Electrical audio signal processing systems and devices – Sound effects
Reexamination Certificate
2001-01-12
2004-12-07
Isen, Forester W. (Department: 2644)
Electrical audio signal processing systems and devices
Sound effects
C381S098000
Reexamination Certificate
active
06829360
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method and an apparatus for expanding a band of an audio signal, capable of reproducing an audio signal pleasant to the human ear by improving the quality of a reproduced sound of an audio signal reproduced by audio equipment, and in particular, the quality of a reproduced sound of high audio frequencies. More particularly, the present invention relates to a method and an apparatus for expanding a band of an input audio signal by performing digital processing for the input audio signal.
BACKGROUND ART
The Japanese patent laid-open publication No. 9-36685 discloses an audio signal reproducing apparatus of the prior art for combining an analog audio reproduced signal with a signal having a frequency spectrum exceeding the highest audio frequency of a reproduction frequency band or the highest limit of the high audio frequency of an audible frequency band. A configuration of the audio signal reproducing apparatus is shown in FIG.
17
. Referring to
FIG. 17
, the audio signal reproducing apparatus is constituted by comprising a buffer amplifier
91
, a filter circuit
92
, an amplifier
93
, a detector circuit
94
, a time constant circuit
95
, a noise generator
96
, a filter circuit
97
, a multiplier
98
and an adder
99
.
First of all, an audio signal is inputted to the buffer amplifier
91
from an input terminal T
1
, and then, is divided into two audio signals. One divided audio signal is inputted directly to the adder
99
, whereas another divided audio signal is inputted to the filter circuit
92
which is of a high-pass filter or a band-pass filter. The filter circuit
92
band-pass-filters only a specific-band signal of the input audio signal, allows the signal to pass through the filter circuit
92
, and then, outputs the same signal to the amplifier
93
. The amplifier
93
amplifies the input audio signal to a predetermined appropriate level, and then, outputs the amplified signal to the detector circuit
94
having the time constant circuit
95
. The detector circuit
94
detects an envelope level of the audio circuit by, for example, envelope detection of the input audio signal. Then, the detector circuit
94
outputs a level signal indicative of the detected envelope level to the multiplier
98
as a level control signal for controlling a level of a noise component to be added to the original audio signal.
On the other hand, a noise component generated by the noise generator
96
is inputted to the filter circuit
97
which is of a high-pass filter or a band-pass filter. The filter circuit
97
allows passage of a noise component of a frequency band of 20 kHz or more, and then, outputs the noise component to the multiplier
98
. The multiplier
98
multiplies the input noise component by the level control signal from the detector circuit
94
, generates a noise component having a level proportional to the level indicated by the level control signal, and then, outputs the generated noise component to the adder
99
.
Furthermore, the adder
99
adds the noise component from the multiplier
98
to the original audio signal from the buffer amplifier
91
, generates the audio signal having the added noise component, and then, outputs the audio signal through an output terminal T
2
. In this case, a time constant of the time constant circuit
95
is selected so as to have a predetermined value, and this leads to adapting the noise component generated by the noise generator
96
to characteristics of the human sense of hearing and thus enhancing an effect of improving sound quality of the audio signal.
As described above, a high-frequency range is expanded by adding random noise proportional to an output level of high audio frequencies of the original audio signal to the original audio signal. However, the above-mentioned audio signal reproducing apparatus of the prior art has the following problems.
(1) The sound is unpleasant to the ear on the quality of sound since a spectral structure of a high-frequency signal of an additional noise component is different from that of a musical sound signal.
(2) Since the audio signal reproducing apparatus of the prior art comprises an analog circuit, the apparatus has the following problems. That is, the performance of the apparatus changes due to variations in parts of the analog circuit and temperature properties. Consequently, deterioration in sound quality occurs each time when an audio signal passes through the analog circuit. Moreover, improvement in precision of the filter circuit constituting the analog circuit causes an increase in the scale of the filter circuit and thus an increase in the manufacturing cost.
(3) Further, when a signal having a single spectrum such as a sinusoidal wave is inputted to the apparatus, a random noise component is added to the signal. Therefore, the measurement of signal characteristics results in marked deterioration in the signal characteristics.
DISCLOSURE OF THE INVENTION
It is an essential object of the present invention to solve the above-mentioned problems, and provide a method and an apparatus for expanding a band of an audio signal, which eliminate the unpleasantness of a sound, cause no deterioration in sound quality, cause little variation in performance of the apparatus, and reduce manufacturing cost as compared to the prior art.
It is another object of the present invention to solve the above-mentioned problems, and provide a method and an apparatus for expanding a band of an audio signal, in which the measurement of signal characteristics does not result in deterioration in a signal even if a sinusoidal signal is inputted to the apparatus.
According to the present invention, there is provided a method for expanding a band of an audio signal including the steps of:
oversampling a digital audio signal of a first band having a predetermined maximum frequency with a sampling frequency that is two or more times the maximum frequency, and low-pass-filtering an oversampled digital audio signal so as to eliminate aliasing noise caused by the oversampling, and outputting a low-pass-filtered digital audio signal;
calculating a spectrum intensity of a predetermined band of the low-pass-filtered digital audio signal, and outputting a signal indicating the calculated spectrum intensity;
generating an expanded signal having frequency components of a second band higher than the first band;
controlling a level of the expanded signal in response to the signal indicating the calculated spectrum intensity; and
adding the expanded signal having the controlled level to the low-pass-filtered digital audio signal, and outputting a digital audio signal of addition result.
In the above-mentioned method, the step of generating the expanded signal preferably includes the steps of:
distorting the digital audio signal by performing non-linear processing on the low-pass-filtered digital audio signal with a non-linear input and output characteristic, and generating a digital signal having higher harmonic components of the digital audio signal; and
high-pass-filtering at least frequency components equal to or higher than the second band, from the digital signal having the higher harmonic components, and outputting a high-pass-filtered signal as an expanded signal.
In the above-mentioned method, the step of generating the expanded signal preferably includes the steps of:
generating a dither signal having a predetermined probability distribution for an amplitude level; and
high-pass-filtering at least frequency components equal to or higher than the second band, from the dither signal, and outputting a high-pass-filtered signal as an expanded signal.
In the above-mentioned method, the step of generating the expanded signal preferably includes the steps of:
distorting the digital audio signal by performing non-linear processing on the low-pass-filtered digital audio signal with a non-linear input and output characteristic, and generating a digital signal having higher harmonic components of the digital audio signal;
high-pass-filtering at least frequency compo
Ejima Naoki
Iwata Kazuya
Sobajima Akira
Chau Corey
Isen Forester W.
Matsushita Electric - Industrial Co., Ltd.
Wenderoth , Lind & Ponack, L.L.P.
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