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JPH09153770

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DESCRIPTION JPH09153770
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
method of reproducing audio frequency signals (hereinafter, music and other acoustic signals are
included) recorded as digital signals such as CDs and DATs, which are cut by the conventional
reproduction method. By reproducing the component from 1/2 of the sampling frequency (SF) to
the sampling frequency (SF) together with the normal reproduction signal, it is richer than the
original music without limiting the frequency band・ It relates to the method of providing the
listener with an auditory effect that results in rich music.
[0002]
2. Description of the Related Art Since Edison invented a wax tube type phonograph, it has
become possible to record and reproduce sound signals such as voice and music. The recording
medium has changed from a disk SP record, a donut disc, and an LP record to a magnetic tape,
but these are methods for recording and reproducing waveform signals in an analog state.
[0003]
However, recently, digital recording / reproduction CDs have become widespread, and
production of LP records has substantially ceased. Digital recording and reproducing systems
have become widely used not only for CDs but also for MDs, CD-ROMs, DATs and the like.
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[0004]
In addition, digital signal transmission is increasing in satellite broadcasting and other
transmission methods. The sound played on a CD is less noisy than the sound played on a normal
LP, and there is no wah caused by the rotation of the record player. Even with relatively
inexpensive CD players, wide frequency response and clear sound quality can be obtained. Good
CD handling and sound quality can be said to be one reason for this widespread adoption.
[0005]
By the way, music by CD has recently been disputed in comparison with LP. That is, the sound
reproduced by the CD lacks richness and moisture and is inferior in musicality to the sound
reproduced by the LP. In the CD, frequency characteristics up to 20 kHz are guaranteed, and a
dynamic range of 90 dB or more is reproduced by the quantization level of 16 bits. The
frequency of 20 kHz is the upper limit of human audio frequencies.
[0006]
On the other hand, production of LP records has a long history, and great efforts have been made
to secure wide frequency characteristics and SN ratio. The recording of the master tape has also
been changed from analog to digital, but a signal with a frequency of 50 kHz or higher is
recorded, and in a carefully created LP record, a signal up to about 100 kHz is recorded. Even in
playback systems, high-quality record players and cartridges can take out recorded signals
without distortion, and the frequency characteristics of speaker systems also cover this.
[0007]
On the other hand, since the sampling frequency is 44.1 kHz in the CD, recording and
reproduction can be performed with distortion only up to half of the frequency component
according to the sampling theorem. At the time of A / D conversion, the signal is band-limited to
20 kHz in advance so as to prevent aliasing distortion due to sampling, and then converted to a
digital signal. After D / A conversion, for the same purpose, high frequency components are cut
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by a 20 kHz low pass filter (LPF) and smoothed to obtain an analog waveform signal. However,
considering that the upper limit of the human audio frequency is 20 kHz, the frequency
characteristics of the CD should be sufficient.
[0008]
About the difference between the reproduction sound of the above LP and CD, the paper by
Ohashi Akira, Nishina Emi, Fuwa Yoshitaka "On the difference in sound quality between LP and
CD-Physiological and Kansei study," Technical Report of the Institute of Electronics, Information
and Communication Engineers HC 94- 18 (June 1994) and others. According to this paper, the
sound of wide band LPs is also listened to in the same tendency as CD when band-limited to 22
kHz. On the other hand, it is concluded that the high frequency components exceeding the
audible frequency band abundantly contained in LP increase the alpha wave of the
electroencephalogram and strengthen human's emotional and physiological responses.
[0009]
[Problems to be Solved by the Invention] As mentioned above, it is said that the reproduction
sound by CD is less rich and moist than the reproduction sound of LP, and is inferior in
musicality, and the reproduction sound of this LP and CD As for the difference between the two,
it is said that high frequency components exceeding the audio frequency band abundantly
contained in LP promote human's emotional and physiological responses, and give richness and
moisture to music.
[0010]
The present invention has been made to solve the above problems, and for example, by
simultaneously presenting a signal of 20 kHz or more related to this signal together with a
reproduction signal of 20 kHz or less of CD, a wide band LP It is an object of the present
invention to provide a method of reproducing a digital audio frequency signal that can be heard
as a rich and moist sound similar to the reproduced sound.
[0011]
According to the present invention, there is provided a method of reproducing digitized audio
frequency signals comprising: an audio frequency signal sampled and recorded as a digital signal
such as CD, DAT or the like; or an audio frequency transmitted by digitization. In a method of
reproducing a signal, an analog waveform signal (A) smoothed by a low pass filter (LPF) having a
cutoff frequency of 1/2 or less of a sampling frequency (SF) after D / A conversion of the
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sampled signal And an analog waveform signal (B) obtained through a band pass filter (BPF)
having a passband from a half of sampling frequency (SF) to the sampling frequency (SF) of the
sampled signal It is a method of reproducing a digitized audio frequency signal that reproduces a
richer or richer audio frequency signal by reproducing.
[0012]
In the method of reproducing an audio frequency signal sampled and recorded as a digital signal
such as a CD or DAT, or an audio frequency signal transmitted by digitizing, usually when the
original sound is digitized, the cutoff frequency of the original sound is sampled first After
passing through a low pass filter (LPF) of 1/2 or less of the frequency, sampling is performed
using a sampling signal (SF).
For example, in the case of a music CD, the sampling frequency is 44.1 kHz, and the cutoff
frequency of the LPF is 20 kHz.
[0013]
Therefore, the digital signal obtained by A / D converting the original sound does not include the
component of 20 kHz or more of the original sound.
However, as shown in FIG. 1 (c), in the above A / D converted digital signal, a spectrum like the
original sound 0 to 20 kHz spectrum folded at sampling occurs infinitely at each sampling
frequency.
Normal CD reproduction uses D / A conversion of the sampled signal and then uses only an
analog signal smoothed by a low pass filter (LPF) whose cutoff frequency is 1/2 or less of the
sampling frequency as reproduced sound There is. Therefore, finally, the signal is reproduced
from only 0 to 20 kHz of the original sound.
[0014]
According to the present invention, from the half of the sampling frequency (SF) within the
spectrum of the original sound that is newly produced at the time of the above sampling, such as
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the 0 to 20 kHz signal that is normally reproduced. The spectrum generated in the pass band up
to the sampling frequency (SF) is subjected to D / A conversion and added to virtually output a
spectrum close to the original sound spectrum.
[0015]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The principles and methods of the
present invention will be described with reference to the drawings.
FIGS. 1 and 2 are frequency spectra of processing steps relating to the method of reproducing
digitized voice frequency signals of the present invention. FIG. 3 shows a first embodiment of the
method for reproducing a digitized voice frequency signal according to the present invention.
[0016]
1 in FIG. 1 (a) shows an outline of the frequency spectrum of musical tones of music such as
orchestra. It is known that the signal exists to a high frequency band beyond the 20 Hz to 20 kHz
which is an audio frequency band. Normal voice occupies a frequency band of about 60 Hz to 10
kHz, but here, all musical tones, noises, mechanical sounds, etc. are collectively referred to as
voice frequency signals. In FIG. 1B, 2 is a frequency spectrum when the frequency band of the
voice frequency signal of the above 1 is limited to a maximum frequency of 20 kHz by a low pass
filter (LPF). This maximum frequency (20 kHz) corresponds to the upper limit of human audio
frequencies. Therefore, in a system that digitizes and records or transmits audio frequency
signals such as CDs and satellite broadcasts, the frequency band is limited in this way so that the
highest audio frequency can be reproduced.
[0017]
FIG. 1C shows the frequency spectrum of a digital signal when the audio frequency signal having
the frequency spectrum of 2 above is sampled at the sampling frequency SF. Here, the sampling
frequency is illustrated as 44.1 kHz used in CD. The frequency spectrum of the sampled signal is
folded symmetrically at SF / 2 (FIGS. 3 and 4), and is also symmetrical with respect to SF, and the
spectrum exists at SF intervals up to theoretically infinity.
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[0018]
The sampled signal is D / A converted by a D / A converter as shown in FIG. 3, but the frequency
spectrum of the analog waveform signal at its output is also almost the same as in FIG. 1 (c).
Usually, for this signal, a signal at a frequency of SF / 2 or less may be extracted. For this
purpose, as indicated by 5 in FIG. 1D, processing (smoothing) is performed by the LPF whose
amplitude characteristic of the required frequency band is flat. The frequency spectrum after
processing is 6 in FIG. 1 (e), which is the same as the voice frequency signal 2 before digitization.
This is called a reproduction signal A. The processing so far is the same as the method of
converting a normal sampled signal into an analog waveform signal.
[0019]
On the other hand, there is provided a band pass filter (BPF) whose passband is a frequency band
between the sampling frequency (SF) and its half SF / 2, and an example of its characteristic is
shown in FIG. It is referred to as 7) in the figure. When the signal of FIG. 1 (c) is processed by the
BPF 1, the frequency spectrum of its output is 9 in FIG. 2 (b). This is an analog waveform signal
having a frequency spectrum that is symmetrical to the voice frequency signal 2 shown in FIG. 1
(b).
[0020]
This signal is perfectly correlated with the 2 speech frequency signal, and both the intensity and
the spectral spread depend on the 2 speech frequency signal. This signal is the reproduction
signal B. When the reproduction signal B is reproduced using a dedicated amplifier and tweeter,
and combined with the reproduction signal A, an acoustic output (sound field) as shown in FIG. 2
(d) is obtained.
[0021]
Usually, the tweeter's frequency characteristic above 20 kHz is not so good, and such a frequency
range far exceeds human's audible range, so the audio frequency signal of FIG. 2 (d) is inputted
to the reproduction system. However, it is more preferable to approach the original sound
spectrum as follows.
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[0022]
That is, the sound output (sound field) shown in FIG. 2D is slightly different in intensity in the
high frequency band from the frequency spectrum (original sound) of 1 described above.
Usually, it is said that musical tones, voices, natural sounds, etc. have a so-called (1 / f)
characteristic in which they are attenuated in inverse proportion to the frequency f. Therefore,
considering these, the characteristic of the BPF is either the (1 / f2) characteristic or the high
frequency It is desirable for the characteristic to be damped by The example of BPF used for that
is "BPF2" (8 of the same figure) of Fig.2 (a). When the signal of FIG. 1 (c) is processed by BPF 2,
the frequency spectrum of the reproduction signal B is as 10 in FIG. 2 (c), and the acoustic output
(sound field) combined with the reproduction signal A is FIG. )become that way. This frequency
spectrum is similar to the frequency spectrum of the above-mentioned 1 audio signal (original
sound). Compared with the signal of the frequency spectrum in Fig. 1 (e) after limiting the
frequency band and digitizing it, it is much closer to the original sound 1, and the richness of the
sound, the sense of presence, etc. I can listen to it.
[0023]
Even if a signal of 20 kHz or more is presented alone, human beings can not perceive it. As is
apparent from the nature of this hearing and the above-mentioned research results, when a
signal of 20 kHz or more related to this signal is simultaneously presented together with the
reproduction signal of 20 kHz or less of CD, it is similar to the reproduced sound of wide band
LP. It can be heard as a rich sound.
[0024]
Also, the characteristics of the tweeter are directional and not flat at high frequencies. And, since
the reproduction signal B is not in the audible frequency band, it is not always necessary to
reproduce the frequency characteristics close to the original sound. Therefore, although the
frequency range of BPF is from SF / 2 to SF, the upper limit frequency may be any frequency
higher than SF. Also, strictly speaking, the signal output reproduced by the present invention
does not necessarily match the spectrum of the original sound in the frequency range of the BPF
from SF / 2 to SF, but this frequency region is originally inaudible to the human ear There is no
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particular difference in the effect because the richness and the richness of the sound are
considered to be sufficiently generated by the amplitude fluctuation correlated with the audible
area.
[0025]
A second embodiment of the present invention is shown in FIG. 4. The method of the second
embodiment is the same as the method of the first embodiment until the reproduced signal A and
the reproduced signal B are obtained. In the configuration of FIG. 3, the sound outputs are
separated for A and B, but here, the reproduction signal A and the reproduction signal B are
added together by the summing circuit 14 and then amplified and reproduced with a wide band
speaker system.
[0026]
The present invention has been described by taking the case where the sampling frequency most
frequently used is 44.1 kHz as an example, but in order to reproduce a high frequency range of
20 kHz or more, for example It is also possible to play in the usual way. However, changing the
sampling frequency results in the construction of a new system, which requires energy for
development and is costly. The present invention makes it possible to easily reproduce rich and
moist music using digital reproduction technology that has already been established.
[0027]
Although the present invention has been described by taking the reproduction of a CD as an
example, it is common as a reproduction method of an audio frequency signal sampled similarly
to the CD. If SF / 2 is higher than the audio frequency band, the scheme of the present invention
is applicable. It can be widely applied as a reproduction system for MD, DAT, satellite
broadcasting, and future digital broadcasting.
[0028]
As described above, according to the present invention, the sampled speech frequency signal
limited to the audio frequency band is reproduced as a signal including a much higher frequency
component exceeding the audio frequency. It can be heard as a realistic sound.
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[0029]
The present invention makes it possible to easily reproduce rich and moist music using digital
reproduction technology that has already been established.
[0030]
Brief description of the drawings
[0031]
1 is a diagram showing the frequency spectrum of the process of the method for reproducing the
digitized voice frequency signal of the present invention.
[0032]
2 is a diagram showing the frequency spectrum of the process of the method for reproducing the
digitized voice frequency signal of the present invention.
[0033]
3 is a block diagram of a method of reproducing the digital audio frequency signal of the present
invention.
[0034]
4 is a block diagram of a method of reproducing the digital audio frequency signal of the present
invention.
[0035]
Explanation of sign
[0036]
1 frequency spectrum of audio frequency signal (original sound) 2 frequency spectrum of bandlimited audio frequency signal 3, 4 frequency spectrum when sampling 2 above 5 frequency
characteristic 6 of low-pass filter 6 above 3 and 4 signals Frequency spectra 7 and 8 when
processed with LPF 5 Frequency characteristics 9 of band filter 9 9 signals with frequency
spectra 10 3 and 4 obtained when BPF 1 of 7 is processed with BPF 2 of 8 Frequency spectrum
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