JPH0964763A - Pilot signal eliminating circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate a pilot signal component included in a stereo signal effectively in the case of receiving an FM broadcast program. SOLUTION: A stereo signal ST fed from an input terminal 1 is given to a subtractor 4 and an HPF 2, the HPF 2 extracts only a harmonic component PL1 including a 19kHz pilot signal, a variable gain amplifier 3 amplifies the signal PL1 to provide an output of a harmonic component PL2 whose level reduction is compensated. The subtractor 4 subtracts the harmonic component PL2 from the stereo signal ST and provides a resulting carrier CW to an output terminal 6 and also to a level detection circuit 5. The level detection circuit 5 converts the signal level of the harmonics resident in the carrier CW into a DC level and gives the result to the variable gain amplifier 3 as a control voltage for its amplification factor. That is, when the much residual component is in existence in the carrier CW, the amplification factor is increased to form a control loop so that the harmonic component including the pilot signal is not left in the subtraction output.

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 for an FM receiver or the like used when receiving a stereo broadcast or a voice multiplex broadcast.

[0002]

2. Description of the Related Art In general FM stereo broadcasting,
1 as a reference signal for separating the original left and right audio signals from the sum signal (L + R) and the difference signal (LR) of the left (L) right (R) audio signals included in the received carrier signal
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.

This pilot signal is P
Although it has an important function in the has Locked Loop (PLL) circuit, it is supplied as it is included in the received stereo signal, so if it is demodulated as it is, it will be output as it is included in the demodulator output. It becomes a problem in hearing. Therefore, it is necessary to remove this 19 KHz pilot signal.

An example of this kind of pilot signal removing circuit is adopted in the product name "μPC1287" of NEC Corporation. Referring to FIG. 4, which shows a block diagram of the pilot signal elimination 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 into a triangular wave SQ1. , The variable gain amplifier 5 for level-amplifying the signal amplitude of the triangular wave SQ1, and 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 wave CW
Is supplied to the output terminal 4, and the carrier signal CW in which this addition is performed and the pilot signal is attenuated is supplied,
The level detection circuit 14 extracts only the pilot signal still remaining from the signal CW and converts the amplitude value thereof into the DC level DC5.

Referring also to FIG. 5, which shows 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 by 90 ° from the square wave signal (the square wave in FIG. 5) that is synchronized with the pilot signal from the input terminal 10 and has the phase shifted by 90 °, that is, 180 ° from the pilot signal ° Phase-shifted triangular wave signal SQ1
(FIG. 5-Triangle 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.

In the adder 13, the pilot signal of the sine wave stereo signal ST and the triangular wave signal SQ2 having a phase difference of 180 ° are added, and the carrier wave CW from which the pilot signal is removed is output to the output terminal 4. At the same time, it is also supplied to the level detection circuit 14.

In the carrier wave CW, when the sine wave and the triangular wave SQ2 whose phase is shifted by 180 ° are added, a residual component always exists in the portion corresponding to the hypotenuse portion of the triangular wave. Only the pilot signals except for are extracted and converted to a DC level DC5.

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

Another example of this type of signal removing circuit is disclosed in Japanese Patent Application Laid-Open No. Hei 4 (1999) -1998
-37373 gazette. 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 are clamped to a predetermined level. And a high-pass filter (H which passes only the luminance component of the video signal).
PF) 17, a limiter 18 that limits the signal amplitude of the output signal HF of the HPF 17, a variable gain amplifier 19 that amplifies the output LM1 of the limiter 18 by the output signal CL of the clamp circuit 16, and The adder 13 outputs the output signal LM2 of the variable gain amplifier 19 and the original video signal VD to the output terminal 4 to reduce the noise reduction amount included in the video signal.

In this circuit, the signal VD supplied from the input terminal passes through the LPF 15 to extract low frequency components. The signal LF output from the LPF 15 is supplied to the clamp circuit 16 and clamped at a predetermined level. On the other hand, the input signal VD is inverted and amplified by the variable gain inverting amplifier 19 as the output signal LM1 of the limiter 18 as the output signal LM1 of the limiter 18 passes through the HPF 17, and the amplification degree is the output signal of the clamp circuit 16. 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 output from the output terminal 4 as a noise-reduced signal.

[0012]

In the conventional pilot signal removing circuit described above, (a) since the PLL itself locks with the pilot signal of 19 KHz, a certain level of pilot signal level remains in the composite signal after the pilot signal is removed. If not done, the PLL circuit cannot be configured, and the 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, the amount of attenuation is limited, and theoretically only a difference of 18 dB can be taken as the difference between the sine wave and the triangular wave. (D) If a circuit for converting a triangular wave, which is mainly used in high-end machines, into a sine wave is used, the circuit scale becomes large.
Furthermore, if the circuit is calibrated with individual parts, adjustment points are required due to variations in the elements. (E) Since only the fundamental wave of the pilot signal is removed by the triangular wave, when the composite signal is demodulated into a stereo signal, the harmonic component of 19 KHz contained in the triangular wave remains. (F) Leakage of the pilot signal component may be output to the output signal due to offset in the variable inverting amplifier 19.

An object of the present invention is to eliminate the pilot signal component included in the output signal after stereo demodulation to the noise level without affecting the PLL circuit, in view of the above-mentioned problems. It is to provide a pilot signal removing circuit capable of

[0014]

The pilot signal detection circuit of the present invention is characterized in 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 are included. In the pilot signal removing circuit for removing the pilot signal, which becomes the reference signal when separating the original left and right audio signals from the signal, respectively, in the carrier signal of each of the composite signal or the signal after stereo demodulation, the pilot signal Is supplied with a stereo signal, the filter means for passing only a predetermined frequency band from the stereo signal, and the signal amplitude value of the output of the filter means is level-amplified, and the amplified signal of the predetermined frequency band is supplied. A subtractor for subtracting the amplitude value from the stereo signal and the subtraction Lies in comprising a signal level control means for controlling the level amplification of the signal amplitude value using any of the signal of the input or output signal of the.

Further, the filter means can be composed of a high-pass filter that removes the carrier signal of the audio signal included in the stereo signal and passes only the harmonic component including the pilot signal.

Further, the signal level control means extracts a level of the harmonic component remaining in the carrier signal output from the subtractor and converts the amplitude value into a first DC level for output, and a level detection circuit for the level detection circuit. A first variable gain amplifier whose amplification degree is controlled when the higher harmonic component is level-amplified with the first direct current level as a control voltage, and the higher harmonic wave is added to the carrier signal of the subtractor output. If left, the first variable gain amplifier can form a control loop that operates to increase the amplification degree by the control voltage.

Furthermore, the signal level control means is
A second variable gain amplifier whose amplification degree when level-amplifying the harmonic component is controlled by a predetermined control voltage, and the amplitude of the harmonic component extracted from the stereo signal supplied to the subtractor. A second level detection circuit for converting and outputting the value into a second direct current level; and a third for converting and outputting the amplitude value of the harmonic component level-amplified by the second variable gain amplifier into a third direct current level. Level detection circuit and a differential amplifier that compares the second and third DC levels and outputs the difference voltage as the predetermined control voltage. The third DC voltage level is higher than the second DC level. The second variable gain amplifier is operated by the predetermined control voltage when the DC level is high, and the third DC level is lower than the second DC level. Wherein by said predetermined control voltage second
The variable gain amplifier can also form a control loop that operates to increase the amplification degree.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION 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 a pilot signal removing circuit of the present invention. Referring to FIG. 1, an input terminal 1 to which a stereo signal including the 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, and an input terminal 1.
HPF that passes only frequencies above 19 KHz by removing audio signals of several wave numbers lower than 19 KHz of the pilot signal included in the stereo signal ST supplied via
2, a variable gain amplifier 3 for level-amplifying the signal amplitude of a harmonic component PL1 including the pilot signal of 19 KHz output from the HPF2, and a harmonic component PL2 including the amplified pilot signal are supplied to the input terminal 1. The subtractor 4 that subtracts the carrier CW from the original stereo signal ST that has been generated and the harmonic level including the pilot signal of 19 KHz remaining in the carrier CW of the subtraction result of the subtracter 4 is detected and the DC level is detected. A level detection circuit 5 for converting into DC1 and controlling the amplification degree of the variable gain amplifier 3 using the outputted DC level DC1, and an output terminal 6 for outputting the output carrier wave CW of the subtractor 3 to the next stage. Composed of and.

Referring also to FIG. 1 and FIG. 2 showing a timing chart for explaining the operation, this figure shows 0 on the vertical axis.
The upper side is a positive voltage and the lower side is a negative voltage with V as the center, and the horizontal axis represents time T. The operation of this circuit is such that, for example, if a stereo signal ST is supplied to the input terminal 1, a pilot signal of 19 KHz is modulated on a carrier of 1 KHz in this signal (FIG. 2-input terminal 1).

The voice signal is removed by the HPF2 connected to the input terminal 1, and the harmonic wave PL1 containing the pilot signal of 19 KHz or more is obtained (FIG. 2-HPF2 output). Since the signal amplitude of this harmonic component PL1 is attenuated by passing through the HPF2, it is level-amplified by the variable gain amplifier 3 inserted in the next stage and supplied to the subtractor 4 (FIG.
Variable amplifier 3 output).

Since the stereo signal ST is supplied from the input terminal 1 to one input terminal of the subtractor 4, 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.

By this subtraction, 19 KHz is output to the output terminal.
The carrier wave CW containing the above pilot signal and having a smaller harmonic component PL2 is output (FIG. 2—output of the subtracter 4). Since the harmonic component PL2 still remains on the carrier wave CW, the level detector 5 first extracts only the 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 DC level DC1 is of course the harmonic component P
Since it fluctuates corresponding to the signal amplitude of L2, it is possible to control the amplification degree of the variable gain amplifier 3 by the fluctuating DC level DC1.

That is, when the harmonic component PL2 remaining in the output of the subtractor 4 is large, the converted DC level DC
1 also increases, the amplification degree of the variable gain amplifier 3 also increases, the amplitude of the harmonic component PL2 supplied to the subtractor 4 also increases, and the value subtracted from the stereo signal ST also increases. The harmonic component PL2 included in the carrier wave CW becomes even smaller.

By the operation described above, since the pilot signal of itself is used as a signal for removing the pilot signal, leakage of harmonic components including the pilot signal due to the offset of the variable gain amplifier 3 is corrected by using the level detection circuit 5. Therefore, the pilot signal component remaining in the subtractor output can be removed up to the noise level.

In the above-described first embodiment, the HPF 2 mounted is a fourth-order 19 KHz HPF.
When the harmonic component including the pilot signal extracted in F is subtracted from the stereo signal ST and removed, the loss of the stereo signal is −0.4 dB at 8 KHz and −0.4 dB at 10 KHz.
The filter characteristic is as steep as about 1.5 dB, but such a lack is a level that does not pose a problem in actual use. In order to implement the same characteristics using the BPF, a high-order filter several tens of times higher is required, and when integrated into a circuit, the number of elements and the capacitance value increase, so that the circuit described in this embodiment is used. Is much more advantageous for cost reduction.

Further, in the above-mentioned conventional removal circuit of this type, the pilot signal is removed by using the triangular wave signal, so that it is necessary to provide the triangular wave conversion circuit, and the chip layout space corresponding to that is required. In the case of the form example, it is unnecessary and the circuit scale can be reduced.

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

That is, 19 KH is added to each of the carrier signals of the composite signal or the signal after stereo demodulation.
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 number of waves lower than 19 KHz of the pilot signal included in the stereo signal ST supplied via the input terminal 1. Of the carrier wave of which the carrier wave is removed to pass a harmonic component of 19 KHz or higher, and a harmonic component PL1 output from this HPF2.
The variable gain amplifier 3 for level-amplifying the signal amplitude of the signal and the amplified harmonic component PL2 are used as the original stereo signal ST.
A subtractor 4 for subtracting the carrier wave CW from the original stereo signal ST, and a level detection circuit 7 for extracting a harmonic component including a pilot signal of 19 KHz from the original stereo signal ST with a built-in HPF 7a, converting it to a DC level DC2, and outputting it. And the harmonic component PL2 amplified by the variable gain amplifier 3, the level detection circuit 8 converting this signal to a DC level DC3 and outputting the DC level DC3, and the DC level DC2 output from these level detection circuits 7 and 8. And DC3, and the difference between them is amplified to output a DC level DC4 to the variable amplifier 3 as a control voltage, and an output terminal 6 for outputting the output carrier CW of the subtractor 3 to the next stage. Composed.

Referring to FIGS. 2 and 3 together, the operation of this circuit is similar to that of the first embodiment, assuming that a stereo signal is supplied to the input terminal 1, for example, this signal has a carrier of 1 KHz. Since the pilot signal of 19 KHz is modulated on (Fig. 2-input terminal 1), input terminal 1
The carrier of the audio signal is removed by the HPF2 connected to the
Harmonic component P including a pilot signal of 19 kHz or higher
L1 is obtained (FIG. 2-HPF2 output).

This harmonic component PL1 has its signal amplitude attenuated by passing through the HPF2 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 into a DC level DC2.

On the other hand, the level detector 8 is a variable gain amplifier 3
The harmonic component PL2 output by the
Convert to 3. That is, since the harmonic component PL2 does not include a voice signal carrier, the level detector 8 does not need to have a built-in 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 which detects the output of the variable gain amplifier 3 is higher than the DC level DC2 of the above, the difference voltage is amplified to be higher than the potential already output. A low-potential DC level DC4 is supplied as a control voltage for the variable gain amplifier 3. The variable gain amplifier 3 controlled by this low voltage has a small amplification degree, and the output harmonic component PL2 also has a small amplitude.

The subtractor 4 is a 19K signal amplified by the variable gain amplifier 3 from the stereo signal ST supplied to one input terminal.
Although the harmonic component PL2 including the pilot signal of Hz or more is subtracted, the amplitude value of the subtracting harmonic component PL2 and the amplitude value of the subtracting harmonic component of the stereo signal ST become substantially equal. The carrier wave 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).

Further, rather than the DC level DC2 of the level detector 7 detected from the original stereo signal ST, the DC level D of the level detector 8 detecting the output of the variable gain amplifier 3 is detected.
If the level of C3 is low, the difference voltage is amplified and a DC level DC4 of a potential higher than the already output potential is supplied as the control voltage of the variable gain amplifier 3. The variable gain amplifier 3 controlled by this high voltage has a large amplification degree and the amplitude of the output harmonic component PL2 also becomes large.

Also in this case, the higher harmonic component PL2 to be subtracted
Since the amplitude value of the harmonic component of the stereo signal ST to be subtracted becomes substantially equal to the amplitude value of, the carrier wave CW from which the harmonic component is removed to the noise level is output to the subtraction result output of the subtractor 4. It

In the case of the second embodiment, the level detection is performed from the output signal of the subtractor 3 in the first embodiment, whereas the level detection is performed from the input signal of the subtractor 3, that is, the subtraction is performed. Since the level detection is performed before the level control, the carrier wave 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 of the variable gain amplifier 3 and the like is eliminated. Since the level detection circuit 5 can be controlled to perform correction, the pilot signal component remaining in the subtractor output can be removed up 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 a composite signal or a signal after stereo demodulation, and the stereo signal is supplied in advance from this stereo signal. Filter means for passing only a predetermined frequency band to be defined, a subtractor for level-amplifying the signal amplitude value of the output of the filter means, and subtracting the amplified signal amplitude value for the predetermined frequency band from a stereo signal, and this subtraction Since it has a 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 instrument, the pilot signal component of the stereo signal can be attenuated to the noise level.

Further, not only the harmonic components of the pilot signal that has passed through the HPF but also other noise components can be removed from the stereo signal.

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

Since the fourth-order 19 KHz HPF is used to remove the 19 KHz pilot signal component, a steep filter characteristic is obtained at a level that does not pose a problem in practical use, and with a normal bandpass filter (BPF). To obtain equivalent characteristics, a tens of times higher order filter is needed,
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 it is a fourth-order 19 KHz HPF, it is possible to provide a lower cost pilot signal elimination circuit.

[Brief description of 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 removing 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]

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 Triangle Wave Converter 12 Variable Gain Amplifier 13 Adder ST Stereo Signal PL1 HPF2 output harmonic component PL2 Variable gain amplifier 3 output harmonic component CW Subtractor output carrier DC1, DC2, DC3, DC5 Level detector 7,
DC level of 8 and 14 outputs 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

Claims (4)

[Claims] 1. A standard for separating an original left / right audio signal from a sum signal and a difference signal of left / right audio signals included in a received carrier signal, which is used in a receiver of stereo broadcasting or audio multiplex broadcasting in FM broadcasting. In the pilot signal removing circuit for removing the pilot signal to be a signal after the separation, a stereo signal including the pilot signal is supplied to each of the carrier signals of the composite signal or the signal after the stereo demodulation, and the stereo signal including the pilot signal is predetermined from the stereo signal. Filter means for passing only the frequency band, a subtractor for level-amplifying the signal amplitude value of the output of the filter means, and subtracting the amplified signal amplitude value of the predetermined frequency band from the stereo signal, and the subtractor The signal swing using either the input or output signal of the A pilot signal removing circuit comprising: a signal level control means for controlling level amplification of a width value. 2. The high-pass filter according to claim 1, wherein the filter means is a high-pass filter that removes the carrier signal of an audio signal included in the stereo signal and passes only a harmonic component including the pilot signal. Pilot signal elimination circuit. 3. The level detection circuit, wherein the signal level control means extracts the harmonic component remaining in the carrier signal output from the subtractor and converts the amplitude value into a first DC level for output, and its output. A first variable gain amplifier whose amplification degree is controlled when the higher harmonic component is level-amplified with the first direct current level as a control voltage, and the higher harmonic wave is added to the carrier signal of the subtractor output. 2. The pilot signal removing circuit according to claim 1, wherein if the residual voltage remains, the first variable gain amplifier forms a control loop that operates to increase the amplification degree by the control voltage. 4. The signal level control means comprises a second variable gain amplifier whose amplification degree when level-amplifying the harmonic component is controlled by a predetermined control voltage, and the stereo supplied to the subtractor. A second level detection circuit for extracting the harmonic component from the signal and converting the amplitude value to a second DC level for output; and an amplitude value of the harmonic component level-amplified by the second variable gain amplifier. To a third DC level and outputs the 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. The second variable gain amplifier operates in a direction to reduce the amplification degree by the predetermined control voltage when the third DC level is higher than the second DC level, and
2. The second variable gain amplifier forms a control loop which operates to increase the amplification degree by the predetermined control voltage when the third DC level is lower than the second DC level. The described pilot signal elimination circuit.