JPH0787390B2 - Noise reduction device for FM stereo tuner - Google Patents

Description

The present invention relates to a noise reduction device for an FM stereo tuner.

[Conventional technology]

Hi-brend circuits are conventionally known as circuits for suppressing noise generated during reception of stereo broadcasts (Japanese Patent Publication No. 45-32282).

In the case of FM stereo broadcasting, this Hi-brend circuit is in reverse phase with each other by N / 2 in the L signal and the R signal in which the noise component N of the stereo difference signal (LR) in the composite signal is demodulated. Paying attention to the existence of noise, the high frequency components of the L and R signals are added together to cancel the noise components that exist in opposite phases by N / 2, and the noise of the high frequency component that is particularly noticeable is removed. It is something that is done.

[Problems to be Solved by the Invention]

However, the above-mentioned conventional Hi-brend circuit exerts a noise reduction effect on the high-frequency signal component, but does not act on the mid-low frequency signal component due to the operating principle. There is a problem that noise cannot be removed over a wide range from low to high frequencies. Further, since the Hi-brend circuit becomes a complete monaural signal in the high frequency range, there is a problem in that the left and right separations are deteriorated and a sufficient stereo feeling cannot be obtained.

Therefore, an object of the present invention is to provide a noise reduction device for an FM stereo tuner that can remove noise components over the entire frequency band with almost no deterioration in separation.

[Means for Solving the Problems]

In order to solve the above problems, the present invention, as shown in FIG. 1, uses a stereo sum signal (L
+ R) and a stereo difference signal (LR) and a circuit 2 for generating the stereo difference signal (LR), the stereo difference signal (LR) is divided into a plurality of frequency bands, and a split stereo difference signal corresponding to a signal level of each of the division bands. A noise removal circuit 3 for outputting a combined signal (LR) 'of (LR) _{d, and} the combined signal (LR)
R) 'and a circuit 4 for reproducing a stereo signal (R, L) from the stereo sum signal (L + R).

[Action]

According to the present invention, the stereo difference signal (LR) output from the circuit 2 that generates the stereo sum signal (L + R) and the stereo difference signal (LR) is input to the noise removal circuit 3 and subjected to noise removal processing. Is applied.

The noise removing circuit 3 decomposes the stereo difference signal (LR) into a plurality (n) of frequency bands, and in each division band, a division stereo difference signal according to the signal level of each division stereo difference signal (LR) _d. Noise removal processing is performed by extracting the signal (LR) _d, and the divided stereo difference signal (L
-R) The combined signal of _d , that is, the combined signal (LR) 'with reduced noise components is output. This composite signal (L
-R) 'is input to the circuit 4 for reproducing a stereo signal.

The circuit 4 for reproducing the stereo signal reproduces the stereo signal (L, R) by the input composite signal (LR) 'and the stereo sum signal (L + R). This reproduced stereo signal (L, R) is in a state in which noise is removed over the entire frequency band, and since the L signal and the R signal are not combined, deterioration of separation can be prevented.

〔Example〕

 Next, an embodiment of the present invention will be described with reference to the drawings.

Since the overall configuration of the present invention is as shown in FIG. 1, the description thereof will be omitted and each embodiment will be described below.

First Embodiment FIG. 2 shows the first embodiment of the present invention.

In FIG. 2, the noise removing circuit 3A includes a plurality of band pass filters for dividing the stereo difference signal (LR) from the circuit 2 for generating a stereo sum signal and a stereo difference signal into a plurality (n) of audible frequency bands. (BPF _{1 to} BPF _n ) 7 _{-1 to} 7 _-n
And a switch control circuit (SWC ₁ -An ₎ that outputs switch control signals A ₁ -A _n based on the split stereo difference signal (LR) _d output from each bandpass filter 7 _-1 -7 _-n.
SWC _n ) 8 _-1 to 8 _-n , switch circuits 9 _{-1 to} 9 _-n for controlling passage of the split stereo difference signal (LR) _d based on the switch control signals A _{1 to} A _n , and And an adder 10 for combining the divided stereo difference signals (LR) _d that have passed through the switch circuits 9 _{-1 to} 9 _-n and outputting a combined signal (LR) '.

The switch control circuits 8 _-1 to 8 _-n have reference level signals V _{th1 to} V _thn corresponding to the noise component levels included in each divided stereo difference signal (LR) _d , and the reference level signals V _th1 ~ V _thn and the split stereo difference signal (LR) _d are compared, and the switch circuits 9 _{-1 to} 9 only when the split stereo difference signal (LR) _d exceeds the reference level signals V _{th1 to} V _{thn.
-Closes n} Outputs switch control signals A _{1 to} A _n . Specifically, it is configured by using an operational amplifier or the like.

Transistor switches or the like are used for the switch circuits 9 _{-1 to} 9 _-n .

The adder 10 uses an operational amplifier or the like.

In the above configuration, the switch control circuits 8 _-1 to 8
_-n and switch circuits 9 _{-1 to} 9 _-n form a noise processing circuit.

Next, the operation will be described.

Circuit 2 for generating a stereo sum signal and a stereo difference signal
The output stereo sum signal (L + R) is sent to the circuit 4 for reproducing the stereo signal as it is, and the stereo difference signal (L-R) is sent to the noise removing circuit 3A.

In the noise removal circuit 3A, the stereo difference signal (LR)
Is divided into _n by band pass filters 7 _{-1 to} 7 _-n . Each divided stereo difference signal (LR) _d is compared with the reference level signals V _{th1 to} V _{thn in} each switch control circuit 8 _-1 to 8 _-n . As a result of the comparison, the switch control signal A having a binary value of ON-OFF only when V _{th1 to} V _thn <(LR) _d
Outputs _{1 to} A _n . That is, the case of V _{th1 to} V _thn <(LR) _d is when the signal component of the stereo difference signal (LR) _d is larger than the noise component level in each divided band. The switch control signals A _{1 to} A _n cause the switch circuits 9 _{-1 to} 9 _-n to close (ON) the switches and pass the split stereo difference signal (LR) _d at that time. _{Therefore, V th1 ~V thn ≧ (L} -R) stereo difference signal sub-band having a relationship of _{d (L-R) d} is blocked output by the switch circuit 9 _-1 to 9 _-n opens (OFF) is To be done.

Each divided stereo difference signal (LR) _{d that} has passed is an adder 10
Is added to form a combined signal (LR) 'and is sent to the circuit 4 for reproducing a stereo signal. At this time, since the combined signal (LR) 'is composed of only the split stereo difference signal (LR) _d having a low noise level, the stereo output from the circuit 2 that generates the stereo sum signal and the stereo difference signal. The noise component is reduced as compared with the difference signal (LR).

In this way, the combined signal (L-
The circuit 4 for reproducing the stereo signal by the R) 'and the stereo sum signal (L + R) generates the L signal and the R signal. Therefore, the noise components included in the generated L signal and R signal can be reduced.

Second Embodiment FIG. 3 shows a second embodiment of the present invention.

The feature of this embodiment is that in the noise removing circuit 3B, the reference level signal V in the switch control circuits 8 _-1 to 8 _-n is
_{The point} is that the levels of _{th1 to Vthn} are variably controlled according to the electric field strength level of the place where the FM stereo tuner is installed.

3 is different from FIG. 2 in that the electric field strength level signal SML is introduced from the input terminal 12 and branched and input to each of the switch control circuits 11 _{-1 to} 11 _-n . Since other parts are the same as those in FIG. 2, the same reference numerals are given and the description thereof is omitted.

In this embodiment, the switch control circuits 11 _{-1 to} 11
_-n has generators (not shown) for the reference level signals V _{th1 to} V _thn , and the electric field intensity level signal SML is input to each of the generators to input the reference level signal V _th1 corresponding to the electric field intensity level signal SML.
~ V _thn is _applied to the reference input terminal of the comparison section.

The reason for configuring in this way is as follows. The noise component in each divided band included in the stereo difference signal (LR) differs depending on the electric field strength. When the electric field strength is low, the noise level is relatively high compared to the stereo difference signal (LR), and when the electric field strength is high, the opposite is true. Therefore, in consideration of the reception state in the field intensity region, uniformly setting the reference level signals V _{th1 to} V _thn may result in an insufficient noise removal effect. Therefore, as in the present embodiment, the reference level signal V _th1 ~
Noise removal by an appropriate reference level signal V _th1 ~V _thn by automatically controlling the V _thn is possible.

Next, the operation will be described.

Field strength level signal SML is for each switch control circuit 1
Given to 1 _{-1 to} 11 _-n . Switch control circuit 11
_{-1 to} 11 _-n sets the reference level signals V _{th1 to} V _thn according to the input electric field strength level signal SML, _compares with the divided stereo difference signal (LR) _d, and is compared with FIG. Similarly, the switch control signals A _{1 to} A _n are given to the switch circuits 9 _{-1 to} 9 _-n and output only when the split stereo difference signal (LR) _d is high to obtain a combined signal (LR) '. .

Third Embodiment Next, FIG. 4 shows a third embodiment of the present invention.

The feature of this embodiment is that the switch control circuits 8 _-1 to 8 _-n in FIG. 2 are turned on and off in the noise removing circuit 3C.
Of the binary switch control signals A _{1 to} A _n are not output, but the control signals B ₁ to B _n according to the deviation amount between the reference level signals V _{th1 to} V _thn and the split stereo difference signal (LR) _d.
A control circuit 15 _-1 to 15 _-n that outputs B _n is used, and the switch circuits 9 _{-1 to} 9 _-n are not a switch circuit but a variable level adjusting circuit (Voltage Controled Amplefie).
r etc.) 13 _{-1 to} 13 _-n . Other parts are first
Since it is the same as the figure, the same reference numerals are given and the description thereof is omitted.

As described above, the control circuit 15 _-1 to 15 _-n and the variable level adjusting circuits 13 _{-1 to} 13 _-n are used to continuously variably control the signal level of the divided stereo difference signal (LR) _d passing therethrough. , The level of the divided stereo difference signal (LR) _d of the frequency component missing over each divided frequency band does not become extreme, and the unnatural feeling of separation in hearing can be reduced.

Fourth Embodiment Next, FIG. 5 shows a fourth embodiment of the present invention.

The feature of this embodiment is that the noise removing circuit 3D is based on the third embodiment of FIG. 4, and like the second embodiment, the reference level signals V _{th1 to} V th in the control circuits 15 _-1 to 15 _-n .
_In order to make it possible to variably set the level of _thn corresponding to the electric field strength of the place where the FM stereo tuner is installed, the electric field strength level signal SML is introduced from the input terminal 12 and each control circuit 16 _{-1 to} 16 _-n This is the point where the branch input is made. Since other parts are the same as those in FIG. 4, the same reference numerals are given and the description thereof is omitted.

With this configuration, variable continuous control of the divided stereo difference signal (LR) _d according to the deviation amount between the noise component level included in each divided band and the divided stereo difference signal (LR) _d is performed. In addition, the reference level signal V _th1 according to the electric field strength
~ V _thn can be variably set, and smoother control becomes possible.

Fifth Embodiment Next, FIG. 6 shows a fifth embodiment of the present invention.

The feature of this embodiment is that in the noise removing circuit 3E, an adder
A dynamic expander 14 is interposed between the output terminal of 10 and the circuit 4 for reproducing a stereo signal. The other parts are similar to those of the first embodiment shown in FIG. 2, and the same parts are designated by the same reference numerals and the description thereof will be omitted.

The reason for interposing the dynamic expander 14 as in this embodiment is as follows.

When the noise elimination in the noise removing circuit 3A, decomposition stereo difference signal (L-R) decomposition stereo difference signal is low split-band signal level of _d (L-R) _d is not outputted,
Therefore, at least the signal level corresponding to the unresolved decomposed stereo difference signal (LR) _{d is} attenuated, and the signal level of the combined signal (LR) 'is generated by the circuit 2 for generating the stereo sum signal and the stereo difference signal. Is lower than the signal level of the stereo difference signal (LR) at the time of the output of. Therefore, as in the noise removal circuit 3E, the combined signal (L-
The signal level of R) ′ can be restored to the original signal level of the stereo difference signal (LR), the noise component can be removed, the deterioration of the separation can be compensated, and the spread of the stereo feeling can be increased. It will be possible.

The interposition of the dynamic expander 14 can be applied not only to the first embodiment but also to the second to fourth embodiments, and the same effect can be obtained. It is not limited to the example.

〔The invention's effect〕

As described above, according to the present invention, the stereo difference signal generated by the circuit that generates the stereo sum signal and the stereo difference signal is divided into a plurality of frequency bands, and the divided stereo difference signal exceeding the noise level for each of the division bands. Since the L and R signals are reproduced by the circuit that synthesizes only the stereo signal and reproduces the stereo signal, noise components can be removed over the entire frequency band with almost no deterioration in separation, and the sound quality is natural and stereo. It is possible to play good FM stereo broadcasts.

[Brief description of drawings]

1 is a schematic block diagram of the present invention, FIG. 2 is a block diagram of a first embodiment of the present invention, FIG. 3 is a block diagram of a second embodiment of the present invention, and FIG. 4 is a third diagram of the present invention. 5 is a block diagram of an embodiment, FIG. 5 is a block diagram of a fourth embodiment of the present invention, and FIG. 6 is a block diagram of a fifth embodiment of the present invention. 1 …… Input terminal 2 …… Circuit for generating stereo sum signal and stereo difference signal 3,3A, 3B, 3C, 3D, 3E …… Noise removing circuit 4 …… Stereo signal reproducing circuit 5 …… Output terminal 6 ...... output terminal 7 _-1 to _7--n ...... bandpass filter 8 _-1 to 8 _-n ...... switch control circuit 9 _-1 to 9 _-n ...... switching circuit 10 ...... adder 11 _-1 to 11 _{- n} ...... Switch control circuit 12 ...... Input terminal 13 _{-1 to} 13 _-n ...... Variable level adjustment circuit 14 ...... Dynamic expander 15 _-1 to 15 _-n ...... Control circuit 16 _{-1 to} 16 _-n ...... Control circuit (L + R) …… Stereo sum signal (LR) …… Stereo difference signal (LR) _d …… Divided stereo difference signal (LR) ′ …… Composite signal V _{th1 to} V _thn …… Reference level signal SML ...... field intensity-level signal A ₁ ~A _n ...... switch control signal B ₁ ~B _n ...... control Issue

Claims (6)

[Claims] 1. A circuit for generating a stereo sum signal and a stereo difference signal in an FM stereo tuner, the stereo difference signal being divided into a plurality of frequency bands, and a divided stereo difference signal according to a signal level of each of the division bands. A noise reduction device for an FM stereo tuner, comprising: a noise removal circuit that outputs a combined signal; and a circuit that reproduces a stereo signal from the combined signal and the stereo sum signal. 2. The noise reduction device for an FM stereo tuner according to claim 1, wherein the noise removal circuit comprises a plurality of band pass filters for dividing the stereo difference signal into a plurality of frequency bands, and a divided stereo in each of the divided bands. A noise processing circuit that compares the difference signal with a certain fixed reference level and outputs the divided stereo difference signal only when the divided stereo difference signal has a signal level higher than the reference level, and a division output from the noise processing circuit. A noise reduction device for an FM stereo tuner, comprising: an adder that synthesizes stereo difference signals and outputs the synthesized signal. 3. The noise reduction device for an FM stereo tuner according to claim 2, wherein the noise processing circuit compares a divided stereo signal output from the band pass filter with a certain reference level, and the divided stereo signal is compared. A switch control circuit that outputs a control signal that passes the split stereo difference signal only when the difference signal exceeds the reference level; and a switch circuit that passes the split stereo difference signal by the control signal. FM stereo tuner noise reduction device. 4. The noise reduction device for an FM stereo tuner according to claim 2, wherein the noise processing circuit has a reference level corresponding to a noise component level included in the divided stereo difference signal and a signal level of the divided stereo difference signal. FM, which is configured to include a control circuit that outputs a control signal according to the deviation level of, and a variable level adjustment circuit that adjusts the output level of the split stereo difference signal according to the control signal. Noise reduction device for stereo tuner. 5. The noise reduction device for an FM stereo tuner according to claim 3 or 4, wherein the noise processing circuit is configured so that its reference level can be variably controlled according to the input electric field strength of the FM tuner. Characterizing
Noise reduction device for FM stereo tuner. 6. The noise reduction device for an FM stereo tuner according to claim 1, 2, 3, 4, or 5, wherein a dynamic expander circuit is interposed between the noise removal circuit and the circuit for reproducing a stereo signal. Noise reduction device for FM stereo tuner.