JP3213495B2 - 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 removing circuit for removing a noise component in an FM composite signal.

[0002]

2. Description of the Related Art Generally, an FM radio receiver is provided with a noise removing circuit for removing pulse noise contained in an FM composite signal obtained by FM detection. By the way, in FM broadcasting, when the electric field intensity of a received signal becomes weak, white noise called weak electric field noise increases. Therefore, when the electric field is weak, the noise elimination circuit detects the weak electric field noise, and the noise elimination circuit may operate erroneously. Therefore, conventionally, a noise elimination circuit including an AGC circuit as shown in FIG. 2 has been proposed.

In FIG. 2, when no pulse noise is included in the FM composite signal, the FM composite signal is supplied to the subtraction circuit (2) via the delay circuit (1).
(+) Terminal. The FM composite signal is applied to an HPF (high-pass filter) (3). Since no noise component is included in the FM composite signal, no output signal is generated from the HPF (3).
No gate signal is generated from the gate signal generation circuit (4). Therefore, the gate circuit (5) is turned on, and the pilot cancel signal is applied to the (-) terminal of the subtraction circuit (2) via the gate circuit (5). Subtraction circuit (2)
Then, the output signal of the delay circuit (1) and the pilot cancel signal are subtracted, and an audio signal is generated at the output terminal. The pilot cancel signal is a signal having the same level, frequency and phase as the pilot signal in the FM composite signal.

When pulse noise is included in an FM composite signal, only the pulse noise
Go through (3). The output signal of the HPF (3) is applied to the gate signal generation circuit (4) via the level control circuit (6). Then, the gate signal generation circuit (4) detects whether or not the output signal of the level control circuit (6) is equal to or higher than a predetermined level. If the output signal is equal to or higher than the predetermined level, a gate signal having a predetermined width is generated. The gate circuit (5) is turned off in response to the gate signal, and the pilot cancel signal is supplied to the subtraction circuit (2).
Will not be applied to the Therefore, FM including pulse noise
The composite signal is equally applied to the two input terminals of the subtraction circuit (2), and no pulse noise is generated at the output terminal of the subtraction circuit (2). At this time, (+) of the subtraction circuit (2)
And the voltage across the capacitor (7) connected to the (-) terminal holds the input level immediately before the gate circuit (5) turns off, so that the capacitor (7)
At the output end of the subtraction circuit (2).

By the way, at the time of a weak electric field, the weak electric field noise increases in the FM composite signal. The weak electric field noise is applied to the level control circuit (6) via the HPF (3). When the level control circuit (6) detects that the weak electric field noise has increased, the level control circuit (6) operates to lower the level of the input signal. Therefore, the gate signal is not generated from the gate signal generation circuit (5) due to the output signal of the level control circuit (6). Therefore, it is possible to prevent malfunction of noise removal due to weak electric field noise at the time of a weak electric field.

[0006]

However, when a pulse signal is included in the composite signal together with the weak electric field noise as shown in FIG. 3A at the time of the weak electric field, the level control circuit (6) Since priority is given to detection of the weak electric field noise, the level control circuit (6) operates so as to lower the level of the input signal. Therefore, the level of the pulse noise in FIG. 3A is reduced as shown in FIG. 3B, and may be lower than a predetermined level in the gate signal generation circuit (b), and the level control is not performed. However, there is a problem that pulse noise to be detected is not detected and noise removal is not performed.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has a high-pass filter for passing a noise component in an input signal, an amplifier circuit for amplifying an output signal of the high-pass filter, and An AGC circuit for generating an AGC signal for varying the gain of the amplifier circuit according to an output signal of the amplifier circuit; and a reference level varying according to the AGC signal for detecting noise equal to or higher than the reference level. A noise detection circuit, a gate signal generation circuit that generates a gate signal according to an output signal of the noise detection circuit, and a cancellation circuit that cancels a noise component in the input signal according to the gate signal. It is characterized by.

[0008]

According to the present invention, the noise component passed through the high-pass filter is amplified in the amplifier circuit. The output signal of the amplifier circuit is applied to the AGC circuit, and an AGC signal corresponding to the output signal of the amplifier circuit is generated. The AGC signal is applied to an amplifier circuit, the gain of which is varied, and also applied to a noise detection circuit, which varies the reference level of the noise detection circuit. The output signal of the amplifier circuit is also applied to a noise detection circuit, and noise equal to or higher than a reference level is detected. Then, a gate signal is generated from the gate signal generation circuit according to the output signal of the noise detection circuit, and the noise component of the input signal is canceled according to the gate signal. Since the reference level of the noise detection circuit changes due to the noise, even if the noise level changes, the noise can always be reliably detected and the noise can be canceled.

[0009]

FIG. 1 shows an embodiment of the present invention.
(8) is a delay circuit for delaying the FM composite signal,
(9) is an HPF for extracting a noise component in the FM composite signal, (10) is an amplifier circuit for amplifying an output signal of the HPF (9), and (11) is an AGC signal according to an output signal of the amplifier circuit (10). AGC circuit that generates an AGC circuit, (12) is a comparison circuit that compares the output signal of the amplification circuit (10) with the reference level of the reference power supply (13), and (14) is the comparison circuit (1).
2) a monostable multivibrator that generates a gate signal having a predetermined width in accordance with the output signal of (2), (15) a gate circuit for passing or blocking a pilot cancel signal by the gate signal, (16) a delay circuit (8) and A subtraction circuit for subtracting the output signal of the gate circuit (15).

In FIG. 1, when a noise component is not included in the FM composite signal, the FM composite signal is applied to a (+) terminal of a subtraction circuit (16) via a delay circuit (8). Even when the FM composite signal is applied to the HPF (9), no output signal is generated from the HPF (9) because there is no noise component. Therefore,
Since the monostable multivibrator (14) does not operate and a gate signal is not generated, the gate circuit (15) turns on as shown in the figure. The pilot cancel signal is sent to the subtraction circuit (16) via the gate circuit (15).
Is subtracted from the output signal of the delay circuit (8), that is, the FM composite signal, which is applied to the (-) terminal and is applied to the (+) terminal. Since the pilot cancel signal has the same level, frequency and phase as the pilot signal in the FM composite signal, an audio signal is generated at the output terminal of the subtraction circuit (16).

When pulse noise is included in the FM composite signal, the FM composite signal is applied to the HPF (9), and only pulse noise is generated from the output terminal. The output signal of the HPF (9) is amplified by an amplifier circuit (10) having a large gain, and then applied to the (+) terminals of the AGC circuit (11) and the comparison circuit (12). In the AGC circuit (11), since the output signal of the amplifier circuit (10) is rectified and the noise is pulse noise, the level of the rectified signal becomes small. Then, an output signal from the AGC circuit (11) corresponding to the rectified signal is applied to the amplifier circuit (10), and the gain of the amplifier circuit (10) is controlled so as to be increased as it is. The output signal of the AGC circuit (11) is applied to a reference power supply (13).
The reference power supply (13) is controlled so as to increase the reference level according to the output signal. (-) Of the comparison circuit (12)
The reference level whose level has increased is applied to the terminal,
Be compared. If the level of the pulse noise is equal to or higher than the reference level, an output signal is generated from the comparison circuit (12). FIG. 4 shows the relationship between the input signal of the comparison circuit (12) and the reference level.
As shown in (a), pulse noise can be reliably detected. The output signal of the comparison circuit (12) is applied to a monostable multivibrator (14), and a gate signal of a predetermined width is generated from the monostable multivibrator (14) according to the output signal. Then, the gate circuit to which the gate signal is applied is turned off (in a state opposite to that shown in the figure), and the pilot cancel signal is cut off.

On the other hand, the FM composite signal containing the pulse noise is delayed by the delay circuit (8) by the same time as the time delayed by the HPF (9) to the monostable multivibrator (14). Thereafter, since the pilot cancel signal is not applied to the subtraction circuit (16), the output signal of the delay circuit (8) is applied to the (+) and (-) terminals of the subtraction circuit and is subtracted. Therefore, no noise is generated at the output terminal of the subtraction circuit (7). At that time, a capacitor (17) is connected between the (+) and (-) terminals of the subtraction circuit (16), and the voltage across the capacitor (17) changes the level of the input signal immediately before the gate circuit (15) turns on. Since the capacitor (1)
A signal corresponding to the voltage at both ends of 7) is generated at the output terminal of the subtraction circuit 16 for a predetermined width of the gate signal. Thereby, pulse noise can be removed.

Further, at the time of the weak electric field, the weak electric field noise increases in the FM composite signal, and the output signal of the HPF (9) to which the FM composite signal is applied is shown in FIG.
(B). The output signal of the HPF (9) is usually amplified by an amplifier circuit (10) having a large gain, and applied to an AGC circuit (11) and a comparison circuit (12). The level of the signal rectified by the AGC circuit (11) becomes large,
Amplification circuit (10) by AGC signal of C circuit (11)
Becomes small, and the output level of the amplifier circuit (10) decreases as shown in FIG. Further, the output level of the AGC circuit (11) causes the reference level of the reference power supply (13) to drop from a dotted line to a dashed line as shown in (b) of FIG. Therefore, in the comparison circuit (12), the relationship between the input signal and the reference level is as shown in FIG. 4, and since the output signal of the amplifier circuit (10) is lower than the reference level, the output signal is generated from the comparison circuit (12). No gate signal is generated. Therefore, the pilot cancel signal is applied to the subtraction circuit (15) via the gate circuit (15), and is subtracted from the FM composite signal by the subtraction circuit (16) applied via the delay circuit. Thereby, an audio signal is generated at the output terminal of the subtraction circuit (16).

When pulse noise is included in the FM composite signal at the time of a weak electric field, the output signal of the HPF (9) is as shown in FIG. The output signal of the HPF (9) is amplified by the amplifier circuit (10), and then amplified by the AGC circuit (1).
It is rectified in 1). Since the FM composite signal contains weak electric field noise, the rectified signal level becomes large. Therefore, the gain of the amplification circuit (10) and the reference level are also reduced by the AGC signal of the AGC circuit (11), and the relationship between the input signal of the comparison circuit (12) and the reference level is shown in FIG. It looks like In FIG. 4D, since only the level of the pulse noise is higher than the reference level, the comparison circuit (12) can generate an output signal only when the pulse noise is applied, and can reliably detect only the pulse noise. Can be detected. Then, a gate signal is generated from the monostable multivibrator (14) according to the output signal of the comparison circuit (12). The gate circuit (15) is turned off by the gate signal, and the application of the pilot cancel signal to the subtraction circuit (16) is stopped. Therefore, in the subtraction circuit (16), the same noise removing operation as when only the pulse noise is included in the FM composite signal is performed, and only the pulse noise can be reliably removed from the FM composite signal.

[0015]

Therefore, by utilizing the AGC signal for controlling the amplification factor of the amplifier circuit for varying the level of the detected noise, the reference level of the circuit for noise detection is varied according to the AGC signal. In addition, even in the case of a weak electric field, only pulse noise can be reliably removed.

[Brief description of the drawings]

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

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

FIG. 3 is a characteristic diagram for explaining the operation of FIG. 2;

FIG. 4 is a characteristic diagram for explaining the operation of FIG. 1;

[Explanation of symbols]

 Reference Signs List 8 delay circuit 9 high-pass filter 10 amplifier circuit 11 AGC circuit 12 comparison circuit 13 reference power supply 14 monostable multivibrator 15 gate circuit 16 subtraction circuit 17 capacitor

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-34727 (JP, A) JP-A-3-270418 (JP, A) JP-A-4-365229 (JP, A) JP-A-56- 169439 (JP, A) JP-A-59-91732 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04H 5/00 H04B 1/10

Claims (1)

(57) [Claims] 1. A high-pass filter for passing a noise component in an input signal, an amplifier circuit for amplifying an output signal of the high-pass filter, and a variable gain of the amplifier circuit according to an output signal of the amplifier circuit. An AGC circuit for generating an AGC signal; a noise detection circuit for varying a reference level in accordance with the AGC signal, and detecting noise equal to or higher than the reference level; and a gate signal in response to an output signal of the noise detection circuit. A noise removing circuit comprising: a gate signal generating circuit that generates the signal; and a cancel circuit that cancels a noise component in the input signal according to the gate signal.