JP3249367B2 - Noise removal circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a noise elimination circuit for eliminating noise in a television audio multiplex signal, and more particularly, for preventing a malfunction when switching from monaural broadcasting to stereo or dual broadcasting.

[0002]

2. Description of the Related Art Conventionally, there is known a noise elimination circuit which is used before a demodulation circuit for demodulating an audio multiplex signal and which can control noise detection sensitivity in accordance with a noise amount as shown in FIG. . In FIG. 2, a voice multiplex signal containing noise is delayed by a delay circuit (1), and only the noise passes through an HPF (2). The noise that has passed through the HPF (2) is amplified by the noise amplifier (3).
Applied to the trigger generation circuit (4). The trigger occurrence time
The path (4) compares the output signal of the noise amplifier (3) with a reference voltage, and generates a trigger having a predetermined width when the output signal level of the noise amplifier ( 3 ) is higher than the reference value. The trigger is applied to the noise removing unit (5), and the noise is removed by the noise removing unit (5). The noise amount detection circuit (6) detects a noise amount per unit time according to the output signal of the noise amplifier ( 3 ). Adjust the gain of the noise amplifier ( 3 ) according to the amount of noise. Here, when a single pulse noise occurs, the amount of noise per unit time is small, so that the gain of the noise amplifier ( 3 ) is large, and the trigger noise generation circuit (4) is caused by the pulse noise. A trigger is generated, and pulse noise can be effectively removed. If the amount of white noise is large, the amount of noise per unit time increases, so the noise amplifier (3)
Becomes smaller, and the output signal level of the noise amplifier ( 3 ) becomes lower than the reference value. Therefore, the noise removal operation is not performed by the white noise. As described above, in the white noise generation state, since the extra noise removing operation is not performed, the waveform of the input signal is not largely disturbed, and it is possible to prevent a sense of incongruity due to the distorted waveform.

[0003]

The frequency spectrum of the television audio multiplex signal and the frequency characteristic of the HPF (2) are as shown in FIGS. 3 (a) and 3 (b), respectively.
There is no sub-channel signal for monaural broadcasting, and there are sub-channel signals for stereo broadcasting and dual broadcasting. The sub-channel signal is a signal obtained by FM-modulating a sub-carrier signal by a stereo difference signal during stereo broadcasting or by a sub-audio signal during double broadcasting. The frequency of the FM modulation signal is attenuated by the HPF ( 2 ), but when the noise amplifier (3) is input, the level of the FM modulation signal is higher than the reference value of the trigger generation circuit (4). Therefore, in the circuit of FIG. 2, when switching from monaural broadcasting to stereo or dual broadcasting, a sub-channel signal is generated abruptly. Since the sub-channel signal is a high-frequency, high-level signal, the noise amount detection circuit (6) reduces the gain of the noise amplifier (3). However, it takes time from the detection of the amount of noise to the gain control, and within this time, the level of the sub-channel signal does not become lower than the reference value of the trigger generation circuit (4). However, there is a problem in that the noise removal operation is performed, and the output signal of the noise removal unit (5) becomes a waveform that largely distorts the waveform of the input signal.

[0004]

SUMMARY OF THE INVENTION The present invention relates to a noise elimination circuit for eliminating noise contained in a television audio multiplex signal, comprising: a noise extraction section for extracting a noise component in the television audio multiplex signal; Depending on the output signal of the circuit,
A noise removal unit that removes a noise component in the television audio multiplex signal, a broadcast detection unit that detects a stereo broadcast or a dual broadcast based on a control channel signal included in the television audio multiplex signal, and a broadcast detection circuit. And a detection sensitivity control unit for lowering the noise detection sensitivity of the noise extraction circuit according to the output signal.

The noise extracting section detects an output signal of the attenuating circuit which attenuates the level of the noise component and an attenuating circuit having a predetermined level or more, and generates a detection signal serving as an output signal of the noise extracting section. Noise detection circuit,
A noise amount detection circuit that detects an amount of noise based on an output signal of the attenuation circuit, and increases the amount of attenuation of the attenuation circuit when the amount of noise increases. It is characterized in that the attenuation of the circuit is forcibly increased.

Further, the attenuation circuit includes a variable resistor composed of a diode and a fixed resistor connected in series, and changes a current flowing through the diode in accordance with an output signal of the noise amount detection section to change the variable resistor. Is characterized by varying the resistance value. Still further, the noise extraction circuit may include a noise amplifier that amplifies the noise component, and a noise detection circuit that detects an output signal of the noise amplifier at a predetermined level or more and generates a detection signal serving as an output signal of the noise extraction unit. And a noise amount detection circuit that detects a noise amount based on an output signal of the noise amplifier, and lowers a gain of the noise amplifier when the noise amount increases, and the noise amplifier detects the noise amount according to an output signal of the detection sensitivity control unit. It is characterized in that the gain is forcibly reduced.

Further, the noise extracting section compares the level of the noise component with a reference level, and generates a signal serving as an output signal of the noise extraction circuit when the level of the noise component is higher than the reference level. And a reference level is increased according to an output signal of the detection sensitivity control circuit. Further, the detection sensitivity control section generates an output signal having a predetermined width, and when the generation of the output signal ends, the noise detection sensitivity of the noise extraction circuit changes with a time constant.

[0008]

FIG. 1 shows an embodiment of the present invention.
It is a figure, (1) is delay which delays a television audio multiplex signal
The first LPF, which is a circuit, (2) is a
HPF that passes the noise component, (3) increases the noise component
Noise amplifier, (4) is a noise detection circuit,
Trigger generation time to generate a trigger according to the noise level
Road, (5) is a noise eliminator that removes noise in response to a trigger
(6) is a noise detection circuit for detecting the amount of noise,
(7) is an attenuation that attenuates the output signal level of the HPF (2).
Circuit, (8) is the main channel signal of the television audio multiplex signal
, A second LPF passing through the sub-channel signal,
The first BPF to be passed, (10) is the first BPF (9)of
A pulse count type FM demodulation circuit for FM demodulating the output signal
(11) is an addition circuit (11a) and a subtraction circuit (11
b) and a switch (11c),2nd LPF
(8) and the output signal of the FM demodulation circuit (10)
Matrix circuit (12) is a matrix circuit (1
Switch (12a) for switching between the two output signals of 1)
And (12b) a switch circuit, and (13) a control circuit.
The second BPF for passing the channel signal, (14)
Envelope detection of the output signal of 2BPF (13)
Envelope detector, (15) and (16) are envelopes
Stereo broadcast by the output signal of the loop detection circuit (14).
Stereo broadcast detection circuit and
(17) is a matrix circuit (1
1) a control circuit for controlling the switch circuit (12);
(18) is a light emitting die that lights up during stereo and dual broadcasting
A drive circuit for driving the odes (19) and (20), (2
1) An output having a predetermined width according to the output signal of the drive circuit (18).
It is an attenuation control circuit that generates a force signal.

In FIG. 1, an audio multiplex signal is a first LPF.
It is applied to (1) and is delayed by a predetermined time. If the voice multiplex signal contains noise, the HPF (2)
Since the noise has an attenuation characteristic with respect to the frequency of the audio multiplex signal as shown in (b), the noise passes through the HPF (2). The noise that has passed through the HPF (2) is attenuated by the attenuation circuit (7).
And is applied to the noise amplifier (3). After the noise is amplified by the noise amplifier (3),
Applied to the trigger generation circuit (4). The trigger occurrence time
The path (4) outputs the output signal of the noise amplifier (3) to the reference voltage V.
Compared with ref, a trigger having a predetermined width is generated when the output signal level of the noise amplifier (3) is higher than the reference value. The trigger is applied to a noise removing unit (5), and the noise removing unit (5), for example, outputs the first LP just before the trigger is applied .
The output signal level of F (1) is held for a predetermined period. No.
Since the delay time of one LPF (1) is substantially equal to the processing time from the HPF (2) to the trigger generation circuit (4), the noise in the voice multiplex signal is removed by the noise removing unit (5).

The noise amount detection circuit (6) is constituted by, for example, a peak value holding circuit and the like.
Since the holding level sequentially increases, the noise amount can be detected according to the output signal of the noise amount detection circuit (6). The output signal of the noise amount detection circuit (6) is applied to the noise amplifier (3), and the gain of the noise amplifier (3) is adjusted according to the noise amount. Here, when a single pulse noise is generated, since the amount of noise per unit time is small, the gain of the noise amplifier (3) is large, and the trigger generation circuit (4) is caused by the pulse noise. A trigger is generated, and pulse noise can be effectively removed. When the amount of white noise is large, the amount of noise per unit time increases, so that the gain of the noise amplifier (3) decreases and the output signal level of the noise amplifier (4) becomes lower than the reference value. Therefore, the noise removal operation is not performed by the white noise.

[0011] Further, the operation of audio multiplex demodulation will be described according to each broadcast. Here, the switch (11
c) turns on when the control signal (a) is applied, and the switches (12a) and (12b) turn on the control signals (b) and (c).
Is applied as shown in FIG. At the time of monaural broadcasting, since there is no control channel signal, the control circuit (17)
Generates only the control signal (c). Therefore, the switch (11c) is turned off, the switch (12a) is in the state shown, and the switch (12b) is in the opposite state as shown.
At the time of monaural broadcasting, the audio multiplex signal includes only the main channel signal. Since the second LPF (8) has a pass band width of 0 to 15 KHz, only the main channel signal is in the second LF.
Passes through the PF (8). The main channel signal is applied to an addition circuit (11a), and the output signal of the addition circuit (11a) is generated from left and right output terminals via switches (12a) and (12b). Since the switch (11c) is turned off, a main channel signal is generated from the left and right output terminals.

In a stereo broadcast, an audio multiplex signal is
A (L + R) main channel signal, and (LR)
It comprises a sub-channel signal obtained by FM-modulating a signal and a control channel signal indicating stereo broadcasting. First, the second BP
Since F (13) has a pass band width that allows a signal of about 55 KHz to pass therethrough, the control channel signal becomes the second signal.
Passes through the BPF (13). The control channel signal is envelope-detected by an envelope detection circuit (14), and an output signal of 982 KHz is generated. The output signal of the envelope detection circuit (14 ) is applied to a stereo broadcast detection circuit (15) and a duplex broadcast detection circuit (16) .
The stereo broadcast detection circuit (15) includes a PLL for locking to a 982 KHz frequency signal and a lock detection circuit for detecting the lock of the PLL. Therefore, an output signal is generated from the stereo broadcast detection circuit (15), and the control circuit (17) generates control signals (a) to (c) according to the output signal. Therefore, the switch (11c) is turned on, and the switches (12a) and (12b) are in the state shown in the figure. Further, a signal indicating a stereo broadcast is generated from the control circuit (17) to the drive circuit (18), and the light emitting diode (19) is turned on.

On the other hand, the (L + R) signal is the second LPF (8)
And an addition circuit (11a) and a subtraction circuit (11b)
Is applied to Furthermore, the first BPF (9) is 16-
Since it has a pass bandwidth of 47 KHz, the sub-channel signal passes through the second BPF (9). The sub-channel signal is demodulated into an (LR) signal by an FM demodulation circuit (10) and applied to an addition circuit (11a) and a subtraction circuit (11b). Therefore, the L signal is generated from the adder circuit (11a) and is generated from the left output terminal via the switch (12a). An R signal is generated from the subtraction circuit (11b),
It is generated from the right output terminal via the switch (12b).

Further, at the time of dual broadcasting, the composite signal includes a main channel signal as a main audio signal, a sub-channel signal obtained by FM-modulating a sub-audio signal, and a control channel signal indicating double broadcasting. The control channel signal that has passed through the second BPF (13) is supplied to the envelope detection circuit (1).
The envelope is detected in 4), and an output signal of 922 KHz is generated. The output signal of the envelope detection circuit (14) is a stereo broadcast detection circuit (15) and a double broadcast detection circuit.
(16) . The double detection circuit (16) has 922
It comprises a PLL that locks to a KHz frequency signal and a lock detection circuit that detects PLL lock. Therefore, an output signal is generated from the dual broadcast detection circuit (16), and when the output signal is generated, the control circuit (17) controls the control signals (b) and / or () according to an external mode signal. c). Therefore, the switch (11c) is turned off, the main audio signal is applied to the adding circuit (11a), and the FM
The sub audio signal obtained by demodulation is applied to a subtraction circuit (11b). Also, the control circuit (17) sends the drive circuit (1).
At 8), a signal indicating a double broadcast is generated, and the light emitting diode (20) is turned on.

Here, when a signal for selecting the main sound is applied as a mode signal, a control signal (c) is generated, and the switch (12a) is set to the state shown in FIG.
Is in a state opposite to that shown in the figure. Therefore, a main audio signal is generated from the left and right output terminals. When a mode signal indicating a sub sound is applied, a control signal (b) is generated, and the switch (12a) is set in a state opposite to that shown in the figure, and the switch (12b)
Is in the state shown in the figure. Therefore, a sub audio signal is generated from the left and right output terminals.

Further, when a mode signal indicating a main sound / sub sound is applied, control signals (b) and (c) are generated, and the switches (12a) and (12b) are in the state shown in the figure. Therefore, a main audio signal is generated from the left output terminal, and a sub audio signal is generated from the right output terminal. When switching from monaural broadcasting to stereo or dual broadcasting, the driving circuit (18) includes a light emitting diode (19) or (20).
To drive the light emitting diode (19) or (2)
0), an "L" level output signal is generated. The attenuation control circuit (21) generates an attenuation control signal having a predetermined width in response to the output signal of "L" level, and the attenuation of the attenuation circuit (7) increases in accordance with the output signal in response to the attenuation control signal. Becomes
Noise detection sensitivity decreases. Therefore, the attenuation circuit (7)
The output signal becomes lower than the reference voltage of the trigger generating circuit (4), a trigger from the trigger generation circuit (4) does not occur. Since the noise removing unit ( 5 ) does not perform the noise removing operation, the signal waveform of the audio multiplex signal is not broken. As described above, when switching from monaural broadcasting to stereo broadcasting or dual broadcasting, the noise removal operation is not forcibly performed for a certain period.

If the attenuation control signal has a time constant,
When the sensitivity reduction of the noise removal operation is released, the attenuation amount changes gently, and it is possible to prevent a sense of incongruity in hearing. Further, as another embodiment, instead of configuring the attenuation circuit (7), the reference voltage of the trigger generation circuit (4) is increased according to the output signal of the attenuation control circuit (21) as shown by the dotted line in FIG. Thus, the sensitivity of the noise removal circuit may be reduced.

FIG. 4 is a diagram showing a specific circuit example of the noise amplifier (3) and the attenuation circuit (7). (22) is a variable resistance circuit which is a diode bridge circuit composed of diodes (22a) to (22d). , (23) are emitter follower transistors for amplifying the output signal of the variable resistance circuit (22), and (24) is an emitter follower transistor (2).
3) a smoothing circuit for smoothing the output signal; (25) a transistor (25a) to which the output signal of the emitter follower transistor (23) is applied to the base; and (25) a smoothing circuit (2).
The transistor (2) to which the output signal of (4) is applied to the base
5b), and (26) a current transistor (26a) and (26b) for generating a current flowing through the variable resistance circuit (22) and a current mirror circuit (26c).
(27) is a current generating circuit (2)
6) a current mirror circuit for drawing a current, (28)
Is a control transistor for controlling the output current of the current mirror circuit (27).

In FIG. 4, the output signal of the HPF (2) at the preceding stage is attenuated by the variable resistor circuit (22) and the resistor (29), and then is attenuated by the emitter follower transistor (23).
Applied to the base. The output signal generated from the emitter of the emitter follower transistor (23) is applied to the base of the transistor (25a) and, after being smoothed by the smoothing circuit (24), is applied to the base of the transistor (25b). The smoothing circuit (24) removes the AC component in the output signal of the emitter follower transistor (23) and extracts only the DC component. Therefore, the differential amplifier circuit (25) amplifies the output signal of the emitter follower transistor (23) and applies the amplified output signal to the next stage noise amplifier (3) via the output terminal OUT. During normal operation,
Since an "H" level signal is applied to the control terminal, the control transistor (28) is on and the current mirror circuit (27) is off. Therefore, the diode (22
Since no current flows through a) to (22d), the internal resistance of the diodes (22a) to (22d) is sufficiently large. Therefore, the amount of attenuation obtained by the variable resistance circuit (22) and the resistance (29) is small, and the output signal of the HPF (2) is not attenuated and is transmitted to a subsequent circuit.

When an "L" level output signal from the attenuation control circuit (21) is applied to the control terminal, the control transistor (28) turns off and the current mirror circuit (27)
Works. When an output signal is generated from the current mirror circuit (27), a part of the collector current of the current transistors (26a) and (26b) is reduced by the current mirror circuit (2).
Flow to 7). Therefore, the diodes (22a) to (2)
2d), the current flowing through the diode (22) increases.
The internal resistances of a) to (22d) are reduced. Therefore,
Since the attenuation rate obtained by the variable resistor circuit (22) and the resistor (29) increases, the HPF is connected to the output terminal OUT.
The signal of (2) having a reduced output signal level is obtained.

As another specific circuit example of the attenuation circuit (7),
In the circuit of FIG. 1, the variable resistance circuit (22) is not configured,
The output signal of the HPF (2) can be attenuated by reducing the operating current of the differential amplifier circuit (25) when the control signal is applied and decreasing the gain of the differential amplifier circuit (25). As described above, the attenuation circuit (7) can be configured by a variable resistance circuit including a diode bridge or a variable operation current type differential amplifier circuit, so that a noise removal circuit suitable for an IC can be configured.

[0022]

Thus, according to the present invention, in a noise elimination circuit for eliminating noise of a television audio multiplex signal, when switching from monaural broadcasting to stereo broadcasting or dual broadcasting, a control channel signal is detected and noise is eliminated. Since the sensitivity of the circuit is deteriorated, it is possible to prevent malfunction of the noise removal operation due to the sub-channel signal at the time of switching, and to improve the sense of incongruity in hearing.

Also, since the sensitivity of the noise elimination circuit can be reduced by inserting an attenuation circuit or changing the reference voltage of the trigger generation circuit, the circuit configuration can be simplified, and the IC can be integrated. It is possible to configure a noise removal circuit that is easy to perform.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram showing a conventional example.

FIG. 3 is a characteristic diagram of a television audio multiplex signal and an HPF.

FIG. 4 is a circuit diagram showing a main part of FIG. 1;

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 first LPF 2 HPF 3 noise amplifier 4 trigger generation circuit 5 noise elimination unit 6 noise amount detection circuit 7 attenuation circuit 8 second LPF 9 first BPF 10 FM demodulation circuit 11 matrix circuit 12 switch circuit 13 second BPF 14 envelope detection circuit 15 stereo broadcast Detection circuit 16 Double broadcast detection circuit 17 Control circuit 18 Drive circuit 21 Attenuation control circuit

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H04B 1/10 H04H 5/00 H04N 5/60

Claims (6)

(57) [Claims] 1. A noise removing circuit for removing noise included in a television audio multiplex signal, comprising: a noise extraction unit for extracting a noise component in the television audio multiplex signal; A noise removal unit that removes a noise component in the television audio multiplex signal; a broadcast detection unit that detects stereo broadcast or double broadcast based on a control channel signal included in the television audio multiplex signal; And a detection sensitivity control unit that reduces the noise detection sensitivity of the noise extraction circuit according to the output signal. 2. The noise extracting section detects an output signal of the attenuating circuit that attenuates a level of the noise component and an output signal of the attenuating circuit having a predetermined level or more and generates a detection signal to be an output signal of the noise extracting section. a trigger generation circuit which, the detected amount of noise by the output signal of the attenuation circuit comprises an attenuation amount of the attenuation circuit when the noise amount is increased and a noise amount detection circuit for a large, the output of the detection sensitivity controller 2. The noise elimination circuit according to claim 1, wherein the attenuation amount of the attenuation circuit is forcibly increased in accordance with a signal. 3. The variable attenuation circuit according to claim 1, wherein the variable resistor includes a variable resistor composed of a diode and a fixed resistor connected in series, and varies a current flowing through the diode in accordance with an output signal of the noise amount detection unit. 3. The noise detection circuit according to claim 2, wherein the resistance value of the noise detection circuit is varied. 4. A noise extraction circuit for amplifying the noise component and a noise for detecting an output signal of the noise amplifier at a predetermined level or more and generating a detection signal serving as an output signal of the noise extraction unit. A noise detection circuit for detecting a noise amount based on an output signal of the noise amplifier, and a noise amount detection circuit for lowering a gain of the noise amplifier when the noise amount increases, wherein the noise amount is determined in accordance with an output signal of the detection sensitivity control unit. 2. The noise removing circuit according to claim 1, wherein the gain of the amplifier is forcibly reduced. 5. A noise detection circuit for comparing a level of the noise component with a reference level, and generating a signal serving as an output signal of the noise extraction circuit when the level of the noise component is equal to or higher than a reference level. 2. The noise removal circuit according to claim 1, wherein the reference level is increased in accordance with an output signal of the detection sensitivity control circuit. 6. The detection sensitivity control section generates an output signal having a predetermined width, and when the generation of the output signal is completed, the noise detection sensitivity of the noise extraction circuit changes with a time constant. The described noise removal circuit.