JPH02148921A - Noise suppression circuit for fm receiver - Google Patents

Abstract

PURPOSE:To reduce noise by detecting pulse noise or multi-path noise from a phase change in an oscillated output of a phase locked loop (PLL) system oscillator for demodulating a switching signal. CONSTITUTION:The circuit consists of a phase change detection means 2 detecting a phase change in an oscillated output of a PLL oscillator 1 and outputting a prescribed noise control signal, a muting control circuit 3 controlled by the noise control signal, a high-cut control circuit 4 and a noise reduction means 6 such as a separation control circuit 5. The phase change detection means 2 monitors the state of the phase of the oscillated output from the PLL oscillator 1 and detects it if the phase of the oscillated output of the PLL oscillator 1 is changed due to pulse noise such as ignition noise or multi-path noise and outputs a prescribed noise control signal. The noise control signal is fed to the noise reduction means 6 to operate each circuit. Thus, noise of any level is detected and the noise is suppressed accurately.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a noise suppression circuit for an FM receiver for reducing pulse noise such as ignition noise and multipath noise.

[Conventional technology]

FM receivers, especially in-vehicle FM receivers that are used when vehicles are moving, are susceptible to multipath noise due to the influence of buildings and terrain, and are also susceptible to pulse noise such as ignition noise from vehicle engines.

Therefore, in-vehicle FM receivers employ noise suppression circuits to reduce these noises.

FIG. 4 shows an example of the configuration of a conventional noise suppression circuit.

In the figure, 41 is a signal meter signal (hereinafter, S
A signal meter circuit (hereinafter referred to as an S meter circuit) generates a meter signal (hereinafter referred to as a meter signal) and displays the received electric field strength on a meter, and 42 is a level detection circuit that detects noise from changes in the level of the S meter signal. 43 is a muting control circuit that reduces the overall signal level of the FM playback signal to suppress noise, and 44 is a high-cut control circuit that attenuates the high frequency components of the FM playback signal to improve the S/N! , 45 is a separation control small circuit that changes stereo separation to reduce noise.

Conventionally, the signal level of the S meter signal output from the S meter circuit 41 is monitored by the level detection circuit 42, and the signal level of the S meter signal drops below the dark value due to pulse noise such as ignition noise or multipath noise. When this is detected, the muting control circuit 43. High cut control circuit 44. The noise was reduced by controlling the separation control circuit 45 and the like.

[Problem to be solved by the invention]

However, when using a conventional noise suppression circuit, the level detection circuit 42 must set a dark value for level detection in order to prevent malfunction, and there is a problem that noise within this dark value cannot be detected. Ta.

The present invention was made based on the above circumstances, and noise reduction is performed by detecting pulse noise and multipath noise from the phase change of the oscillation output of a phase comparison (PLL) type oscillator for switching signal demodulation. The present invention aims to provide a noise suppression circuit for an FM receiver.

[Means to solve the problem]

In order to achieve the above object, the present invention employs a configuration as shown in FIG. 1.

That is, in an FM receiver that uses a phase comparison (PLL) type oscillator 1 as an oscillator for demodulating a switching signal synchronized with a pilot signal, a phase change in the oscillation output of the phase comparison type oscillator 1 is detected and a predetermined noise is detected. A phase change detection means 2 that outputs a control signal, and a muting control circuit 3 that is controlled by the noise control signal. High cut control circuit 4. It consists of a noise reduction means 6 such as a separation control circuit 5.

[For production]

When pulse noise such as ignition noise or multipath noise occurs during FM reception, the FM received wave undergoes amplitude modulation and phase modulation. Amplitude modulation is intermediate frequency (IF)
It can be removed by the limiter action of the amplification stage, but
Phase modulation cannot be removed. Therefore, the intermediate frequency signal undergoes phase modulation as shown from the solid line to the dotted line in FIG. 2(a), and the phase of the pilot signal also changes as shown from FIG. 2(b) to FIG. 2(C).

By the way, in the FM receiver, in order to perform stereo demodulation (MPX), the
An 8KHz switching signal is required.

As an oscillator to demodulate this switching signal,
Some use a phase comparison (PLL) type oscillator (hereinafter referred to as a PLL oscillator). This PLL oscillator uses the above pilot signal as a reference input signal and generates a 38KHz switching signal while comparing the phase with the oscillation output.If there is no phase modulation due to noise, the phase difference between the pilot signal and the oscillation output is kept constant (π/2).

However, as mentioned above, when the intermediate frequency signal undergoes phase modulation due to pulse noise such as ignition noise or multipath noise, the pilot signal demodulated from this phase modulated intermediate frequency signal is shown in Figure (C). Since the phase of the pilot signal changes as shown in FIG. The present invention focuses on this phenomenon and detects pulse noise such as ignition noise and multipath noise from the phase change of the oscillation output of the PLL oscillator.

That is, in FIG. 1, the phase change detection means 2
The phase state of the oscillation output of the L oscillator 1 is monitored, and if the phase of the oscillation output of the PLL oscillator 1 changes due to pulse noise such as ignition noise or multipath noise, this is detected and a predetermined noise control signal is output. do.

This noise control signal is transmitted to the muting control circuit 3. High cut control circuit 4. The signal is sent to a noise reduction means 6 consisting of a separation control circuit 5 and the like, and a predetermined noise reduction is performed by operating each circuit.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 3 shows an embodiment of the noise suppression circuit of the present invention. P
The LL oscillator 1 is composed of a phase comparator 7, a low pass filter (LPF) 8, a voltage controlled oscillator (VCO) 9, and a frequency divider 10, as is well known. The phase change detection means 2 is
It is composed of a comparator 11 for voltage comparison and a time constant circuit 12 that delays the input signal by a predetermined time. The output of the low-pass filter 8 is directly input to 14. The noise reduction means 6 is composed of a muting circuit 3, a high-cut control circuit 4, a separation control circuit 5, etc., as in the conventional case.

Next, the operation of the above embodiment will be explained. During normal operation, when there is no pulse noise such as ignition noise or multipath noise, 19K is input to PLL oscillation II.
Hz pilot signal and 19 output from frequency divider 10
It oscillates with a phase difference of π/2 (=90°) from the KHz oscillation output. Therefore, the phase comparison output of the phase comparator 7 is kept constant, and the rectified output voltage of the low-pass filter 8 that rectifies it is kept constant.

The rectified output of the low-pass filter 8 is transmitted through a time constant circuit 12.
is input to the input terminal 13 of the comparator 11 through the input terminal 13, and is directly input to the input terminal 14. However, in the above-mentioned normal state, the rectified output of the low-pass filter 8 is kept constant, so the input terminal 13 and The signal potentials of the signals 14 and 14 are the same potential, and the comparator 11 does not operate.

Under the above normal reception conditions, 19KH due to pulse noise such as ignition noise and multipath noise.
When the phase of the pilot signal z changes as shown in FIG. 2(C), the comparison output of the phase comparator 7 increases or decreases in proportion to the phase change, and the rectified output of the low-pass filter 8 changes. This rectified output is the input terminal 13.1 of the comparator 11.
However, since the time constant circuit 12 is connected to the input terminal 13, the level change of the rectified output is delayed by a predetermined delay time, and only the signal level of the input terminal 14 changes immediately, and the signal level of the comparator 11 changes immediately. The input potential becomes unbalanced. Therefore, at the same time as the phase change occurs, the comparator 11 operates and a noise control signal is output, and the muting control circuit 3, high cut control circuit 4. Noise reduction means 6 such as separation control circuit 5 operates to reduce noise.

Note that the noise reduction means 6 by the phase change detection means 2
As a control method, the level of the noise control signal may be changed in accordance with the change in the output of the low-pass filter 8, and the noise control operation may be performed continuously, or a predetermined darkness value may be set, and the level of the noise control signal may be changed in accordance with the output change of the low-pass filter 8. A noise control signal may be output when the output change of No. 8 exceeds this dark value, and the optimum control method is adopted depending on the purpose.

(Effects of the Invention) As explained above, the present invention detects pulse noise such as ignition noise and multipath noise from the phase change of the oscillation output of the PLL oscillator.
It is not necessarily necessary to set a dark value to prevent malfunctions as in the past, and it has the excellent effect of being able to detect noise of all levels and perform accurate noise suppression.

[Brief explanation of the drawing]

Fig. 1 is a diagram of the principle of the present invention, Fig. 2 is a waveform diagram of an intermediate frequency signal and a pilot signal for explaining the present invention, Fig. 3 is a block diagram of an embodiment of the present invention, and Fig. 4 is a conventional example. FIG. DESCRIPTION OF SYMBOLS 1... Phase comparison (PLL) type oscillator, 2... Phase change detection means, 6... Noise reduction means. Figure 2

Claims (1)

[Claims] In an FM receiver using a phase comparison (PLL) type oscillator as an oscillator for demodulating a switching signal synchronized with a pilot signal, a phase change in the oscillation output of the phase comparison type oscillator is detected. 1. A noise suppression circuit for an FM receiver, comprising: a phase change detection means for outputting a predetermined noise control signal; and a noise reduction means controlled by the noise control signal.