JPH0787391B2 - 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]

Conventionally, a Hi-brend circuit is known as a circuit for suppressing noise generated during reception of stereo broadcasting (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.

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. Therefore, there is a problem that the noise cannot be removed in 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, the present inventors can remove noise components over the entire frequency band without deteriorating the separation.
A noise reduction device for FM stereo tuner was proposed.

Fig. 7 shows the outline of the noise reduction device for this FM stereo tuner. As shown in FIG. 7, the noise reduction device of the FM stereo tuner includes a circuit 2 for generating a stereo sum signal (L + R) and a stereo difference signal (LR) in the FM stereo tuner, and the stereo difference signal (L-R). R) that divides R) into a plurality of frequency bands and outputs a combined signal (LR) 'of the divided stereo difference signal (LR) _d that exceeds the noise component level for each divided band, And a circuit 4 for reproducing a stereo signal (R, L) from the combined signal (LR) 'and the stereo sum signal (L + R).

In FIG. 7, the noise removing circuit 3 outputs the stereo difference signal (LR) from the circuit 2 for generating the stereo sum signal (L + R) and the stereo difference signal (LR) to a plurality (n) of audible frequency bands. Multiple bandpass filters (BPF _{1 to} BPF _n ) 7 _{-1 to} 7 _-n and each bandpass filter 7 _-1
~ 7 _-n output split stereo difference signal (LR) _d
Switch control circuit _{(SWC 1 ~SWC n) 8 -1} ~8 -n and, dividing the stereo difference signal based on the switch control signal A ₁ to A _n for outputting a switch control signal A ₁ to A _n based on (L −
R) and the switch circuit 9 _-1 to 9 _-n controlling passage of _d, split stereo difference signal passed through the switch circuits 9 _-1 ~9 _-n (L
-R) _d and an adder 10 for outputting a combined signal (LR) '.

The switch control circuits 8 _-1 to 8 _-n have reference level signals V _{th to} V _thn corresponding to the noise component levels included in the split stereo difference signal (LR) _d , and the reference level signals V _th1 to V _th1 to V _thn and split stereo difference signal (L-R) only when it is compared with the _d which divide the stereo difference signal (L-R) _d exceeds the reference level signal V _th ~V _thn switch circuits _9-1 to 9 _{- n}
The switch control signals A _{1 to} A _n are output.

Next, the operation will be described.

Stereo sum signal (L + R) and stereo difference signal (L-
The stereo sum signal (L + R) output from the circuit 2 that generates R) is sent to the circuit 4 that reproduces the stereo signal as it is, and the stereo difference signal (LR) is the noise removal circuit 3.
Sent to.

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 signal component. The switch control signals A _{1 to} A _n close the switches of the switch circuits 9 _{-1 to} 9 _-n (ON) and the split stereo difference signal (LR) at that time.
Pass _d . _{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
Are added together to form a combined signal (LR) ′,
Circuit 4 for reproducing a stereo signal as a stereo difference signal of
Sent to. At this time, the combined signal (LR) 'is only the split stereo difference signal (LR) _{d having a} noise level lower than that of the stereo difference signal (LR) from the circuit 2 which generates the stereo sum signal and the stereo difference signal. Therefore, the noise component is reduced as compared with the stereo difference signal (LR) output from the circuit 2 that generates the stereo sum signal and the stereo difference signal.

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 of the generated L signal and R signal can be reduced.

[Problems to be Solved by the Invention]

However, the noise reduction apparatus of the stereo difference signal (L-R) _d obtained by band division, since it is intended to remove the low signal levels divided stereo difference signal (L-R) _d, amount corresponding removed This will be accompanied by a loss of signal.

As a result, the reproduced sound is slightly deteriorated in terms of audibility and becomes a sound with reduced stereo feeling.

It is an object of the present invention to provide a noise reduction device for an FM stereo tuner that can compensate for the loss of the stereo difference signal that occurs due to the removal of noise components and improve the stereo feeling.

[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 the stereo difference signal (LR), and the stereo sum signal (L + R) and the stereo difference signal (LR) are divided into a plurality of frequency bands at the same division frequency, A low-level split stereo difference signal (LR) _d is removed for each time to generate a second stereo difference signal, and the second stereo difference signal and the removed split stereo difference signal (LR) _d. And a noise removal circuit 3 for outputting a combined signal of the delayed signal (L + R) 'of the divided stereo sum signal in the same divided band, and a stereo difference signal (L, R) from the combined signal and the stereo sum signal (L + R). And a circuit 4 for reproducing.

[Action]

According to the invention, the circuit 2 for generating the stereo sum signal and the stereo difference signal generates the stereo sum signal (L + R) and the stereo difference signal (LR) in the FM stereo tuner. The generated stereo sum signal (L + R) and stereo difference signal (LR) are each divided into a plurality of frequency bands at the same division frequency, and the noise removal circuit 3 performs noise removal processing.

The noise removal circuit removes a low level split stereo difference signal for each split band to generate a second stereo difference signal. The noise component is removed from the second stereo difference signal.
While the signal has a good S / N ratio, its signal level is only as much as the noise component is removed compared to the stereo difference signal (LR) output from the circuit 2 that generates the stereo sum signal and the stereo difference signal. Decay. However, the noise removing circuit 3 delays the divided stereo sum signal in the same divided band as the removed divided stereo difference signal (LR) _d (L +).
Since R) ′ is added to the second stereo difference signal to generate the combined signal, it is possible to compensate for the loss of the stereo difference signal due to the removal of the noise component. As a result, the level of the composite signal input to the circuit 4 for reproducing the stereo signal does not decrease,
Therefore, the circuit 4 for reproducing the stereo signal can prevent the stereoscopic feeling of the stereo signal (L, R) reproduced from the combined signal and the stereo sum signal (L + R) from being deteriorated.

〔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 In FIG. 2, the same parts as those in FIG. 7 are designated by the same reference numerals and will be described below.

The difference between FIG. 2 and FIG. 7 is the noise removing circuit.

The noise removing circuit 3A in the present embodiment outputs a signal (hereinafter, simply referred to as a delayed signal) (L + R) ′ obtained by delaying the stereo sum signal (L + R) for a predetermined time, and a delay circuit 12.
Delay signal (L + R) 'and stereo difference signal (LR)
Noise processing circuits that perform noise processing with inputs as inputs
11 _{-1 to} 11 _-n .

There is only one delay circuit 12, and the noise processing circuits 11 _{-1 to} 11
Commonly used for _-n .

The delayed L + R signal level can be set to an arbitrary level.

Each of the noise processing circuits 11 _{-1 to} 11 _-n is provided by the number of signal division bands to be described later, and the individual configurations are the same except that the frequency bands they handle are different. To simplify the explanation,
Hereinafter, the noise processing circuit 11 _-1 will be described, and description of the noise processing circuits 11 _{-2 to} 11 _-n will be omitted.

The noise processing circuit 11 _-1 forms a signal division circuit that divides the delayed signal (L + R) 'and the stereo difference signal (LR) at the same division frequency in the audible frequency range, respectively. Bandpass filter 14
And a control signal A based on the split stereo difference signal (LR) _d output from the second bandpass filter 14.
And a first switch circuit 16 and a second switch circuit 15 which are ON / OFF controlled by a control signal A.
And a switch circuit 17.

The control circuit 15 controls the reference level V _th corresponding to the noise component level of the divided stereo difference signal (LR) _{d in} the divided band and the divided stereo difference signal (LR) _d.
Compared to the reference level V _th the dividing stereo difference signal bets (L
-R) The control signal A is output when _d is small.
The control circuit 15 can be composed of, for example, a comparator including an operational amplifier.

The first switch circuit 16 is closed when the control signal A is applied to allow the divided delay signal (L + R) 'from the first bandpass filter 13 to pass, and at the same time, the second switch circuit 17 is provided.
_Opens to block the split stereo difference signal (LR) _d from the second bandpass filter 14. The first switch circuit 16 and the second switch circuit 17 can be configured by transistor switches or the like.

Next, the operation will be described.

Circuit 2 for generating a stereo sum signal and a stereo difference signal
Is sent to the stereo sum signal (L + R) delay circuit 12 and the circuit 4 for reproducing the stereo signal, and the stereo difference signal (LR) is sent to the second bandpass filter 14.

In the noise removal circuit 3A, the stereo sum signal (L + R)
Is converted into a delayed signal (L + R) 'by the delay circuit 12,
It is input to the bandpass filter 13.

The first band-pass filter 13 and the second band-pass filter 14 each have a delay signal (L +
R) 'and the stereo difference signal (LR) are divided, and the division delay signal (L + R)' and the division stereo difference signal (L-) are divided.
R) Output _d .

Now, when the signal level of the split stereo difference signal (LR) _d is higher than the reference level V _th , the control circuit 1
Does not output the control signal A, therefore the second switch circuit 17 passes the divided stereo difference signal (LR) _d to the adder 10, and the first switch circuit 16 outputs the divided delay signal (L + R) '. Block.

On the other hand, when the signal level of the split stereo difference signal (LR) _d is smaller than the reference level V _th (that is, when the noise component is larger than the signal component), the control circuit
15 outputs the control signal A, and the first switch circuit 1
6. Switch the second switch circuit 17. As a result, the second switch circuit 17 blocks the split stereo difference signal (LR) _d , and instead the first switch circuit 16 delays the delayed signal (L-R) _d.
+ R) 'is output to the adder 10. That is, here, the divided stereo difference signal (L−R) _d becomes a loss, but since the delay signal (L + R) ′ is output, the above loss is compensated.

The above operation is performed in each of the noise processing circuits 11 _{-1 to} 11 _-n , and the respective output signals thereof, that is, the division delay signal (L + R) 'and / or the division stereo difference signal (L-
R) _d is added in the adder 10. The adder 10 outputs the combined signal [(L + R) '+ (LR) _d ] to the circuit 4 for reproducing a stereo signal. This combined signal [(L +
R) ′ + (L−R) _d ] is a delayed signal (L + R) ′ corresponding to the amount of the lost signal as the noise component is reduced by removing the divided stereo difference signal (L−R) _{d having} a low signal level.
Means that it is covered by. Therefore, the combined signal [(L + R) '+ (LR) _d ] is restored to the signal level of the stereo difference signal (LR) output from the circuit 2 that generates the stereo sum signal and the stereo difference signal. It will be.

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

The feature of this embodiment is that the level of the reference level signal V _th in the control circuit 15 is variably controlled in the noise removing circuit 3B 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 18 and each control circuit 1
This is the point where the input is branched to 5. 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 control circuit 15 has generators (not shown) for the reference level signal V _th , and the electric field strength level signal SML is input to each of these generators to input the electric field strength level signal MSL.
_{Is applied} to the reference input terminal of the comparison section.

The reason for configuring in this way is as follows. The level of the noise component 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, considering the reception states in various electric field intensity areas, uniformly setting the reference level signal V _th may result in an insufficient noise removal effect. Therefore, as in this embodiment, by automatically controlling the reference level signal V _th according to the electric field strength level, it is possible to remove noise by the appropriate reference level signal V _th .

Next, the operation will be described.

The electric field strength level signal SML is given to each control circuit 15. The control circuit 15 sets the reference level signal V _th in accordance with the input electric field strength level signal SML, compares it with each divided stereo difference signal (LR) _d, and, similarly to FIG. 1, switches control signal. A is applied to the switch circuits 16 and 17 to obtain a combined signal [(L + R) '+ (LR) _d '] in which noise components are reduced and restored to the original signal level.

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

The feature of this embodiment is that, in the noise removing circuit 3C, the control circuit 15 in FIG. 2 is controlled by the reference level signal V _th and the split stereo difference signal (LR) instead of the binary ON / OFF switch control signal A. ) The control circuit 19 is configured to output the control signal B according to the deviation amount from _d, and the delay signal (L +
R) ′ and the divided stereo difference signal (LR) _d are passed through a variable level adjusting circuit (Voltage Controled Amplefier).
Etc.) It is a point that 20 and 21 are continuously variable. Since other parts are the same as those in FIG. 1, the same reference numerals are given and the description thereof is omitted.

In this way, the split stereo difference signal (L- that passes through the control circuit 19 and the variable level adjusting circuits 20 and 21) is passed.
By continuously variably controlling the signal level of R) _d, the levels of the frequency components missing over the respective divided frequency bands do not become extremely unbalanced, and it is possible to reduce the unnatural feeling of separation in the sense of hearing.

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 shown in FIG. 4, and the level of the speed level signal V _th in the control circuit 19 is controlled by the FM stereo tuner as in the second embodiment. The point is that the electric field strength level signal SML is introduced from the input terminal 18 and is branched and input to the control circuit 19 so that the electric field strength can be variably set according to the electric field strength of the installed place. 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, in addition to the 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 each divided stereo difference signal, the electric field strength is increased. The reference level signal V _th can be variably set according to the above, and smoother control can be performed.

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
The dynamic expander 22 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.

By interposing the dynamic expander 22 as in the present embodiment, the separation can be improved, and the sense of presence and the sense of stereo can be improved.

The interposition of the dynamic expander 22 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 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 stereo difference lost due to the noise removal processing for each of the divided bands. Since the signal is compensated by the delayed signal of the stereo sum signal, the signal level input to the circuit for reproducing the stereo signal can be restored to the output level of the circuit for generating the stereo sum signal and the stereo difference signal. In this way, it is possible to reproduce FM stereo broadcast with good sound quality as well as a natural stereo feeling.

[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 a fourth embodiment of the present invention, FIG. 6 is a block diagram of a fifth embodiment of the present invention, and FIG. 7 is a block diagram of a conventional example. 1 ... Input terminal 2 ... Circuit for generating stereo sum signal and stereo difference signal 3,3A, 3B, 3C, 3D, 3E ... Noise elimination circuit 4 ... Circuit for reproducing stereo signal 5 ... Output terminal 6 Output terminal 10 Adder 11 _{-1 to} 11 _-n Noise processing circuit 12 Delay circuit 13 Bandpass filter 14 Bandpass filter 15 Control circuit 16 Switch circuit 17 Switch circuit 18 Input terminal 19 Control circuit 20 Variable level adjustment circuit 21 Variable level adjustment circuit 22 Dynamic expander (L + R) Stereo sum signal (LR) Stereo difference Signal (LR) _d ... Split stereo difference signal (L + R) '... Delay signal _Vth ... Reference level signal SML ... Electric field strength level signal A ... Switch control signal B ... Control signal

Claims (5)

[Claims] 1. A circuit for generating a stereo sum signal and a stereo difference signal in an FM stereo tuner, and a circuit for dividing the stereo sum signal and the stereo difference signal into a plurality of frequency bands at the same division frequency. A low-level split stereo difference signal is removed to generate a second stereo difference signal, and the second stereo difference signal and a delayed signal of the split stereo sum signal in the same split band as the removed split stereo difference signal. A noise reduction device for an FM stereo tuner, comprising: a noise removal circuit that outputs the combined signal of 1 .; 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 divides the stereo sum signal and the stereo difference signal into a plurality of frequency bands at the same division frequency, respectively. A reference level signal corresponding to the noise component level of the divided stereo difference signal in each divided band is compared with the divided stereo difference signal, and when the divided stereo difference signal is smaller than the reference level signal, the divided stereo difference signal is removed. A control circuit for outputting a control signal for controlling the divided stereo difference signal, and a switch circuit for blocking the divided stereo difference signal by the control signal and passing a delayed signal of the divided stereo sum signal in the same divided band as the removed divided stereo difference signal. Of an FM stereo tuner characterized by It is reduction apparatus. 3. The noise reduction device for an FM stereo tuner according to claim 2, wherein the noise removing circuit divides the stereo sum signal and the stereo difference signal into a plurality of frequency bands at the same division frequency, respectively. A control circuit that compares the reference level signal corresponding to the noise component level of the divided stereo difference signal in each divided band and the divided stereo difference signal, and outputs a control signal corresponding to the comparison deviation amount, and the control signal, Variable to attenuate the second stereo difference signal level and increase the delay signal level of each divided stereo sum signal belonging to the same division band as the removed stereo difference signal by a level corresponding to this attenuation level. An FM stereo tuner node characterized by a level adjustment circuit and 'S reduction apparatus. 4. The noise reduction device for an FM stereo tuner according to claim 2 or 3, wherein the noise removal circuit is variably controllable in accordance with the input electric field strength of the FM stereo tuner. Noise reduction device for FM stereo tuner. 5. The noise reduction device for an FM stereo tuner according to claim 1, 2, 3 or 4, wherein a dynamic expander circuit is interposed between the noise removal circuit and the circuit for reproducing a stereo signal. Characterized by
Noise reduction device for FM stereo tuner.