generation circuit 36.
[0043]
The control signal generation circuit 36 increases the level of the control signal when the sign
detected by the sign detection circuit 35 is a positive sign, holds when the sign is zero, and the
level of the control signal when the sign is negative. Let it decrease.
[0044]
Therefore, from the control signal generation circuit 36, when the sign detected by the code
detection circuit 35 is a positive code, that is, when the level control signal in the increasing
direction of left audio signal> right audio signal is zero, that is, left When the audio signal = right
audio signal, the level control signal holding the previous value is output, and when the left audio
signal <right audio signal, a decreasing level control signal is output.
[0045]
An output signal of the control signal generation circuit 36 is supplied to a level control signal
output terminal 38 for supplying a level control signal to the level control circuit 20 through a
smoothing circuit 37 formed of a low pass filter or the like.
[0046]
In this case, the level control signal repeatedly increases and decreases by comparing the levels
of the left audio signal and the right audio signal, but the change period is every audio sampling,
and a level difference is generated. Be done.
[0047]
The level control signal obtained at the level control output terminal 38 is supplied to the level
control circuit 20, and the level control circuit 20 controls the level of the right audio signal to
form a feedback loop circuit. For example, this level control When the level of the signal
increases, the output level of the level control circuit 20 is increased. Conversely, when the level
of the level control signal decreases, the output level of the level control circuit 20 is decreased.
And the level of the right audio signal are the same.
[0048]
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The left and right audio signals having no level difference are supplied to the stereo processing
circuit 14 through the AD conversion circuits 13L and 13R.
In this stereo processing circuit 14, the digitized left and right audio signals are processed by
processing directional stereo signals L and R from nondirectional left and right audio signals
through matrix circuits and equalizing circuits. To get it done.
[0049]
The stereo signals L and R obtained at the output side of the stereo processing circuit 14 are
recorded on a recording medium such as a magnetic tape via a recording system processing
circuit.
[0050]
According to this embodiment, the level control circuit 20 for controlling the level of the right
audio signal is provided, and the low frequency below the frequency F corresponding to the half
period λ / 2 between the microphones 10L and 10R of the left and right audio signals. The level
control circuit 20 is controlled by the level control signal corresponding to the level difference of
the components to eliminate the level difference between the left and right audio signals. Level
difference between the left and right audio signals can be eliminated even if the characteristic
microphones 10L, 10R, amplification circuits 11L, 11R, etc. have variations in characteristics,
and the stereo obtained at the output side of the stereo processing circuit 14 When reproducing
the recording medium in which the signals L and R are recorded, the localization of the stereo
sound field is not disturbed.
[0051]
Further, according to the present embodiment, even if the non-directional microphones 10L and
10R, the amplifier circuits 11L and 11R, etc. have variations in characteristics, the level
difference between the left and right audio signals can be eliminated. The tolerance of the
variation of the components such as the acoustic microphones 10L and 10R and the
amplification circuits 11L and 11R can be taken large, the inexpensive components can be used,
and the cost can be reduced.
[0052]
Although the level control circuit 20 for controlling the level of the right audio signal is provided
in the example of FIG. 1, it can be easily understood that a level control circuit for controlling the
level of the left audio signal may be provided instead. You see.
10-04-2019
12
[0053]
4, 5 and 6 show other examples of the embodiment of the present invention.
In FIG. 4, FIG. 5 and FIG. 6, the same reference numerals are given to parts corresponding to FIG.
[0054]
In the example of FIG. 4, the level control is performed by the digital processing with the audio
signal after digital conversion in the example of FIG. 1, and the left and right audios obtained at
the output side of the automatic gain control circuit 12 in the example of FIG. The signal is
supplied to AD conversion circuits 13L and 13R for converting an analog signal into a digital
signal, respectively, and converted into digital left and right audio signals, respectively.
[0055]
The digital left audio signal obtained on the output side of the A-D conversion circuit 13L is
supplied to the stereo processing circuit 14, and the digital right audio signal obtained on the
output side of the A-D conversion circuit 13R is digitally configured. Are supplied to the stereo
processing circuit 14 through the level control circuit 20 of FIG.
The level control circuit 20 is configured of, for example, a multiplier in digital processing.
[0056]
Also, the digital left audio signal obtained at the output side of the A-D conversion circuit 13L
and the digital right audio signal obtained at the output side of the level control circuit 20 are
digitally configured in the same manner as in FIG. The level control signal supplied to the circuit
21 and supplied to the output side of the level control signal forming circuit 21 is supplied to the
level control circuit 20 to control the level of the right audio signal.
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13
Others are configured in the same manner as in FIG.
[0057]
It can be easily understood that the same function and effect as those in FIG. 1 can be obtained
also in this example in FIG.
Further, even if there are variations in the characteristics of the nondirectional microphones 10L
and 10R, the amplification circuits 11L and 11R, the A-D conversion circuits 13L and 13R, etc. in
the previous stage of the stereo processing circuit 14, the levels of the left and right audio signals
It is possible to eliminate the difference.
[0058]
Further, in the example of FIG. 4, since the stereo processing circuit 14, the level control circuit
20 and the level control signal forming circuit 21 can be configured by digital circuits, they can
be easily incorporated in an LSI, and the semiconductor miniaturization and density increase in
the future In the above, the increase in the circuit scale is hardly a problem and can be realized.
[0059]
5, the level control circuit 20 has a feedforward loop circuit configuration as compared with the
example of FIG. 4, and the digital left and right audio signals obtained at the output sides of the
AD conversion circuits 13L and 13R are leveled. The level control signal supplied to the control
signal forming circuit 21 and supplied to the output side of the level control signal forming
circuit 21 is supplied to the level control circuit 20, and the other configuration is the same as
that shown in FIG.
[0060]
In this FIG. 5 example, it can be easily understood that the same function and effect as the FIG. 4
example can be obtained.
[0061]
Further, FIG. 6 shows an example in which three nondirectional microphones 10L, 10R and 10C
are used as in FIG. 7C in FIG.
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14
[0062]
In the example shown in FIG. 6, the nondirectional microphones 10C are provided in the digital
camera at the positions of the apexes of an equilateral triangle with respect to the nondirectional
microphones 10L and 10R arranged at an interval d.
[0063]
The nondirectional left, right and middle audio signals obtained by the microphones 10L, 10R
and 10C are supplied to the automatic gain control circuit 12 through the amplifier circuits 11L,
11R and 11C respectively composed of operational amplifiers.
[0064]
The automatic gain control circuit 12 controls the signal level in order to optimize the dynamic
range of the left, right and middle audio signals from the microphones 10L, 10R and 10C to the
signal processing in the subsequent stage.
[0065]
The left, right and medium audio signals obtained at the output side of the automatic gain control
circuit 12 are respectively supplied to A-D conversion circuits 13L, 13R and 13C for converting
analog signals into digital signals, and digital left, right and Convert to medium audio signal.
[0066]
The digital audio signal obtained on the output side of the A-D conversion circuit 13L is supplied
to the stereo processing circuit 14, and the digital right and middle audio signals obtained on the
output side of the A-D conversion circuits 13R and 13C. Are supplied to the stereo processing
circuit 14 through level control circuits 20a and 20b of digital configuration, respectively.
[0067]
Further, the digital left audio signal obtained at the output side of the A-D conversion circuit 13L
is supplied to two level control signal forming circuits 21a and 21b configured in the same
manner as in FIG. 2 in digital configuration, and this A-D conversion is also performed. The
digital right audio signal obtained at the output side of the circuit 13R is supplied to the level
control signal forming circuit 21a, and the level control signal obtained at the output side of the
level control signal forming circuit 21a is supplied to the level control circuit 20a. Control the
level of the right audio signal.
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[0068]
Further, the digital medium audio signal obtained on the output side of the A-D conversion circuit
13C is supplied to the level control signal forming circuit 21b, and the level control signal
obtained on the output side of the level control signal forming circuit 21b is level controlled. It is
supplied to the circuit 20b to control the level of the middle audio signal.
[0069]
In this case, the right audio signal and the middle audio signal are controlled to have the same
level as the left audio signal respectively, and the left, right and middle audio signals are
controlled such that there is no level difference between them.
The other components in FIG. 6 are the same as in FIG.
[0070]
It can be easily understood that the same effects as those in the example of FIG. 4 can be
obtained in the example of FIG. 6 as well.
[0071]
Although in the example of FIG. 6 the level control circuits 22a and 22b are constituted by feed
forward loop circuits, it goes without saying that they may be constituted by feedback loop
circuits.
[0072]
It goes without saying that the present invention is not limited to the above-described example
and that various other configurations can be adopted without departing from the scope of the
present invention.
[0073]
According to the present invention, level control means for controlling the level of the first or
second audio signal is provided, and the level according to the level difference between the low
frequency components of the first and second audio signal. Since the level control means is
controlled by the control signal to eliminate the level difference between the first and second
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audio signals, the omnidirectional microphone in the front stage of the stereo processing circuit,
various circuits, etc. There is an advantage that the level difference between the levels of the first
and second audio signals can be eliminated regardless of the variation of the characteristics of
(1), and a stereo signal without disturbance in stereo sound field localization can be obtained.
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