JP2845782B2 - Pilot signal removal circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pilot signal removing circuit, and more particularly to a pilot signal removing circuit such as an FM receiver used for receiving a stereo broadcast or an audio multiplex broadcast.

[0002]

2. Description of the Related Art In general FM stereo broadcasting,
As a reference signal for separating the original left and right voice signals from the sum signal (L + R) and the difference signal (LR) of the left (L) right (R) voice signals included in the received carrier signal, respectively.
A composite signal in which a pilot signal composed of a 9 KHz sine wave is included in each carrier signal or a stereo signal composed of a signal after stereo demodulation is used.

[0003] The pilot signal is transmitted to a P
It plays an important role in a case locked loop (PLL) circuit. However, since it is included in a received stereo signal, if it is demodulated as it is, it is output as it is included in the demodulator output. This is a problem in hearing. Therefore, it is necessary to remove the 19 KHz pilot signal.

[0004] An example of this kind of pilot signal removal circuit is employed in a product name "μPC1287" of NEC Corporation. Referring to FIG. 4 showing a block diagram of a pilot signal removing circuit mounted on this product, a triangular wave conversion circuit 11 for converting a square wave having the same frequency as the pilot signal supplied to the square wave input terminal 10 to a triangular wave SQ1. A variable gain amplifier 5 for level-amplifying the signal amplitude of the triangular wave SQ1, the amplified triangular wave SQ2 and the stereo signal ST supplied to the input terminal 1 are added to attenuate the pilot signal included in the stereo signal ST. Carrier CW
Is supplied to the output terminal 4 and a carrier signal CW having the added and attenuated pilot signal is supplied,
A level detection circuit 14 extracts only the pilot signal still remaining from the signal CW and converts the amplitude value into a DC level DC5.

Referring to FIG. 5 showing a timing chart for explaining the operation of the pilot signal removing circuit,
The stereo signal ST is supplied from the input terminal 1 to one input terminal of the adder 4 (FIG. 5-Pilot signal). On the other hand, the triangular wave conversion circuit 11 further shifts the phase of the square wave signal (square wave shown in FIG. 5) synchronized with the pilot signal by 90 degrees from the input terminal 10 by 90 degrees. ° Triangular wave signal SQ1 out of phase
(FIG. 5-triangular wave signal SQ1) is generated, and the signal SQ2 whose level is amplified by the variable gain amplifier 12 is supplied to the other input terminal of the adder 13.

The adder 13 adds the pilot signal of the sine wave stereo signal ST and the triangular wave signal SQ2 whose phase is shifted by 180 °, and outputs the carrier CW from which the pilot signal has been removed to the output terminal 4. At the same time, it is also supplied to the level detection circuit 14.

In the addition of the sine wave and the triangular wave SQ2 whose phase is shifted by 180 ° to the carrier wave CW, a residual component always exists in a portion corresponding to the hypotenuse portion of the triangular wave. Are extracted and converted to DC level DC5.

The DC level DC5 controls the gain of the variable gain amplifier 12 to increase, and the amplified triangular wave and the pilot signal are further added to remove the pilot signal.

Another example of this kind of signal elimination circuit is disclosed in
-37373. Referring to FIG. 6 which shows a block diagram of the signal processing device described in the publication, a low-pass filter (LPF) 15 that passes only a low-frequency portion of a video signal VD, and an output signal LF of the LPF 15 is clamped at a predetermined level. And a high-pass filter (H) that passes only the luminance component of the video signal.
PF) 17, a limiter 18 for limiting the signal amplitude of the output signal HF of the HPF 17, a variable gain amplifier 19 for controlling the amplification of the output LM1 of the limiter 18 with the output signal CL of the clamp circuit 16, and amplifying the output. An adder 13 adds the output signal LM2 of the variable gain amplifier 19 and the original video signal VD to reduce the amount of noise reduction contained in the video signal and outputs the reduced noise to the output terminal 4.

In this circuit, the signal VD supplied from the input terminal passes through the LPF 15 to extract a low-frequency component. The signal LF output from the LPF 15 is supplied to a clamp circuit 16 and clamped to a predetermined level. On the other hand, the input signal VD is inverted by the variable gain inverting amplifier 19 as the output signal LM1 of the limiter 18 by inverting and amplifying the signal HF from which the wide-range component is extracted by passing through the HPF 17. Controlled by level.

The input signal VD is added to the output signal LM2 of the variable gain inverting amplifier 19 in the adder 13, and is output from the output terminal 4 as a signal with reduced noise.

[0012]

In the above-mentioned conventional pilot signal removal circuit, (a) Since the PLL itself locks with the 19 KHz pilot signal, a certain pilot signal level remains in the composite signal after the pilot signal removal. Otherwise, a PLL circuit cannot be formed, and a pilot signal remains in the output signal after stereo demodulation. (B) Since the 19 KHz pilot signal, which is a sine wave, is equivalently removed by a triangular wave, its attenuation is limited, and the difference between a sine wave and a triangular wave can theoretically be limited to 18 db. (D) If a circuit for converting a triangular wave used mainly in a high-end machine into a sine wave is used, the circuit scale becomes large.
Further, when the circuit is calibrated with individual components, an adjustment point is required due to the variation of the elements. (E) Since only the fundamental wave of the pilot signal is removed by the triangular wave, when demodulating the composite signal to the stereo signal, the 19 KHz harmonic component contained in the triangular wave remains. (F) Leakage of the pilot signal component may be output to the output signal due to an offset or the like in the variable inverting amplifier 19.

An object of the present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to remove a pilot signal component included in an output signal after stereo demodulation to a noise level without affecting a PLL circuit. It is another object of the present invention to provide a pilot signal removing circuit that can perform the following.

[0014]

A feature of the pilot signal removing circuit according to the present invention is that it is used in a receiver for stereo broadcasting or audio multiplex broadcasting in FM broadcasting, and a sum signal and a difference of left and right audio signals included in a received carrier signal. in the pilot signal removing circuit a pilot signal dividing <br/> removed by from the carrier signal after the separation as a reference signal when each separated original to the left and right audio degree from signal, the pilot
The frequency of the pilot signal is converted from the stereo demodulated stereo signal including the carrier signal on which the pilot signal is superimposed.
Filter means that to extract only the number or more harmonic components, the extracted first signal amplitude level amplification of harmonic components
Variable amplification means, and the amplified first harmonic
Subtraction means for subtracting the component from the stereo demodulated stereo signal; and taking the carrier signal from the subtraction result.
However, the pilot signal remaining on this carrier signal is
Convert the extracted second harmonic component to DC level
And the level amplification is variably controlled at this DC level.
1 level control means or the stereo demodulated stereo
The harmonic component including the pilot signal from the E signal
The first DC level extracted and detected only and the level
The amplified output is compared with the detected second DC level.
Level amplification at a third DC level obtained by amplifying the difference
And any one of the second level control means for variably controlling .

Further, the first level control means, said
The pilot signal is obtained from the stereo signal after stereo demodulation.
Is input from the subtractor, and this signal
The pilot signal remaining in
And the second harmonic component with a high-pass filter
And converts the amplitude value to the first DC level.
And outputs the first DC voltage.
A variable control circuit for amplifying the level of the first harmonic component
Output to the first variable gain amplifier of the variable amplifying means.
The first level detection circuit can be configured with the first level detection circuit .

Further, the second level control means inputs the stereo signal after the stereo demodulation, and
Built-in pilot signal included with harmonic components
And the amplitude value is extracted by the first
A first level detection circuit that converts and outputs a DC level of
Level amplification by the first variable gain amplifier of the variable amplification means
The amplitude value of the first harmonic component obtained is converted to the second DC
A second level detection circuit for converting and outputting the signal to a level;
Comparing the first DC level with the second DC level
The difference voltage is used to amplify the level of the first harmonic component.
Output to the first variable gain amplifier as a variable control voltage
And a differential amplifier .

Still further, the signal level control means includes:
A second variable gain amplifier whose amplification degree when level-amplifying the harmonic component is controlled by a predetermined control voltage; and extracting the harmonic component from the stereo signal supplied to the subtractor, and extracting the amplitude of the harmonic component. A second level detection circuit for converting the value to a second DC level and outputting the converted value to a third DC level, and a third level detecting circuit for converting the amplitude value of the higher harmonic component level-amplified by the second variable gain amplifier to a third DC level. And a differential amplifier that compares the second and third DC levels and outputs the difference voltage as the predetermined control voltage, wherein the third DC level is higher than the second DC level. The second variable gain amplifier operates in a direction to decrease the amplification degree by the predetermined control voltage when the DC level is high, and when the third DC level is lower than the second DC level. Wherein by said predetermined control voltage second
Can form a control loop that operates in the direction of increasing the amplification.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing a first embodiment of the pilot signal removing circuit of the present invention. Referring to FIG. 1, an input terminal 1 to which a stereo signal including a pilot signal having a frequency of 19 KHz is supplied to each of the carrier signals of a composite signal or a signal after stereo demodulation,
HPF that removes an audio signal of a pilot signal included in the stereo signal ST supplied through the FPC through a frequency lower than 19 KHz and passes only a frequency equal to or higher than 19 KHz
2, a variable gain amplifier 3 for level-amplifying the signal amplitude of a harmonic component PL1 including a 19 KHz pilot signal output from the HPF 2, and a harmonic component PL2 including the amplified pilot signal to an input terminal 1. A subtractor 4 for subtracting from the original stereo signal ST to extract a carrier CW, and detecting only a harmonic component including a 19 KHz pilot signal remaining on the carrier CW resulting from the subtraction of the subtracter 4 to obtain a DC level. DC1 and a level detection circuit 5 for controlling the amplification of the variable gain amplifier 3 using the output DC level DC1, and an output terminal 6 for outputting the output carrier CW of the subtractor 3 to the next stage. It is composed of

Referring to FIG. 1 and FIG. 2 showing a timing chart for explaining the operation, FIG.
The upper side around V is a positive voltage, the lower side is a negative voltage, and the horizontal axis represents time T. The operation of this circuit is, for example, assuming that a stereo signal ST after stereo demodulation is supplied to the input terminal 1 and a 19 KHz pilot signal is modulated on a 1 KHz carrier wave in this signal (FIG. 2).
Input terminal 1).

The audio signal is removed by the HPF 2 connected to the input terminal 1 and a harmonic PL1 including a pilot signal of 19 KHz or more is obtained (FIG. 2-HPF 2 output). Since the signal amplitude of the harmonic component PL1 is attenuated by passing through the HPF 2, the level is amplified by the variable gain amplifier 3 inserted in the next stage and supplied to the subtractor 4 (FIG. 2).
Variable amplifier 3 output).

Since a stereo signal ST is supplied to one input terminal of the subtractor 4 from the input terminal 1, this signal S
The component of the harmonic component PL2 including the pilot signal of 19 KHz or more amplified by the variable gain amplifier 3 is subtracted from T.

Due to this subtraction, 19 KHz is applied to the output terminal.
A carrier wave CW having a small harmonic component PL2 including the above pilot signal is output (FIG. 2—output of the subtractor 4). Since the higher harmonic component PL2 still remains in the carrier CW, the level detector 5 first extracts only the higher harmonic component PL2 by the built-in HPF 5a. Further, the extracted harmonic component PL2 is converted into a DC level DC1 (FIG. 2).
Level detector 5 output).

This direct current level DC1 naturally has a harmonic component P
Since the amplitude varies in accordance with the signal amplitude of L2, the degree of amplification of the variable gain amplifier 3 can be controlled by the fluctuating DC level DC1.

That is, when the harmonic component PL2 remaining at the output of the subtractor 4 is large, the DC level DC
Therefore, the amplitude of the harmonic component PL2 supplied to the subtractor 4 also increases, and the value to be subtracted from the stereo signal ST also increases. The harmonic component PL2 included in the carrier wave CW is further reduced.

Since the above operation uses its own pilot signal as a signal for removing the pilot signal, the leakage of harmonic components including the pilot signal due to the offset or the like of the variable gain amplifier 3 is corrected using the level detection circuit 5. Therefore, the pilot signal component remaining in the output of the subtractor can be removed to the noise level.

In the above-described first embodiment, the mounted HPF 2 is a fourth-order 19 KHz HPF.
When the harmonic component including the pilot signal extracted by F is subtracted from the stereo signal ST and removed, the lack of the stereo signal is −0.4 db at 8 KHz and − at 10 KHz.
Although the filter characteristic is as steep as about 1.5 db, the lack of such a level is a level that does not cause a problem in practical use. In order to implement similar characteristics using a BPF, a high-order filter of several tens of times is required, and when an integrated circuit is used, the number of elements and the capacitance value are increased. Is much more advantageous for cost reduction.

In the above-described conventional removing circuit, the pilot signal is removed by using the triangular wave signal. Therefore, it is necessary to provide a triangular wave converting circuit, and the chip layout space corresponding to this is required. In the case of the embodiment, it becomes unnecessary and the circuit scale can be reduced.

Next, referring to FIG. 3 which shows a block diagram of the second embodiment, the difference from the first embodiment is that two level detection circuits 7 and 8 and two And a differential amplifier 9 for comparing the outputs of

That is, the carrier signal of each of the composite signal and the signal after the stereo demodulation has a frequency of 19 KH.
an input terminal 1 to which a stereo signal ST including the pilot signal having a frequency of z is supplied, and an audio signal having a wave number lower than 19 KHz of the pilot signal included in the stereo signal ST supplied via the input terminal 1 And a harmonic component PL1 output from the HPF 2 that removes the carrier wave and passes a harmonic component of 19 KHz or more.
And a variable gain amplifier 3 for level-amplifying the signal amplitude of the original stereo signal ST2.
And a level detector 7 which extracts only the harmonic component including the pilot signal of 19 KHz from the original stereo signal ST by the built-in HPF 7a, converts it into the DC level DC2, and outputs it. And a harmonic component PL2 amplified by the variable gain amplifier 3 is supplied. The level detector 8 converts this signal to a DC level DC3 and outputs the DC level. The DC level DC2 output from the level detectors 7 and 8 And a differential amplifier 9 for comparing the DC3 and amplifying the difference and outputting a DC level DC4 as a control voltage to the variable amplifier 3 and an output terminal 6 for outputting the output carrier CW of the subtractor 3 to the next stage. Be composed.

Referring to FIGS. 2 and 3, the operation of this circuit is similar to that of the first embodiment. For example, assuming that a stereo signal after stereo demodulation is supplied to input terminal 1, this signal Is a 19 KHz pilot signal modulated on a 1 KHz carrier (Fig. 2-input terminal 1)
Therefore, the carrier wave of the audio signal is removed by the HPF 2 connected to the input terminal 1, and a harmonic component PL1 including a pilot signal of 19 KHz or more is obtained (FIG. 2-HPF 2 output).

The signal amplitude of the harmonic component PL1 attenuated by passing through the HPF2 is level-amplified by the variable gain amplifier 3 in the next stage and supplied to the subtractor 4 (FIG. 2).
Variable amplifier 3 output).

On the other hand, the level detector 7 has a built-in HPF 7
First, only the harmonic component PL2 component is extracted from the stereo signal ST input to the subtractor using a. Further, the extracted harmonic component PL2 is converted to a DC level DC2.

On the other hand, the level detector 8 is a variable gain amplifier 3
Output the harmonic component PL2 directly to the DC level DC
Convert to 3. That is, since the harmonic component PL2 does not include the audio signal carrier, the level detector 8 does not need to include the HPF.

A differential amplifier 9 to which the DC levels DC2 and DC3 converted by the level detectors 7 and 8 are supplied.
Is a level detector 7 detected from the original stereo signal ST.
If the level of the DC level DC3 of the level detector 8 that detects the output of the variable gain amplifier 3 is higher than the DC level DC2, the differential voltage is amplified to be higher than the potential already output. The low potential DC level DC4 is supplied as a control voltage of the variable gain amplifier 3. The gain of the variable gain amplifier 3 controlled by this low voltage is reduced, and the amplitude of the output harmonic component PL2 is also reduced.

The subtractor 4 has a 19K variable gain amplifier 3 which amplifies the stereo signal ST supplied to one input terminal.
The harmonic component PL2 including the pilot signal of Hz or more is subtracted. However, since the amplitude value of the harmonic component PL2 to be subtracted and the amplitude value of the harmonic component of the stereo signal ST to be subtracted become substantially equal. The carrier CW from which the harmonic components have been removed to the noise level is output to the subtraction result output of the subtractor 4 (FIG. 2—output of the subtractor 4).

The DC level D of the level detector 8 that detects the output of the variable gain amplifier 3 is higher than the DC level DC2 of the level detector 7 that is detected from the original stereo signal ST.
If the level of C3 is low, the difference voltage is amplified and a DC level DC4 having a higher potential than the already output potential is supplied as a control voltage of the variable gain amplifier 3. The gain of the variable gain amplifier 3 controlled by the high voltage is increased, and the amplitude of the output harmonic component PL2 is also increased.

Also in this case, the harmonic component PL2 to be subtracted is used.
And the amplitude value of the higher harmonic component of the stereo signal ST to be subtracted is substantially equal, so that the subtraction result output of the subtracter 4 outputs the carrier CW from which the higher harmonic component has been removed to the noise level. You.

In the case of the second embodiment, the level detection is performed from the input signal of the subtractor 3, whereas the level detection is performed from the output signal of the subtractor 3 in the first embodiment, that is, the subtraction is performed. Since the level is controlled by performing the level detection before, the carrier CW from which the pilot signal component of the stereo signal ST is completely removed is output to the output terminal 6 at the output of the subtractor 3.

Also in the case of the above-described second embodiment, since its own pilot signal is used as a signal for removing the pilot signal, leakage of harmonic components including the pilot signal due to the offset or the like of the variable gain amplifier 3 is reduced. Since control can be performed so as to perform correction using the level detection circuit 5, the pilot signal component remaining in the output of the subtractor can be removed to the noise level.

[0040]

As described above, according to the pilot signal removing circuit of the present invention, a stereo signal including a pilot signal is supplied to each carrier signal of the composite signal or the signal after the stereo demodulation, and the stereo signal is preliminarily obtained from the stereo signal. A filter means for passing only a predetermined frequency band to be determined, a signal amplitude value of an output of the filter means being level-amplified, and a subtracter for subtracting the amplified signal amplitude value of the predetermined frequency band from the stereo signal, Signal level control means for controlling the level amplification of the signal amplitude value using either the input signal or the output signal of the mixer, so that the pilot signal component of the stereo signal can be attenuated to the noise level.

Further, it is possible to remove not only the harmonic components of the pilot signal passed through the HPF but also other noise components from the stereo signal.

Further, since it is not necessary to generate a triangular wave unlike the conventional removing circuit, the circuit scale can be reduced.

Further, since the 19 KHz pilot signal component is removed by using the fourth-order 19 KHz HPF, a steep filter characteristic can be obtained at a level that does not cause a problem in practical use. In a normal band-pass filter (BPF), To obtain equivalent characteristics, several tens of times higher order filters are required.
In the case of an integrated circuit, the number of elements and the capacitance value increase, but in the case of the present invention, since the HPF is of the fourth order of 19 KHz, a lower cost pilot signal removing circuit can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention.

FIG. 2 is a timing chart for explaining the operation.

FIG. 3 is a block diagram showing a second embodiment of the present invention.

FIG. 4 is a block diagram of a conventional pilot signal removal circuit.

FIG. 5 is a timing chart for explaining the operation of the circuit in FIG. 4;

FIG. 6 is a block diagram of another example of a conventional removal circuit.

[Explanation of symbols]

Reference Signs List 1 input terminal 2, 5a, 7a HPF 3 variable gain amplifier 4 subtractor 5, 7, 8, 14 level detector 9 differential amplifier 10 square wave input terminal 11 triangular wave converter 12 variable gain amplifier 13 adder ST stereo signal PL1 Harmonic component of HPF2 output PL2 Harmonic component of variable gain amplifier 3 output CW Carrier of subtracter output DC1, DC2, DC3, DC5 Level detector 7,
DC level of 8,14 output DC4 DC level of differential amplifier 9 output SQ1 Triangle wave of triangular wave converter output SQ2 Triangle wave of variable gain amplifier output LF LPF15 output HF HPF17 output CL Clamp circuit 16 output LM1 Limiter circuit 18 output LM2 Variable gain inversion Amplifier 19 output

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H04B 1/10 H04H 5/00 H04N 5/60 102

Claims (3)

(57) [Claims] 1. A reference for use in a receiving apparatus for stereo broadcasting or audio multiplex broadcasting in FM broadcasting and for separating the original left and right audio signals from a sum signal and a difference signal of left and right audio signals included in a received carrier signal. in the pilot signal removing circuit for removing from said carrier signal a pilot signal is a signal after the separation, the pilot signal
From the stereo demodulated stereo signal including the carrier signal on which the pilot signal is superimposed.
Filter means you extract only harmonic components above, the extraction
The signal amplitude of the output first harmonic component is level-amplified .
Variable amplifying means and the amplified first harmonic component
And a subtraction means for subtracting from a stereo signal after the stereo demodulation, it takes the carrier signal from the subtraction result
The pilot signal remaining on the carrier signal is
The second harmonic component extracted from the above is converted into a DC level,
A first method for variably controlling the level amplification at this DC level
Level control means or the stereo signal after the stereo demodulation
Only the harmonic components including the pilot signal from the signal
And the first DC level detected and extracted and the level amplification
The detected output is compared with the detected second DC level, and the difference is calculated.
The level amplification is variable at the third DC level that has been amplified
Pilot signal removing circuit according to claim Rukoto provided and either the second level control means for controlling. 2. The first level control means inputs a carrier obtained by subtracting the pilot signal from the stereo demodulated stereo signal from the subtractor, and converts the pilot signal remaining in the signal into a harmonic component. And a built-in high-pass filter to further extract as the second harmonic component and convert and output the amplitude value to the first DC level, and convert the output first DC level to the first harmonic level. 2. The pilot signal elimination circuit according to claim 1, comprising a first level detection circuit that outputs a variable control voltage for wave component level amplification to a first variable gain amplifier of said variable amplification means. 3. The second level control means inputs a stereo signal after the stereo demodulation, extracts the pilot signal contained in the signal together with a harmonic component by a built-in high-pass filter, and extracts an amplitude value of the pilot signal. A first level detection circuit for converting and outputting the first direct current level, and an amplitude value of the first harmonic component level-amplified by the first variable gain amplifier of the variable amplifying means. A second level detection circuit that converts the output into a level, compares the first DC level and the second DC level, and uses the difference voltage as a variable control voltage for amplifying the level of the first harmonic component. 2. The pilot signal removing circuit according to claim 1, further comprising a differential amplifier for outputting to said first variable gain amplifier.