JP2005286582

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DESCRIPTION JP2005286582
PROBLEM TO BE SOLVED: To provide a modulator for superdirective speaker capable of
suppressing a decrease in modulation factor even with a small level audio signal input and
obtaining a desired level of a self-generated demodulated sound emitted from an ultrasonic
vibration element into air. provide. SOLUTION: An audio signal is converted from an analog
signal to a digital signal by an A / D converter 2, and period information and level information
are generated by a signal level control unit 3 based on this digital signal. The output level of the
sound wave band oscillator 8 is controlled. Also, the output of the A / D converter is input to the
envelope 4 and the coefficient unit 5, which respectively output a signal multiplied by m and the
envelope level. These outputs are added by the adder 6 and square rooted by the square root
converter, and then multiplied by the output of the ultrasonic band oscillator 8 by the multiplier
9. This signal is converted back to an analog signal by the D / A converter 10, and then
multiplied by the value generated by the gain variable unit on the basis of the output of the
signal level control unit 3 by the multiplier 12 to obtain a modulated signal output. [Selected
figure] Figure 1
Modulator for superdirective speaker
[0001]
The present invention relates to a modulator for superdirective speaker which radiates audible
sound in a directional manner.
[0002]
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The conventional superdirective speaker modulator multiplies the audio signal of the modulated
signal by m with a coefficient unit, adds a DC bias to the signal, and then performs square root
conversion before multiplying it with the output carrier signal of the ultrasonic band oscillator To
obtain a modulated signal.
This modulated signal is amplified by a power amplifier and supplied to an ultrasonic transducer
array to emit a sound wave. That is, in the processing from the above coefficient unit to the
multiplier, processing is performed in which the sound signal of modulation degree m is
subjected to square root processing and amplitude modulation is performed (for example, see
Patent Document 1).
[0003]
Japanese Examined Patent Publication No. 4-58758 (Pages 2 to 3, FIG. 1)
[0004]
In the conventional superdirective speaker modulator, there is a problem that the carrier signal is
output from the radiator as it is when there is no sound source signal.
Further, since amplitude modulation is adopted as the modulation method, the modulation rate is
lowered when an audio signal of a low level is inputted. For this reason, there is a problem that
the demodulation sound emitted from the ultrasonic vibration element into air and self-generated
can not obtain a desired sound pressure level. In addition, in order to obtain a desired sound
pressure level for various types of audio signals, there is also a problem that it is necessary to
increase the gain in the subsequent amplification apparatus.
[0005]
The present invention has been made to solve the problems as described above, and enables
optimum modulation output by controlling the carrier signal level according to the input audio
signal level. Furthermore, when the audio signal level is low, the sound pressure level is raised by
raising the modulation signal gain, and furthermore, it is easy to listen to the audio by changing
the modulation signal gain according to the input audio signal. It is possible.
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[0006]
A superdirective speaker modulator according to the present invention comprises an A / D
converter for converting an audio signal from an analog signal to a digital signal, an envelope for
obtaining an envelope level from an output signal of the A / D converter, and A coefficient unit
for multiplying the output of the A / D converter by a coefficient, an adder for adding the output
of the envelope unit and the output of the coefficient unit, a square root converter for performing
square root processing on the output of the adder; A signal level control unit that generates
period information and level information based on an output signal of a D converter, an
ultrasonic band oscillator controlled by an output signal of the signal level control unit, the
square root converter, and the ultrasonic band A multiplier for multiplying the output signal of
the oscillator, a D / A converter for converting the output signal of the multiplier from a digital
signal to an analog signal, and an output signal of the signal level control unit A gain variable
unit for generating a gain value based on the value set in mind, those comprising a multiplier for
multiplying an output of said variable gain across the output of the D / A converter.
[0007]
According to the present invention, since it is possible to adjust the carrier wave signal level in
accordance with the input audio signal level, it is possible to enable an optimal modulation
output and obtain a desired sound pressure level.
[0008]
Embodiment 1
FIG. 1 is a diagram showing the configuration of a superdirective speaker modulator according to
a first embodiment of the present invention.
Description will be made based on FIG. This superdirective speaker modulator includes a
modulation signal source 1 for outputting an audio signal, an A / D converter 2 for converting an
audio signal from an analog signal to a digital signal, and a signal output from the A / D
converter 2 It is supplied to the level control unit 3 and is also supplied to the envelope unit 4
and the coefficient unit 5 arranged in parallel. The outputs of the envelope 4 and the coefficient
5 are input to the adder 6. The added output is input to the square root converter 7. The output
of the square root converter 7 is multiplied by the output of the ultrasonic band oscillator 8 that
outputs a carrier wave of a fixed frequency higher than the audio signal controlled by the signal
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level control unit 3 output signal and then D / A conversion 10 is input to the The output of the
D / A converter 10 is multiplied by the signal generated by the gain variable unit 11 in the
multiplier 12 based on the output signal of the signal level control unit 3 to obtain an output
signal.
[0009]
The superdirective speaker is provided with a radiator (not shown) which is a radiating portion of
a highly directional speaker that emits ultrasonic waves to obtain an audio signal by parametric
action. The parametric speaker, which is a superdirective speaker, utilizes ultrasonic waves that
are inaudible to humans, and a distortion component is generated in the process of strong
ultrasonic waves propagating through the air. The principle of obtaining (nonlinearity) is
adopted. Although the conversion efficiency for obtaining an audible sound is low, it can exhibit
"superdirectivity" in which sound is concentrated in a beam shape in a narrow area in the sound
radiation direction. Although conventional low-directivity speakers create a sound field in a large
area including the back, like the light of a bare bulb, the area can not be controlled, but speakers
used in parametric speakers are It is possible to limit the area that sounds like a spotlight,
making it a highly directional speaker.
[0010]
Next, the operation will be described based on FIG. An audio signal output from the modulation
signal source 1 is converted by the A / D converter 2 from an analog signal to a digital signal of
quantized data value. The sampling frequency of the A / D converter 2 can be sampled at a
frequency sufficiently higher than the audio signal output from the modulation signal source 1,
and the sampling frequency which is sampling can be adjusted in accordance with the input
audio signal. Do. The quantized data value is input to the signal level control unit 3 and is also
input to the envelope 4 and the coefficient unit 5 provided in parallel. First, the quantized data
value input to the envelope 4 and the coefficient unit 5 is determined the envelope level by the
envelope 4 and is multiplied by m by the coefficient unit 5 and output. The outputs of the
envelope 4 and the coefficient 5 are added by the adder 6 and then input to the square root
converter 7. In the square root converter 7, parallel root processing of the added signal is
performed and output. On the other hand, the signal level control unit 3 generates period
information and level information from the quantized data value. FIG. 2 is a block diagram
showing details of the signal level control unit. The operation of the signal level control unit 3
will be described based on FIG. The input quantized data value is input to and stored in the
memory 13 and is output to the difference unit 14. The memory 13 incorporates a program for
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managing data to be stored. The difference processing is performed by the difference unit 14,
and the result is output to the increase / decrease detector 15. The increase / decrease detector
15 determines the increase / decrease tendency of each boundary value from the difference
result data. For example, the level is determined based on the period (time) and the two
decreasing boundary values and the maximum value between the two decreasing boundary
values, and the period information and the level information are output to the pulse generator 16
via the memory 13 . Pulse generation is performed based on the period information and level
information sent to the pulse generator 16, and is output to the ultrasonic band oscillator 8. The
ultrasonic band oscillator 8 outputs a carrier wave, which is an ultrasonic wave corresponding to
the time and level of the pulse, to the multiplier 9 based on the pulse information sent, and is
amplitude-modulated by the input audio signal. The amplitude modulated signal is converted to
the original analog signal by the D / A converter 10. After conversion, the output of the gain
variable unit 11 is multiplied by the multiplier 12 to change the gain of the modulation signal,
resulting in a modulation signal output. When the gain variable unit 11 is smaller than an
arbitrary reference, it outputs a numerical value to be increased to increase the modulation
signal gain.
[0011]
For example, the level A output from the signal level control unit 3 is set to a value of 0 <A ≦ 1.
The gain varying unit 11 varies the modulation signal gain according to the equation 1 / (X + A).
It is assumed that X in this equation can be set to an arbitrary value based on the audio signal
output from the modulation signal source 1. By substituting the level output from the signal level
control unit 3 into the 1 / (X + A) equation of the gain variable unit 11, the modulation gain can
be increased at a small audio signal level. For example, assuming that the value of X is 0.3, when
the level output from the signal level control unit 3 is 0.7 for a large audio signal, the output
value of the gain variable unit 11 is 1 ×. On the other hand, when the level output from the
signal level control unit 3 of the small audio signal is 0.1, the output value of the gain variable
unit 11 is 2.5 times and the modulation signal gain is increased when the audio signal is small. It
becomes possible.
[0012]
Further, according to the signal level control unit 3 to the output level, the gain variable unit 11
uses an a as a constant, an exponential function such as a <(1 / X + A)>, a logarithmic function
such as loga <X>, etc. You may behave as a monotonically increasing function.
[0013]
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It is a block diagram of the modulator for superdirective speakers which shows Embodiment 1 of
this invention.
It is a block diagram showing composition of a signal level control part of this invention.
Explanation of sign
[0014]
Reference Signs List 1 modulation signal source, 2 A / D converter, 3 signal level control unit, 4
enveloper, 5 coefficient unit, 6 adder, 7 square root converter, 8 ultrasonic band oscillator, 9
multiplier, 10 D / A Converter, 11 gain variable, 12 multiplier, 13 memory, 14 differencer, 15
increase / decrease detector, 16 pulse generator.
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