JP2000101462A - Receiver provided with noise canceller - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve sound quality by reducing a noise generated through switching control due to noise canceller or remaining noise which is not removed by switching control. SOLUTION: A noise canceller 22 is provided to eliminate a pulse noise mixing into an antenna 23 of the receiver 21. A gate signal generating circuit 33 of the noise canceller 22 generates a gate signal corresponding to the detected pulse noise, a switch circuit 34 shuts off an audio signal with a low frequency outputted from a detection circuit 28, so as to prevent the pulse noise from mixing into the audio output. The cut-off frequency of a low-pass filter 41 of an output circuit 40 is changed into lower frequencies by controlling a switching circuit 45 with a gate signal from the gate signal generating circuit 33 so as to reduce the noise generated, when the switch circuit 34 is subject to switching control and the remaining pules noise which is not removed by the switching control, thereby improving the sound quality.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a receiver provided with a noise canceller for reducing pulse noise when receiving broadcast or communication.

[0002]

2. Description of the Related Art FIG. 5 shows a schematic electrical configuration of a receiver 1 on which the present invention is based. The receiver 1 includes a noise canceller 2 and can remove a pulse noise mixed in the antenna 3 to obtain an audio output. A high-frequency signal received by the antenna 3 is supplied to a high-frequency amplifier (hereinafter referred to as “RF
AMP) 4, is amplified by a mixing circuit (hereinafter sometimes abbreviated as “MIX”) 5, and is generated from an oscillation circuit (hereinafter sometimes abbreviated as “OSC”) 6. It is mixed with the local oscillation signal. The output of the mixing circuit 5 includes a frequency component of a sum or a difference between the high-frequency signal and the local oscillation signal. After that, the signal is selectively amplified at 7) and then demodulated at a detector 8 to extract an audio signal.

The pulse noise mixed from the antenna 3 is input from the mixing circuit 5 to the noise canceller 2.
In the noise canceller 2, after being amplified at a constant amplification rate by an amplification circuit (AMP) 9, a voltage-controlled amplification circuit (hereinafter referred to as "V
CA) and an automatic gain control circuit (hereinafter may be abbreviated as "AGC") 11 such that the gain is small if the input signal level is large, and the gain is large if the signal level is small. And amplify. The comparator 12 discriminates a portion of the output of the voltage control amplifier circuit 10 where the output level largely changes as pulse noise. The output of the comparator 12 is supplied to a gate signal generation circuit 13, and a gate signal is generated according to the pulse detected by the comparator 12. The gate signal is
When the gate signal is at a predetermined level, the switch circuit 14 cuts off the audio output from the detection circuit 8. While the switch circuit 14 cuts off the audio output, the sample and hold circuit 15 operates,
The audio output is maintained at a level immediately before the switch circuit 14 is cut off. Gate signal generation circuit 13 includes an NPN transistor 16 for switching control, a constant current source 17 connected to the collector side, and a capacitor 18 for integration.
The sample and hold circuit 15 also includes a hold capacitor 19.

FIG. 6 shows an outline of a process of removing pulse noise in the noise canceller 2 shown in FIG. Fig. 6 (1)
Indicates a high-frequency signal mixed with pulse noise received by the antenna 3 as shown in FIG. Pulse noise is superimposed on the high frequency signal subjected to amplitude modulation (AM). FIG. 6B shows a waveform of the audio signal demodulated by the detection circuit 8 as shown in FIG. Although the pulse signal is included in the audio signal, the pulse noise is delayed in time and wider than the state mixed with the antenna 3 shown in (1). Such a time delay and the widening of the width mainly occur in the intermediate frequency amplifier circuit 7 shown in FIG. In the intermediate frequency amplifying circuit 7, a filter having a steep characteristic is used in order to increase the selectivity of the received signal. For example, in a ceramic filter or the like, a considerable delay occurs in a passing signal.

In the noise canceller 2, in order to avoid a signal delay caused by the intermediate frequency amplifier 7, the amplifier 9 and the voltage control amplifier 10 amplify mainly the intermediate frequency signal component in the output of the mixer 5. Since a filter having selectivity characteristics is not used in the steep case, the input signal level is discriminated by the comparator 12 in a state close to the pulse noise shown in FIG. 6A, and as shown by a virtual line in FIG. Output. When the detection output of the pulse noise as shown by the virtual line in FIG. 6C is generated as the output of the comparator 12, the gate signal generation circuit 13 in FIG.
Thus, a gate signal indicated by a solid line in FIG. 6C is supplied to the control input side of the switch circuit 14 illustrated in FIG.
When the comparator output is at a high level, the NPN transistor 16 conducts and its output goes to a low level.
When the comparator output returns to the low level, the NPN transistor 16 is shut off, a constant current flows from the constant current source 17 to the output integrating capacitor 18, and the NPN transistor 1
The output voltage of No. 6 rises. The fall time of the gate signal is when the NPN transistor 16 is turned on. Since the capacitor 18 is rapidly discharged with a sufficiently low impedance, the fall time is short. Therefore, the gate signal sharply falls and rises with a slope. Switch circuit 14 conducts when the level of the gate signal exceeds reference level Vref shown in, for example, FIG. 6 (3), and the level of the gate signal is changed to reference level Vref.
An operation of shutting off is performed when: As a result, the waveform shown in FIG. 6D is obtained as the audio output shown in FIG.

In the noise canceller 2 shown in FIG. 5, a switching circuit 14 shown in FIG.
However, problems such as the occurrence of high-frequency noise due to the ON / OFF operation associated with the switching control and the occurrence of uncut noise occurring after the gate signal as shown in FIG.

A prior art for effectively removing extraneous pulse noise and noise generated inside the receiver is disclosed in, for example, Japanese Patent Application Laid-Open No. 56-84036. In this prior art, a high-cut device is provided to reduce internal noise when a weak electric field is applied, and the operation of the high-cut device is stopped when pulse noise is detected in order to prevent rounding of a pulse waveform.

[0008]

Problems to be Solved by the Invention
In the prior art of No. 6, nothing is shown about the removal of high frequency noise accompanying the operation of the switch circuit 14 of the noise canceller 2 as shown in FIG. In the switch circuit 14, since the switching element operates at high speed to perform ON / OFF control of the audio signal, high-frequency noise is generated due to the switching. In addition, FIG.
Even if the noise signal as shown in FIG. 6 is output from the detection circuit 8 in a state where it is blurred as shown in FIG.
Does not necessarily correspond to the width of the pulse noise. The comparator 12 detects a portion where the input signal exceeds the threshold value as pulse noise, and the gate signal generation circuit 13 charges the integrating capacitor 18 at the rising edge of the signal to widen the pulse width. For this reason, in a certain range or condition regarding the waveform or level of the pulse noise mixed into the antenna 3, FIG.
Even if the gate signal shown by the solid line exceeds the reference level Vref, the pulse noise waveform remains, and the uncut noise always occurs. These noises are mixed into the audio output,
It degrades the audible sound quality.

An object of the present invention is to provide a receiver including a noise canceller capable of improving noise quality by reducing noise which is generated due to the operation of a switch circuit of the noise canceller or which is left out upon removal. That is.

[0010]

SUMMARY OF THE INVENTION The present invention provides a noise canceller that performs switching control so as to detect pulse noise in a received signal and cut off the output of the received signal during the detection period to remove the noise. A noise canceller, comprising: a low frequency output circuit that derives a received signal output by changing a high-frequency attenuation of a frequency characteristic according to whether the switching control by the noise canceller is in operation or in non-operation. It is a receiver provided with.

According to the present invention, the noise canceller controls so as to cut off the output of the received signal when pulse noise is detected in the received signal by the switching control. The low-frequency output circuit operates during switching operation by the noise canceller and within a certain time after the operation ends.
Since the high-frequency attenuation of the frequency characteristics is changed so that the output signal level in the high frequency range is reduced, noise or uncut noise that occurs with the operation of the switch circuit of the noise canceller is reduced, and sound quality is improved. be able to.

Further, in the present invention, the low frequency output circuit includes a frequency characteristic changing element and a switching circuit for switching connection and disconnection of the frequency characteristic changing element in response to a switching control signal of the noise canceller. It is characterized by including.

According to the present invention, the frequency characteristic changing element provided in the low frequency output circuit is a switching circuit that responds to a switching control signal of the noise canceller, and is in a state of being connected to the low frequency output circuit and a state of being disconnected. Are switched. The frequency characteristic of the low-frequency output circuit can be easily changed by adding a frequency characteristic changing element and a switching circuit.

Further, in the present invention, the receiver includes an AM high-cut circuit for receiving an AM signal, which attenuates a high frequency component of a low-frequency output signal demodulated when an electric field level is small, and shuts off output by the noise canceller. A high cut control circuit for operating the AM high cut circuit also at the time of switching control is further included.

According to the present invention, the AM high-cut circuit for attenuating the high frequency component of the low frequency output signal demodulated when the electric field level is small when receiving the AM signal is provided even when the switching of the output cutoff by the noise canceller is controlled. Because it operates, the noise and uncut noise due to switching control can be reduced by the AM high cut circuit,
In a receiver including an AM high cut circuit, noise generated due to switching control of the noise canceller can be reduced and sound quality can be improved without adding a new configuration.

[0016]

FIG. 1 shows a schematic electrical configuration of an embodiment of the present invention. Receiver 21 of the present embodiment
Includes a noise canceller 22, an antenna 23, a high-frequency amplifier circuit 24, a mixing circuit 25, an oscillation circuit 26, an intermediate frequency amplifier circuit 27, a detection circuit 28, and an amplifier circuit, which are basically equivalent to the receiver 1 shown in FIG. 29, a voltage control amplifier circuit 30, an automatic gain control circuit 31, a comparator 32, a gate signal generation circuit 33, a switch circuit 34, a sample hold circuit 35, and the like. Further, the gate signal generation circuit 33 includes an NPN transistor 36, a constant current source 37, and a capacitor 38 in the same manner as the gate signal generation circuit 13 of FIG. The sample and hold circuit 35 includes
Of the capacitor 39 as in the sample-and-hold circuit 15 of FIG.
including. The operation of each of these components is basically equivalent to that of FIG.

In the receiver 21 of this embodiment, an output circuit 40 is provided on the output side of the switch circuit 34, and the output circuit 40 includes a low-pass filter (LPF) 41. The low-pass filter 41 is composed of a resistor 42 connected in series with the passage of the audio signal, and a capacitor 43 connected between the output side of the resistor 42 and the ground. The frequency characteristics of the low-pass filter 41 are determined based on the resistance value of the resistor 42 and the capacitance of the capacitor 43. In a low frequency region where the impedance of the capacitor 43 is larger than the resistance value of the resistor 42, if the impedance on the output side is higher than the resistance value of the resistor 42, the audio signal is hardly attenuated. For a component having a high frequency, the capacitor 43
And the level of audio output drops at a rate of 6 dB per octave. The switch circuit 45 connects the capacitor 44 in parallel with the capacitor 43 according to the gate signal from the gate signal generation circuit 33. When the capacitor 44 is connected in parallel with the capacitor 43, the high-frequency attenuation characteristic of the low-pass filter 41 shifts to the low-frequency side, and higher attenuation is given to the high-frequency signal.

FIG. 2A shows the switching circuit 45 of FIG.
The operation timing of the switching circuit 4 is shown in FIG.
5 shows a change in the frequency characteristic of the low-pass filter 41 according to FIG. As shown in FIG. 2A, the switching circuit 45 conducts at time t0 when the level of the gate signal from the gate signal generation circuit 33 rises to the high level, and the capacitor 44
Are connected in parallel with the capacitor 43. When the noise detection ends and the gate signal rises, it exceeds the reference level Vref at time t1, the cutoff state of the switch circuit 34 in FIG. 1 ends, and the audio signal from the detection circuit 28 is derived as an audio output. Become so. The switching circuit 45
Even after the time t1, the connection state of the capacitor 44 is continued,
At time t2, the gate signal is cut off after reaching the high level, and the capacitor 44 is disconnected from the low-pass filter 41. That is, the switching circuit 45 cuts off the period indicated by A between the period before the time t0 and the period after the time t2, disconnects the capacitor 44 from the low-pass filter 41, and turns off the capacitor B from the time t0 to the time t2. The capacitor 44 is connected to the low-pass filter 41 during the period shown.

As shown in FIG. 2B, A in FIG.
The frequency characteristics of the output circuit 40 change between the period B and the period B. In the period A, a high-frequency attenuation characteristic is set such that the cut-off frequency is 4 kHz for AM broadcast reception, for example. At the cutoff frequency, the resistance value of the resistor 42 becomes equal to the impedance value of the capacitor 43,
The output level drops by 3 dB. At frequencies after the cutoff frequency, as described above, 6d per octave
The output level decreases at the rate of B. Switching circuit 4
5 conducts, as shown in B, the low-pass filter 4
The cutoff frequency of 1 shifts to the lower frequency side, and the amount of attenuation to high frequency components increases. As a result, at time t0
In addition, high-frequency noise caused by switching control generated near time t1 can be reduced, and uncut noise can be reduced in a period from time t1 to time t2.
In general, when the attenuation in the high frequency range is increased from the state indicated by A to the state indicated by B, the high frequency component in the audio signal is reduced, and the sound quality is reduced. However, under the condition that noise is included, the effect of sound quality deterioration due to noise is greater than the sound quality deterioration due to narrow frequency characteristics,
By reducing the noise component, it is possible to improve the sound quality in audibility.

FIG. 3 shows the configuration of another feature of the embodiment of the present invention. In the present embodiment, a low-frequency output circuit whose frequency characteristics can be changed is configured by combining the switch circuit 34 and the sample-and-hold circuit 35 with the capacitor 44 and the switching circuit 45. Other components are the same as those of the embodiment of FIG. 1, and portions corresponding to those of the embodiment of FIG. 1 are denoted by the same reference numerals, and redundant description is omitted. The switch circuit 34 includes a bias power supply 5 on the audio input side.
0 is provided, and a switching operation is performed by a differential circuit including a pair of PNP transistors 51 and 52. The emitters of the PNP transistors 51 and 52 are commonly connected, and receive a gate signal from the gate signal generation circuit 33. The audio input is applied to the base of one of the PNP transistors 51, and the base changes the audio input signal around a constant voltage level provided by the output voltage of the bias power supply 50. The collectors of the PNP transistors 51 and 52 forming the differential circuit are respectively connected to the collectors of the NPN transistors 53 and 54 forming the current mirror circuit. The bases of the NPN transistors 53 and 54 are commonly connected, and further connected to the collector of the NPN transistor 53 on the audio input side. The emitters of the NPN transistors 53 and 54 are grounded via resistors 55 and 56, respectively. The collector of NPN transistor 54 is connected to the base of PNP transistor 52, and is connected to holding capacitor 39 via resistor 57. The switching circuit 45 and the capacitor 44 are connected in parallel with the holding capacitor 39. When the capacitors 44 are connected in parallel, the frequency characteristics can be attenuated on the high frequency side, as in the embodiment of FIG. 1, so that noise accompanying switching control can be reduced.

FIG. 4 shows a partial configuration of a low-frequency output circuit as still another embodiment of the present invention. The receiver according to the present embodiment includes an AM high cut circuit 60. The AM high cut circuit 60 is generally used in an AM radio receiver, and changes a frequency characteristic according to an electric field level received by an antenna. For example, the circuit is ON / OFF controlled by the output of a signal meter (S meter) provided in the AM radio so that the characteristic of FIG. A
When the M high cut circuit 60 is turned on, the AM high cut capacitor 61 is connected to the audio signal system, and lowers the output level of high frequency components. In the present embodiment, the OR circuit 62 is provided so that the AM high cut circuit 60 operates even in response to the gate signal generated from the gate signal generation circuit 33 in FIG.

In the present embodiment, an existing circuit can be used in a receiver having the AM high cut circuit 60 originally. Since the AM high-cut capacitor 61 already exists, the sound quality can be improved only by providing the OR circuit 62 without increasing the circuit scale and the manufacturing cost. Further, a capacitor 44 and a switching circuit 45 as shown in FIG. 1 can be further connected in parallel to the AM high cut capacitor 61 so that the frequency characteristics at the time of AM high cut and the frequency characteristics at the time of noise canceler operation can be changed.

In each of the embodiments described above, the cut-off frequency of the passive low-pass filter constituted by the resistor 42 and the capacitor 43 is changed to reduce high-frequency noise accompanying switching control of the noise canceller. However, other types of low-pass filters can be used, such as active filters.

[0024]

As described above, according to the present invention, noise generated by switching control of the noise canceller and uncut noise are reduced by attenuating the high frequency range of the low-frequency output circuit, and the operation of the noise canceller is reduced. In addition to the elimination of the pulse noise accompanying the above, the sound quality can be further improved.

According to the present invention, the frequency characteristic changing element and the switching circuit are added to the low frequency output circuit,
Noise accompanying switching control of the noise canceller can be reduced, and sound quality can be improved with a simple configuration.

Further, according to the present invention, in a receiver including an AM high-cut circuit, noise accompanying switching control of a noise canceller can be reduced, and an increase in the number of components can be minimized to improve sound quality.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic electrical configuration of a receiver 21 according to an embodiment of the present invention.

FIG. 2 is a time chart showing a timing at which a cutoff frequency of a low-pass filter 42 of an output circuit 40 is switched in the receiver 21 of FIG. 1, and a graph showing a frequency characteristic that is changed by switching the cutoff frequency. .

FIG. 3 is a simplified electric circuit diagram showing a partial configuration of a low-frequency output circuit according to another embodiment of the present invention.

FIG. 4 is a simplified block diagram showing a partial configuration of a low-frequency output circuit as still another embodiment of the present invention.

FIG. 5 is a block diagram showing a schematic electrical configuration of a receiver 1 on which the present invention is based.

FIG. 6 is a time chart showing an operation of the noise canceller 2 in the receiver 1 of FIG.

[Explanation of symbols]

 Reference Signs List 21 receiver 22 noise canceller 23 antenna 25 mixing circuit 27 intermediate frequency amplifying circuit 28 detection circuit 29 amplifying circuit 30 voltage control amplifying circuit 31 automatic gain control circuit 32 comparator 33 gate signal generating circuit 34 switch circuit 40 output circuit 41 low-pass filter 42 Resistance 43, 44 Capacitor 45 Switching circuit 60 AM high cut circuit 61 AM high cut capacitor 62 OR circuit

Claims (3)

[Claims] 1. A receiver provided with a noise canceller for performing switching control so as to detect pulse noise in a received signal and cut off the output of the received signal during a detection period to remove the noise. During control operation,
A receiver comprising a noise canceller, comprising: a low-frequency output circuit that derives a received signal output by changing a high-frequency attenuation of a frequency characteristic according to a time of non-operation. 2. The low frequency output circuit includes: a frequency characteristic changing element; and a switching circuit for switching connection and disconnection of the frequency characteristic changing element in response to a switching control signal of the noise canceller. Claim 1 characterized by the following:
A receiver comprising the noise canceller according to any one of the preceding claims. 3. The receiver according to claim 1, further comprising: an AM high-cut circuit for attenuating a high frequency component of a low-frequency output signal demodulated when an electric field level is low, for receiving an AM signal; 2. The receiver according to claim 1, further comprising a high-cut control circuit for operating the AM high-cut circuit.