JP3086602B2 - 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 suitable for use in an audio multiplex demodulation circuit of a television.

[0002]

2. Description of the Related Art Generally, in a vehicle-mounted FM radio receiver, a noise removal circuit for removing the ignition noise is provided so that an S / N ratio is not adversely affected by ignition noise generated from an engine. It is provided in the stereo demodulation circuit of the FM radio receiver. The noise removal circuit of such an FM radio receiver is configured as shown in FIG.

In FIG. 2, a composite signal generated from a sound detection circuit (not shown) at a preceding stage is applied to a noise detection circuit (2) via a high-pass filter (1). In the noise detection circuit (2), when a signal of a predetermined level or more is detected from the output signal of the high-pass filter (1), it is determined that noise has been detected, and the noise detection circuit (2) generates a detection signal. Then, a gate signal having a predetermined width is generated from the monostable multivibrator (3) according to the detection signal, and is applied to the switch (4). On the other hand, the composite signal is delayed in the delay circuit (5) by the same time as the delay time due to the circuit operation of the high-pass filter (1) to the switch (4).
The (−) terminal is applied to the (+) terminal of the subtraction circuit (7) in which the capacitor (6) is connected between the (+) terminal and the (−) terminal via the switch (4) in the ON state. Is subtracted from the pilot cancel signal applied to still,
The pilot cancel signal is a signal having the same frequency, the same phase, and the same level as the 19 KHz stereo pilot signal included in the composite signal. When the switch (4) is turned on, the (-) terminal of the subtraction circuit (7) is grounded in an AC manner.

If the composite signal does not contain noise, no output signal is generated from the noise detection circuit (2).
No gate signal is generated from the monostable multivibrator (3). Therefore, the switch (4) is in the ON state,
Since the pilot cancel signal is applied to the (-) terminal of the subtraction circuit (7) via the switch (4), the low frequency signal in the composite signal is generated from the subtraction circuit (7).

When noise is detected, an output signal is generated from the noise detection circuit (2), and a gate signal is generated from the monostable multivibrator (3) by a predetermined width. The switch (4) is turned off by the gate signal,
The pilot cancel signal is obtained by subtracting (−) of the subtraction circuit (7).
It is not applied to the terminal. Therefore, the composite signal including noise is equally applied to the (+) and (-) terminals of the subtraction circuit (7) and is subtracted by the subtraction circuit (7). No noise is generated. At that time, a capacitor (6) is connected between the (+) and (-) terminals of the subtraction circuit (7), and the voltage between both ends of the capacitor (6) is the input signal level immediately before the switch (4) is turned off. Therefore, only an output signal corresponding to the voltage between both ends of the capacitor (6) is generated at the output terminal of the subtraction circuit (7).

[0006]

However, when the noise elimination circuit of the FM radio receiver as shown in FIG. 2 is used for an audio multiplex demodulation circuit of a television, a sub signal is extracted from a composite signal generated from the noise elimination circuit. Signal F
If an attempt is made to obtain a sub-sound signal by performing M demodulation, the sub-signal in the composite signal disappears due to the noise elimination operation of the noise elimination circuit of FIG. 1. There was a problem that it became worse.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has a first canceling circuit for removing noise contained in a composite signal, and a sub-signal from the composite signal. A filter to be extracted, a sub-signal demodulation circuit for demodulating an output signal of the filter, and a second signal for removing noise of an output signal of the sub-demodulation circuit.
, A noise detection circuit for detecting noise included in the composite signal, and a first operation for canceling the first cancellation circuit for a first predetermined time in accordance with an output signal of the noise detection circuit. A gate circuit, and a second gate circuit that causes the second cancel circuit to perform a cancel operation for a second predetermined time in accordance with an output signal of the noise detection circuit.

[0008] The second predetermined time is set to be longer than the first predetermined time. Further, the sub signal demodulation circuit is an FM detection circuit, and the noise elimination circuit is used for an audio multiplexing circuit for a television. The sub-signal demodulation circuit is an AM demodulation circuit, and the noise removal circuit is used for a stereo demodulation circuit of an FM radio receiver.

The first gate circuit performs a gate operation according to an output signal of the noise detection circuit, and the second gate circuit performs a gate operation according to an output signal of the first gate circuit. . The audio multiplexing circuit includes a control channel signal detection circuit for detecting a control channel signal from the composite signal, and applies an input signal of the first gate circuit to the control channel signal detection circuit.

[0010]

According to the present invention, when noise in a composite signal is detected, an output signal is generated from the first gate circuit for a first predetermined time, and the first cancel circuit is operated by the output signal. , Noise included in the composite signal is removed. In addition, when noise is detected, an output signal is generated from the second gate circuit for a second predetermined time, and the output signal generates a demodulated signal of a predetermined frequency signal extracted from the composite signal applied to the second cancel circuit. Is removed. Therefore, since the noise removal processing is performed on the predetermined frequency signal after the sub-signal demodulation, the sub-signal does not disappear, and the deterioration of the distortion factor can be prevented.

[0011]

FIG. 1 shows an embodiment of the present invention.
(8) a second monostable multivibrator that generates an output pulse according to an output signal of the noise detection circuit (2);
(9) is a second switch that operates according to the output signal of the second monostable multivibrator (8), (10) is a first bandpass filter for extracting a sub-signal from a composite signal, and (11) is a first bandpass. An FM detection circuit that performs FM detection on an output signal of the filter (10), (12) is a low-pass filter that allows the output signal of the FM detection circuit (11) to pass in a low band, and (13) is a second switch (9) and an FM detection circuit. A second subtraction circuit for subtracting the output signal from (11),
(14) is a low-pass filter for low-passing the output signal of the first subtraction circuit (7), (15) is a first de-emphasis circuit for performing de-emphasis on the output signal of the low-pass filter (14), and (16) is a first de-emphasis circuit. A second de-emphasis circuit for performing de-emphasis on the output signal of the subtraction circuit (13); (17) a first and a second de-emphasis circuit (1);
A matrix circuit for obtaining L and R signals from the output signals of (5) and (16). (18) reproduces only main sound, only sub sound, and simultaneous reproduction of main sound and sub sound at the time of sound multiplexing. (19) is a second bandpass filter for extracting a control channel signal from a composite signal, and (20) is a second bandpass filter (1).
9) an envelope detection circuit for envelope detection of the output signal, (21) a PLL circuit for generating a signal synchronized with the output signal of the AM detection circuit (20), and (22) a PLL.
The matrix circuit (1) according to the output signal of the circuit (21)
7) and a counter for supplying a switching signal to the switch circuit (18). The second monostable multivibrator (8) uses the output signal of the noise elimination circuit (2) as a trigger as shown by the solid line in FIG. 1 and triggers the output signal of the first monostable multivibrator (3) as shown by a dashed line. It may be a trigger. Also, the same circuits as those in the conventional example of FIG. 2 are denoted by the same reference numerals as those in the conventional example, and description thereof will be omitted.

In FIG. 1, an audio FM detection circuit (not shown) at the preceding stage detects a signal and outputs a main audio signal for television, F
A composite signal on which a sub-signal composed of an M-modulated sub-audio signal and a control channel signal are superimposed is applied to a high-pass filter (1) and a delay circuit (5). still,
The composite signal has a frequency spectrum as shown in FIG. The high-pass filter (1) allows only the noise pulse to pass through. If no noise pulse is included in the composite signal, no output signal is generated from the high-pass filter (1). Therefore, no noise is detected in the noise detection circuit (2), so that no detection signal is generated from the noise detection circuit (2) and no output pulse is generated from the first monostable multivibrator (3).
Also, no output pulse is generated from the second monostable multivibrator (8) to which the detection signal is not applied. First and second monostable multivibrators (3) and (8)
The first and second switches (4) and (9) to which no output pulse is applied are turned on, that is, in a state opposite to the state shown in the figure, and the first and second subtraction circuits (7) and (13) ( -) A bias is applied to the terminal.

On the other hand, in the delay circuit (5), after the composite signal is delayed by a predetermined time, it is applied to the (+) terminal of the first subtraction circuit (7). Since a bias is applied to the (-) terminal of the first subtraction circuit (7), an output signal having the same waveform as the input signal is generated from the output terminal of the first subtraction circuit (7). The bias is at the same level as the bias in the composite signal. The output signal of the first subtraction circuit (7) passes through the low-pass filter (14), and further the first de-emphasis circuit (15) performs de-emphasis on the output signal of the low-pass filter (14). A signal generated from the first de-emphasis circuit (15) is applied to a matrix circuit (17).

The output signal of the delay circuit (5) is applied to the first band pass filter (10), and only the sub signal passes through the first band pass filter (10). The sub-signal is detected by an FM detection circuit (11) and then applied to a (+) terminal of a second subtraction circuit (13) via a low-pass filter (12). Since the bias is also applied to the (-) terminal of the second subtraction circuit (13), the output signal of the second subtraction circuit (13) has the same waveform as the input signal. The output signal of the second subtraction circuit (13) is applied to a second de-emphasis circuit (16), and the output signal is de-emphasized. The signal generated from the second de-emphasis circuit (16) is transmitted to the matrix circuit (1).
7) is applied.

Further, the output signal of the delay circuit (5) is applied to a second band pass filter (19), and only the control channel signal passes through the second band pass filter (19). The control channel signal is subjected to envelope detection in an envelope detection circuit (20). The frequency of the output signal of the envelope detection circuit (20) differs depending on monaural broadcasting, stereo broadcasting, and audio multiplex broadcasting. Thereafter, the output signal of the envelope detection circuit (20) is applied to a PLL circuit (20), and a signal equal to the frequency of the output signal is generated from the PLL circuit (20). The output signal of the PLL circuit (20) is counted by a counter (22), and a monaural broadcast, a stereo broadcast and an audio multiplex broadcast are discriminated based on the count value, and the switching signals (a) and (b) corresponding to each broadcast are determined. )
And (c) are the counter (22) to the matrix circuit (1).
7) and the switch circuit (18).

Here, since the control channel signal is not applied to the counter (22) during monaural broadcasting, the switching signal (a), (b) and (c) is not generated, and the 982 Hz control channel signal is transmitted during stereo broadcasting. Applied to generate only the switching signal (a).
z control channel signal is applied and the control terminal (25)
Are set so as to generate switching signals (b) and (c) in response to a control signal applied to the counter (22) via the control circuit (1).

At the time of monaural broadcasting, the composite signal comprises only the main audio signal shown in FIG. 3 and does not include the control channel signal.
(B) and (c) do not occur. An output signal of the first de-emphasis circuit (15), which is a main audio signal, is applied to the matrix circuit (17), and no output signal is generated from the second de-emphasis circuit (16). Not applied to The main audio signal is not matrixed in the matrix circuit (17) to which the switching signal (a) is not applied, and the main audio signal is generated at two output terminals of the matrix circuit (17). The main audio signal passes through the switch circuit (18) to which the switching signals (b) and (c) are not applied as it is and is generated at the output terminals (23) and (24).

In stereo broadcasting, the composite signal is composed of a main audio signal (L + R) and an (L-R) signal.
It is composed of a sub-signal obtained by FM-modulating a signal and a control channel signal indicating stereo broadcasting. The signal obtained by envelope detection of the control channel signal is a 982 Hz signal. By counting the output signal of the PLL circuit (21) according to this signal, only the switching signal (a) is counted by the counter (2).
2), the (L + R) signal from the first de-emphasis circuit (15) and the second de-emphasis circuit (16)
(LR) signals are matrixed in a matrix circuit (17). The matrixed L signal and R signal are generated at two output terminals of the matrix circuit (17), and the switching signals (b) and (c) are not applied to the switch circuit (18). Is generated at an output terminal (23), and the R signal is generated at an output terminal (24).

Further, at the time of audio multiplexing, the composite signal includes a main audio signal, a sub-signal serving as a sub audio signal, and a control channel signal indicating audio multiplexing. As a result of envelope detection of a control channel signal indicating audio multiplexing, 9
An 82 Hz signal is generated from the envelope detection circuit (20), and the switching signal (a) is not generated from the counter (22). Therefore, the main audio signal from the first de-emphasis circuit (15) and the sub-audio signal from the second de-emphasis circuit (16) are not matrixed by the matrix circuit (17), and the main audio signal and the sub-audio signal are not changed by the switch circuit. (1
8). By the way, at the time of voice multiplexing,
There are cases where a main sound is generated, cases where a sub sound is generated, and cases where a main sound and a sub sound are simultaneously generated. When a control signal for selecting the main sound is applied to the control terminal (25), the switching signal (b) is generated from the counter (17), so that the main sound signal is selected in the switch circuit (18) and the output terminal ( A main voice signal is generated in (23) and (24). When the control signal for selecting the sub-audio is applied, the switching signal (c) is generated, so that the sub-audio signal is selected, and the sub-audio signal is generated at the output terminals (23) and (24). Further, when a control signal for selecting the main sound and the sub sound is applied, the switching signals (b) and (c) are not generated, and the main sound signal and the sub sound signal pass through the switch circuit (18) as they are. The main audio signal is generated at the output terminal (23), and the sub audio signal is generated at the output terminal (24).

Now, assuming that pulse noise is included in the composite signal, the noise passes through a high-pass filter (1) and is applied to a noise detection circuit (2). In the noise detection circuit (2), when the output signal level of the high-pass filter (2) is equal to or higher than a predetermined level,
Since the noise has been detected, the noise detection circuit (2)
Is generated and applied to the first monostable multivibrator (3). Then, a pulse having a predetermined width is generated from the first monostable multivibrator (3). The first switch (4) to which the output pulse of the first monostable multivibrator (3) is applied is turned off, that is, the state shown in the drawing, and a bias is applied to the (-) terminal of the first subtraction circuit (7). Disappears. Therefore, the composite signal including noise is equally applied to the (+) and (-) terminals of the subtraction circuit (7) and is subtracted by the subtraction circuit (7). No noise is generated. At that time, a capacitor (6) is connected between the (+) and (-) terminals of the subtraction circuit (7), and the voltage between both ends of the capacitor (6) is the input signal level immediately before the switch (4) is turned off. Therefore, only an output signal corresponding to the voltage between both ends of the capacitor (6) is generated at the output terminal of the subtraction circuit (7). Therefore, when noise occurs, the noise can be removed by setting the level to a level immediately before the noise occurs for a predetermined time.

Further, a pulse longer in time than the output pulse of the first monostable multivibrator (3) is generated from the second monostable multivibrator (8) in accordance with the output signal of the noise detection circuit (2). 2 applied to switch (9). In response to the pulse, the second switch (9) turns off, that is, the state shown in the figure, and the second subtraction circuit (13)
No bias is applied to the (−) terminal of the above. For that reason,
The sub signal including noise is calculated by (+) of the subtraction circuit (13).
And a subtraction circuit (13) applied equally to the
, No noise is generated at the output terminal of the subtraction circuit (13). At this time, a capacitor (26) is connected between the (+) and (-) terminals of the subtraction circuit (13).
Is connected, and the voltage across the capacitor (26) holds the input signal level immediately before the switch (9) is turned off. Therefore, only the output signal corresponding to the voltage across the capacitor (26) is applied to the subtraction circuit. This occurs at the output end of (13). As described above, by performing the FM detection on the FM-modulated sub-signal included in the composite signal and then performing the noise removal, the distortion rate can be improved without largely destroying the FM-detected sub-signal. As shown by the dashed line in FIG. 1, the output signal of the first monostable multivibrator (1) is applied as a trigger of the second monostable multivibrator (8), and the output pulse of the first monostable multivibrator (3) is applied. Is set to a level at which the second monostable multivibrator (8) can operate satisfactorily, especially when the IC is implemented, even if the characteristics of the elements vary, the second monostable multivibrator (8) is surely detected when noise is detected. The stable multivibrator (8) can be operated.

By the way, the first band-pass filter (1
0) is composed of a plurality of band-pass filters (10a) to (10c) having different center frequencies as shown in FIG. 4, whereby the pass band of the band-pass filter can be made flat and the attenuation characteristic can be made steep. However, on the other hand, a delay occurs in the sub signal path, and the noise becomes longer in time. Therefore, if the output pulse generation time of the second monostable multivibrator (8) is made longer than the output pulse generation time of the first monostable multivibrator (3), the first band-pass filter (10) and the low-pass filter (12). The noise can be removed even if the noise generation time in the detected sub-signal becomes long due to the delay caused by the above.

Although the control channel signal is extracted from the output signal of the delay circuit (5), the control channel signal can be reliably detected when noise is generated. Thus, the control channel signal is not lost due to the above, and the malfunction of the matrix circuit (17) and the switch circuit can be prevented. Also, FM
The detection circuit (11) is replaced with an AM detection circuit, and the first and second
If the bias and the pilot cancel signal are applied from the bias applied to the (-) terminals of the subtraction circuits (7) and (13), the present invention can be applied to the stereo demodulation circuit of the FM radio receiver. .

[0024]

Therefore, according to the present invention, not only the noise is removed from the composite signal, but also the sub-signal in the composite signal is extracted, and the noise is removed after detecting the sub-signal. The loss of the sub signal can be prevented, and the distortion rate of the sub signal can be improved.

Since the noise elimination time of the sub-signal is longer than the noise elimination time of the composite signal, even if the noise generation time becomes longer due to the delay time due to the signal path for extracting the sub-signal, the noise can be reduced. Can be 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 showing a frequency spectrum of a composite signal.

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

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 High-pass filter 2 Noise detection circuit 3 1st monostable multivibrator 4 1st switch 5 Delay circuit 7 1st subtraction circuit 8 2nd monostable multivibrator 9 2nd switch 10 1st bandpass filter 11 FM detection circuit 13 2nd Subtraction circuit

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-56-143740 (JP, A) JP-A-58-183533 (JP, U) JP-A-57-198139 (JP, U) JP-A-61-1981 113476 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04B 1/10-1/12 H04B 15/00 H04H 5/00

Claims (6)

(57) [Claims] A first canceling circuit for removing noise contained in a composite signal; a filter for extracting a sub-signal from the composite signal; a sub-signal demodulating circuit for demodulating an output signal of the filter; A second canceling circuit for removing noise from an output signal of the sub-demodulation circuit; a noise detection circuit for detecting noise included in the composite signal; A first gate circuit for canceling the first cancel circuit for a predetermined time; a second gate circuit for canceling the second cancel circuit for a second predetermined time in accordance with an output signal of the noise detection circuit; A noise removal circuit comprising: a gate circuit; 2. The noise elimination circuit according to claim 1, wherein the second predetermined time is longer than the first predetermined time. 3. The noise elimination circuit according to claim 1, wherein said sub-signal demodulation circuit is an FM detection circuit, and said noise elimination circuit is used for an audio multiplexing circuit for a television. 4. The noise elimination circuit according to claim 1, wherein said sub signal demodulation circuit is an AM demodulation circuit, and said noise elimination circuit is used for a stereo demodulation circuit of an FM radio receiver. 5. The semiconductor device according to claim 1, wherein the first gate circuit performs a gate operation in accordance with an output signal of the noise detection circuit, and the second gate circuit performs a gate operation in accordance with an output signal of the first gate circuit. The noise removal circuit according to claim 1, wherein 6. The audio multiplexing circuit includes a control channel signal detection circuit for detecting a control channel signal from the composite signal, and applies an input signal of the first gate circuit to the control channel signal detection circuit. The noise removing circuit according to claim 3, wherein