Electrical audio signal processing systems and devices – Binaural and stereophonic – Broadcast or multiplex stereo
Reexamination Certificate
2001-04-02
2004-11-23
Mei, Xu (Department: 2644)
Electrical audio signal processing systems and devices
Binaural and stereophonic
Broadcast or multiplex stereo
C381S003000
Reexamination Certificate
active
06823071
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a frequency modulation (hereinafter, FM) multiplex demodulation device, by which a (L−R) signal multiplexed by a subcarrier at 38 kHz is demodulated to a (L+R) signal contained in a composite signal (FM detected signal) so that a L signal and R signal are obtained.
(2) Description of the Related Art
As to a FM stereo broadcast, a (L−R) signal is demodulated to a (L+R) signal by a subcarrier at 38 kHz in a FM-detected baseband as shown in FIG.
5
A.
So far, a FM multiplex demodulation device shown in
FIG. 6
has been used to obtain a L signal and R signal by demodulating such a multiplexed signal.
As shown in
FIG. 6
, the conventional FM multiplex demodulation device consists of a demodulator
50
for demodulating the multiplexed (L−R) signal shown in
FIG. 5A
, a noise detecting section
51
for detecting a noise level contained in the composite signal, an attenuator (ATT)
52
for attenuating the demodulated (L−R) signal, an adding section
53
, and a subtracting section
54
.
When the composite signal is received, in a state that a multipass fading exists, noise in a FM-detected output increases, the noise detecting section
51
detects the noise level and changes an attenuation quantity by the attenuator
52
in response to the detected noise level. Therefore, when the noise is not detected, the attenuation quantity by the attenuator
52
becomes to be zero.
As to the demodulator
50
, when the subcarrier at 38 kHz is added to the signal shown in
FIG. 5A
, the (L−R) signal shown in
FIG. 5B
is demodulated.
At the adding section
53
, the signal shown in
FIG. 5B
is added to the signal shown in
FIG. 5A
to obtain 2L signal, while the signal shown in
FIG. 5B
is subtracted from the signal shown in
FIG. 5A
to obtain 2R signal, each of which is input into a speaker (not shown in the figure).
The added or subtracted output contains multiplexed subcarrier components, which are not in an audio frequency band and not reproduced from a speaker.
When a noise is detected at the noise detecting section
51
, an output at the adding section
53
is equal to
(
L+R
)+
k
′(
L−R
)=(1+
k
′)
L
+(1−
k
′)
R
and an out put at the subtracting section
54
is equal to
(
L+R
)−
k
′(
L−R
)=(1−
k
′)
L
+(1+
k
′)
R
where k′ (0≦k′≦1) is the attenuation quantity at the attenuator
52
. Consequently, it takes effect a reduction in multipass noise, however a feeling of stereo deteriorates at the same time, causing a deterioration of the tone quality.
SUMMARY OF THE INVENTION
It is therefore an objective of the present invention to solve the above problem and to provide a FM multiplex demodulation device, in which the feeling of stereo is maintained excellent and the offensive noise to the ears can be reduced.
In order to attain the above objective, the present invention is to provide a FM multiplex demodulation device comprising: a filter for selecting a signal multiplexed by a subcarrier at 38 kHz from a composite signal; a noise detecting section for detecting noises from the composite signal; a ratio changing section for changing a composition ratio between an output signal from the filter and the composite signal, and for outputting a composed signal, the composition ratio being determined in response to the noises detected by the noise detecting section; a demodulating section for demodulating the multiplexed signal with multiplying the output signal from the ratio changing section by the subcarrier; an adding section for adding an output signal from the demodulating section to the composite signal; and a subtracting section for subtracting the output signal from the demodulating section from the composite signal.
The FM multiplex demodulation device further comprises a delaying section for delaying the composite signal for a period of time corresponding to a delay time of the filter, wherein the delayed signal instead of the composite signal is input to the ratio changing section, the adding section and the subtracting section.
The filter filters out a part of a multiplexed signal.
The filter consists of a second-order band-pass infinite impulse response (hereinafter IIR)-type digital filter, a sampling frequency of the digital filter is set to be six times of a center frequency of the pass band, a first-order input feed back coefficient b
1
is set as b
1
=−1+2
−n
, while a second-order input feed back coefficient b
2
is set as b
2
=1−2
−(n−1)
, where n is 3 or an odd number more than 3.
A zero-order output coefficient a
0
is set as a
0
=2
−n
and a second-order output coefficient a
2
is set as a
2
=−2
−n
.
A second-order output is subtracted from a zero-order output and the resultant output is multiplied by 2
−n
.
REFERENCES:
patent: 4032717 (1977-06-01), Mallon
patent: 5068896 (1991-11-01), Short
patent: 5740523 (1998-04-01), Nakajima et al.
Mei Xu
Michalski Justin
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