JPS6366087B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a noise and distortion reduction circuit for an FM receiver, and more particularly to a circuit for reducing noise and distortion specific to a vehicle-mounted FM receiver.

[Prior art] When an FM receiver is mounted on a vehicle and moved,
Noise and distortion are caused by various external factors.
For example, first, there is noise due to variations in electric field strength based on Rayleigh distribution that occur in weak electric field regions. That is, in a weak electric field region, when the electric field strength changes from a relatively large state to a small state, for example, if the input electric field strength decreases by 30 dB, the S/N ratio deteriorates by about 55 dB. then,
The noise level (white noise) increases by 55 dB, and this increase in noise level causes discomfort to the listener. Second, there is noise due to so-called multipath distortion due to the influence of reflected waves. It is known that the ratio of the former to the latter appears at a ratio of about 3:7.

Conventional in-vehicle FM receivers mainly determine the output level of the intermediate frequency signal circuit, detect the received electric field strength, and detect when the electric field strength falls below a certain level in order to alleviate the noise caused by the former Ray-Ray fluctuation. In this case, the high frequency range of the demodulated output is suppressed and the stereo separation is reduced, thereby improving the apparent S/N ratio.

[Problems to be Solved by the Invention] However, in the conventional device, noise is reduced based on the level of the intermediate frequency signal, so when the level of the intermediate frequency signal (carrier level) is higher than a predetermined level, However, there was a problem in that no countermeasures could be taken for the generation of noise due to multi-pulse distortion, and the noise would be output as is.

Therefore, the main object of the present invention is to significantly reduce noise and distortion mainly due to multipath distortion, regardless of the strength of the electric field strength of the received signal.
An object of the present invention is to provide a noise and distortion reduction circuit for an FM receiver.

[Means for Solving the Problems] The present invention detects only the noise and distortion components contained in the output components of the demodulation circuit, charges the charging/discharging circuit based on the detected output, and adjusts the charging voltage to Accordingly, part or all of the output of the demodulation circuit including high frequency components is reduced.

[Operation] According to the present invention, only the noise and distortion components contained in the demodulated output are extracted regardless of the output level of the intermediate frequency signal, and the reduction means is operated based on them, so that the noise and distortion components are substantially reduced. The circuit operates only when distortion components are included.

[Embodiments] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of the present invention. In the car-mounted FM receiver 1, FM radio waves are received by the antenna 3, and the FM front end 5
given to. As is well known, the FM front end 5 includes a high frequency amplifier, a local oscillation circuit, a frequency conversion circuit, etc., and derives an intermediate frequency signal as its output. The intermediate frequency signal from the FM front end 5 is given to an IF amplification/demodulation circuit 7.
An example of the configuration of the IF amplification/demodulation circuit 7 is shown in FIG.

Referring to FIG. 2, the IF/demodulation circuit 7 receives an intermediate frequency signal from the FM front end 5.
For example, it includes six stages of IF amplifiers 71. The output from the IF amplifier 71 is amplitude limited by a limiter 72 and given to a quadrature detector 73.
demodulated. The output of this detector 73 is
The signal is applied to the subsequent pulse noise removal circuit 9 via the AF miute amplifier 74. On the other hand, the level of the output from the IF amplifier 71 is further detected by a level detector 75, and the detected level output is given to a signal strength indicator 76.
It is applied to the terminal 7a of the IF/demodulation circuit 7.

Returning to FIG. 1, the output of the IF amplification/demodulation circuit 7 is given to a pulse noise removal circuit 9. Here, with reference to FIG. 3, pulse noise removal circuit 9
I will briefly explain about.

The demodulated signal from the IF amplification/demodulation circuit 7 is applied to a gate circuit 93 via a low-pass filter 91 and a low-pass amplifier 92. On the other hand, the demodulated signal is further provided to a high pass filter 95 via an AGC circuit 94. The high pass filter 95 is
The high frequency band of the demodulated signal is passed through, and its output is applied to the noise detector 96 and also to the terminal 9a.
The noise detector 96 detects noise (pulse noise) according to the high frequency signal, and triggers the monostable multivibrator 97. The output of the monostable multivibrator 97 is given as a gate control signal to the gate circuit 93 described above. Therefore, the gate circuit 93 is a monostable multivibrator 97
gates the demodulated output that has passed through the low-pass amplifier 92 and supplies it to the output circuit 98. The output circuit 98 provides an output to the subsequent stereo demodulation circuit 11 and is connected to a pilot signal holding circuit 99.

Returning to FIG. 1 again, the pulse noise removal circuit 9
The output from the output circuit 98 is
The signal is applied to the stereo demodulation circuit 11. The stereo demodulation circuit 11 derives a left signal and a right signal from the demodulated output from which pulse noise has been removed. The balance between the left signal and the right signal is adjusted by the variable resistor 13 for balance, the left signal is given to the low frequency amplifier 19 via the variable resistor 15, and the right signal is given to the low frequency amplifier 19 via the variable resistor 17. The signal is applied to an amplifier 21. The low frequency amplifiers 19 and 21 are each
The corresponding speakers 23 and 25 are driven.

The terminal 9a of the pulse noise removing circuit 9, that is, the output of the high-pass filter 95 (FIG. 3), is applied via a capacitor 201 to the base of a transistor 203 as a gain element. The transistor 203 has its base supplied with a bias voltage from the power supply +B, and its collector connected to the power supply +B via a resistor. Further, the emitter of transistor 203 is grounded via switching transistor 205, a resistor, and a capacitor. switching transistor 2
The base of 05 is connected to resistor 7b by wire 205a.
IF amplification/
Terminal 7a of demodulation circuit 7, that is, level detector 75
(Fig. 2).

The collector of transistor 203 is further connected to transistors 209, 215 and diode 21.
The voltage is applied to the base of a transistor 217 through a voltage doubler rectifier circuit 207 including 1,213. The collector of transistor 217 is connected to power supply +B, and the emitter is grounded via resistor 219. At the same time, the emitter of the transistor 217 is connected to the base of a switching transistor 229 through a series connection of a diode 221 and a resistor 225, and further connected to the base of a switching transistor 231 through a series connection of a diode 221 and a resistor 227. has been done. A connection point between diode 221 and resistors 225 and 227 is grounded via capacitor 223.
This capacitor 223 constitutes a charging/discharging circuit.
A capacitor 233 is connected between the collector of the switching transistor 229 and the left signal output of the stereo demodulation circuit 11, and its emitter is grounded. Similarly, switching transistor 231
A capacitor 235 is connected between the collector and the right signal output of the stereo demodulation circuit 11, and its emitter is grounded.

The output of the high pass filter 95 (FIG. 3) included in the pulse noise removal circuit 9 is applied to the base of the transistor 203, as described above. In this embodiment, the high-pass filter 95 functions in addition to the function in the original pulse noise removal circuit 9 to detect the high-order level of the distortion component. Therefore, if the pulse noise removal circuit 9
If the FM receiver does not include a filter, a separate high-pass filter can be installed.

An example of the characteristics of the high-pass filter 95 is shown in FIG. If another independent high-pass filter is provided and one with good characteristics, that is, one with steep attenuation, is used, the performance of distortion detection, which will be described later, will be improved accordingly.
In this embodiment, the cutoff frequency of the high-pass filter 95 is selected to be an appropriate value of about 100 kHz. The cutoff frequency must be chosen appropriately. This is because if the cutoff frequency is too high, distortion cannot be detected with sufficient accuracy, and if the cutoff frequency is too low, erroneous detection may occur.

Next, a charging/discharging circuit including the capacitor 223, which is one of the features of the present invention, will be explained. The charging path of the capacitor 223 is composed of +B-transistor 217-diode 221. Therefore, the resistance component on the circuit related to this charging time constant is the internal resistance of each element. Furthermore, when the capacitor 223 is discharged, the reverse resistance R221 of the diode 221, the resistors 225, R22
5. Base-emitter resistance Rbe of transistor 229, resistance 227, R227 and transistor 2
31 base-emitter resistances R'be act in parallel. Here, the diode 221
The relationship between the reverse resistance R221 and other resistance components is expressed by the following equation (1).

R221＞(R225+Rbe)×(R227+R′be)/(R225+Rbe
)+(R227+R'be)...(1) Here, the resistance value R2 of resistors 225 and 227
25 and R227 are each 56 kΩ, and the base-emitter resistance Rbe of transistor 229 and the base-emitter resistance of transistor 231 are
Assuming that R'be is equal, the right side of equation (1) above becomes (56/2)kΩ+Rbe, and transistors 229 and 231
When is in a conductive state, the resistance Rbe is extremely smaller than 56kΩ, so the right side of equation (1) above is approximately 22.5kΩ.
Therefore, the discharging time constant of the discharging path of the capacitor 223 is extremely large compared to the charging time constant of the charging path. That is, the charging and discharging characteristics of the capacitor 223 are as shown in FIG.

In FIG. 5, line A shows the charging characteristics, and line B
indicates the discharge characteristics. It can be seen that the line B showing the discharge characteristics has a longer tail as time passes compared to the characteristics in the case of only an ordinary resistor and capacitor. This is the base-emitter resistance Rbe of transistor 229 and transistor 231.
This is because R'be increases as the base voltage decreases. However, this characteristic does not affect the switching characteristics of the transistors 229 and 231, that is, the recovery time, but rather has the effect of helping quick response when distortion occurs continuously.

Next, the operation of the circuit according to the embodiment of the present invention described above will be explained.

First, a case where received radio waves are accompanied by multipath distortion will be described. The composite wave of FM radio waves that has entered the antenna 3 is demodulated with distortion by the IF amplification/demodulation circuit 7 via the FM front end 5, and is further input to the pulse noise removal circuit 9. The demodulated signal containing distortion input to this circuit 9 passes through a high-pass filter 95 (Fig. 3),
It is output from the terminal 9a of the circuit 9. The output level of the high-pass filter 95 increases as the distortion (distortion rate) included in the demodulated signal increases and as the demodulation frequency ωm increases.

The output of the high pass filter 95 is the capacitor 2
01 to the base of the transistor 203, and is amplified by the transistor 203.
However, this is when the switching transistor 205 is in a conductive state, that is, when the voltage at the terminal 7a of the IF amplification/demodulation circuit 7 is at a high level. Note that the switching operation of the transistor 205 will be described later.

The noise and distortion components amplified by transistor 203 are rectified by voltage doubler rectifier circuit 207 and applied to the base of switching transistor 217. switching transistor 217
is conductive when the output voltage of the voltage doubler rectifier circuit 207 is equal to or higher than a predetermined value, and current flows from the power supply +B at that time. The current flowing through the transistor 217 is applied to a capacitor 223 (the capacitance of this capacitor can be selected, for example, 47 μF/16 V) via a diode 221, and the capacitor 223 is rapidly charged according to the charging characteristic shown by line A in FIG. . When the charging voltage of capacitor 223 reaches a predetermined value, switching transistors 229 and 231 become conductive at the same time. Therefore, a capacitor 233 is inserted between the left signal output of the stereo demodulation circuit 11 and the ground, and a capacitor 233 is inserted between the right signal output and the ground.
35 will be inserted. Therefore, high frequency components of the output of the stereo demodulation circuit 11 are reduced.
Note that by appropriately selecting the values of the capacitors 233 and 235, it is possible to completely cut out the high frequency, or it is also possible to perform a muting operation in which the high frequency is cut off by a necessary amount. In this way, the high frequencies of the respective signals input to the low frequency amplifiers 19 and 21 are reduced, thereby making it possible to reduce the adverse effects of noise and distortion on auditory sensation.

Next, a case where the demodulated signal contains very little or no distortion will be described. In this case, the high pass filter 95 obtained at the terminal 9a of the circuit 9
The output voltage of is almost zero. Even if the output of the high-pass filter 95, which is approximately zero, is amplified by the transistor 203, it does not reach a level sufficient to be rectified by the diodes 211 and 213 of the voltage doubler rectifier circuit 207, so the switching transistor 217 is non-conductive. Therefore, no charging current is applied to the capacitor 223, and the transistor 22
9 and 231 are both non-conductive. Therefore, capacitors 223 and 235 are disconnected from the ground, and the above-mentioned high frequency component reduction operation is not performed.

In the case of a vehicle-mounted FM receiver, the multipath distortion described above varies instantaneously in proportion to the speed of the vehicle. Therefore, immediate response is required for a change from a state with no distortion to a state, and conversely, delay characteristics are required for a change from a state with distortion to a state without distortion. This is because it needs to be restored naturally. In other words, when the state changes from a state with distortion to a state without distortion, if the high frequency reduction is immediately canceled in response, the signals input to the low frequency amplifiers 19 and 21 will be instantaneously increased, which is rather This results in audibly unpleasant noise. Therefore, in this embodiment, the charging time constant of the charging path of the capacitor 223 and the discharging time constant of the discharging path are different, and the charging and discharging characteristics are set as shown in FIG. To this end, as soon as distortion occurs, it operates to reduce the effects of noise and distortion on the sense of hearing, and when the demodulated signal changes to a state where no distortion is included, it is set to return naturally with a delay time. The additional circuit is designed to cause almost no audible discomfort.

Next, referring to FIG. 6, depending on the magnitude of the output level of the high-pass filter 95 provided in this circuit (as mentioned earlier, this is correlated with the magnitude of distortion), A description will be given of how the low frequency signals input to the frequency amplifiers 19 and 21 are given a high frequency reduction effect.

First, the high frequency reduction effect of low frequency signals is R _A (=impedance between the low frequency signal variable resistor 15 or 17 and ground), R _B (=emitter-collector impedance of transistors 229 and 231), and C _A (= capacitors 233 and 23
5).

In Figure 1, an impedance of impedance R _B + (1/jωC _A ) is connected in parallel to impedance R _A , and impedance R _B changes from the value Ra to Rb
Suppose that it has changed to. However, the value Ra is the impedance when the transistors 229 and 231 are completely non-conductive, that is, when there is no distortion at all. Further, the value Rb is the impedance when the transistors 229 and 231 are completely conductive.

When transistors 229 and 231 are completely non-conductive, impedance R _A is the output impedance of stereo demodulation circuit 11, the impedance of variable resistors 15 and 17, and variable resistor 1.
This value is obtained by connecting three impedances in parallel, and is several kΩ. On the other hand, the value Ra mentioned above is several 100k
It is Ω. Therefore, the total impedance Z when impedance R _A and impedance Ra + (1/jωC _A ) are connected in parallel is the impedance
R is almost equal to _A. That is, in this case (when transistors 229 and 231 are completely non-conducting), the effect of reducing the high frequency range of the low frequency signal is not achieved, and the frequency characteristic of the low frequency signal is as shown by line C in FIG. It will be shown as follows.

Next, when transistors 229 and 231 are completely conductive, the total impedance Z is
It is expressed by the following equation (2).

Z=R _A (Rb+(1/jωC _A ))/R _A +(Rb+(1/jω
C _A ))...(2) The fact that the transistors 229 and 231 are completely conductive means that the above value Rb is close to zero, and this equation (2) can be approximated by the following equation (2' )
It is expressed as

Z=R _A /(jωC _A )/R _A +(1/jωC _A )……(2)
′ Therefore, the high frequency reduction effect of a low frequency signal is expressed by the following equation (3).

Attenuation = 20log (R _A /Z) [dB] ...(3) In this equation (3), the total impedance Z is a function of the frequency f, so at any frequency f
The amount of attenuation at is known. The high frequency reduction effect when switching transistors 229 and 231 are completely conductive is shown by line F in FIG. Note that if you want to increase the high-frequency attenuation amount beyond line F, use capacitors 233 and 23.
The capacitance value C _A of No. 5 may be increased. As shown in Fig. 6, when there is no distortion, the frequency characteristic is normal as shown by line C, and as the distortion increases, impedance R _B gradually decreases.
The frequency characteristics change as shown by line D, line E, or line F.

In the embodiment shown in FIG. 1, sensitivity adjustment to noise and distortion can be set by appropriately adjusting the amplification gain of the transistor 203.

Next, the operation of switching transistor 205 will be explained. Transistor 205 is provided to prevent malfunction.

FIG. 7 shows the input level of the antenna 3, the output level of the high-pass filter 98, and the level detector 7.
5 is a graph showing the output level of No. 5. In FIG. 7, line G represents the output level of high-pass filter 95, and line H represents the output level of level detector 75. As can be seen from line G, white noise increases as the antenna becomes smaller. Since the white noise contains a high frequency level high enough to pass through the high pass filter 95, if it is assumed that the switching transistor 205 shown in FIG. 229 and 231
becomes conductive, and a high frequency reduction operation is performed. Therefore, in this embodiment, switching transistor 205 becomes conductive and transistor 203 becomes conductive only when the antenna input exceeds a certain level.
Thus, transistor 217 is activated.

FIG. 8 shows line 205a or transistor 2.
Transistor 203 for base voltage of 05b
The collector current characteristics and gain characteristics of 8th
In the figure, the line shows the current characteristics, and the line J shows the gain characteristics. In this example, transistor 203 is
As shown by line J in FIG.
The base voltage starts to conduct around 0.4V, and the voltage decreases to 0.6V.
It becomes completely conductive in the vicinity. Therefore, the seventh
An arbitrary antenna input level is selected based on the output level of the level detector 75 shown by line H in the figure, and this arbitrary point is set as a point K. When the base voltage becomes higher than this point K, the switching transistor 205 becomes conductive. The values of the resistors 7b and 7c shown in FIG. 1 may be selected so as to achieve the desired result. In this way, transistor 203 and therefore transistor 217 will only be activated when the antenna input level is higher than or equal to an arbitrary antenna input level, for example, higher than point K.

Note that the circuit related to the switching transistor 205 may not be necessary. That is,
If the influence of noise caused by fluctuations in electric field strength according to the Ray-Ray distribution in weak electric field regions is to be reduced, the circuit related to the switching transistor 205 is unnecessary. In this case, as shown in FIG. 9, switching transistor 205, line 205a, resistors 7b and 7c, etc. in FIG. 1 may be removed, and the emitter of transistor 203 may be directly grounded via a resistor or capacitor. .

As explained above, the electric field strength in the weak electric field region follows the Rayleigh distribution. For this reason, the antenna input level of the FM receiver mounted on vehicles traveling in this area fluctuates according to its distribution, and noise is generated each time this fluctuation occurs. Therefore, if a circuit for reducing the high frequency components of a low frequency signal is operated below the antenna input level at which noise due to this Rayray distribution occurs, noise will occur due to fluctuations in electric field strength due to this Rayray distribution. However, the influence of the noise is reduced, and reception is possible with less audible discomfort. For example, if the noise effect due to the Ray-Ray distribution begins to occur below the antenna input level at point K in FIG. good.
Then, at an antenna input level below this point K, even if noise due to Ray-Ray distribution occurs, its high frequency is reduced by capacitors 233 and 235, so its influence becomes extremely small. Moreover, this effect is more effective as the antenna input level is lower. This is because the lower the antenna input level, the greater the white noise, that is, the output of the high-pass filter 98.

As shown in FIG. 9, according to the embodiment in which the circuit related to the switching transistor 205 in the circuit of the embodiment described above (the circuit in FIG. 1) is removed, the electric field strength is reduced based on the Ray-Ray distribution in weak electric field regions. The magnitude of noise due to fluctuations and the magnitude of white noise when the antenna input level drops to a certain extent, and when the composite wave is accompanied by noise and distortion due to multipath interference in medium electric field regions. The magnitude of this distortion and noise can be detected accurately and automatically, and a high frequency reduction operation can be performed according to the magnitude of each white noise, noise, distortion, etc. Therefore, in such a case,
It does not make the listener feel uncomfortable. Moreover, if the magnitude of white noise, noise, or distortion is reduced, the high-frequency reduction effect is naturally canceled and restored naturally, so that a good noise and distortion reduction circuit can be obtained.

[Effects of the Invention] As described above, according to the present invention, it is possible to provide an FM receiver in which the influence of noise and distortion in a received signal due to multipath distortion is reduced and a good reception condition can be obtained. .

Further, it is possible to provide an FM receiver that can also reduce noise caused by fluctuations in electric field strength.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention. FIG. 2 is a block diagram showing the IF amplification/demodulation circuit 7 in FIG. 1 in detail. FIG. 3 is a block diagram showing in detail the pulse noise removal circuit 9 in FIG. 1. FIG. 4 is a graph showing the characteristics of the high-pass filter 95, in which the horizontal axis shows frequency and the vertical axis shows attenuation. FIG. 5 is a graph showing the charging and discharging characteristics of the capacitor 223, with time on the horizontal axis and terminal voltage on the vertical axis, with line A showing the charging characteristics and line B showing the discharging characteristics. Figure 6 shows the first
This is a graph for explaining the high frequency reduction effect according to the embodiment shown in the figure, in which the horizontal axis shows frequency and the vertical axis shows attenuation amount, line C shows when there is almost no noise or distortion, and lines D, E, and F. shows a case where noise or distortion gradually increases. FIG. 7 is a graph showing the output level of the high-pass filter 95 and the output level of the level detector 75 with respect to the antenna input level (horizontal axis), and line G indicates the output level of the high-pass filter 95.
The line H shows the output level of the level detector 75. FIG. 8 is a graph for explaining the action of the switching transistor 205. The horizontal axis represents the base voltage of this transistor 205, the vertical axis represents the current and the amount of attenuation, and the line represents the collector current characteristics of the transistor 203. , and line J shows the gain characteristic of the transistor 203. FIG. 9 is a circuit diagram showing another embodiment of the invention. In the figure, 1 is the FM stereo receiver, 3 is the antenna, 5 is the FM front end, and 7 is the IF amplification/
demodulation circuit; 9 is a pulse noise removal circuit; 95 is a high-pass filter; 11 is a stereo demodulation circuit;
9, 21 are low frequency amplifiers, 23 are amplification transistors (gain elements), 205, 217, 22
9,231 is a switching transistor, 207
is a voltage doubler rectifier circuit, 223 is a capacitor (charging/discharging circuit), and 233 and 235 are capacitors (for reducing demodulated output).

Claims (1)

[Claims] 1. An antenna, a front end that receives an FM signal received by the antenna and derives an intermediate frequency signal at its output, an intermediate frequency signal circuit that receives the intermediate frequency signal, and an intermediate frequency signal circuit that receives the intermediate frequency signal; and a demodulation circuit that demodulates the output of the frequency signal circuit.
A circuit for reducing noise and distortion caused by multipath in an FM receiver, comprising: a detection means for detecting only noise and distortion components included in the output component of the demodulation circuit; a charging/discharging circuit; charging path means for supplying a charging current to the charging/discharging circuit based on the noise and distortion components generated by the charging/discharging circuit; Noise and distortion reduction circuit comprising a reduction means for reducing the percentage. 2. includes a discharging path means through which the electric charge of the charging/discharging circuit is discharged, and a discharging time constant of the discharging path means is selected to be larger than a charging time constant of the charging path means;
A noise and distortion reduction circuit according to claim 1. 3. The charging path means supplies a charging current to the charging/discharging circuit when the output of the detecting means is a certain value or more.
Noise and distortion reduction circuit as described in Section. 4. The noise and distortion reduction according to claim 3, wherein the charging path means includes a gain element receiving the output of the detection means, and supplies charging current to the charging/discharging circuit according to the output of the gain element. circuit. 5. The noise and distortion according to claim 4, wherein the charging path means includes a rectifier circuit receiving the output of the gain element, and the output of the rectifier circuit supplies charging current to the charging/discharging circuit. mitigation circuit. 6. The noise and distortion reduction circuit according to any one of claims 1 to 5, wherein the detection means includes a high-pass filter that receives the output of the demodulation circuit. 7. The noise and distortion reduction circuit according to any one of claims 1 to 6, wherein the reduction means includes a high frequency component reduction means for reducing high frequency components included in the output of the demodulation circuit. 8. The high-frequency component reducing means includes: a switching element that becomes conductive or non-conductive depending on the voltage of the charge/discharge circuit; and a capacitor selectively connected between the demodulation circuit output and ground by the switching element. 8. The noise and distortion reduction circuit according to claim 7, comprising: 9. The FM receiver is configured as a stereo receiver, the demodulation circuit includes a stereo demodulation circuit, and the reduction means is provided in each of the left signal path and the right signal path of the stereo. 9. The noise and distortion reduction circuit according to any one of Items 8 to 9.