JPS6228109Y2 - - Google Patents

Description

[Detailed description of the invention] This invention relates to an FM stereo receiver, especially for in-vehicle use.
This invention relates to a noise suppression device for an FM stereo receiver.

FM broadcast waves are easily affected by the influence of topography, etc., and the electric field strength changes widely. Therefore, especially in a car-mounted FM stereo receiver, there is a drawback that skip noise occurs and the signal-to-noise ratio is significantly reduced especially at a weak electric field strength due to the influence of multipath.

Furthermore, pulse noise such as the ignition noise of an automobile has a very large influence on the S/N ratio, causing auditory discomfort.

In order to suppress such noise and improve the S/N ratio, an FM stereo receiver as shown in FIG. 1 has been used.
The level of the IF signal amplified by amplifier 1 is IF
The level detector 2 detects the level, and when it falls below a predetermined level, the monaural/stereo mode switching signal generator 3 generates a switching signal, which switches the operation of the MPX (multiplex) demodulator 4 from stereo mode to monaural mode, thereby maintaining a certain degree of S/N for weak input. On the other hand, the output of the IF amplifier 1 is converted to a composite signal by the FM detector 5, and the impulse noise is suppressed by the next stage, the impulse noise elimination circuit 6, and the signal becomes the MPX demodulation input.
As is well known, the conventional multi-channel amplifier comprises, for example, a high-pass filter for detecting impulsive noise components from a composite signal, a noise amplifier for amplifying the noise components output from the filter, and a monostable multi-channel amplifier for generating a gate signal from the noise amplifier output.
This gate signal creates the MPX of the composite signal.
This includes a gate circuit that blocks transmission to the demodulation circuit, and a hold circuit that samples and holds the level of the composite signal just before the gate circuit is closed. When pulse noise is present, the gate circuit cuts off the composite signal, and during that time the hold output of the hold circuit is transmitted to the MPX demodulation circuit. This configuration eliminates pulse noise such as ignition pulses and improves the S/N ratio.

However, in the noise suppression device shown in Fig. 1, it is impossible to completely eliminate noise caused by sudden changes in the electric field and distortion components and noise due to multipulses, and it is also impossible to completely eliminate the effects of multipath and electric field strength. If sudden changes occur frequently, switching between the stereo mode and the monaural mode will occur each time, resulting in an undesirable feeling of discomfort for the listener.

An object of the present invention is to provide an in-vehicle FM stereo receiver that eliminates the above-mentioned drawbacks, suppresses noise more completely, and does not give a sense of discomfort to the listener.

The receiver of the present invention includes an electric field strength detection means that generates a first control signal according to the electric field strength when the electric field strength becomes less than a predetermined value, and detects a noise component included in the FM detection output. noise detection means for generating a second control signal according to the noise component level when the noise component exceeds a predetermined value;
and separation degree control means for continuously changing the degree of separation between both channels of the reproduced signal by each of the second control signals, and a frequency characteristic for the reproduced signal being continuously changed by each of the first and second control signals. The present invention is characterized in that it includes a tone control circuit and a level control means for continuously changing the reproduced signal level using a second control signal.

FM which is preferably the input of the noise component detection means
The output of the pulse noise removal circuit 6 shown in FIG. 1 is used as the detection output. This makes it possible not only to remove pulse noise, but also to remove distortion and noise components due to multipath, and to smoothly switch from stereo mode to monaural mode.

The present invention will be explained below with reference to the drawings.

FIG. 2 is a block diagram showing an embodiment of the present invention, and parts equivalent to those in FIG. 1 are designated by the same reference numerals.
An electric field strength detection circuit 10 is provided to detect the electric field strength, and since the IF signal level of the IF amplifier 1 is proportional to the electric field strength, the IF signal level is detected by the IF level detector 11. Generates a corresponding detection output and converts this output to DC
The signal is amplified in an amplifier 12 to provide a control signal A corresponding to the IF level.

Next, a noise detection circuit 20 is provided to detect various noise components contained in the output of the pulse noise removal circuit 6 which removes pulse noise from the composite signal which is the FM detection output. That is, the FM detection output (composite signal) from which pulse noise has been removed is converted into a stereo composite signal component and the second harmonic of the pilot signal by a trap circuit 21 that traps the 38KHz subcarrier and an HPF (high pass filter) 22. is 38KHz
The noise component and the noise component near this 38KHz component are removed, and only the noise component is amplified in the next stage noise amplifier 23. The noise detector 24 detects the level of this amplified output, the peak hold circuit 25 holds the peak value, and the DC amplifier 26 controls the level to be sufficient to drive the next stage circuit, and the control signal B becomes.

The control signal A of the electric field strength detection circuit 10 and the control signal B of the noise component detection circuit 20 are input to the separation degree control circuit 7 and the tone control control circuit 8, respectively. Both control circuits 7 and 8 receive both control signals A
and B, and has a so-called OR gate-like function. The output of the separation degree control circuit 7 is configured to generate a separation degree control signal corresponding to each level of both control signals A and B, and the separation between the left and right channel signals is determined by the MPX demodulation circuit in accordance with this signal. Change within 4. In this case, even if the noise component is a little large when the electric field strength is large, the S/N will be good,
Since the S/N deteriorates when the noise component becomes large to a certain extent, a circuit configuration is used that relatively compares the levels of both control signals A and B and controls the degree of separation according to the difference. .

The tone control control circuit 8 receives both control signals A.
It generates tone control control signals corresponding to the levels of and B, and MPX
The tone control circuits 9a and 9b inserted into the output line of the demodulation circuit 4 are configured to continuously control the tone control characteristics, that is, the frequency characteristics with respect to the reproduced channel signal. This control circuit 8 is also configured to have the same function as the separation degree control circuit 7 described above.

The left and right channel signals of tone control circuits 9a and 9b are further applied to electronically operated volume circuits 11a and 11b, respectively, and their levels are controlled. A volume control circuit 12 is provided for this control, and a noise component detection circuit 2
It is controlled by control signal B of 0.

Figure 3A shows the output spectrum distribution of the pulse noise removal circuit 6, in which the main channel signal (L+R) up to 15KHz, the pilot signal of 19KHz, and the subcarrier of 38KHz are AM-modulated to form both side bands. It has a sub channel signal (LR). Since pulse noise of approximately 100KHz or more is suppressed by the pulse noise removal circuit 6, the distortion components due to multipath including components of 100KHz or less, noise associated with it, FM noise peculiar to FM, etc. are represented by the spectrum shown by the dotted line. has been done. Therefore, in order to detect these noise components, the noise detection circuit 20 has a frequency characteristic as shown in FIG.
It includes.

By doing this, when the electric field strength decreases to a certain extent and the nozzle becomes noticeable, the IF level decreases and the FM noise increases, so control signals A and B having corresponding levels are output, and thus separation is performed. A separation control circuit 7 generates a separation control signal corresponding to these control signals to lower the separation and bring it closer to the monaural mode.

Furthermore, a tone control control signal corresponding to these control signals is generated from the tone control control circuit 8, and the frequency characteristics of the tone control circuits 9a and 9b are such that the high frequency component continuously decreases as shown in FIG. 4A, for example. It changes like this. Furthermore, in addition to reducing the high-frequency components, the low-frequency components may also be reduced to continuous points as shown by the dotted line.

Further, the electronic volume circuits 11a and 11b are operated to reduce the reproduction output level when the S/N deteriorates below a predetermined value (approximately 30 dB). This operation is similar to the muting operation in a normal FM receiver, but differs in this example in that the output signal is not controlled on and off, but the level is continuously controlled.

More preferably, when the degree of separation is controlled to decrease continuously from a point below the electric field strength _E1 as shown in curve b of FIG. 4B, the tone control is controlled as shown in curve a. The output level
Slightly higher electric field strength than the electric field strength that decreases by 3dB
It is more effective to operate continuously from the point _E2 . Note that the dotted line in the figure indicates the noise level.

When the electric field strength is large and there is no multipath, the output of the IF level detector is high and the output of the noise level detector is low, so that both the separation degree control circuit and the tone control circuit function as normal separation circuits and tone control circuits. to maintain good reception. In addition, even if the average electric field strength is large, if there are many multipaths, distortion components of the composite signal will be detected by the noise detection circuit, so the degree of separation will be reduced accordingly, and the tone control circuit will be By reducing frequency components or high and low frequency components, noise can be reduced and suppressed. Further, by gradually decreasing the output level from the point of field strength _E3 at which the S/N becomes worse than about 30 dB, it is possible to suppress a decrease in the audible S/N.

As described above, according to the present invention, it is possible to obtain an FM stereo receiver that prevents the S/N from decreasing due to weak input and multipath, etc., and does not give any audible discomfort, and is especially suitable for use in automobiles. It can be an opportunity.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of a conventional FM stereo receiver.
Figure 2 is a block diagram of an embodiment of the present invention, Figure 3A
Figure 4 shows the frequency spectrum of the composite signal output, B shows the detection characteristics of the noise component detection circuit, Figure 4A shows the control characteristics of the tone control circuit, and B shows the electric field strength, tone control circuit, and degree of separation. FIG. 3 is a diagram illustrating the operation of a control circuit and a volume circuit. Explanation of symbols of main parts, 5...FM detection circuit,
6...Pulse noise reduction circuit, 7...Separation degree control circuit, 8...Tone control control circuit, 10
... electric field strength detection circuit, 12 ... volume control circuit, 20 ... noise component detection circuit.

Claims (1)

[Scope of utility model registration request] an electric field strength detection means for generating a first control signal according to the electric field strength when the electric field strength becomes less than a predetermined value; noise detecting means for generating a second control signal according to the noise component level when a separation degree control means that is operated by the controller and continuously changes the degree of separation between both channels of the reproduced signal according to the magnitude of each signal, and receives each of the first and second control signals as an input. a tone control circuit that operates when one or both of these signals is input, and whose frequency characteristics with respect to the reproduced signal continuously change according to the magnitude of each signal; and the second control circuit. 1. A vehicle-mounted FM stereo receiver comprising: level control means for continuously changing the level of the reproduced signal according to a signal, and each of the predetermined values being set as desired.