Electrical audio signal processing systems and devices – Binaural and stereophonic – Broadcast or multiplex stereo
Reexamination Certificate
2001-04-02
2004-03-30
Mei, Xu (Department: 2644)
Electrical audio signal processing systems and devices
Binaural and stereophonic
Broadcast or multiplex stereo
C381S003000
Reexamination Certificate
active
06714651
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an FM stereo signal demodulating apparatus and method for demodulating an (L−R) signal multiplexed on an (L+R) signal included in an FM detected signal to acquire an L signal and an R signal.
2. Description of the Related Art
In the reception of FM stereo broadcasting, when a received FM wave is FM detected, as seen from
FIG. 6A
, an (L −R) signal has been multiplexed on an (L+R) signal. In order to demodulate such a multiplexed signal to an L signal and an R signal, an FM stereo signal demodulating apparatus as shown in
FIG. 4
has been proposed.
In
FIG. 4
, the FM stereo signal demodulating apparatus includes a 19 KHz bandpass filter (BPF)
1
for extracting a signal at 19 KHz from the FM detected signal, an approximated sine wave generator
30
for generating a carrier signal at 38 KHz from the signal at 19 KHz, a multiplier
31
for multiplying the FM detected signal by the carrier signal at 38 KHz to demodulate the (L−R) signal, an adder
7
and a subtracter
8
.
The carrier signal at 38 KHz and the FM detected signal are multiplied so that the (L−R) signal is demodulated as shown in FIG.
6
B.
The signals shown in
FIGS. 6A and 6B
are subjected to addition in the adder
7
to produce an L signal of 2L. The signals shown in
FIGS. 6A and 6B
are also subjected to subtraction in the subtracter
8
to produce an R signal of 2R.
The L signal and the R signal are supplied to a speaker (not shown) in which these signals are converted into an audio signal.
The other multiplexed signal than 2L is also produced from the adder
7
and the other multiplexed signal than 2R is also produced from the subtracter
8
. However, these multiplexed signals are outside an audio frequency range and hence are not produced as sound from the speaker.
The FM stereo signal demodulating apparatus explained with reference to
FIG. 4
is operated through digital signal processing and the approximated sine wave generator
30
the details of which is shown in
FIG. 5
generates a signal at 38 KHz.
In
FIG. 5
, the approximated sine wave generator
30
includes a 90-degree phase shifter
21
for 90-degree phase shifting the phase of the 19 KHz signal, an XOR
22
for taking an exclusive OR between the 19 Hz signal and the output signal from the 90-degree phase shifter
21
, another 90-degree phase shifter
23
for 90-degree shifting the phase of the output signal from the XOR
22
, another XOR
24
for taking an exclusive OR between the output from the XOR
23
and the output from the 90-degree phase shifter
23
, an inverter
25
, still another 90-degree phase shifter
26
for 90-degree phase shifting the output signal from the inverter
25
, still another XOR
27
for taking an exclusive OR between the outputs from XOR
22
and from the 90-degree phase shifter
90
, a multiplier
32
for multiplying the output signal from the XOR
27
by {square root over (2)}−1, and an adder
33
for adding the output from the XOR
22
and the output from the multiplier
32
.
An explanation will be given of the operation of the approximated sine wave generator
30
.
When the 19 KHz signal shown in
FIG. 3A
is fed to the approximated sine wave generator
30
, the signal shown in
FIG. 3B
is produced from the 90-degree phase shifter
21
, and the signal shown in
FIG. 3C
is produced from the XOR
22
.
The signal shown in
FIG. 3D
is produced from the 90-degree phase shifter
23
, and the signal shown in
FIG. 3E
is produced from the XOR
24
.
When the output from the XOR
24
is inverted by the inverter
25
and 90-degree phase-shifted by the 90-degree phase shifter
26
, the signal shown in
FIG. 3F
is produced from the 90-degree phase shifter
26
.
The exclusive OR between the output from the XOR
22
and the output from the 90-degree phase shifter is taken by the XOR
27
so that the signal shown in
FIG. 3G
is produced from the XOR
27
.
The output from the XOR
27
is multiplied by {square root over (2)}−1 in the multiplier
32
, the signal having an amplitude of 0.414 indicated by dotted line in
FIG. 3H
is produced from the multiplier
31
.
The output from the XOR
22
and output from the multiplier
32
are added in the adder
33
so that the signal shown in
FIG. 3I
is produced from the adder
33
.
The signal shown in
FIG. 3I
has a period of 38 KHz that is twice as long as that of the 19 KHz signal and an amplitude approximated to a sine wave.
As described above, in the above conventional FM stereo signal demodulating apparatus, as shown in
FIG. 4
, the multiplier
31
multiplies the m bit signal from the approximated sine wave generator
30
and the FM-detected n bit signal.
The multiplication of m bits×n bits is carried out by adding m bits by n-times. This takes a relatively long time. In order to complete the multiplication within a prescribed time, the multiplier which can be operated at a high speed is required. This leads to an increase in the production cost.
SUMMARY OF THE INVENTION
An object of this invention is to provide an FM stereo signal demodulating apparatus and method which can be operated at a low speed.
In order to attain the above object, in accordance with a first aspect of this invention, there is provided an FM stereo signal demodulating apparatus comprising:
a filter for extracting a 19 Hz signal from an FM detected signal;
a first phase-shifter for phase-shifting the 19 Hz signal by 90 degree;
first logic circuits for taking an exclusive OR between an output from the first phase-shifter and the 19 KHz signal;
a second phase-shifter for phase-shifting by 90 degree an output signal from the first logic means;
second logic circuits for taking an exclusive OR between an output from the first logic means and an output from the second logic means;
an inverter for inverting an output from the second logic means;
a third phase-shifter for phase-shifting an output signal from the inverting means;
third logic circuits for taking an exclusive OR between an output from the third phase-shifter and an output from the first logic circuits;
a first multiplier for multiplying an output from the first logic circuit and the FM detected signal;
a second multiplier for multiplying an output from the third logic circuit and the FM detected signal;
a third multiplier for multiplying an output from the second multiplier by {square root over (2)}−1;
a first adder for adding an output from the first multiplying means and an output from the third multiplier;
a second adder for adding the FM detected signal and an output from the first adder; and
a subtracter for subtracting an output from the first adder from the FM detected signal.
Preferably, the third multiplier makes multiplication by 2
−1
instead of that by {square root over (2)}−1.
Preferably, the first multiplier is adapted so that if the output from the first logic circuits is “1”, the FM detected signal is passed, whereas if the output from the first logic circuits is “0”, the FM detected signal is inverted, or otherwise if the output from the first logic circuits is “0”, the FM detected signal is passed, whereas if the output from the first logic circuits is “1”, the FM detected signal is inverted.
Preferably, the second multiplier is adapted so that if the output from the third logic means is “1”, the FM detected signal is passed, whereas if the output from the first logic circuit is “0”, the FM detected signal is inverted, or otherwise if the output from the third logic circuit is “0”, the FM detected signal is passed, whereas if the output from the third logic means is “1”, the FM detected signal is inverted.
In accordance with a second aspect of this invention, there is also provided a method of demodulating an FM stereo signal using the apparatus according to the first aspect.
In accordance with this invention, the 2 bits produced from the approximated sine wave source and the FM detected signal are individually multiplied and thereafter the multiplied results are added. Therefore,
Mei Xu
Pendleton Brian T.
Pioneer Corporation
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