JPH01108802A - Fm signal demodulator - Google Patents

Abstract

PURPOSE:To reduce impulsive noise caused when C/N of a signal is lower then the threshold value by using plural FM demodulation circuits and noise reduction circuits whose high frequency transmission characteristic is made different so as to reduce only the non-correlation noise component relatively. CONSTITUTION:A circuit varying slightly a high frequency transmission characteristic is arranged to a pre-stage of an amplitude limiter circuit. Thus, demodulation signals at demodulation outputs 10, 11 have nearly similar waveform but the production time with respect to pulse noise generated is not correlative. Since the pulse occurrence frequency is small when the C/N reaches the vicinity of the threshold point, the probability of both output pulses to be overlapped is small and the state has nearly no correlation. Thus, only noise component is reduced by a noise reduction circuit 12 by utilizing that the two signals at the demodulation outputs 10, 11 are the same and the pulse noise has no correlation. Then the pulse noise caused when the signal C/N is lower than the threshold value is reduced.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to an FM signal demodulator, and in particular, the present invention relates to an FM signal demodulator, and in particular, the FM signal has Gaussian-distributed noise whose power spectral density is almost uniformly distributed in the frequency band of the signal. The present invention relates to an FM signal demodulation device that reduces pulse-like noise that appears in a demodulated signal when an FM signal is added to the demodulated signal.

(Summary of the Invention) This invention relates to pulse noise that occurs when the CN ratio of a signal <*ratio of transmitted power to thermal noise power) falls below a specific value (threshold) in an FM signal demodulator such as a satellite broadcasting receiver. This method uses multiple FM demodulation circuits with different high-frequency transfer characteristics for one input signal to obtain a demodulated signal in which the signals are highly correlated with each other and pulse noise is uncorrelated, and further noise reduction is achieved. The present invention relates to an FM signal demodulator that reduces pulse noise through a circuit.

(Prior Art) Pulse noise occurs in an FM signal when the C/N reaches ζ9 dB, and as the C/N falls below this value, the S/N of the demodulated signal rapidly decreases. The value of C/N -9 dB mentioned above is called the threshold point, and there are some threshold improvement methods for lowering this value, such as a method that increases the C/N in an isometric manner (some proposals have been made (for example, Ito Yuya et al. [Easy to understand FM technology J 1982, 6.10. Hodo Publishing p. 1
37~). Examples include the FM feedback method and the PLL negative feedback phase detection method, all of which utilize the principle of a tracking narrowband filter.

Since the instantaneous frequency of the FM signal changes in accordance with the modulation signal, these devices perform feedback control using the demodulated signal so that the instantaneous frequency passes through a specified narrowband filter. The noise power (N) that influences the C/N of the input FM signal is proportional to the frequency bandwidth occupied by the FM signal, so by narrowing the band of the filter as described above, the effective C/N can be reduced. This increases the threshold and improves the threshold. These methods are mainly used in FM signal demodulation devices for audio transmission.

When a modulated signal has high frequency components, such as a television signal, it becomes difficult to perform a negative feedback operation because the delay time of various filters used as circuit components can be ignored. Further, even if the negative feedback method is not used, there are problems such as the need for highly accurate circuit installation for adjusting the delay time due to the group delay characteristics of the filters and the circuit configuration.

(Problem to be solved by the invention) When random noise such as Gaussian distribution noise is added to the FM signal, the demodulated signal will also contain random noise, which is usually called white noise. When the C/N (ratio of carrier wave power to noise effective power) of the FM signal becomes 9 dB or less, positive and negative pulse-like noises having high peak values are also derived from the demodulated signal.

If the demodulated signal is an audio signal, this pulsed noise becomes a clicking noise that causes an unpleasant disturbance. Furthermore, if the demodulated signal is a television video signal, it appears in the image as irregular white and black point-like sharp pulse noise.

This pulsed noise is more noticeable than the random noise (commonly called snow noise) that is also included, resulting in an extremely poor image quality. Image quality can be improved simply by removing this pulse noise.

Further, threshold improvement techniques in FM transmission have been studied for a long time, and various methods have been proposed as described in the previous section. However, a highly effective method for improving television video signals has not been put into practical use. Satellite television broadcasting receivers that have been sold in Japan since 1981 also use a PLL-based FM detection method or an injection-locked oscillator method for the FM carrier extraction circuit to improve the S/ Although some methods have been adopted that are expected to be effective in improving N, none of them have had a significant effect in improving threshold. In satellite television broadcasting, the strength of the terrestrial electric field is weak, so there is a strong desire to improve receiver sensitivity performance.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to solve the above-mentioned problems and to eliminate pulse noise that occurs when the CN ratio of a signal falls below a threshold value in an FM signal demodulator such as a satellite broadcasting receiver using a relatively simple configuration. It is an object of the present invention to provide an FM signal demodulator that can reduce the amount of noise.

(Means for solving the problem) In order to achieve this object, the FM signal demodulation device of the present invention has the following features:
In other words, an FM signal demodulation device that demodulates an FM signal to which noise is applied and whose power spectral density is distributed almost uniformly includes a plurality of FM demodulation circuits with different high frequency transfer characteristics and a noise reduction circuit. However, the plurality of FMfi modulation circuit modulation path signals have substantially the same waveforms and are highly correlated, and a plurality of demodulated signals are obtained which are mutually uncorrelated with respect to noise, and are simultaneously input to these by the noise reduction circuit. The present invention is characterized in that one demodulated output signal is obtained by reducing uncorrelated noise components from the plurality of demodulated signals.

A preferred embodiment of the present invention further provides a plurality of demodulation circuits in which the plurality of FM demodulation circuits have substantially the same main frequency bandwidth and have mutually different amplitude frequency characteristics or phase frequency characteristics within the frequency bandwidth. It is characterized by being composed of.

Furthermore, in a preferred embodiment, the plurality of FM demodulation circuits share a common frequency bandwidth occupied by the predetermined FM signal, and include surplus frequency bands on the lower side or the higher side adjacent to the frequency band. The present invention is characterized in that it is composed of a plurality of demodulation circuits whose sides are different from each other.

(Function) By using multiple FM demodulation circuits and noise reduction circuits with different high-frequency transfer characteristics in this way, the demodulated waveform of the FM signal is almost the same, and the noise contained in the FM signal is generated by multiple demodulated circuits that are uncorrelated with each other. A demodulated output signal in which only uncorrelated noise components are relatively reduced can be obtained from the signals.

(Embodiments) The present invention will be described in detail below using a satellite television broadcasting (FM signal) receiver as an example with reference to the accompanying drawings.

FIG. 1 shows a block diagram of the configuration of the first embodiment of the present invention.

A satellite television broadcast signal is a signal obtained by frequency modulating a 12 GHz band carrier wave. As shown in Fig. 1, the receiving device receives a radio wave 1 from a broadcasting satellite with a planar receiving antenna (BS antenna) 2, and converts the radio wave 1 into the first one using a frequency converter (BS converter) 3 installed integrally with the antenna.
A BS tuner that is converted to an intermediate frequency (BS-IF: IGHz) and installed indoors using a connection cable 4 (FM signal demodulation device consisting of 5 or less tuning circuits in the figure)
and its output is received by a separately provided picture tube display device.

The present invention relates to a part of the above-mentioned component equipment related to an FM signal demodulator.

First, the BS-IF double signal input to the channel selection circuit 5 in the figure has a bandwidth of 300 MHz within the frequency bandwidth that includes 8 channels of satellite broadcasting allocated to Japan, and is a received signal of multiple channels. It is included. A predetermined channel is then selected by the channel selection circuit 5. This typically involves converting the signal of the selected channel to a second intermediate frequency using a frequency converter. A 130 MHz band or a 400 MHz band is used for this second intermediate frequency.

The configuration and operation of the present invention will be explained below in comparison with a conventional general FM signal demodulation circuit configuration example.

The conventional circuit configuration includes a band pass filter (BPF) 6 that passes only the signal selected from the output of the channel selection circuit 5, a second intermediate frequency signal amplifier 7, and one FM signal detection circuit. There are various types of FM detection circuits, and depending on the type, an amplitude limiter may be placed before the detection circuit to keep the signal amplitude constant. That is, conventionally, one set of FM signal demodulation circuits has been used.

In the first embodiment of the present invention, normally used circuits can be used as they are for the channel selection circuit 5, BPF 6, and amplifier circuit 7, and the FM signal demodulation circuits 8 and 9 have a new circuit configuration.
, a noise reduction circuit 12. The output signal of amplifier circuit 7 is supplied in parallel to FM signal demodulation circuits 8 and 9 to obtain demodulated signals 10 and 11, respectively.

The meaning of providing two FM signal demodulation circuits here is as follows.

The input signal of the FM signal demodulation circuit shown in the figure is a frequency-converted FM signal from a broadcasting satellite to which thermal noise incident on the BS antenna 2 and noise mainly generated inside the BS converter 3 are added. These noises can be regarded as random noises having a Gaussian distribution in which the normal Casvector density is distributed at approximately -like levels over a wide frequency band. Since the noise has a limited bandwidth by the BPF 6, the amplitude and phase changes of only the noise wave are random, and it is known that the instantaneous value probability distribution of the amplitude envelope is approximated by a Rayleigh distribution.
The envelope may momentarily reach a peak four times its noise average power value. On the other hand, the FM signal is a signal having a constant amplitude value, but at the instant when the amplitude value of this signal and the amplitude value of the noise sound wave become almost equal, pulse noise unrelated to the modulation signal is generated in the demodulated output. This is caused by the sudden phase change imparted to the signal component by the noise wave at that instant. The polarity, amplitude, and pulse width of the generated pulsed noise are determined by the relative relationship between the frequency of the signal and the frequency of the noise sound at the moment when the amplitudes of the signal and the noise sound almost match.

The conditions under which such pulsed noise starts to occur start from the point where the ratio of the signal component power value (C) to the noise wave average power value (effective value, N) is C/N-9db, and the C/N value decreases. As the number of pulses increases, the number of pulses generated also increases, and the rate of increase can be considered to follow a Rayleigh distribution curve.

Here, if we consider that the total vector sum of the noise distributed within a certain frequency bandwidth determines the instantaneous and instantaneous envelope value and phase of the noise wave, then if we make the signal transfer characteristics within the same band different It is also possible to change the time at which the noise wave envelope shows a high peak, and the present invention focuses on this point.

FM signal demodulation circuits 8 and 9 used in the present invention each have an amplitude limiter and an FM detection circuit, and
A circuit that can slightly vary the amplitude frequency characteristic, phase frequency characteristic, or amplitude and phase frequency characteristic, that is, the high frequency transfer characteristic of the high frequency stage up to the FM detection circuit is arranged in the preceding stage of the amplitude limiter circuit. The amplitude limiter may not be necessary depending on the method of the FM detection circuit. Characteristic curves 18 and 19 of characteristic B in FIG. 3 show examples in which the transfer characteristics are made different from each other by this configuration. Characteristic A in the same figure shows the spectral distribution 16 of the FM signal and spectral distribution 17 of Gaussian noise, and curve 1 of characteristic B in the figure
8 and 19 are overall characteristics that are passed through the frequency transfer characteristic variable circuit after the main bandwidth is limited by the BPF 6.

By making the transfer characteristics different in this way, the demodulated signals 10 and 11 shown in the first diagram have almost the same waveform, but the generation times of the generated pulse noise waves are uncorrelated with each other. In addition, even if the generation times overlap, the polarity, magnitude, and width of the pulse waveforms will be different.

When the C/N value is around 9 db (threshold point), the frequency of pulse generation is low, so the probability that the re-output pulses overlap is also small, resulting in a state that can be said to be almost uncorrelated.

Utilizing the fact that the two demodulated output signals are the same and the pulse noises are uncorrelated, it is possible to reduce only the noise component by the noise low-band circuit 12 shown in FIG.

In order to effectively implement the present invention, the two demodulation outputs 10 and 11 should have the same waveform as possible for signals, and the outputs of both circuits should be as uncorrelated as possible for pulse noise. The purpose is to provide a difference in transfer characteristics, and characteristic curves 18 and 19 of characteristic B in FIG. 3 are specific examples of characteristics that satisfy this.

In general, if the demodulated signal itself passes through circuits with different transfer characteristics, there may be a slight difference in waveform, but in reality, unless there is a large difference in transfer characteristics, there will be a slight difference in the waveform of the demodulated signal. It is possible to relatively easily reduce the correlation with respect to pulsed noise without causing any difference. However, as the C/N value decreases and the number of pulses increases for both the re-demodulation output and the re-demodulation output, highly correlated pulse noise also increases. Considering this case, it is better to increase the difference in transfer characteristics between the two circuits. Although this is desirable, since distortion will occur in the demodulated signal, the balance between the two is a matter to be determined at the time of design.

An example of the characteristic difference shown in characteristic B in Figure 3 is when the main bands are the same and the gain characteristics of both have opposite slopes.
An example of the demodulated signal obtained in the case of this characteristic is shown in waveform a in Fig. 4.
Shown in b. The synchronization signal of the television synchronized video signal is on the low frequency side (left side of the figure) with the original polarity of the FM signal in characteristic A in Figure 3.
, the demodulated signal obtained by the characteristic of curve 18 of characteristic B in the figure corresponds to the output of waveform S in figure 4, and the generated pulses are proportionally more negative pulses, and the demodulated signal obtained by the characteristic of curve 18 of characteristic B in the figure corresponds to the output of waveform S in figure 4. The characteristic No. 19 corresponds to the output of waveform a in FIG. 4, and the number of positive pulses increases. However, the absolute number of pulses included in each output is approximately equal.

Next, an embodiment of the noise reduction circuit 12 shown in FIG. 1, which inputs the two demodulated signals thus obtained and reduces or eliminates the amplitude of the pulse noise component, will be described.

As the simplest first circuit example, there is a method of adding re-input signals. Although the signal amplitude is doubled, the amplitudes of mutually uncorrelated pulse noises are not added, and the pulse amplitude in the combined output can be relatively reduced to 1/2. The effect on image quality in this case is that although the number of pulses increases, the amplitude of each pulse decreases by 6 dB, so that the image quality is improved by about one rank in terms of subjective evaluation value.

Next, a method for removing prominent pulses will be described as a second circuit example. By taking the difference between two input signals, only the noise component contained in both signals can be extracted. This difference signal is rectified, aligned to have the same polarity, and sliced to obtain a key signal. This signal gates off the period of the pulse signal portion included in the sum signal of the two input signals, and
By filling in (holding) this off time with a potential immediately before gate off, it is possible to remove sharp pulse components. Although some of the resulting image quality may remain unnatural depending on the pattern, it is possible to obtain an image with less noticeable pulse noise than in the first circuit example.

Although two examples have been described above, the noise reduction circuit 12 can also reduce or remove noise by other means such as converting it into a digital signal and performing signal processing, and the present invention It does not specifically specify the noise reduction circuit.

As an embodiment of the present invention, the case where two FM signal demodulation circuits are provided has been described above, but by having three or more FM signal demodulation circuits and making the high frequency signal transfer characteristics before demodulation of each different from each other, It can also be used by reducing the correlation between the pulse noises of the respective demodulated signals.

Depending on the method of the noise reduction circuit that reduces or eliminates noise components, it may be appropriate to obtain three or more outputs.

However, as the number of demodulated outputs increases, it becomes difficult to reduce the correlation between pulse noises.
It is necessary to consider that it becomes difficult to expect a noise reduction effect in proportion to the increase in the number.

In addition to the above, when implementing the present invention, the following aspects may be implemented in any manner.

In the case of a satellite broadcasting receiver, each demodulation output of a plurality of FM signal demodulation circuits includes an audio subcarrier (5,
73MHz band Q-PSK signal), but there are two methods: one is to separate both signals after performing noise reduction with a noise reduction circuit while including this audio subcarrier, and the other is to separate both signals immediately after the FM signal demodulation circuit. In some cases, only the video signal is input to the noise reduction circuit, but which one to adopt is arbitrary. Furthermore, the de-emphasis circuit used for demodulation signal equalization in the FM transmission system may be placed before or after the noise reduction circuit.

Next, a second embodiment of the present invention will be described with reference to FIG.

The circuit configuration up to the channel selection circuit 5 shown in the figure is the same as that of the first embodiment.

Both the FM signal demodulation circuits 14 and 15 shown in the second embodiment have the same main circuit configuration, but have different high frequency transfer characteristics in order to achieve the object of the present invention.
The main circuits that constitute the FM signal demodulator include a BPF, an amplifier circuit, an amplitude limiter, and an FM signal demodulator.
Regarding the M detection circuit, the feature of this embodiment is to vary the frequency band selection characteristics of the BPF as a means for varying the high frequency transfer characteristics. Examples of frequency selection characteristics of the same BPF are shown in curves 20 and 21 of characteristic C in FIG.

The same side has a characteristic that commonly includes the frequency band occupied by the FM signal shown in characteristic A in the same figure, but includes a slight portion of the adjacent low-frequency side surplus band, and the other side has a characteristic that includes a slight portion of the high-frequency surplus band. It is a characteristic that includes parts. By including some surplus bands in this way, conditions are created in which the noise waves tend to become uncorrelated.

In the case of this embodiment, the C/N value of the signal before demodulation is slightly lowered in order to include the surplus band, but compared to the first embodiment, each demodulated signal does not contain waveform distortion and It becomes possible to obtain a demodulated signal with a small degree of noise correlation.

When including surplus bands, it is not a good idea to unnecessarily widen them because there is a risk of interference waves such as spurious waves from unnecessary bands being mixed in.Also, since it lowers the C/N value of the signal before demodulation, the overall Appropriate values are required. In the case of a satellite broadcasting receiver, the occupied bandwidth of the FM signal is 27 MHz, and an appropriate surplus bandwidth of about 2 to 3 MHz for both the low-frequency side and the high-frequency side.

The embodiment of the characteristic C in FIG. 3 shown in this embodiment is an example in which the in-passband amplitude-frequency characteristic is made almost flat, but the slope may be varied to have a slight slope.

The noise reduction circuit 12 only needs to have a function that achieves the same purpose as the first embodiment.

(Effect of the invention) The effects of the present invention include the following points.

(1) No control circuit system such as feedback control is required, and circuit operation is stable and a reliable noise reduction effect can be expected.

(2) Generates a noise reduction effect with a relatively low-layer circuit configuration.

(3) Regarding image signals, it is possible to obtain an improvement effect of one rank or more by subjective evaluation of pulse noise disturbance images below the threshold point.

When applied to a satellite broadcast receiver, it is effective in suppressing pulse noise disturbances and preventing image quality deterioration even if the C/N value of the received signal is 9 dB (threshold point) or less, thereby improving effective reception. This improves the noise limiting sensitivity of the machine.
This has a significant improvement effect when satellite broadcasting waves are affected by rainfall attenuation. This will greatly contribute to the promotion of the spread of satellite broadcast reception, such as making small-area satellite broadcast reception antennas practical.

Furthermore, the present invention can be applied not only to satellite broadcasting receivers, but also to broadcasting receiving equipment such as FM audio broadcasting and digitized code broadcasting, or receiving terminal equipment in various communication fields using the FM system, and has industrial applications. The field is wide.

[Brief explanation of the drawing]

FIG. 1 shows a block diagram of the configuration of the first embodiment of the present invention, FIG. 2 shows a block diagram of the configuration of the second embodiment of the invention, and FIG. 3 shows the block diagram of the configuration of the second embodiment of the invention. An example of operating characteristics is shown, and FIG. 4 shows an example of a signal waveform in an embodiment of the present invention. 1... Input radio wave 2... BS antenna 3.
...BS converter 4...Connection cable 5...
Tuning circuit 6...Band pass filter 7...
Second intermediate frequency signal amplifier 8.9...FM signal demodulation circuit 10.11...Demodulated signal 12...Noise reduction circuit 1
4.15...FM signal demodulation circuit 16...spectral distribution of FM signal

Claims (1)

[Scope of Claims] 1. An FM signal demodulation device for demodulating an FM signal to which noise is applied and whose power spectral density is approximately uniformly distributed, comprising: a plurality of FM demodulation circuits having mutually different high frequency transfer characteristics; , a noise reduction circuit, and the plurality of FM demodulation circuits obtain a plurality of demodulated signals whose waveforms are substantially the same and highly correlated, and whose noise is uncorrelated with each other, the noise reduction circuit An FM signal demodulation device characterized in that one demodulation output signal is obtained by reducing uncorrelated noise components from the plurality of demodulation signals inputted simultaneously to the FM signal demodulation device. 2. The demodulation device according to claim 1, wherein the plurality of FM demodulation circuits have substantially the same main frequency bandwidths and have different amplitude frequency characteristics or phase frequency characteristics within the frequency bandwidths. 1. An FM signal demodulation device comprising a plurality of demodulation circuits. 3. In the demodulation device according to claim 1, the plurality of FM demodulation circuits have a common frequency bandwidth occupied by the predetermined FM signal, and a low frequency band or a high frequency band adjacent to the frequency band. 1. An FM signal demodulation device comprising a plurality of demodulation circuits each including a surplus frequency band in a different manner.