FMX stereophonic broadcast receiver

An improved FMX sterophonic broadcast receiver provided with countermeasures against transient noises, which includes a level detection circuit for detecting level of a detected stereo difference signal, and a level control circuit for controlling level of an expanded stereo difference signal according to an output signal of the level detection circuit. By the above arrangement of the present invention, since it is so arranged to cause the level control circuit to function when the degree of modulation becomes large so as to control the level of the stereo difference signal to be low, a level difference is produced between the stereo sum signal and the stereo difference signal for deterioration of the stereo separation degree upon matrixing at a matrix circuit for reduction of noises accordingly.

BACKGROUND OF THE INVENTION 
The present invention generally relates to a radio receiver for receiving 
an FMX stereophonic broadcasting and more particularly, to an FMX 
stereophonic broadcast receiver provided with countermeasures against 
transient noises. 
FMX stereophonic broadcasting has been proposed as one means for expanding 
the service area and improving the signal-to-noise ratio characteristics 
of FM stereophonic broadcasts. The transmission signal of the 
aforementioned FMX stereophonic broadcast includes a compressed stereo 
difference signal (L+R)' broadcast simultaneously with the transmission 
signal of conventional FM stereophonic broadcasting, for example, a stereo 
sum signal (L+R) and stereo difference signal (L-R). The transmission 
signal can be expressed as: 
EQU f(t)=(L+R)+Psin(.omega./2)t+(L-R)sin.omega.t+(L-R)'cos.omega.t(1) 
where L+R is a stereo sum signal, L-R is a stereo difference signal P, is a 
stereo pilot signal, and .omega. is the subcarrier angular frequency. As 
shown by the aforementioned Equation (1), compressed stereo difference 
signal (L-R)' is quadrature modulated from uncompressed stereo difference 
signal (L-R), resulting in an FMX stereophonic broadcast transmission 
signal spectrum shown in FIG. 4. 
Furthermore, the relationship between the uncompressed stereo difference 
signal (L-R) and the compressed stereo difference signal (L-R)' is as 
shown in FIG. 5 which expresses the compression characteristics. In FIG. 
5, when the input signal level is low, the aforementioned signal (L-R)' is 
20 dB greater than the uncompressed stereo difference signal (L-R) and, at 
the same time, input/output characteristics become linear, and also the 
compression ratio becomes 1 :1. When the level of the input signal is 
medium (approximately -30 dB), the compression ratio becomes .infin.:1, 
and input/output characteristics are flat over a range of approximately 10 
dB. When the input signal level becomes high, the aforementioned signal 
(L-R)' rapidly attenuates. Therefore, compressed stereo difference signal 
(L-R)' is as shown by solid line (b) in FIG. 5 with respect to stereo 
difference signal (L-R) (solid line (a)), and the sum signal of the 
aforementioned signal (L-R) and the aforementioned signal (L-R)' is as 
shown by dotted line (c) in FIG. 5. 
The transmission signal for FMX stereophonic broadcasting as described 
above is received by a conventional receiver as shown in FIG. 7. In FIG. 
7, the FMX stereophonic broadcast transmission signal received by antenna 
1 is received by a receiving circuit 2 of the same construction as a 
conventional FM stereophonic receiver in which stereo sum signal (L+R) 
(hereinafter referred to as M), stereo difference signal (L-R) 
(hereinafter referred to as S), and compressed stereo difference signal 
(L-R)' (hereinafter referred to as S') are each demodulated. When the 
received signal is detected by the FM detection circuit (not shown) 
included in the receiving circuit 2, the stereo sum signal M is 
demodulated. When the stereo composite signal is subjected to synchronous 
detection through employment of the 38-KHz subcarrier signal obtained from 
a PLL in the receiving circuit 2, uncompressed stereo difference signal S 
is demodulated. And when the stereo composite signal is subjected to 
quadrature detection, the compressed stereo difference signal S' is 
demodulated. 
The uncompressed and compressed stereo difference signals S and S' obtained 
from the receiving circuit 2 are added by an adder 3, and the result is 
supplied to a VCA (voltage control amplifier) 4 operating as an 
attenuator. When the stereo difference signal S and output signal (S+S') 
of the VCA 4 are greater than a specified level (a knee-point level), 
first and second level detection circuits 5 and 6, each having a threshold 
level, operate in such a manner that the level of stereo difference signal 
S and the level of the aforementioned output signal (S+S') of the VCA 4 
are respectively detected by the first and second level detection circuits 
5 and 6, and are compared by a comparison circuit 7. Next, a signal 
according to the level difference obtained from the aforementioned 
comparison circuit 7 is rectified and smoothed by a rectifying circuit 8, 
and the rectified signal is applied to the VCA 4 as a control signal. The 
output signal (S+S') of the aforementioned VCA 4 is controlled by this 
control signal to be equal to the level of the stereo difference signal S. 
However, when the aforementioned stereo difference signal S and output 
signal (S+S') of the VCA 4 are below the knee-point level, the first and 
second level detection circuits 5 and 6 do not operate, and attenuation at 
the VCA 4 is fixed at approximately 20 dB. 
Although the stereo sum signal M obtained from the receiving circuit 2 is 
applied directly to a matrix circuit 9, and the stereo difference signal S 
and output signal (S+S') of the VCA 4 are selected by a switch 10, and 
applied to the matrix circuit 9. A 10Hz ID signal is included in the FMX 
stereophonic broadcast transmission signal, and FMX stereophonic 
broadcasts are differentiated from conventional FM stereophonic broadcasts 
by the aforementioned ID signal. In addition, because an ID detection 
circuit (not shown) which detects the aforementioned ID signal is 
incorporated in the receiving circuit 2, whether the broadcast is FMX 
stereo or not can be determined by the output signal of the ID detection 
circuit. The switch 10 is controlled by the aforementioned ID signal. When 
the ID signal (such as a HIGH level signal) is present, the switch 10 is 
switched to a position as shown in FIG. 7. Accordingly, the stereo sum 
signal M and output signal (S+S') from the level controlled VCA 4 are 
matrixed, and left and right stereo signals L and R are generated at left 
and right output terminals 11 and 12. Furthermore, when the ID signal is 
not present, the switch 10 is switched to a position opposite to that 
shown in FIG. 7, and stereo sum signal M and stereo difference signal S 
are matrixed in the matrix circuit 9. As described above, because FMX 
stereophonic broadcast system uses compressed and expanded stereo 
difference signal S, it is possible to achieve significant improvements in 
the S/N ratio, and the service area can be expanded generally equal to 
that of the conventional monaural FM broadcast system. 
It is to be noted that the FMX stereophonic broadcast transmission signal 
can be accurately received by a conventional FM stereophonic receiver. In 
this case, the compressed stereo difference signal S' is quadrature 
modulated with respect to stereo difference signal S, and reception is not 
adversely affected. 
Details concerning FMX stereophonic broadcasting are disclosed, for 
example, in an article "Improving the Signal-to-Noise Ratio and Coverage 
of FM Stereophonic Broadcasts" by Emil L. Torick and Thomas B. Keller in 
"JOURNAL OF THE RADIO ENGINEERING SOCIETY", volume 33, number 12, issued 
December 1985. 
Incidentally, in the reception of the FMX stereophonic broadcasting, the 
noise level is raised by approximately 20 dB at the maximum in the degree 
of modulation above the knee point (point A) as shown by a solid line (a) 
in FIG. 6, and there has been a problem that such rising of the noise 
level tends to be offensive to the ear, thus spoiling pleasant feeling in 
listening during reception of a signal having a level low on the average 
but increasing temporarily, e.g. playing sound of a piano or the like. 
Meanwhile, it is known that the noise level of a receiving signal increases 
as the field strength of said receiving signal becomes weak. Therefore, 
influence of the noise level becomes remarkably larger as the field 
strength becomes weaker, with respect to the variation of the noise level 
corresponding to the degree of modulation referred to earlier. 
SUMMARY OF THE INVENTION 
Accordingly, an essential object of the present invention is to provide an 
improved FMX stereophonic broadcast receiver which is provided with 
countermeasures against transient noises, with substantial elimination of 
disadvantages inherent in the receivers of this kind. 
Another object of the present invention is to provide an FMX stereophonic 
broadcast receiver of the above described type which is simple in 
construction and reliable in functioning. 
In accomplishing these and other objects, according to one preferred 
embodiment of the present invention, there is provided a receiver which is 
arranged to receive an FMX stereophonic broadcasting signal including a 
stereo sum signal, a stereo difference signal and a compressed stereo 
difference signal modulated in a quadrature relation with respect to said 
stereo difference signal, and comprises a synchronous detection circuit 
for detecting the stereo difference signal in an FM detection output 
signal, a quadrature detection circuit for detecting the compressed stereo 
difference signal in said FM detection output signal, an expansion circuit 
for obtaining an expanded signal through employment of output signals of 
said synchronous detection circuit and said quadrature detection circuit, 
a level detection circuit for detecting level of the output signal of said 
synchronous detection circuit, and a level control circuit for controlling 
level of the output signal of said expansion circuit according to the 
output signal of said level detection circuit. The FMX stereophonic 
broadcasting signal receiver is adapted to effect control of stereophonic 
separation degree by controlling the level of the output signal of said 
expansion circuit according to the degree of modulation of the FMX 
stereophonic broadcasting signal. 
By the above arrangement of the present invention, since it is so arranged 
to cause the level control circuit to function when the degree of 
modulation becomes large so as to control the level of the stereo 
difference signal to be low, a level difference is produced between the 
stereo sum signal and the stereo difference signal for deterioration of 
the stereo separation degree upon matrixing at the matrix circuit, and 
thus, the noises may be reduced according thereto. 
In another aspect of the present invention, there is provided a receiver 
which is arranged to receive an FMX stereophonic broadcasting signal 
including a stereo sum signal, a stereo difference signal and a compressed 
stereo difference signal modulated in a quadrature relation with respect 
to said stereo difference signal, and comprises a first signal generation 
circuit for generating an output signal corresponding to field strength of 
the receiving signal, a first demodulation circuit for demodulating the 
stereo difference signal in the FM detection output signal, a second 
demodulation circuit for demodulating the compressed stereo difference 
signal in the FM detection output signal, an expansion circuit for 
obtaining an expanded signal through employment of the output signals of 
said first and second demodulation circuits, a second signal generation 
circuit for generating an output signal corresponding to level of the 
output signal of said first demodulation circuit, a level control circuit 
for controlling level of the stereo difference signal according to the 
output signals of said first and second signal generation circuits, and a 
stereo demodulation circuit for generating left and right stereophonic 
signals through employment of a stereo sum signal in the FM detection 
output signal and the stereo difference signal subjected to the level 
control by said level control circuit. 
In the above arrangement of the present invention, since the level of the 
stereo difference signal is controlled according to the field strength of 
the receiving signal and the degree of modulation of the demodulated 
stereo difference signal, it is possible to reduce the noise to a large 
extent by lowering the level of the stereo difference signal when the 
degree of modulation is large and the field strength is weak. Meanwhile, 
during strong field strength, the noise is small and thus, deterioration 
in the degree of stereophonic separation may be prevented without 
effecting level control of the stereo difference signal even when the 
degree of modulation becomes large.

DETAILED DESCRIPTION OF THE INVENTION 
Before the description of the present invention proceeds, it is to be noted 
that like parts are designated by like reference numerals throughout the 
accompanying drawings. 
Referring now to the drawings, there is shown in FIG. 1 an FMX stereophonic 
broadcast receiver according to one preferred embodiment of the present 
invention. 
The FMX stereophonic broadcast receiver of FIG. 1 generally includes an FM 
detection circuit 13 for subjecting an FMIF signal applied to an input 
terminal 14 to FM detection, a synchronous detection circuit 15 for 
subjecting a stereo difference signal S contained in an output signal of 
said FM detection circuit 13 to synchronous detection, a quadrature 
detection circuit 16 for subjecting a compressed stereo difference signal 
S' in said output signal to quadrature detection, an expansion circuit 17 
for effecting expansion of a sum signal (S+S') in which the output signal 
S of the synchronous detection circuit 15 and the output signal S' of the 
quadrature detection circuit 16 are added to each other, a level detection 
circuit 18 including a diode 19, a capacitor 20 and a resistor 21 for 
detecting the level of the stereo difference signal S, a level control 
circuit 22 for controlling the level of the output signal (S+S') of said 
expansion circuit 17 according to the output signal of said level 
detection circuit 18, a switch 23 for selecting either one of the output 
signal S of the synchronous detection circuit 15 or the output signal 
(S+S') of the level control circuit 22 according to ID signals, and a 
matrix circuit 24 for matrixing a stereo sum signal (L+R) contained in the 
output signal of the FM detection circuit 13 and the stereo difference 
signal obtained at the output side of the switch 23 so as to provide left 
and right stereo signals at left and right output terminals 25 and 26, all 
of which are connected to each other as shown. 
It is to be noted here that the expansion circuit 17 described above 
represents in a simplified form the adder 3, voltage control amplifier 4, 
level detection circuits 5 and 6, comparison circuit 7, and rectifying 
circuit 8 referred to earlier in the conventional circuit of FIG. 7. 
When the degree of modulation of the receiving signal becomes large, the 
level of the output signal S of the synchronous detection circuit 15 also 
becomes high, and the level of said output signal S is detected by the 
level detection circuit 18. Thus, since DC voltage obtained at one end of 
the capacitor 20 of said level detection circuit 18 is applied to the 
level control circuit 22 as a control signal, the level of the expanded 
stereo difference signal (S+S') to be obtained by the expansion circuit 17 
is suppressed when the degree of modulation is large. The stereo 
difference signal (S+S') subjected to the level control is applied to the 
matrix circuit 24 through the switch 23 so as to be matrixed with the 
stereo sum signal M. In the case where the degree of modulation of the 
receiving signal is large, the level of the stereo sum signal M becomes 
larger than that of the stereo difference signal (S+S'), with consequent 
lowering of the degree of stereophonic separation, thus making it possible 
to reduce the noise by that extent. Accordingly, if the circuit of FIG. 1 
is employed, undesirable rising of the noise level may be advantageously 
prevented even when the degree of modulation becomes large during 
reception of the FMX stereophonic broadcasting. 
Reference is made to FIG. 2 showing an FMX stereophonic broadcast receiver 
according to a second embodiment of the present invention. 
In the construction of FIG. 2, the level detection circuit 18 including the 
diode 19, capacitor 20 and resistor 21, and the level control circuit 22 
in the circuit of FIG. 1 are replaced by a level detection circuit 27 for 
detecting the level of the output signal S of the synchronous detection 
circuit 15 and a level control circuit 28 for controlling the level of 
said output signal S and the output signal of the level control circuit 28 
is applied to a control circuit 29 inserted between said level control 
circuit 28 and the expansion circuit 17 as shown so as to control the 
expanding function at the expansion circuit 17, while other circuit 
constructions are generally similar to those in the first embodiment of 
FIG. 1. 
In the case where the level of the output signal S of the synchronous 
detection circuit 15 is low, the level control at the level control 
circuit 28 is not effected, and the output signal S is applied to the 
control circuit 29 as it is for comparison with the level of the sum 
signal (S+S'). Therefore, normal expansion is effected at the expansion 
circuit 17, and the level of the sum signal (S+S') becomes equal to the 
level of the stereo sum signal M, so that the degree of stereophonic 
separation at the matrix circuit 24 is not deteriorated. When the output 
signal of the synchronous detection circuit 15 becomes large according to 
the signal having the degree of modulation above the knee point, the level 
control circuit 27 is actuated, and the level of the stereo difference 
signal S is lowered. Accordingly, the level of the output signal (S+S') of 
the expansion circuit 17 is also lowered to correspond to the level of the 
stereo difference signal S so as to be lower than the level of the stereo 
sum signal M. Therefore, degree of stereophonic separation of the matrix 
circuit 24 is deteriorated to reduce noises for the improvement. 
Referring further to FIG. 3, there is shown an FMX stereophonic broadcast 
receiver according to a third embodiment of the present invention, which 
generally includes an input terminal 43 to which an IF (intermediate 
frequency) signal is applied, an IF amplification circuit 44 for 
amplifying the IF signal, an FM detection circuit 45 for subjecting the 
amplified IF signal to FM detection, a first demodulation circuit 46 for 
demodulating the stereo difference signal S in the FM detection output 
signal, a second demodulation circuit 47 for demodulating the compressed 
stereo difference signal S' in the FM detection output signal, an addition 
circuit or adder 48 for adding the output signals of said first and second 
demodulation circuits 46 and 47, a voltage control amplifier (VCA) 49 for 
expanding the output signal of said addition circuit 48, a level detection 
circuit 50 for generating control signal to control gains of the voltage 
control amplifier 49 by comparing the output signal level of the first 
demodulation circuit 46 with the output signal level of said voltage 
control amplifier 49, a change-over circuit 51 for changing over between 
the output signal of said first demodulation circuit 46 and the output 
signal of said voltage control amplifier 49 for output, a level control 
circuit 52 for further subjecting the output signal of said voltage 
control amplifier 49 to level control, a matrix or stereo modulation 
circuit 53 which matrixes the stereo sum signal M obtained by the FM 
detection circuit 45 and the output signal of said change-over circuit 51 
for producing left and right stereo signal at left and right output 
terminals 54 and 55, a first signal generation circuit 56 including a 
detection diode 57, a smoothing capacitor 58 and an inverting amplifier 59 
for producing an output signal corresponding to the level of the IF 
signal, a second signal generation circuit 60 including a detection diode 
61 and a smoothing capacitor 62 for producing an output signal 
corresponding to the level of the stereo difference signal S obtained at 
the output end of said first demodulation circuit 46, and an addition 
circuit or adder 63 for adding the output signals of said first and second 
signal generation circuits 56 and 60, all of which are coupled to each 
other as illustrated, with the output signal of said addition circuit 63 
being arranged to be applied to the level control circuit 52 as a control 
signal. 
Subsequently, functioning of the FMX stereophonic broadcast receiver in 
FIG. 3 as described above will be explained hereinafter. 
The signal received by an antenna (not shown) is converted into the IF 
signal, and after having been amplified by the IF amplification circuit 
44, is subjected to FM detection by the FM detection circuit 45. The 
stereo sum signal M in the FM detection output signal is applied to the 
stereo demodulation circuit 53. In the case where the receiving signal is 
based on the FM stereophonic broadcasting, the change-over circuit 51 is 
brought into the state opposite to that shown in FIG. 3, and the stereo 
difference signal S demodulated by the first demodulation circuit 46 is 
applied to the stereo demodulation circuit 53 through the change-over 
circuit 51. Therefore, in the stereo demodulation circuit 53, the stereo 
sum signal M and the stereo difference signal S are matrixed to provide 
the left and right stereophonic signals L and R on the left and right 
output terminals 54 and 55. 
Meanwhile, in the case where the receiving signal is based on the FMX 
stereophonic broadcasting, the change-over circuit 51 takes the position 
as shown in FIG. 3, and the stereo difference signal S demodulated at the 
first demodulation circuit 46 and the compressed stereo difference signal 
S' demodulated at the second demodulation circuit 47 are added to each 
other by the addition circuit 48. The sum signal (S+S') thus obtained is 
subjected to level control at the voltage control amplifier 49, and is 
applied to the stereo demodulation circuit 53 through the change-over 
circuit 51 and the level control circuit 52. Therefore, in the 
stereophonic demodulation circuit 53, the stereo sum signal and the 
expanded stereo difference signal (S+S') are matrixed to provide the left 
and right stereophonic signals L and R at the left and right output 
terminals 54 and 55. It is to be noted here that, since the change-over 
circuit 51 is automatically changed over by an identification signal of 
10Hz contained in the FMX stereophonic broadcasting signal, the circuit of 
FIG. 3 is capable of receiving the FM stereophonic broadcasting and FMX 
stereophonic broadcasting through automatic ahange-over. 
When the field strength of the receiving signal is lowered during reception 
of the FMX stereophonic broadcasting, the noise level is raised. In such a 
state, if the degree of modulation of the stereo difference signal is 
rapidly changed from a small degree to a large degree, the gain of the 
voltage control amplifier 49 is rapidly increased to amplify the noise at 
a high gain, with a result that S/N ratio is quickly deteriorated. 
Meanwhile, if the receiving signal is of a strong field strength, the 
noise contained in the receiving signal is small, even when the degree of 
modulation of the stereo difference signal becomes large, with an increase 
of the gain for the voltage control amplifier 49, and therefore, the S/N 
ratio is not rapidly deteriorated. According to the present invention, 
attention has been directed to the above point, and it is intended to 
improve the S/N ratio through deterioration of the stereophonic separating 
degree by controlling the level of the stereo difference signal, with the 
use of the signal corresponding to the field strength of the receiving 
signal and the signal corresponding to the degree of modulation of the 
receiving signal. 
Now, if it is assumed that the field strength of the receiving signal is 
large, the level of the signal to be obtained from the IF amplification 
circuit 44 becomes high, and consequently, the voltage across the 
smoothing capacitor 58 becomes large, with a decrease of the output signal 
level of the inverting amplifier 59. Therefore, the output signal level of 
the addition circuit 63 becomes low irrespective of the extent of the 
voltage across the smoothing capacitor 62 indicating the degree of 
modulation of the stereo difference signal S, and thus, the level control 
circuit 52 does not effect the level control function. Accordingly, the 
output signal of the voltage control amplifier 49 passes through the level 
control circuit 52 as it is so as to be applied to the stereo demodulation 
circuit 53 for being matrixed thereat. Therefore, deterioration of the 
degree of stereophonic separation is prevented, with a less degradation of 
the S/N ratio. 
Upon lowering of the field strength of the receiving signal, the level of 
the signal obtained from the IF amplification circuit 44 becomes low, and 
therefore, the terminal voltage or the voltage across the smoothing 
capacitor 58 is lowered, with the level of the output signal of the 
inverting amplifier 59 being raised. In the above state, if the degree of 
modulation of the stereo difference signal S becomes large, the voltage 
across the smoothing capacitor 62, is raised, with rising of the output 
signal level of the addition circuit 63. Since the output signal of the 
addition circuit 63 is applied to the level control circuit 52 as a 
control signal, the gain of said level control circuit 52 is lowered, with 
a consequent lowering of the level of the stereo difference signal (S+S') 
applied to the stereo demodulation circuit 53. As a result, the stereo 
separating degree in the stereo demodulation circuit 53 is deteriorated, 
and the S/N ratio can be improved by that extent. 
Even in the case where the field strength of the receiving signal is 
lowered, no control signal is produced from the addition circuit 63 if the 
degree of modulation of the stereo difference signal S is small. When the 
modulation degree of the stereo difference signal S is small, since there 
is a level difference of about 20 dB between the stereo difference signal 
S and the compressed stereo difference signal S' so as to be utilized in 
the noise suppressing effect for the FMX stereophonic broadcasting system, 
it is not necessary to attempt to improve the S/N ratio by deteriorating 
the separating degree. However, when the field strength of the receiving 
signal is extremely lowered, the output level of the inverting amplifier 
59 is excessively raised, and since the output signal of the addition 
circuit 63 is also strengthened to a large extent, the control by the 
level control circuit 53 is effected so as to improve the S/N ratio by 
deteriorating the degree of stereophonic separation. 
It is to be noted here that, in the foregoing embodiment, although the 
level control circuit 52 is controlled by generating the control signal 
through employment of the addition circuit 63, the arrangement may be so 
modified, for example, that the inverting amplifier 59 and the level 
control circuit 52 are connected via a switch, which is adapted to be 
subjected to on/off control according to the voltage across the smoothing 
capacitor 62. 
As is clear from the foregoing description, according to the present 
invention, unnatural variation of noises during reception of FMX 
stereophonic broadcasting may be reduced to a large extent. Therefore, 
since noises as in piano playing signals or the like low in an average 
level, but high in a momentary level can be lowered, it is possible to 
improve feeling in the listening. 
Furthermore, according to the present invention, an improved FMX 
stereophonic broadcast receiver capable of receiving the FMX stereophonic 
broadcasting under favorable conditions may be provided. Particularly, by 
the present invention, an FMX stereophonic broadcast receiver in which S/N 
ratio has been improved can be provided, whereby the FMX stereophonic 
broadcasting in a weak field strength can be favorably received without 
being obstructed by noises. 
Although the present invention has been fully described by way of example 
with reference to the accompanying drawings, it is to be noted here that 
various changes and modifications will be apparent to those skilled in the 
art. Therefore, unless otherwise such changes and modifications depart 
from the scope of the present invention, they should be construed as 
included therein.