JPS5945303B2 - FM receiver noise removal device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a noise removal device for an FM stereo receiver, and more particularly to a pulse noise removal device.

In FM receivers, pulse noise caused by automobile spark plugs and noise caused by wipers are particularly problematic.

In order to remove such pulse noise, for example, the following pulse noise suppression circuit is provided. That is, a high-pass filter is provided to detect pulse noise from the composite signal output from the FM detector, and a monostable multi is triggered to amplify the noise component output from this filter and generate a gate signal for a predetermined period. This gate signal closes the gate circuit and temporarily cuts off the transmission of the composite signal to the MPX (altiplex) demodulation circuit. During this time, a hold circuit is provided to sample and hold the composite signal level immediately before the gate cutoff operation, and this hold output is generated. This is intended to be used as a substitute for MPX demodulation input. In such a pulse noise suppression circuit,
Because processing is performed using a composite signal after FM detection, phase shifts occur when the signal passes through multi-stage circuits and filters from the RF (high frequency) stage to the FM detection stage, resulting in the width of the noise pulse being reduced. As a result, it becomes necessary to lengthen the cut-off period of the gate circuit. Therefore, noise due to the operation of the noise suppression circuit becomes noticeable in the audio output signal that has passed through the LPF after MPX demodulation, which is undesirable (see FIG. 2B). Furthermore, if a stereo pilot signal is superimposed on the composite signal and this pilot signal is larger than the amplitude of the information signal, the hold circuit will effectively hold the level of the birod signal during the cutoff period of the gate circuit. Hold and MP
The signal will be applied to the X demodulation circuit, and noise proportional to the pilot signal level will also be generated in the audio output.

To solve this problem, we remove the pilot signal from the composite signal and then apply this composite signal to the noise suppression circuit to solve the above problem.However, in order to accurately transmit this pilot signal to the MPX demodulation circuit, a 19KHz pilot signal processing circuit is required. There are drawbacks that make it necessary. The object of the present invention is to be able to perform noise removal more completely and to
It is an object of the present invention to provide a pulse noise removing device for an FM stereo receiver that does not require a Z-pilot signal processing circuit. The present invention will be explained below with reference to the drawings.

FIG. 1 is a schematic block diagram illustrating the principle of the invention. A received signal from an antenna 1 is amplified by an RF amplifier 2 and mixed with a local signal from a local oscillator 4 in a mixer 3 to become an IF (intermediate frequency) signal. This F signal is selectively amplified in the IF amplifier 5 and input to the noise removal section 6. This circuit 6 is provided with a variable control type frequency information signal generator 61 which receives an IF signal as one input and receives a predetermined control signal as an external input. This signal generator 61 is configured to generate a reference signal whose reference signal changes according to a control signal, and to generate a frequency information signal corresponding to the frequency deviation of the input 1F signal with respect to this reference signal. That is, the frequency information signal is derived in the form of a DC voltage whose level changes according to the input frequency, and this output is outputted via the gate 62 as an audio output or a stereo composite output to an audio amplifier or MP.
It is applied to the X demodulation circuit 8. Further, the output of the gate 62 is fed back through a feedback loop to serve as a control input to the information signal generator 61 to control the reference signal. Further, a capacitor 63 is provided as a holding means for holding the output of the gate 62. A noise detector 7 is provided to control the gate 62, and this includes a noise detector 71 that detects and amplifies pulse noise in the IF signal.
and a control signal generator 72 that is triggered by this detection output and generates a gate signal for a predetermined period.

In such a configuration, if there is no noise in the IF signal, no gate signal is generated and gate 62 is open.

Therefore, the variable control type frequency information signal generator 61 forms a proper feedback loop and generates a DC voltage having a level corresponding 1:1 to the frequency of the F signal. This is the FM detection output, and therefore gate 6
2 to the amplifier 8. On the other hand, when pulse noise is mixed into the F signal, a gate signal is generated from the noise detector 7 and the gate 62 is closed for a certain period of time.

At this time, the feedback loop is disconnected, but since the control signal immediately before the gate is closed is held by the holding means 62, an output corresponding to the IF signal frequency immediately before the gate is closed is generated while the gate is closed. It turns out. In other words, if this output is used as an audio output, it will maintain its previous level during the gate closing period. Figure 2 shows the audio signal waveform after reproduction. In the present invention, as shown in A, the noise is removed in the FM detection stage itself, so the width of the noise pulse is smaller than that in the conventional FM detection stage. Since this is smaller than when processing noise (see B), the gate closing period T1 can be made shorter than the conventional one T2. Therefore, it is possible to suppress noise to the extent that it does not cause any problems with hearing. FIG. 3 is a block diagram showing one embodiment of the present invention, and parts equivalent to those in FIG. 1 are designated by the same reference numerals.

The noise removal section 6, that is, the variable control type frequency information signal generator 61 of the FM detection section consists of a phase comparator 64, an LPF (low pass filter) 62, and a VCO (voltage controlled oscillator) 66. It has an FM detection configuration using a well-known PLL (phase lock loop) circuit serving as a control input. Note that the reference signal in this case is the VCO output signal. This PLL loop is provided with a gate 62 as shown in the figure to cut off the loop during the noise period and instead use the hold output of the hold means 63 as a control input and as a detection output.

In this case, it goes without saying that the hold capacitor 63 may also be used as part of the capacitor of the LPF 65. In the above embodiment, the input of the noise detection section 7 is derived from the stage before the IF amplifier 5, and the input of the noise removal section 6 is passed through the IF amplifier 5, so that the gate signal generation timing and the noise removal timing are adjusted appropriately. There is no need to add a delay circuit or the like for separate timing control. According to the present invention, since noise processing is performed in the FM detection stage itself, unlike the processing after the FM detector that significantly affects the noise pulse width, the gate closing period can be shortened as explained in Fig. 2. This has the advantage that noise can be reduced by the sound removal operation and there is no need for pilot signal processing.

In addition, the low-pass filter 65 has a single-stage CR configuration,
If this capacitor is shared with the hold capacitor 63, no capacitor will be included before the gate 62, and therefore there will be almost no element that causes the signal waveform to vibrate during the after-sound removal operation, making it possible to further shorten the gate closing period. . Further, since the circuit can be easily integrated into an IC, it can be made smaller.

[Brief explanation of the drawing]

Figure 1 is a block diagram explaining the principle of the present invention, Figure 2A
, B are waveform diagrams illustrating the effects of the present invention, and FIG. 3 is a block diagram illustrating an embodiment of the present invention.

Claims (1)

[Scope of Claims] 1. Noise detection means for detecting noise components from a received signal on a signal line at a stage before the FM detection stage and generating a detection signal for a predetermined period; a frequency information signal generation means for generating a frequency information signal corresponding to the frequency deviation of the received signal, a feedback loop for feeding back this frequency information signal to the control input, and a frequency information signal generating means inserted into the feedback loop for generating a frequency information signal corresponding to the frequency deviation of the received signal; A gate that disconnects the loop during its existence period, and a hold means that receives the frequency information signal immediately before the loop disconnection operation of the gate as the control input,
A noise removal device for an FM receiver, characterized in that the gate output is an FM detection output. 2. The frequency information signal generating means compares the phase and frequency of the oscillation output of the voltage-controlled oscillator whose oscillation frequency is controlled by the control input and the received signal and generates a signal according to the difference between the voltage-controlled oscillator and the received signal. 2. The apparatus according to claim 1, further comprising a phase comparator, wherein the control input is a signal based on the output of the phase comparator.