Pulsive noise removing apparatus

A pulsive noise removing apparatus comprises a subtracting circuit (35) receiving one input thereto a stereo composite signal directly from a delay circuit (31). A stereo switching signal is generated as a function of a stereo pilot signal included in the stereo composite signal and a cancel signal of the same level, the same frequency and the same phase as those of the stereo pilot signal is generated as a function of the stereo switching signal. A gate (67) is interposed between the output of the cancel signal generating circuit (65) and the other input of the subtracting circuit and the gate is interrupted responsive to detection of a pulsive noise included in the composite signal. The pulsive noise is removed due to interruption of the gate and the stereo pilot signal is prevented from appearing at the output of the subtracting circuit irrespective of conduction or interruption of the gate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to a pulsive noise removing 
apparatus. More specifically, the present invention relates to a pulsive 
noise removing apparatus such as employed in an FM stereo receiver, for 
example, for removing a pulsive noise included in a stereo composite 
signal and for preventing a stereo pilot signal from being applied to a 
stereo demodulating circuit. 
2. Description of the Prior Art 
It is well-known that a pulsive noise such as a motor noise, an ignition 
noise generated by an automobile or the like could exert an influence upon 
normal reception by an FM receiver. Since such pulsive noise phase 
modulates the FM signal, the pulsive noise cannot be removed even by a 
limiter. Furthermore, since the frequency spectrum of such pulsive noise 
is distributed throughout a wide range from a low frequency to a high 
frequency, the same cannot be removed even by employment of a filter. 
Accordingly, a particular circuit for removing such pulsive noise has 
already been proposed and put into practical use. 
FIG. 1 is a block diagram showing one example of an FM stereo receiver 
which constitutes the background of the invention. An FM stereo receiver 
comprises an antenna 1 for receiving FM broadcasting, a high frequency 
amplifier 3 for amplifying an FM signal received by the antenna 1, a local 
oscillator 5 for generating a local oscillation signal, a mixer 7 for 
mixing the amplified high frequency signal obtained from the high 
frequency amplifier 3 and the local oscillation signal obtained from the 
local oscillator 5 for converting the high frequency signal into an 
intermediate frequency signal, an intermediate frequency amplifier 9 for 
amplifying the intermediate frequency signal, and an FM detector 11 for 
demodulating the intermediate frequency signal amplified by the 
intermediate frequency amplifier 9 for providing a stereo composite 
signal. The stereo composite signal obtained from the FM detector 11 is 
applied to a stereo demodulating circuit 15 through a noise removing 
circuit 13. The stereo demodulating circuit 15 demodulates the stereo 
composite signal to provide a left signal and a right signal, which are 
applied to low frequency amplifiers 17 and 19. Speakers 21 and 23 are 
driven with the outputs from the low frequency amplifiers 17 and 19. The 
present invention is directed to an improvement in a pulsive noise 
removing apparatus such as inserted between the FM detector and the stereo 
demodulating circuit, as shown in FIG. 1. 
One example of such pulsive noise removing apparatus is disclosed in 
Japanese Patent Publication No. 15710/1964 published for opposition Aug. 
5, 1964, for example. The apparatus disclosed in the reference patent 
publication comprises a gate in a signal transfer path, which gate is 
interrupted for a given time period responsive to detection of a pulsive 
noise, whereby a pulsive noise is prevented from passing through the 
signal transfer path. A capacitor is also connected to the output point of 
the gate for the purpose of holding a level. The capacitor serves to 
maintain the level of a signal immediately before the gate is interrupted, 
whereby the signal is compensated with the signal maintained in the 
capacitor when the gate is interrupted. The pulsive noise is thus removed 
and distance of the signal during the gate interruption period is 
minimized. However, the above referenced Japanese Patent Publication No. 
15710/1964 involves a problem that a stereo pilot signal is lost when the 
same is employed in an FM stereo receiver. 
A pulsive noise removing apparatus which can prevent loss of a stereo pilot 
signal and can advantageously remove a pulsive noise has been proposed. 
One example is disclosed in U.S. Pat. No. 3,739,285 issued June 12, 1973. 
The apparatus disclosed in the referenced U.S. patent also comprises such 
gate and capacitor as described previously and a parallel resonance 
circuit is connected between the capacitor and the ground. The parallel 
resonance frequency of the parallel resonance circuit is selected to be 
the frequency of the pilot signal of the FM stereo broadcasting, say 19 
kHz. Accordingly, the signal level immediately before the gate is 
interrupted is maintained in the capacitor and the pilot signal obtained 
from the parallel resonance circuit is applied to the stereo demodulating 
circuit. Thus, when the pulsive noise is detected, the gate is interrupted 
and the pulsive noise is prevented from being applied to the stereo 
demodulating circuit. Furthermore, when the gate is again closed, the 
continuity of the signal is maintained, inasmuch as the signal level has 
been maintained in the capacitor. At the same time, the pilot signal for 
the stereo demodulating circuit is prevented from being lost. Although the 
referenced United States patent can be advantageously utilized in an FM 
stereo receiver, still a further problem as set forth in the following is 
involved. More specifically, another series resonance circuit is formed 
with the capacitor and the parallel resonance circuit, whereby the signal 
being applied to the stereo demodulating circuit gives rise to distortion 
with such series resonance frequency. The series resonance frequency is 
necessarily smaller than the parallel resonance frequency and accordingly 
the same in an audible region and the distortion is outputted as a sound. 
Therefore, the same assignee as the present invention previously proposed a 
noise removing apparatus directed to an improvement over the above 
referenced U.S. Pat. No. 3,739,285 and U.S. Pat. No. 4,066,845 issued Jan. 
3, 1978 on the above described improvement. The last referenced U.S. 
patent teaches that an oscillation circuit is provided so that a stereo 
pilot signal is prevented by the oscillation circuit from being lost 
during the interruption period of the gate. A further pulsive noise 
removing apparatus is disclosed in Japanese Utility Model Laying-Open No. 
106608/1978 laid-open Aug. 26, 1978. The referenced Japanese utility model 
laying-open discloses that a trap circuit is provided to a combination of 
the gate and capacitor so that the stereo pilot signal is removed by the 
trap circuit. Accordingly, a problem of losing the stereo pilot signal is 
eliminated; however, the interruption period of the gate is necessarily 
prolonged by the trap circuit, whereby discontinuity of the stereo 
composite signal is increased. 
The same assignee as the present invention further proposed an improved 
noise removing apparatus in U.S. patent application, Ser. No. 133,932 and 
European patent application No. 80102251.8 (European Patent Publication 
No. 0018608). The last mentioned noise removing apparatus comprises two 
sets of a combination of a gate and a capacitor for maintaining the level. 
One of the gates receives a stereo composite signal, while the other 
receives a pseudo-pilot signal, and both are interrupted responsive to 
detection of a pulsive noise included in a composite signal. The outputs 
from the gate are applied to a signal synthesizing circuit such as an 
adding or subtracting circuit, for example. If and when a pulsive noise is 
included in the composite signal, the first gate is interrupted, whereby 
the pulsive noise is prevented from being applied to the adding or 
subtracting circuit, while the second gate is interrupted and the 
pseudo-pilot signal is also prevented from being applied to the adding or 
subtracting circuit. Accordingly, the signal maintained in the 
corresponding capacitor during the interruption period of the first and 
second gates is applied to the two inputs of the adding or subtracting 
circuit, whereby the stereo pilot signal is prevented from being obtained 
from the adding or subtracting circuit for that period. Meanwhile, when 
the first and second gates are rendered conductive, the stereo pilot 
signal is removed with the adding or subtracting circuit. In other words, 
the proposed noise removing apparatus simultaneously performs removal of a 
pulsive noise and cancellation of the pilot signal. Accordingly, an 
adverse influence upon the stereo pilot signal in the stereo demodulating 
circuit can be completely eliminated in an FM stereo receiver. 
However, the proposed noise removing apparatus necessitates a combination 
of two sets of gates and capacitors, which makes complicated a circuit 
structure. In addition, since the signals from the two sets of circuits 
are utilized for offsetting the stereo pilot signals upon addition or 
subtraction thereof, both need have the same characteristics. Accordingly, 
diversification of the characteristics and capacitances of the transistors 
and capacitors constituting the two sets of circuits must be minimized and 
accordingly mass productivity is poor. 
Furthermore, any of the above described prior art circuits comprises a gate 
implemented by a transistor, for example, in a transfer path of a stereo 
composite signal. Insertion of a gate in the signal path is one of the 
causes giving rise to distortion in the composite signal. The reason is 
that a transistor constituting the gate is not a completely linear device 
and hence involves non-linear portion. Accordingly, an apparatus including 
a gate inserted in the signal path necessitates another means for removing 
distortion caused by such non-linear portion. Without any means for 
removing such distortion, it becomes necessary not to insert a gate in a 
signal path; however, any apparatus of such structure which simultaneously 
performs removal of a pulsive noise and cancellation of pilot signal has 
not yet been proposed. 
SUMMARY OF THE INVENTION 
A pulsive noise removing apparatus in accordance with the present invention 
performs removal of a pulsive noise and removal of a reference signal such 
as a pilot signal. The present invention comprises a subtracting circuit 
having two inputs, with an electric charge storing device such as a 
capacitor connected between the two inputs of the subtracting circuit. One 
input of the subtracting circuit is supplied with a composite signal 
directly; the other input of the subtracting circuit is supplied with a 
cancel signal for cancelling the reference signal through a gate. The 
output voltage of the subtracting circuit is a difference voltage of the 
two inputs and the same is equal to the terminal voltage of the electric 
charge storing device. When the gate for the cancel signal is rendered 
conductive, one input of the subtracting circuit is supplied with the 
composite signal and the other input of the subtracting circuit is 
supplied with the cancel signal, whereby only an information signal such 
as a sound signal with the reference signal such as the pilot signal 
removal is obtained as the output of the subtracting circuit. When the 
gate is interrupted responsive to detection of the pulsive noise, the 
electric charge storing device can be deemed in a short-circuited state in 
terms of the alternating current and therefore the signal waveforms being 
applied to the two inputs of the subtracting circuit become the same. 
Accordingly, when a pulsive noise is included in the composite signal, the 
pulsive noise appears not only at one input but also at the other input. 
Therefore, the pulsive noise is cancelled in the subtracting circuit and 
only the voltage maintained by the electric charge storing device appears 
at the output thereof. The voltage from the electric charge storing device 
is the terminal voltage immediately before the gate is interrupted and 
accordingly no reference signal is included therein. 
According to the present invention, a pulsive noise removing apparatus 
performing removal of a pulsive noise and cancellation of a reference 
signal such as a pilot signal is provided with a simple circuit structure. 
In addition, since no such gate as implemented by a non-linear device such 
as a transistor is inserted in a transfer path of the composite signal, no 
distortion is caused in the composite signal due to such non-linear 
device. Such respect is entirely different from any of the previously 
described prior art apparatuses. 
In a preferred embodiment of the present invention, such pulsive noise 
removing apparatus as described above is applied in an FM stereo receiver 
and such FM stereo receiver comprises a switching signal generating 
circuit for generating a switching signal for stereo demodulation 
responsive to a stereo pilot signal included in a composite signal and 
therefore the above described cancel signal is generated based on the 
output from the switching signal generating circuit. Accordingly, any 
particular circuit need not be provided for the purpose of generating the 
cancel signal. 
In another preferred embodiment of the present invention, means is provided 
for compensating a signal component associated with a subcarrier in 
addition to provision of an electric charge storing device such as a 
capacitor between the two inputs of a subtracting circuit. One example of 
such compensating means comprises an LC parallel resonance circuit the 
parallel resonance frequency of which is selected in association with the 
frequency of the subcarrier. According to the embodiment, no disturbance 
of the subcarrier associated signal is caused and hence a stereo receiver 
of a better characteristic is provided. 
Accordingly, a principal object of the present invention is to provide a 
pulsive noise removing apparatus that can perform removal of a pulsive 
noise and cancellation of a reference signal with a simple circuit 
structure and with a better characteristic. 
One aspect of the present invention resides in a pulsive noise removing 
apparatus without interposition of a gate implemented by a non-linear 
device in a path of the composite signal. 
Another aspect of the present invention resides in a pulsive noise removing 
apparatus suited for an FM stereo receiver. 
These objects and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2 is a block diagram showing one embodiment of the present invention. 
The FM detector 11 receives the output of the intermediate frequency 
amplifier 9 in the same manner as that in FIG. 1 and the detected output, 
i.e. a stereo composite signal is applied to a delay circuit 31 and a gate 
control circuit 33. The delay circuit 31 serves to delay the received 
stereo composite signal for a predetermined time period (td) and the 
delayed composite signal is applied directly to one input, i.e. a plus 
input, of a subtracting circuit 35. The subtracting circuit 35 comprises a 
subtracter 37 and buffer circuits 39 and 41 connected to the two inputs of 
the subtracter 37. Accordingly, the input of the buffer circuit 39 becomes 
the plus input of the subtracting circuit 35 and the input of the buffer 
circuit 41 becomes the minus input of the subtracting circuit 35. The 
subtracter 37 and the buffer circuits 39 and 41 constituting the 
subtracting circuit 35 are shown in more detail in FIG. 4. The buffer 
circuits 39 and 41 each comprises an emitter follower and accordingly the 
plus input and the minus input of the subtracting circuit 35 both exhibit 
a very large input impedance. An electric charge storing device or a 
capacitor 43 is connected between the plus input and the minus input of 
the subtracting circuit 35. Since the output of the subtracting circuit 35 
is a difference voltage between the plus input and the minus input, the 
same becomes equal to the terminal voltage of the capacitor 43. The output 
of the subtracting circuit 35 is applied to the stereo demodulating 
circuit 15. 
The composite signal that passed the buffer circuit 39 is further applied 
to a pilot signal extracting circuit 45. The circuit 45 serves to extract 
a reference signal, i.e. a pilot signal from the composite signal and the 
extracted output is applied to a phase locked loop 47. The phase locked 
loop 47 serves to generate a switching signal of 38 kHz in synchronism 
with the pilot signal (19 kHz). The phase locked loop 47 comprises a phase 
comparator 49, a low-pass filter 51, a voltage controlled oscillator 53 
and two frequency dividers 55 and 57, as well-known. The output of the 
first frequency divider is applied to the stereo demodulating circuit 15 
as a stereo switching signal of 38 kHz and is also applied to the second 
frequency divider 57. The output of the second frequency divider 57 is 19 
kHz and a cancel signal to be described subsequently is generated based on 
the output of the second frequency divider 57. More specifically, the 
output of the second frequency divider 57 is applied to a wave shaping 
circuit 59. The wave shaping circuit 59 serves to convert the signal of 19 
kHz from the second frequency divider 57 to a signal of a triangle wave, a 
trapezoidal wave or a sine wave, for example. The signal of 19 kHz from 
the wave shaping circuit 59 is applied to a synchronous detecting circuit 
61 and is also applied to an amplitude adjusting circuit 47. The 
synchronous detecting circuit 61 is supplied with the output from the 
subtracting circuit 39, i.e. a low frequency signal. The synchronous 
detecting circuit 61 synchronous detects the signal applied to the input 
of the stereo demodulating circuit 15 with the output signal of the 
waveform shaping circuit 59 and provides at the output terminal thereof a 
signal associated with the magnitude of the stereo pilot signal remaining 
at the input terminal of the stereo demodulating circuit 15. The output of 
the synchronous detecting circuit 61 is converted into a direct current 
voltage by means of a low-pass filter 63. More specifically, the output 
from the low-pass filter 63 turns to a direct current voltage of a level 
associated with the magnitude of the stereo pilot signal remaining at the 
output terminal of the subtracting circuit 35. The direct current voltage 
from the low-pass filter 63 is applied to the amplitude adjusting circuit 
65 has a control input. Specifically, the amplitude adjusting circuit 65 
has a circuit structure as shown in FIG. 5, for example. Accordingly, the 
output impedance at the output stage of the amplitude adjusting circuit 65 
is extremely small. Thus, a cancel signal of the same frequency and phase 
as those of the stereo pilot signal included in the composite signal is 
obtained from the amplitude adjusting circuit 65. The cancel signal is 
applied through the gate circuit 67 to the minus input of the subtracting 
circuit 35. 
The gate circuit 67 is controlled by the gate control circuit 33. The gate 
control circuit comprises a high-pass filter 69, a noise detector 71 and a 
waveform shaping circuit 73. The high-pass filter 69 extracts only a high 
frequency component from the composite signal. The noise detector 71 
provides to the wave shaping circuit 73 the detected output when the level 
of the high frequency signal as extracted exceeds a predetermined value. 
The wave shaping circuit 73 comprises a monostable multivibrator, for 
example, and is triggered with a noise detecting signal, thereby to 
provide a pulse of a predetermined time duration to the gate circuit 67 as 
a gate control signal. Accordingly, the gate circuit 67 is responsive to 
the control signal from the gate control circuit 33 to be interrupted for 
that period, thereby to prevent transfer of the cancel signal from the 
amplitude adjusting circuit 65 to the minus input of the subtracting 
circuit 35. 
Meanwhile, of those circuits shown in FIG. 2, the subtracting circuit 35, 
the waveform shaping circuit 59, the synchronous detecting circuit 61, the 
low-pass filter 63 and the amplitude adjusting circuit 65 are disclosed in 
detail in European Patent Publication No. 0018608. Accordingly, the above 
described European patent publication is incorporated by reference thereto 
and any detailed description of these circuits will be omitted. 
Now referring to FIG. 3, an operation of the FIG. 2 embodiment will be 
described. Referring to FIG. 3, (A) shows a composite signal at the output 
point A of the FM detecting circuit 11, (B) shows a cancel signal at the 
output point B of the amplitude adjusting circuit 65, (C) shows a signal 
at the minus input point C of the subtracting circuit 35, (D) shows a 
signal at the output point D of the subtracting circuit 35, and (E) shows 
a gate control pulse at the output point E of the gate control circuit 33. 
First an operation in the case where no pulsive noise is included in the 
composite signal will be described. In such a case, no gate control pulse 
is obtained from the gate control circuit 33 and accordingly the gate 
circuit 67 is in a conductive state. Therefore, the cancel signal from the 
amplitude adjusting circuit 65 is applied through the gate circuit 67 to 
the minus input of the subtracting circuit 35. On the other hand, the 
composite signal from the delay circuit 31 is applied to the plus input of 
the subtracting circuit 35. The composite signal is represented as VL+VP 
in brief, where VL is a low frequency signal and VP is a stereo pilot 
signal while a subcarrier is omitted. Assuming the cancel signal to be VC, 
the output voltage Vout given by the following equation (1) is obtained at 
the output of the subtracting circuit 35, 
EQU Vout=VL+VP-VC . . . (1) 
where the cancel signal VC has the same amplitude and the same frequency 
and the same phase as those of the pilot signal VP and accordingly VP=VC. 
Therefore, the output signal of the subtracting circuit 35 becomes VL, 
i.e. only a low frequency signal. Thus, removal of a pilot signal is 
achieved. More specifically, the pilot signal included in the composite 
signal is canceled as a function of the cancel signal by means of the 
subtracting circuit 35 and hence only a low frequency signal appears at 
the output of the subtracting circuit 35. At that time the terminal 
voltage of the capacitor 43 also is a difference voltage between the 
composite signal and the cancel signal, i.e. a low frequency signal. 
Meanwhile, as seen from the FIG. 5, the output impedance of the amplitude 
adjusting circuit 65 is small and therefore, when the gate circuit 67 is 
rendered conductive, the input impedance of the minus input of the 
subtracting circuit 35 is decreased irrespective of existence of the 
buffer 41. Accordingly, the input signal applied to the minus input point 
C of the subtracting circuit 35 through the capacitor 43 from the delay 
circuit 31 becomes an extremely small level enough to be negligible. 
Accordingly, substantially the cancel signal is only applied to the minus 
input. 
Now an operation in the case where the pulsive noise superposed on the 
composite signal is described. In such a case, such pulsive noise is 
detected by the noise detector 71 and a gate control pulse is obtained 
from the gate control circuit 33, as shown as (E) in FIG. 3. Accordingly, 
the gate circuit 67 is interrupted. On the other hand, the composite 
signal from the FM detector 11 is delayed by a predetermined time period 
(td) by the delay circuit 31. Accordingly, the pulsive noise is applied to 
the subtracting circuit 35 for an interruption period of the gate circuit 
67. When the gate circuit 67 is interrupted, the cancel signal from the 
amplitude adjusting circuit 65 is not applied to the minus input of the 
subtracting circuit 35 any more. Accordingly, the composite signal with a 
pulsive noise superposed from the delay circuit 31 is applied to both of 
the plus input and the minus input of the subtracting circuit 35. More 
specifically, in terms of the alternating current, the capacitor 43 can be 
deemed as in a short-circuited state, whereas when the gate circuit 67 is 
interrupted the subtracting circuit 35 is not subjected to any influence 
of a low output impedance of the amplitude adjusting circuit 65 at all, 
whereby the input impedance of the minus input becomes high. Accordingly, 
the signal waveforms at the plus input and the minus input of the 
subtracting circuit 35 become equal to each other as shown as (A) and (C) 
in FIG. 3, apart from the low frequency signal. Assuming a pulsive noise 
to be VN and a terminal voltage of the capacitor 43 to be VL1, then the 
signal Vplus applied to the plus input and the signal Vminus being applied 
to the minus input of the subtracting circuit 35 are expressed by the 
following equations (2) and (3), respectively. 
EQU Vplus=VL+VP+VN . . . (2) 
EQU Vminus=VL+VP+VN-VL1 . . . (3) 
Accordingly, through subtraction of the signals represented by these two 
equations by the subtracting circuit 35, only the terminal voltage VL1 of 
the capacitor 43 is obtained at the output thereof. Since the voltage VL1 
is the terminal voltage of the capacitor 43 immediately before the gate 
circuit 67 is interrupted, the same is only the low frequency signal 
immediately before the gate circuit 67 is interrupted. Since the input 
impedance of the subtracting circuit 35 is high, a charging/discharging 
current does not flow to or from the capacitor 43 during the time period 
when the gate circuit 67 is interrupted and accordingly the capacitor 43 
can maintain the terminal voltage VL1. Thus, during the interruption 
period of the gate circuit 67, removal of the pulsive noise and 
cancellation of the stereo pilot signal are simultaneously performed, as 
shown as (D) in FIG. 3. 
FIG. 6 is a block diagram of a major portion of a modification of the FIG. 
2 embodiment. In the embodiment shown, the capacitor 43 and an LC 
resonance circuit 75 are provided between the plus input and the minus 
input of the subtracting circuit 35. The resonance circuit 75 has the 
resonance frequency of 38 kHz, for example, which is the same frequency as 
that of the subcarrier of the FM stereo broadcasting. The resonance 
circuit 75 serves to compensate the signal level associated with the 
subcarrier. More specifically, since the resonance circuit 75 makes 
resonance at the frequency of 38 kHz, both the low freqency signal and the 
signal associated with the subcarrier are obtained at the output of the 
subtracting circuit 35. 
FIG. 7 is a block diagram showing a major portion of another embodiment of 
the present invention. The embodiment shown is particularly suited for an 
FM monaural receiver. More specifically, in the FIG. 2 embodiment, the 
switching signal was generated responsive to the pilot signal included in 
the stereo composite signal and the cancel signal was generated responsive 
to the switching signal. Since a monaural receiver need not comprise such 
a circuit for generating a switching signal, no cancel signal is generated 
with the same structure as that of the FIG. 2 embodiment. Therefore, as 
seen in the FIG. 7 embodiment, a parallel resonance circuit 77 making 
resonance to the frequency of 19 kHz is employed. The resonance circuit 77 
functions as a cancel signal generator. More specifically, since the 
resonance circuit 77 makes resonance to the pilot signal (19 kHz) included 
in the composite signal, a cancel signal is obtained from the circuit 77 
when the gate circuit 67 is rendered conductive. When the gate circuit 67 
is interrupted, the same operation as that of the FIG. 2 embodiment is 
performed. 
As described in the foregoing, according to the present invention, a 
pulsive noise can be effectively removed and an undersired reference 
signal such as a stereo pilot signal can also be effectively removed. In 
addition, according to the present invention, no gate circuit need be 
inserted in the transfer path of the composite signal and therefore no 
distortion occurs due to non-linearity of a device constituting the gate 
circuit. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.