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DESCRIPTION JPH05265478

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DESCRIPTION JPH05265478
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
echo addition circuit and an audio apparatus using the same, and more particularly to an echo
addition circuit used for audio equipment such as a karaoke apparatus, component stereo
apparatus, radio cassette apparatus, VTR, The present invention relates to improvement of a
circuit that generates an echo signal from a so-called original audio signal (hereinafter referred to
as an original signal), adds the echo signal to the original signal, and outputs the echo signal.
[0002]
2. Description of the Related Art Most current audio devices have a so-called karaoke function
that can mix the audio input from a microphone into another reproduction signal or combine it
with another reproduction signal and output it. . This requires an echo addition function to add
reverberation effects, for which an echo signal is generated internally. An echo signal is usually
generated by delaying an original signal obtained by converting audio information such as voice
or music into an electrical signal for a predetermined time. Then, in order to enhance the
reverberation effect, the echo addition circuit attenuates the original signal and the echo signal,
feeds it back to the original signal side, and generates a signal mixed with the original signal.
Repeat the mixing.
[0003]
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An example of a conventional configuration of such an echo addition circuit is shown in FIG. In
the figure, 8 is an echo addition circuit, 1 is its waveform synthesis circuit, 2 is a low pass filter
(LPF), 3 is a delay circuit using BBD (Bucket Brigade Device), 4 is a low pass filter (LPF) , 5 and 6
are attenuators (ATT), and 7 is a waveform synthesis circuit.
[0004]
The original signal A input to the echo addition circuit 8 is synthesized by the waveform
synthesis circuit 7 with a signal in which the echo signal B is attenuated to an appropriate level
by the attenuator 6. As a result, an echo-added audio signal C is added and output. The echoadded audio signal C is amplified and finally output as sound from the speaker to the outside or
is recorded on a tape or the like through a recording circuit.
[0005]
Here, the echo signal B is generated by a feedback loop including an attenuator. That is, the
output echo signal B is attenuated to an appropriate level by the attenuator 5 and returned to the
input side, and is synthesized by the original signal A and the waveform synthesis circuit 1. Next,
it passes through the low pass filter 2 and becomes the signal D. This signal D is delayed for a
fixed time in a delay circuit (BBD) 3 to generate a delayed signal E, which is passed through a low
pass filter 4 to generate an echo signal B. The low pass filters 2 and 4 are filters provided to
prevent the occurrence of aliasing distortion, and are inserted before and after the BBD 3. In this
way, aliasing distortion is prevented by passing only frequency components of half or less of the
sampling frequency of BBD 3. In addition, the low pass filter 4 removes the noise component of
the delay signal E to become an echo signal B. By further attenuating and feedbacking the echo
signal B generated by the delay in this way, the original signal becomes an echo signal, and is
circulated while being attenuated, and this is repeated to produce a reverberation effect.
[0006]
However, since the BBD 3 configured to delay the analog signal itself is susceptible to noise and
is likely to change with age, it suffers from a lack of precision and reliability. On the other hand,
demands for improved performance of audio equipment from consumers, customers, etc. are
becoming more severe. For example, in the karaoke function, there is a demand for a dynamic
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range of about 90 dB as an audio signal to which an echo signal is added. In order to satisfy such
a requirement, recently, the delay circuit 3 is being replaced with a BBD, and is now configured
with a digital circuit capable of high performance.
[0007]
FIG. 7 is a block diagram of the delay portion when the delay circuit 3 is replaced with a digital
circuit. 31 is an A / D converter, 32 is a memory control circuit (CTRL), 33 is a memory, and 34
is a D / A converter. In this delay circuit, first, the signal D received from the low pass filter 2 is
converted into a digital value by the A / D converter 31. Then, the memory control circuit 32
having received it stores it in the memory 33 in order. Next, the memory control circuit 32
sequentially reads out the digital values stored from the memory 33 a certain period ago and
sends them to the D / A converter 34. The D / A converter 34 outputs it as a delay signal E to the
low pass filter 4 instead of analog. Adoption of such a digital circuit can prevent the influence of
noise and aging.
[0008]
However, in the delay circuit using digital circuits as described above, when it is desired to
increase the accuracy, the number of bits of digital data becomes extremely large in principle.
For example, in order to secure a dynamic range of 90 dB, since the accuracy of one bit
corresponds to about 6 dB, the number of bits of digital values handled by the circuits 31, 32, 33,
34 must be 15 bits or more. As described above, when the delay is performed by digital
processing, the number of processing bits is large, and actually, a large-capacity memory and a
high-precision A / D converter and D / A converter are required. For this reason, as a result, the
cost becomes high, and in an audio device with severe cost constraints, a delay circuit by digital
processing can not be easily adopted even though it is high precision.
[0009]
Therefore, an object of the present invention is to provide an echo addition circuit that can
secure a dynamic range equivalent to a larger number of bits while using a delay circuit
configured by digital processing of a small number of bits. . Another object of the present
invention is to provide an acoustic device which can secure a large dynamic range and is
provided with an inexpensive echo addition circuit. Furthermore, another object of the present
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invention is to provide an audio device with a karaoke function provided with an echo addition
circuit capable of securing a large dynamic range.
[0010]
SUMMARY OF THE INVENTION The configuration of the echo addition circuit of the present
invention for achieving such an object receives an audio signal and any signal of a signal
including an audio signal and an echo signal, and has a large amplitude region. An analog
compression conversion circuit for compressing any one of the signals according to a
compression function having a larger compression ratio than a compression ratio of a lower
amplitude than this, an A / D conversion circuit for converting an output signal of the analog
compression conversion circuit into a digital signal; A memory for storing a digital signal, a D / A
conversion circuit for converting a digital signal stored for a predetermined delay time in the
memory into an analog signal, and outputting the analog signal as a delay signal; And an analog
expansion conversion circuit for performing expansion conversion according to the inverse
function and outputting it as an echo signal.
[0011]
Thus, according to the present invention, first, the delay circuit is converted to a digital circuit,
and the analog signal to be delayed is compressed and converted according to, for example, a
simple increase and an upwardly convex compression conversion function. Send out.
Then, after a certain delay time has elapsed, the digital value data stored in the memory is read
out, converted back to an analog signal by the D / A conversion circuit, and this analog signal is
then expanded according to the inverse function of the compression conversion function. It is
converted and restored as an echo signal.
[0012]
Here, if a simple increase and convex function conversion is performed, the sound quality of a
large amplitude signal, that is, the sound quality of a large volume is degraded by the nonlinearity of this function, in particular, the effect of the convex characteristic. Even if digital
conversion by the number of bits is adopted, it becomes possible to convert a signal of minute
amplitude, that is, to a smaller one for a small volume without much degrading the sound quality.
In other words, the dynamic range can be improved by the amount at the expense of the large
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sound quality, as compared with the case where the compression conversion is not performed in
the digitization of the same number of bits. Moreover, stable delay processing can be performed
by converting it into a digital value and storing it in a memory. Furthermore, since the
compression conversion function is a simple increase, its inverse function can be easily obtained.
Therefore, even though this echo signal loses the sound quality at the expense of the loud volume
sound compared to the original signal, the large dynamic range can be maintained. Also, since
the small volume side is not sacrificed, the echo is beautiful. This is because when a person sings
a song, the range of the volume remains within a certain level range, and the volume is not very
large.
[0013]
The echo signal in the echo addition circuit is not the final output signal. It is always output after
being added to the original signal. For this reason, if the accuracy of the original signal is
maintained while the original signal has a large amplitude, even if the accuracy of the sound
quality of the additional signal, the echo signal, is considerably reduced, it can be heard by the
human ear. It is difficult to reach the echo side because attention is directed to the original signal.
The sound quality of the loud audio signal is less careful than that of the small audio signal when
the audio signal is output and a person listens to it.
[0014]
Therefore, in the echo addition circuit of the configuration of the present invention, compression
conversion and expansion conversion are performed while maintaining the dynamic range, and
even if the number of bits in the digital circuit is reduced, the karaoke function is obtained as the
entire output signal. The required sound quality is maintained. Therefore, the A / D conversion
circuit and the D / A conversion circuit can be simplified by using a delay circuit with a small
number of bits to ensure a dynamic range equivalent to a larger number of bits. be able to. In
addition, the memory capacity can be reduced, and an appropriate circuit can be realized as an
echo addition circuit of an audio apparatus with a karaoke function.
[0015]
DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, 9 is an echo addition circuit, 1 is a
waveform synthesis circuit, 2 and 4 are low pass filters (LPF), 5 and 6 are attenuators (ATT), and
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7 is a waveform synthesis circuit, Since the components are the same as conventional ones, they
are indicated by the same reference numerals and the description thereof is omitted. 300 is an
analog compression conversion circuit (CMP), 301 is an A / D conversion circuit, 302 is a
memory control circuit (CTRL), 303 is a memory, 304 is a D / A conversion circuit, and 305 is an
analog expansion conversion circuit (EXP). These circuits constitute a digital delay circuit.
[0016]
The analog compression conversion circuit 300 is an analog signal F from the waveform
synthesis circuit 1 (this is a signal including the original signal A and a signal in which the echo
signal B is appropriately attenuated by the attenuator 5). ) Is compressed and converted
according to the compression conversion function 300a or its approximation function to
generate an analog signal G. Here, as the compression conversion function, as shown in the
figure, the inclination at the origin (see the alternate long and short dashed line in contact with
the compression conversion function 300a) is steep, and the inclination gradually decreases. Is
used. By using this function, for example, a dynamic range of 90 dB (equivalent to 15 bits) can be
secured with 8 bits. The specific function form will be described later.
[0017]
The A / D conversion circuit 301 converts the compression-converted analog signal G into a
digital signal H. In this case, since the digital signal H is 8 bits, a circuit configuration simpler
than that of the high precision configuration requiring 15 bits as described above is sufficient.
The memory 303 is controlled by the memory control circuit 302 to sequentially store an 8-bit
digital signal H. Since the number of bits per word of the memory 303 is also 8 bits, it can be
about half that of the memory capacity of 15 bits. The memory control circuit 302 further reads
out 8-bit stored data after a predetermined delay time has elapsed since the digital signal H was
stored in the memory 303, and sends it to the D / A conversion circuit 304 as a digital signal I.
[0018]
The D / A conversion circuit 304 converts this digital signal I into an analog signal and outputs it
as a delay signal J. Since the digital signal I is also 8 bits, a simple circuit is sufficient. Also, in the
case of mere linear conversion, 8 bits are equivalent to 48 dB, but digital signal I is digitized at
the expense of local sound quality (sound quality of a large amplitude signal) by non-linear
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conversion, The dynamic range of the restored delayed signal J is 90 dB or more (about 96 dB).
[0019]
The analog expansion conversion circuit 305 expands and converts the delay signal J in
accordance with the inverse function of the compression conversion function 300 a or its
approximate function, and outputs it as an echo signal B. Here, the analog compression
conversion circuit 300 and the analog expansion conversion circuit 305 may be a circuit for
generating a normal logarithmic function or square root function using an amplifier. Therefore,
these are simple and do not complicate the overall configuration of the echo addition circuit.
[0020]
Also, in order to prevent the occurrence of aliasing, low-pass filters 2 and 4 passing only
frequency components of half or less of the sampling frequency of the delay circuit are arranged
in front of analog compression conversion circuit 300 and D / It is provided after the A
conversion circuit 304. Here, the positional relationship between the low pass filter 2 and the
analog compression conversion circuit 300 and the positional relationship between the low pass
filter 4 and the analog expansion conversion circuit 305 may be interchanged.
[0021]
Under such a circuit configuration, the signal F including the original signal A and the echo signal
B is converted into the analog compression conversion circuit 300 (CMP), the A / D conversion
circuit 301, the memory control circuit 302 (CTRL), and the memory 303. The echo signal B is
generated by being delayed for a fixed time by a delay circuit including the D / A conversion
circuit 304 and the analog expansion conversion circuit 305 (EXP). The generated echo signal B
is attenuated to an appropriate level by the attenuator 6 as in the prior art, and the signal is
added to the original signal A by the waveform synthesis circuit 7. In this manner, the original
signal A is circulated as the echo signal B while being attenuated, and is repeated to generate an
echoed audio signal C, thereby obtaining a reverberation effect. Finally, the audio signal C is
amplified and output from the speaker as sound, or recorded on a tape or the like through a
recording circuit.
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[0022]
At this time, since maintaining the dynamic range of the original signal A itself at 90 dB or more
is easily realized by processing on the circuit side that handles this signal, the dynamic range of
both the original signal A and the echo signal B becomes 90 dB or more. The dynamic range of
the audio signal C can also be 90 dB. Moreover, even if the sound quality of the echo signal is
degraded, the sound quality as a whole is maintained by the original signal A, and there are
almost no problems with the sound quality.
[0023]
Next, the simple increase and upward convex compression conversion function 300a will be
described. First, a logarithmic function that may be used in the field of digital communication and
the like will be described as a specific example, and then a square root function will be
mentioned. In order to simplify the description, the range of possible values of the analog signal
F which is an input signal of the analog compression conversion circuit 300 and the analog
signal G which is an output signal is respectively set to the minimum value “0” and the
maximum value The compression conversion function normalized as "1" will be described. Then,
this compression conversion function is limited to the function form passing through the origin
(0, 0) and the point (1, 1) as shown in FIG.
[0024]
The logarithmic function is one of simple increasing and upward convex functions, but the
function is fixed even if it is normalized by adding the restriction passing through the origin (0,
0) and the point (1, 1) as described above I will not. One degree of freedom, the inclination,
remains. Therefore, if this degree of freedom is defined by the inclination at the origin (0, 0) (see
the dashed line in FIG. 1), the logarithmic function is fixed. On the other hand, the slope at the
origin of the compression conversion function means the amplification factor of the small signal
or the minute input signal. This slope corresponds to the rate of increase with respect to the
dynamic range when linear conversion is performed with slope "1". Therefore, if compression
conversion is performed according to the steep logarithmic function of the slope at the origin, the
sound quality at high volume is sacrificed, but the dynamic range can be improved accordingly.
[0025]
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Incidentally, in the field of digital communication, “μ-255 logarithmic companding method” is
used as one of methods for obtaining a wide dynamic range with a small amount of
communication. In this method, the slope at the origin is about 16 to match the sound quality
and the dynamic range (see 300b in FIG. 1), and even in 8-bit digital communication, the
dynamic is 72 dB equivalent to 12 bits effectively. It is said that a range can be obtained.
However, this is a case where the original signal itself is compressed and reproduced. The
conditions are different from those of the echo addition circuit that generates an echo
component. That is, since the echo signal is not used alone, in the present invention, the dynamic
range is prioritized to further sacrifice the sound quality. This further increases the dynamic
range. For this purpose, as indicated by the one-dot chain line in FIG. 1, the slope of the output
signal with respect to the input signal at the origin (= output signal level / input signal level)
should be 32 or more. This saves 5 bits or more. For example, when the number of bits of the
digital signal is 8 bits and the inclination at the origin is 128 (see 300a in FIG. 1), a dynamic
range of 90 dB equivalent to 15 bits is effectively obtained.
[0026]
Next, the case where a square root function is adopted as the compression conversion function
will be described. The normalization described above uniquely determines the function form of
the square root function. That is, the slope at the origin is theoretically infinite. However, because
of the conversion to digital values, the slope through the minimum value represented by the
digital value is a substantial slope. That is the amplification factor for small and small signals.
Therefore, if the digital signal has n bits, the minimum value is (1 / (2 n)). The effective slope is
approximately (n-th power of 2) because it is a square root function. If this is considered, for
example, for the number of bits of the digital signal to be 8 bits, the effective slope is 256. That
is, when the square root function is employed, a dynamic range of 96 dB equivalent to 16 bits
can be effectively obtained.
[0027]
The specific function form of the compression conversion function is not limited to the
logarithmic function and the square root function mentioned here. It may be a cube root function
or a fourth root function. Although not practical in terms of circuit configuration, it may be a 1.5th root function. In short, in order to sacrifice the conversion accuracy on the large amplitude
side of the audio signal, it may be a simple increase and a convex function. The compression
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conversion function and the extension conversion function may be approximated by broken line
approximation or the like. The use of an approximation function often makes the circuit
configuration simpler.
[0028]
In the above embodiment, the attenuator 5 is inserted between the output of the analog
expansion conversion circuit 305 and the input of the analog compression conversion circuit
300 in order to make the comparison with the prior art clear. However, the attenuator 5 may be
connected between the output of the D / A conversion circuit 304 and the input of the A / D
conversion circuit 301. In this case, since the signal fed back locally for echo signal generation is
not subjected to extra compression and decompression, it is possible to prevent the echo signal
from being degraded by that much. Further, the attenuator 5 is constituted by a digital circuit, is
connected between the output of the memory control circuit 302 and its input, and reduces or
attenuates the output digital value by dividing, multiplying or bit shifting etc. It may be returned
to the input side and added to the input of the A / D conversion circuit 301. In this case, the
waveform synthesis circuit 1 becomes unnecessary. As a result, not only the signal fed back
locally for echo signal generation is not subjected to extra compression and expansion, but also it
is not subjected to disturbance such as crosstalk noise since it is a digital signal. As a result,
deterioration of the echo signal can be prevented.
[0029]
By the way, the echo addition circuit is used as part of a circuit for realizing the karaoke function.
Microphones used in karaoke are usually prone to noise and pick up disturbance noise. If this is
mixed into the echo addition circuit side, the echo will not be clean. Therefore, a logarithmic
amplification circuit that suppresses this type of noise will be described next with reference to
FIG. 10 is a logarithmic conversion circuit, 11 is a differential amplifier circuit of the first stage,
12 is a differential amplifier circuit of the second stage, 13 is an output stage amplifier circuit,
and 14 is a detection circuit.
[0030]
The differential amplifier circuit 11 is provided downstream of the pair of differential transistors
Q1 and Q2 whose current loads are supplied to the collectors by the transistor loads connected
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in a current mirror, and the pair of differential transistors. A current control circuit for
controlling the operating current I1 is connected in this order between the power supply line Vcc
and the ground line GND. Here, in the transistor Q1, the base receives the input signal F through
the resistor R1, and the transistor Q2 receives the predetermined reference voltage. The base and
the collector of the transistor Q1 are connected to have a diode characteristic, and the
exponential characteristic by this is used, but instead of being diode-connected as shown in FIG.
4, a resistor is inserted on the emitter side A feedback amount of exponential function may be
generated.
[0031]
The differential amplifier circuit 12 is also provided downstream of a pair of differential
transistors Q3 and Q4 which are similarly current mirror-connected, a pair of differential
transistors Q3 and Q4 whose current values are supplied to the collectors by the transistor load.
And a current control circuit for controlling the operating current I2 in this order between the
power supply line Vcc and the ground line GND. The base of the transistor Q4 receives the
collector voltage of the transistor Q1 on the side of the input signal F of the differential amplifier
circuit 11, and the base of the transistor Q3 receives the reference voltage.
[0032]
Output stage amplifier circuit 13 receives current signal F ′ output from differential amplifier
circuit 12 as an input signal, amplifies this, and converts it into a voltage signal to generate and
output output signal G. The detection circuit 14 detects the output signal G to detect its
amplitude level to generate a detection signal G '. The detected signal G 'is fed back to the current
control circuit of the differential amplifier circuit 11 as a control signal, whereby the operating
current I1 is controlled in accordance with the level of the output signal G.
[0033]
Under such a configuration, first, the input signal F, which is a voltage signal, is converted to a
current signal through the resistor R1. In the differential amplifier circuit 11 receiving this, a
voltage difference in accordance with the current signal is generated as a difference between the
base-emitter voltages of the transistors Q1 and Q2. Next, the voltage difference is converted into
a differential current by the differential amplifier circuit 12 and output as a current signal F ′.
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Then, the current signal F 'is output as an output signal G which is a voltage signal by passing
through the resistor R2 of the output stage amplifier circuit 13. At this time, the amplification
factor of the entire circuit from the input signal F to the output signal G is usually determined by
the resistance ratio between the resistors R2 and R1 and the current ratio between the currents
I2 and I1.
[0034]
Here, since the amplification factor is a current ratio between the current I2 of a constant value
and the current I1 controlled by the level of the output signal G, the whole circuit responds to the
level of its own output signal G fed back. It becomes a circuit in which its own amplification
factor changes. The change in amplification factor is that the amplification factor of the
differential amplifier circuit 11 changes exponentially with the operating current I1 and that the
operating current I1 is controlled by the feedback amount (voltage value or current value) So, it's
logarithmic.
[0035]
Also, since the resistors R1 and R2 are fixed resistors, the resistance ratio between the resistors
R2 and R1 is usually a constant value. Here, when the signal level of the input signal F becomes
minute and the level of the output signal G also becomes minute accordingly, the operation
resistance of the transistors Q1 and Q2 can not be ignored if the operation current I1 becomes
smaller. This operating resistance acts as if apparently in series with the resistance R1 with
respect to the amplification factor. For this reason, as shown in the characteristic diagram of the
origin portion in FIG. 3, a dead zone portion K is provided for the minute signal input of the input
signal F. When the signal level of the input signal F is very small, the amplitude level of the
output signal G is suppressed to a smaller value than in the original logarithmic conversion based
on the current ratio. As a result, the output signal B is generated only after the input signal F of a
certain level or more is input.
[0036]
Microphones used in karaoke usually pick up human voice. As for the voice of the person at this
time, as described above, it is unlikely to be a large amplitude signal. Moreover, it is not a very
small signal. On the other hand, the microphone picks up even a minute signal. Therefore, by
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further suppressing the amplitude level of the output signal G, disturbance noise smaller than the
voice of the person picked up by the microphone is suppressed by the characteristics shown in
FIG. As a result, it is possible to prevent noise from mixing into the echo addition circuit side and
circulating. As a result, the echo can be cleaned. FIG. 4 shows an example of another
configuration, but the operation and effect are the same as those of the above example. The
specific explanation is omitted.
[0037]
FIG. 5 shows an embodiment in which the audio apparatus of the present invention is applied to
a VTR 20 with a karaoke function. The audio signal picked up by the microphone 21 is amplified
by the input amplifier (AMP) 22 via the microphone input jack 20 a. Then, it passes through the
volume control volume 23 and is added to the echo addition circuit 9 shown in FIG. 1 as an
original signal A. The echo addition circuit 9 adds an echo signal B to the original signal A to
generate an echo-added audio signal C, and outputs this to the addition circuit 24. Reference
numeral 27 denotes an echo adjusting circuit, which can adjust the attenuator 6 of FIG. 1
manually from the outside as a variable resistor.
[0038]
On the other hand, in the video reproduction system 25, the signal of the back music portion
reproduced from the tape 25a together with the video by the rotary head 25b is inputted to the
audio processing circuit 25c and reproduced as an audio signal. This reproduction signal is
applied to the addition circuit 24. The adder circuit 24 adds and synthesizes the audio signal C
(sound signal with echo) and the back music signal, and sends this synthesized signal to the
amplifier (AMP) 26. The amplifier 26 amplifies the input signal and outputs the amplified signal
to the line output terminal 20b. The signal thus output is input to the main amplifier (AMP) 28
and output from the speaker 29 as a voice with echo + back music.
[0039]
As can be understood from the above description, in the echo addition circuit and the audio
apparatus of the present invention, compression conversion and expansion conversion are
performed while maintaining the dynamic range in accordance with the function of simple
increase and convexity. . As a result, even if the number of bits in the digital circuit for delay is
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reduced, the dynamic range is satisfied, and further, the overall quality of the final output signal,
such as the sound quality required by the original signal, is maintained. As a result, since a
dynamic range equivalent to a larger number of bits can be exhibited equivalently by using a
delay circuit with a small number of bits, the A / D converter and the D / A converter can be
simplified. The memory capacity can also be reduced.
[0040]
In addition, the proportion of memory circuits in the echo system is high, and the cost when this
is reduced can be significantly reduced. On the other hand, circuits for compression and
expansion can be provided at relatively low cost except for high-precision circuits, and waveform
synthesis circuits, attenuator circuits, etc. can be realized at a small cost if integrated with other
peripheral circuits. . Therefore, when viewed as a total, significant cost reduction is possible.
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