JPS6243610B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pulse noise detection circuit that is particularly adapted to easily detect pulse noise in an FM receiver or the like.

In FM receivers, a signal meter is often connected to the intermediate frequency amplification stage to detect the electric field strength of the received signal.

Figure 1 is a block diagram showing the main parts of an FM receiver, where 1 is an antenna, 2 is a front end section, and 3 is a block diagram showing the main parts of an FM receiver.
IF filter section, 4 is IF amplifier section, 5 is FM detection section, _T1 is signal meter connection terminal, _T2 is audio signal (detection output signal) output terminal. In the above, the output of the signal meter connected to terminal _T1 includes not only signal information regarding the electric field strength but also information regarding pulse noise and the modulation state of FM. Figures 2 and 3 show the signal waveforms of the signal meter output signal and detection output signal obtained from the terminals T ₁ and T ₂ , where the horizontal axis represents time t and the vertical axis represents the output voltage v. . In FIG. 2, the DC voltage V increases in electric field strength level in proportion to the electric field strength. Further, this DC voltage V changes due to pulse noise, and when a strong pulse noise is mixed in, ripples are instantaneously generated in the direction A. Furthermore, if an IF signal exceeding the bandwidth of the IF filter section is input, the receiver enters an overmodulated state, and the DC voltage V momentarily generates ripples in the direction B.

Correspondingly, distortions A' and B' occur in the waveform of the detection output signal shown in FIG. 3, and are output as pulse noise and overmodulation noise, respectively.

That is, from the signal meter output provided for detecting the electric field strength, it is also possible to know the occurrence state of pulse noise and overmodulation noise.

This shows the details of the signal waveform when pulse noise is mixed in Figure 4. By mixing pulse noise like b into the FM signal (IF carrier signal) like a, the composite waveform becomes c. As a result, as long as it is not saturated by envelope detection, the part mixed with pulse noise increases in the direction d
A waveform like this will be obtained by the signal meter.

FIG. 5 shows details of the signal waveform when overmodulation occurs. Peak P ₁ for time t as in a
The FM signal that changes between ~P ₂ with a sine curve is b
The peak above is centered around the receiving frequency _O as in
Considering a field where the IF band is received with the IF band characteristics consisting of the upper frequency _H and lower frequency _L corresponding to P ₁ and P ₂ respectively, and the IF bandwidth W is particularly narrow, the reception frequency
In _O , the carrier level of IF is e ₁ ,
The envelope-detected DC voltage V ₁ is generated at the corresponding signal meter output of c. On the other hand, the FM signal levels at the upper frequency _H and lower frequency _L corresponding to the above peaks P ₁ and P ₂ are P″ ₁ and P″ ₂ because they deviate from the above reception bandwidth W.
Since the level is at point _e2 , the signal meter output at this time drops to DC voltage _V2 . Therefore, when an overmodulation condition occurs, the signal meter output will decrease in the direction.

At this time, the detection output signal d is output through the S-curve waveform of e, but since the peaks P ₁ and P ₂ are on both sides of the S-curve, _{the two} points P' ₁ and P' are signal distortion occurs, and
Overmodulation noise occurs because the FM signal level decreases at points P″ ₁ and P″ ₂ . In other words, when the IF bandwidth is narrow as described above, the input IF signal frequency falls outside the band of the IF filter, which lowers the input level of the IF amplifier, and as the IF limiter decreases, the degree of AM component suppression decreases. As a result, the S/N of the detection output deteriorates and overmodulation noise occurs. Particularly in the United States, where there are many FM broadcasting stations, widening the IF filter band can cause problems such as adjacent interference, so it is restricted and the above-mentioned problems are unavoidable.

By the way, overmodulation noise is considered a type of pulsed noise, so in applications such as car radios that are equipped with a pulsed noise canceller for ignition noise, this canceller may malfunction due to overmodulation noise. This caused problems with sound quality deterioration.

On the other hand, in conventional FM receivers, in order to detect pulse noise prior to pulse noise removal, a particularly high frequency component of the detection output signal is used, and a pulse signal with a level sufficiently higher than the average level of this signal component is used. The detection method used was to treat the noise as sexual noise. For this reason, especially in the case of pulsed noise containing many low frequency components, such as the detection signal obtained in a weak electric field by the quadrature detection method, which has recently been adopted as it is advantageous for IC implementation, this pulsed noise Since the above-mentioned high-frequency components are hardly included, and because the amplitude level ratio with respect to other internal noise levels is small, there is a problem that they are not detected and the noise removal effect is reduced.

The present invention has been made in response to the above problems, and by utilizing the high frequency components of the detection output signal and changes in the DC voltage of the signal meter output, the noise of the detection output signal is reduced by overmodulation of pulsed noise. It is an object of the present invention to provide a pulse noise detection circuit configured to eliminate the conventional drawbacks by determining whether the noise is noise and detecting only pulse noise. Embodiments of the present invention will be described below with reference to the drawings.

FIG. 6 is a block diagram showing a pulse noise detection circuit according to an embodiment of the present invention, where IN is an input terminal for a detection output signal (audio signal), OUT is an output terminal, 1 is a high-pass filter, 2 is a low-pass filter, and 3 is a low-pass filter. Noise amplifier circuit, 4 is an AGC level detection circuit, 5 is a smoothing circuit, 6 is a first pulse generation circuit, 7 is an input terminal for the signal meter output signal,
8 is a first gate circuit, 9 is a level holding circuit, 10
11 is a comparison circuit, 11 is a noise level detection circuit, and 12 is a comparison circuit.
1 is a second pulse generating circuit, and 13 is a second gate circuit.

With the above configuration, the detection output signal applied to the input terminal IN is branched into the a direction and the b direction, and the signal component branched in the a direction passes through the low-pass filter 2 and then is filtered by the second gate circuit 13. After pulse noise is removed, it is output from the output terminal OUT.

On the other hand, pulse noise is detected by the signal component branched in the b direction. First, pulse noise is applied to high-pass filter 1 as shown in Figure 7a.
Assuming that the detection output signal containing A' and overmodulation noise B' is added, the signal meter output will have directional ripple A and directional ripple A and directional ripple A and directional ripple A and directional ripple A and directional ripple A and directional ripple B', and the DC voltage v changes due to the noise. The output waveform of the high-pass filter 1 is as shown in c, and the amplitudes of both portions corresponding to pulse noise and overmodulation noise are increased and output. In the past, there was a possibility that both of these noises were considered to be pulse noise and removed without being discriminated.

However, the present invention operates as follows.

The signal amplified by the noise amplifier circuit 3 is
An AGC operation is applied by the AGC level detection circuit 4 and the smoothing circuit 5 to obtain a predetermined average level.

FIG. 7d shows the output waveform of the AGC level detection circuit 4, in which the pulse noise and overmodulation noise portions have a large amplitude so as to exceed the preset operating level L. Only when this level L is exceeded, the first pulse generating circuit 6 is operated to generate an output pulse such as e, and the first gate circuit 8
control off. The signal waveform shown in FIG.
b' and is applied to the comparator circuit 10. Further, since a level holding circuit 9, for example, a capacitor circuit, is connected to the output side of the first gate circuit 8, the level holding circuit 9 is used during the period when the first pulse generating circuit 6 is turned off. Since the level is maintained, the output of the first gate circuit 8 has a DC voltage waveform as shown in FIG. 7f. Comparison circuit 1
0 is the f waveform signal from the first line and the second line.
A comparison operation is performed with the waveform signal b using the line. As a result, if the signal on the second line (signal meter output signal) is relative to the first line signal, it can be determined that it is pulse noise for the above-mentioned reason, and if it is, it can be determined that it is overmodulation noise. In each case, comparator circuit 10 applies its output signal to noise level detection circuit 11 to control its operation. Since the noise level detection circuit 11 is connected to the noise amplifier circuit 3, the output waveform of the noise amplifier circuit 3 as shown in FIG. .

At this time, when the output of the comparator circuit 10 detects a voltage (corresponding to pulsed noise), it decreases the detection level L' as shown by the dotted line to increase the detection sensitivity; equivalent)
When detected, the noise level detection circuit 11 is controlled so as to increase the detection level L' and lower the detection sensitivity.

Therefore, the noise level detection circuit 11 performs a detection operation only when the output voltage particularly corresponds to the pulsed noise portion of the output signal from the comparison circuit 10, and operates the second pulse generation circuit 12 only at this time. A signal as shown in Fig. 7h is output. As a result, the second pulse generating circuit 12 applies its output to the second gate circuit 13 and is controlled so as to remove particularly the pulse noise portion of the detected output signal as shown in FIG. 7a.

Therefore, from the output terminal OUT, an audio output signal having a waveform as shown in FIG. 7i from which only pulse noise has been accurately removed is obtained.

Even when the peak value of pulsed noise is not large enough, such as in the quadrature detection method,
If there is a level component that activates the first pulse generation circuit in AGC level detection, the noise detection level can be further lowered by changing the DC voltage input to the signal meter, making it possible to detect pulsed noise.

As is clear from the above explanation, according to the present invention,
By using the high frequency components of the detection output signal and changes in the DC voltage of the signal meter output, it is possible to determine whether the noise in the detection output signal is pulsed noise or overmodulation noise, and to detect only pulsed noise. Since it is composed of the following components, pulse noise can be easily detected. Furthermore, by improving the detection sensitivity of pulsed noise, it is possible to reduce differences in noise removal performance due to differences in detection methods.

Overmodulation noise occurs instantaneously, but according to the present invention, only pulse noise can be accurately detected without detecting overmodulation noise immediately in response to this instantaneous operation. Malfunctions due to noise can be prevented. In addition, this allows you to apply strong AGC as in the past, or
Problems such as time delays due to time constants are no longer necessary and are also eliminated, making reception unaffected by pulse noise possible.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a conventional example, FIGS. 2, 3, 4 a to d, and 5 a to e are waveform diagrams showing a conventional example, and FIGS. 6 and 7 a ~
i is a block diagram and a waveform diagram both showing an embodiment of the present invention. 1...High pass filter, 3...Noise amplifier circuit, 4...AGC level detection circuit, 6, 12...Pulse generation circuit, 7...Signal meter input terminal,
8, 13...gate circuit, 9...level holding circuit, 1
0... Comparison circuit, 11... Noise level detection circuit.

Claims (1)

[Claims] 1. A noise amplifier circuit and a noise amplifier circuit connected to the circuit.
An AGC level detection circuit, a pulse generation circuit connected to the circuit, a gate circuit controlled by the output pulse of the circuit, a noise level detection circuit connected to the noise amplifier circuit, and controls the operation of the circuit. a first line that applies the output of the signal meter to the comparator circuit; a second line that applies the output of the signal meter to the comparator via the gate circuit; and a level holding circuit connected to the output of the gate circuit, and operates the pulse generation circuit only when the detection output signal is applied to the noise amplifier circuit and the output of the AGC level detection circuit reaches a certain level. A pulse noise detection circuit characterized in that the noise level detection circuit is configured to detect only pulse noise by controlling a gate circuit. 2. The pulse noise detection circuit according to claim 1, wherein the pulse noise detection circuit is controlled so that the detection level in the noise level detection circuit becomes small when a positive voltage is detected in the output of the comparison circuit. . 3. The pulse noise detection circuit according to claim 1, wherein the detection level in the noise level detection circuit is controlled to increase when a negative voltage is detected in the output of the comparison circuit. . 4. The pulse noise detection circuit according to claim 1, wherein a detection output signal is applied to the noise amplifier circuit via a high-pass filter.