JPS6370628A - Noise reduction circuit for fm stereo - Google Patents

Abstract

PURPOSE:To improve the S/N at the reception with a weak electric field and to improve the separation of right/left channel signals by providing a signal processing circuit comprising an amplitude comparator or the like, a synchronizing detector, an LPF, a BPF and a full wave rectifier or the like. CONSTITUTION:A stereo band component is separated from an output of an FM demodulator by the BPF 5, the result is subject to synchronizing detection by synchronism detectors 6, 7 and a signal Es' is outputted and the envelope of the signal Es' is obtained via the full wave rectifier 9 and the LPF 10. Then the output of the LPF 10 and the detector 6 is given to the signal processing circuit 11, which eliminates a signal having a prescribed amplitude or below from the output of the LPF 10 being the envelope of outputs Es' and the signal Es' itself of the detector 6 via the built-in amplitude comparator 16 and the gate pulse generator 17 or the like, and a right channel signal ER13 and the left channel signal EL14 are obtained at the output of the matrix 12. Thus, the S/N at the reception with a weak electric field is improved and the separation of the right/left channel signal is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION A. Field of Industrial Application The present invention relates to a noise reduction circuit in FM stereo.

B0 Summary of the Invention During stereo reception with an FM receiver, a carrier wave CO8ωt is used to detect a stereo signal, and 23 to 53 kl (when synchronously detecting the z band, a carrier wave sin ωp orthogonal to the same band is used)
Perform synchronous detection at t, compare the amplitude of the envelope with the envelope of the signal obtained by synchronous detection at CO9ωp1, and calculate the sin
Components whose level is lower than the component detected by ωp1 are 0.
As a result, interference signals distributed in small amplitudes are removed. Furthermore, when the interference signal is large, discontinuities caused by signal processing are interpolated using sawtooth waves.

C0 Conventional technology In the frequency distribution of the baseband signal of the FM broadcast currently being broadcast, as shown in Fig. 10, the monaural signal (
The right channel signal ER2, left channel signal Et, monaural signal EM = ER + EL) is below 15kHz, and this signal has fp/2 = 19kHz and fp = 3
A stereo signal (ES = E p E L ) of f, ± 15kl (z) that is amplitude-modulated is multiplexed on 8. This multiplexed signal is used for FM modulation, and the receiver transmits F
The signal is demodulated into the signal shown in Fig. 10, and the ER and EL channel signals are separated from the EM and ES by means of a 71-rix.

Noise from the transmission system is added to the demodulated signal of the FM signal, and this noise is so-called triangular noise that increases in proportion to the frequency. Therefore, between the monaural signal EM and the stereo signal ES in Figure 10, the ES is 20 dB lower.
The S/N ratio has deteriorated. Therefore, when receiving in a weak electric field, the S/N ratio of the stereo signal Es deteriorates significantly, and even if it is sufficiently audible as a monaural signal, it may become unlistenable as a stereo signal.

D0 Problems to be Solved by the Invention As a countermeasure to this problem, for example, in the 5ASC circuit, a method is adopted in which stereo reception is switched to monaural reception in a weak electric field, but this has the drawback that stereo reception is no longer possible.

In addition, in the United States, CBS is experimenting with adding a signal modulated with a different orthogonal carrier wave to the stereo signal, called FMX, to improve the S/N ratio of the stereo signal, but in Japan, such a method has not been implemented. There is no chance of putting it into practical use.

The purpose of the present invention is to improve the S/N ratio of a stereo signal in an FM receiver, improve the S/N ratio when receiving in a weak electric field, and improve the degree of separation between the left channel signal and the right channel signal. An object of the present invention is to provide an FM stereo noise reduction circuit that makes it possible to perform FM stereo noise reduction.

Means for Solving the E0 Problem In order to achieve the above object, the FM stereo noise reduction circuit according to the present invention includes means for separating a monaural component from a demodulator output signal, and a means for separating a monaural component from a demodulator output signal. a carrier wave oscillator that is driven by the means for extracting the predetermined frequency pilot component and transmits a carrier wave; first synchronous detection means for synchronously detecting the stereo band components obtained by the means for separating the stereo band components;
means for shifting the phase of the carrier wave by 90°; second synchronous detection means for synchronously detecting the stereo band component by means for shifting the phase of the carrier wave by 90°; means for obtaining an envelope of the resulting signal;
a signal processing means for removing a signal having a predetermined amplitude or less from a signal obtained by the means for obtaining an envelope from a signal obtained by the first synchronous detection means; a signal produced by the means for separating the monaural component; and means for obtaining left and right channel signals by signals according to the present invention.

In an advantageous embodiment of the invention, the signal processing means consist of a full-wave rectifier, an amplitude comparator, a gate pulse generator and a gain control circuit.

F1 Effect FIG. 2 is an example in which an interference component is mixed in the stereo band of the FM baseband signal of FIG. 10. Let the carrier frequency be fp (angular frequency ω.

=2πfp), and the frequency of the interference component is fN=f.

If it is 18 fN, the distribution will be as shown in FIG. 2(a).

(a) is decomposed into 005 (11pt) in-phase component in (b) and orthogonal component in (c).

cos (ω, + ΔωN)t = 1. /2 C08((+Jp−ΔωN)t”],
/2 Co5(C,1,+ΔωN)t-1/2 cos
(ωp−ΔωN)t+ 1/2 cos(ω, +Δ(1
111) t・・・・・・(1) The upper part of equation (1) is shown in Fig. 2(b), and the lower part is shown in Fig. 2(b).
c).

After FM demodulation at the receiver, the stereo band components are converted to cosω
Perform synchronous detection with pt. In other words, the amplitude of the jamming signal is
20, the stereo band signal of the received signal is expressed by the formula (2
).

es=Scosωp1 + 2n CO8(CIIFI+Δ(,IN)
t −=・−(2) Expression (2) as CO3ω
, t, synchronous detection, that is, taking the low frequency component of C5cos ωp1, yields Es'.

Es' = S + n cos△(+) N
Figure 3 ('a) shows the waveform of Esl.

On the other hand, the component of equation (2) is synchronously detected using sinωpt orthogonal to the carrier wave cosω9t, that is, C5sinω
If we take the low frequency component of p1, it becomes R5'.

Es' = −n sinΔ ωN t −−・−
(4) FIG. 3(b) shows the interference component of equation (4). formula(
4) does not include the stereo signal S, but only the interference signal.

The present invention utilizes the fact that the interference component shown in equation (4) corresponds to the interference component shown in equation (3) to remove a small-level interference signal from a received stereo signal. In terms of waveforms, components in the amplitude range of 2n are removed from the synchronous detection signal E sl shown in FIG. 3(a) as shown in FIG. 4, and components with large amplitudes are left.

Figure 4 (a) is an example in which low amplitude components are removed, and (b)
) is clipped, so the overall amplitude is reduced by 2n, and the stereo signal S also suffers from discontinuity due to clipping, so (a) is preferable. Here, a method for realizing the waveform shown in FIG. 4(a) is proposed.

G. EXAMPLES The present invention will be explained in more detail below using examples with reference to the drawings, but these are merely illustrative and various modifications and improvements can be made without going beyond the scope of the present invention. Of course it is possible.

FIG. 1 is a block diagram showing the basic configuration of an FM stereo noise reduction circuit according to the present invention. In the figure, 1 is an FM demodulator, 2 is an fI, /2 separation filter, 3 is an fp generator, and 4 is a 15kl (z low-pass filter, 5 is 23-53k) Iz band-pass filter, 6, 7 are synchronous detectors, 8 is 90° phase shifter, 9 is full-wave rectifier, 1
0 is a low-pass filter, 11 is a signal processing circuit, 12
is the matrix, 13 is the right channel output, 14
represents the left channel output.

A monaural component EM is separated from the output of the FM demodulator by a 15 kl (z bandpass filter).

The fp/2 separation filter 2 is a narrow band filter, extracts a 1.9 kHz pilot component, drives a carrier wave oscillator 3, and obtains CO8ωpt.

A stereo band component is separated from the output of the FM demodulator 1 by a band pass filter 5 of 23 to 53 kl (z), and a synchronous detector 6 performs synchronous detection using cosωp1 of the output of the fp generator 3, and E in equation (3) , 1. On the other hand, the output cosωpt of fp generator 3 is taken out as 90
'The phase shifter 8 sets sinωpt, the output of the low-pass filter 5 is synchronously detected by the synchronous detector 7, and E of equation (4) is
Take out s″.

T2S' is rectified by a full-wave rectifier 9 to obtain an envelope of Es'' by a low-pass filter 1o.Signal processing circuit 11
The details will be described later. The signal processing circuit 11 is an envelope of Es' and Es', which are the outputs of the synchronous detector 6.
The output of the low-pass filter 10 is used to remove signals with an amplitude of n or less, resulting in the waveform shown in FIG. 4(a), and the matrix 12
A right channel signal ER13 and a left channel signal EL14 are obtained as outputs.

Details of the signal processing circuit 11 are shown in FIG. 5, and waveforms of the signal processing process are shown in FIG. Figure 6(a) shows sin
This is the output Es' of the synchronous detector 7 detected at ωp1, (b) is the output of the full-wave rectifier 9, and (c) is the output of the low-pass filter 10, that is, the envelope of Es+. (d) is the output of the synchronous detector 6, ie, Es', which is rectified by the full-wave rectifier 15 in FIG. 5 to form (e).

The amplitude comparator 16 detects the time when the level is equal to or lower than the level n shown by the broken line in (e). Goo 1 ~ Pulse generator 17
A gay 1 to pulse corresponding to this time is generated. 6th
Figure (f) shows the waveform of the gate pulse. gain control circuit
When the gain is 1 at 1 of the pulse (f) in 18 and the gain is O at O, the output E of the gain control circuit 18 is
In sII, as shown in FIG. 6(g), signals with an amplitude of 2n or less are removed.

When n is small, the configuration shown in FIG. 5 is sufficient. However, as n becomes larger, discontinuity in the waveform of S becomes a problem. When n becomes large, discontinuity occurs in the process shown by the solid line, so continuity occurs when the process shown by the broken line in FIG. 7 is used. FIG. 8 shows the signal processing process.

FIG. 8(a) shows the output of the gate pulse generator 17 shown in FIG. 5, the gain 1~pulse, and the time at which the gain becomes 1 is defined as T. (b) is a differential waveform of (a), and time information of the leading edge and trailing edge is extracted. (c) shows the generation of a sawtooth wave, with length T and amplitude k. (d) shows a sawtooth wave with an amplitude of 2n due to the gain control in (c). on the other hand,(
(e) is also obtained by setting the amplitude of g) as n. (In Figure 9, the polarity of (e) is also reversed.) (f) is (d)
+(e), resulting in the desired waveform, and interpolation of the broken line is performed as shown in (g). (e) may be used at point A in FIG. 7, but at point B, interpolation (i) is performed using a signal with the polarity inverted from (f) as shown in (h).

FIG. 9 is a system diagram of an interpolated waveform generation circuit. The gate pulse generation output at time T of the gate pulse generator 17 is the eighth
This is the waveform in the figure (,). Differentiate using the differentiating circuit 19 to obtain the result shown in Fig. 8(b), and use the sawtooth wave oscillator (multivibrator) 20 to obtain the time of 11 TT and the amplitude k (k changes with T).
A sawtooth wave with an amplitude of 2n is generated in the gain control circuit 21 due to the envelope of the interference component of the low-pass filter 10, as shown in FIG. 8(d). On the other hand, the output of the gate pulse generator 17 is transferred to the polarity inverter 22.
The polarity is inverted by the gain control circuit 23, the amplitude is controlled to n by the gain control circuit 23, as shown in FIG.
The outputs of 1 and 23 are combined to obtain the interpolated signal shown in FIG. 8(f).

The polarity of the output is inverted by a polarity inverter 25 to obtain (h).

On the other hand, the differential circuit 27
Differentiate with and use gate circuit 28 as gate pulse generator.
Gate with the gate pulse of 17, polarity discrimination circuit 29
Add to. That is, the output of gate =12-28 is a positive output of the differentiating circuit 27 in FIG. 7A, and a negative output in FIG.

Therefore, if it corresponds to A, the polarity discrimination circuit 29
is positive, set electronic switch 26 to a, and set B
In this case, the electronic switch 26 is set to b, and the adder circuit 3
Gain at 0! In addition to the output of the II control circuit 18, (
g) or (i).

As explained in detail about the H1 invention, when receiving FM stereo, the S/N ratio of the stereo signal deteriorates in a weak electric field and the sound quality deteriorates, but the signal processing of the present invention eliminates interference noise at low amplitudes. , S/N J4 can be improved.

The circuits shown in FIGS. 5 and 9 can be applied to NG (noise canceller). For example, the discontinuity in Figure 7 occurs not only near the zero level but also at any location.
Since the control pulse of the low-pass filter corresponds to noise in the IF band, discontinuities caused by noise removal can be smoothly interpolated using the method shown in FIG. In this way, by linearly interpolating waveform discontinuities caused by pulse noise removal in NG (noise canceller), the auditory sense of abnormality caused by noise removal is reduced.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the basic configuration of the FM stereo noise reduction circuit according to the present invention, Fig. 2 is an explanatory diagram of interference components, Fig. 3 is a synchronous detection output waveform diagram, and Fig. 4 is a low amplitude noise A waveform diagram for removal, Figure 5 is a block diagram showing the configuration of the signal processing circuit, Figure 6 is a waveform diagram showing the signal processing process for noise suppression, Figure 7 is an output waveform diagram when n is large,
Figure 8 is an explanatory diagram of linear interpolation, Figure 9 is a block diagram showing the configuration of the interpolation waveform generation circuit, and Figure 10 is stereo FM.
FIG. 2 is a frequency distribution diagram of a baseband signal of FIG. 1...FM demodulator, 2......f,
/2 separation filter, 3... f, generator,
4・・・・・・・・・ 15kl (z low-pass filter,
5......23-53 upper band pass filter,
6, 7...... Synchronous detector, 8...
・90° phase shifter, 9...Full wave rectifier, 10.
...Low pass filter, 11...
Signal processing circuit, 12...71~Rix, 1
3...Right channel output, 14...
...Left channel output, 15...Full wave rectifier, 16...Amplitude comparator, 17...
...Gate pulse generator, 18...Old...Gain control circuit, 19...Differential circuit, 2o...
...Sawtooth wave oscillator (multivibrator), 2
1゜23...・Gain control circuit, 22.25...Old...Polarity inversion circuit, 24...Addition circuit, 26
......Electronic switch, 27...Differential circuit, 28...Old gate circuit, 29...
...Polarity discrimination circuit, 3o...Addition circuit.

Claims (2)

[Claims] (1) (a) means for separating a monaural component from a demodulator output signal; (b) means for extracting a predetermined frequency pilot component from a demodulator output signal; (c) means for separating a stereo band component from a demodulator output signal; (d) a carrier wave oscillator driven by the means for extracting the predetermined frequency pilot component and transmitting a carrier wave; (e) a first carrier wave oscillator for synchronously detecting the stereo band component obtained by the means for separating the stereo band component by the carrier wave oscillator; synchronous detection means, (f) means for shifting the phase of the carrier wave by 90 degrees, and (g) second synchronous detection means for synchronously detecting the stereo band component by means for shifting the phase of the carrier wave by 90 degrees. (h) means for obtaining the envelope of the signal obtained by the second synchronous detection means; (i) a signal obtained by the means for obtaining the envelope from the signal obtained by the first synchronous detection means, which has a predetermined amplitude or less; An FM stereo noise reduction circuit characterized in that it includes a signal processing means for removing the signal, and (j) means for obtaining left and right channel signals from the signal produced by the monaural component separating means and the signal produced by the signal processing means. . (2) The FM stereo noise reduction circuit according to claim 1, wherein the signal processing means comprises a full-wave rectifier, an amplitude comparator, a gate pulse generator, and a gain control circuit.