JPS6250027B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a high-sensitivity demodulation method that has a simple configuration, improves the threshold level for wideband TV (video)-FM modulated waves at low reception input, and improves the noise characteristics of the demodulated signal. Conventionally, as the simplest method for demodulating a frequency-modulated signal, a frequency demodulation method using a discriminator using an LC circuit or a delay line has been used. In this case, the S/N (signal to noise ratio) of the FM demodulated (detected) demodulated signal to the C/N (carrier power to noise power ratio) of the FM modulated input signal is S/N = C/N - Expressed as FI (FI: constant), demodulation S/N is proportional to C/N of input signal. On the other hand, this C/N is determined by the passband width of a bandpass waveformer used on the input side of the demodulator in order to limit noise and signal bandwidth. Normally, according to this method, the above-mentioned relationship is maintained up to a C/N of about 10 dB, and the S/N deteriorates rapidly at a C/N of less than that. This point is called the threshold point. Generally, in communications that transmit TV (video) signals, such as satellite communications, FM modulation is often used to transmit the signals. In this case, the operating point of the communication line for reception is often set near the threshold due to limitations on satellite transmission power, stability of the satellite communication propagation path, and economic efficiency of ground reception equipment. As a result, the reception input sometimes decreases due to changes in environmental conditions, the reception level falls below the threshold, the demodulated image on the TV monitor is significantly disturbed by impulse noise peculiar to TV transmission, and furthermore, the demodulated image becomes difficult to obtain. It reaches the point where it doesn't exist. Therefore, in satellite communications, improving impulse noise using a simple method is considered to be a very important issue in terms of improving TV demodulated image quality and, ultimately, the economics of receiving equipment.Especially in satellite receiving equipment such as broadcasting satellite communications. In this field, a method for improving noise (improving demodulated image quality) using a simple configuration is considered to be an extremely important issue. Incidentally, there have been various methods for improving the threshold, and as an example, a dynamic tracking filter FM demodulation method, which is somewhat related to the present invention, is shown in FIG. 1 in the same figure
is a narrow-band variable bandpass waveform whose center frequency changes, 2 is a limiter circuit, 3 is a frequency discriminator circuit, 4 is a low-pass waveform, 5 is a signal input terminal, and 6 is an FM signal detection output terminal. be. The FM signal wave entering from the input terminal 5 passes through the center frequency variable band pass wave generator 1, passes through the limiter circuit 2, and is demodulated by the discriminator 3. The demodulated signal passes through a low-frequency wave generator 4 to remove high-frequency noise other than the modulated signal frequency, and then controls the center frequency of the variable wave generator 1. At this time, the center frequency of the variable wave generator 1 is controlled to completely follow the instantaneous frequency change of the input signal from the variable wave generator 5. That is, the signal band component of the frequency discriminator output is faithfully fed back in both amplitude and phase to control the center frequency of the variable waver. By the way, as is well known, a color video signal is composed of a luminance signal and a color subcarrier component, and is an extremely wide-band signal with a baseband bandwidth of up to 4.2MHz. Furthermore, the frequency spectrum of the baseband signal of the video signal changes significantly depending on the type of image (subject) to be transmitted. In particular, the amplitude of the color subcarrier component changes significantly depending on the color depth (saturation degree) of the image. Due to the unique properties of color video signals,
A number of difficult problems arise when demodulating this type of FM modulated signal using the conventional method shown in FIG. First, due to the wideband nature of the baseband signal, the signal components of the discriminator 3 can be stably and faithfully processed in terms of phase and amplitude through a feedback circuit consisting of an amplifier, a low frequency amplifier, an internal circuit of the variable wave generator 1, and other attached circuits. , and it is a very difficult problem to apply the voltage to one center frequency variable element so as to match the instantaneous frequency change of the input signal. If this state cannot be set,
The FM change component of the input signal for the modulation frequency component that has been correctly fed back effectively passes through the variable narrowband transducer, but it may act in the opposite direction on the modulation frequency component that has not been properly fed back. the result,
For example, if a modulated frequency component with a large frequency deviation is not fed back correctly, this component with large FM deviation power will be removed according to the pass characteristics of the variable waveform with a narrow bandwidth, and will eventually be rejected by the limiter circuit or discriminator. Degrading the C/N entering the circuit,
Therefore, it may have the opposite effect, such as falling into a state below the threshold. Furthermore, even if these feedback circuits are set correctly, the variable bandpass waveformer is controlled by the wideband noise accompanying the input signal passing through this wideband feedback circuit, and the noise modulation component also controls the narrowband waveform converter. Pass validly. As a result, when accompanied by large noises, the demodulated image is distorted, and especially when the level of the modulated signal component is low, the demodulated image quality is significantly affected by these noises. For example, when the saturation of the color signal of the modulated video is low and the color subcarrier component is small, broadband noise is effectively demodulated and becomes very noticeable as color noise on the monitor. The present invention aims to solve these problems and provides a demodulation method that improves the threshold of an FM signal modulated with video by utilizing characteristics specific to color video signals. That is, in the present invention,
Narrowband variable wave generator whose center frequency mainly changes,
Using a phase detection circuit, a color subcarrier control circuit, etc., the input/output signal of the variable waveform is phase-detected, the color subcarrier component is extracted from the obtained detected signal, and the center frequency of the variable waveform is set as the input frequency. Change in response to changes. Furthermore, the variable wave generator is controlled using a nonlinear circuit whose input/output ratio is nonlinear with respect to the input level for the subcarrier.
Thereby, the output C/N of the variable wave generator is improved compared to the input C/N. Therefore, by demodulating this output signal with a frequency discriminator, the threshold level is improved and highly sensitive demodulation becomes possible. This will be explained in detail below. FIG. 2 shows an embodiment of the present invention, in which numeral 7 is a variable bandpass transducer whose passing center frequency changes depending on an external signal, 8 is a phase detection circuit, and 9 is a color subcarrier that passes color subcarrier frequency components. 10 is a color subcarrier adjustment circuit capable of varying the amplitude and phase of the color subcarrier component; 11 is a limiter circuit; 12 is a frequency discriminator circuit; 13 is an FM signal input terminal; 14 is the FM detection signal output terminal, 1
5 is a phase shifter used as necessary. The signal from the input terminal 13 passes through the variable wave generator 7 and further passes through the limiter circuit 11, and then is demodulated by the discriminator 12. Then, the demodulated (detected) signal is taken out from the output terminal 14. The input and output signals of the variable waveform generator are phase-compared by the phase detection circuit, and among the detected output signals, the video color subcarrier component (3.58MHz) is extracted and amplified by the waveform generator 9, and then passed through the variable waveform generator 7.
control the center frequency of the pass. In this configuration, the center frequency of the variable wave generator 7 is demodulated by the phase detection circuit 8 as described later, and the extracted color subcarrier frequency component is used to match the instantaneous frequency deviation due to the color subcarrier modulation component of the input FM signal. It is controlled so that it changes accordingly. At this time, the adjustment circuit 10 and phase shifter 15 are used as necessary so that this condition is satisfied. As a result, the output C/N of the variable wave generator 7 is improved compared to its input C/N, and the threshold level for the input signal is improved. This effect will be discussed later. By the way, in the demodulation method using a normal frequency discriminator, the signal demodulation bandwidth is determined by the FM frequency deviation (peak value) of the input signal in order to sufficiently pass the energy of the modulated signal and demodulate the waveform well. Δ, the highest frequency of the modulating signal is _h
Then, it can be determined as B ₀ 〓2(Δ+ _h ). The narrower the demodulation bandwidth is under certain conditions, the worse the waveform characteristics of the demodulated signal will be, but since the noise accompanying demodulation will be reduced, the threshold level can be lowered. However, if the band is made narrower casually, the signal component with a large frequency shift cannot be passed by the narrowband waver due to the degree of frequency deviation of the input FM signal, and as a result, the narrowband waveform On the output side of the waveform generator, the signal power decreases more than the noise removal effect of the waveform generator, resulting in deterioration of the threshold level due to C/N deterioration. Therefore, in the present invention, in a unique circuit configuration, when narrowing the band of a bandpass waver, the modulation component that gives the largest frequency shift, that is, the largest frequency of the color video signal to which pre-emphasis is applied on the modulation side. Focusing on color subcarrier components that are likely to cause deviations,
In response to the instantaneous frequency change caused by this component, a method is adopted to match the center frequency of the wave generator. This also facilitates implementation of the circuit configuration. The present invention is effective in video transmission, particularly in systems to which emphasis is applied. Now, let's consider a video signal to which emphasis is applied. A color video signal usually consists of a luminance signal and a color signal, and includes frequency components up to about 4.2MHz. Among them, the luminance signal is mainly concentrated in a low frequency region that is a multiple of the horizontal scanning frequency (15.75KHz), and the color component is concentrated in the vicinity of 3.58MHz. This type of video signal is, for example, CCIR
When the pre-emphasis circuit specified by Rec.405-1 is applied, the low frequency part of the signal is approximately -10dB.
A power weighting of about +3 dB is given to the high frequency components. Looking at the typical standard color bar signal, which has the highest degree of saturation as a video signal, the maximum amplitude of the signal is
For 140IRE, the maximum amplitude of the luminance signal is
The amplitude of the 77IRE, 3.58MHz color subcarrier component is 88IRE. Therefore, when this signal is weighted by the pre-emphasis circuit described above, the amplitude of the color subcarrier component becomes 127IRE, which is close to the maximum amplitude of the original signal, 140IRE. Therefore, the instantaneous frequency change (deviation) of the FM signal modulated with the video signal to which pre-emphasis is applied is the largest, and becomes a problem with respect to the C/N change due to the narrowing of the passband. may be considered to be due to this color subcarrier component. For these reasons, the 3.58MHz component changes the center frequency of the narrowband variable waver to follow the input frequency deviation, effectively transmitting the signal power to the frequency discriminator, and removing a lot of noise. This improves the C/N. In addition, since the color subcarrier control system handles only the frequency component (3.58MHz) close to a single signal, it is easy to create a stable and simple circuit configuration, and it is easy to adjust the optimal control phase and amplitude. In the present invention, these are realized by a special configuration. First, the phase detection circuit 8 performs a phase comparison of the input and output signals of the variable bandpass waveform 7 shown in FIG. Having described what can be obtained, this operation will now be explained. Now, for the sake of simplicity, it is assumed that the variable bandpass transducer is composed of a single resonant system as shown in Figure 3, and a small portion of the input/output signal of the transducer is branched to the phase detection circuit. (A series resonant circuit is shown here, but a parallel resonant circuit has the same effect.) Further, it is assumed that the input/output voltage and current of the wave detector are not affected by the phase detection circuit. At this time, the input/output voltage E〓 _i of the bandpass waveformer,
The following relationship holds true between E〓 and ₀ . Here, Δ is the number of detuned waves from the center frequency of the wave generator, b ₀ corresponds to the 3 dB bandwidth of the wave generator, and the phase detection circuit 8 is supplied with a signal equivalent to the input/output voltage of the wave generator 7. If the input voltage E〓 _i is taken as E〓 _i =E _l e ^j 〓, then the detection output e of the phase detection circuit is as follows. Here K is a constant. When the fixed phase Φ is set to 0 and the characteristic of the output signal of the phase detection circuit with respect to the normalized detuning frequency (2Δ/b ₀ ) is determined, it becomes as shown in FIG. In FIG. 4, the output e of the phase detection circuit 8 exhibits a frequency discriminator characteristic with respect to the frequency change of the input signal of the wave generator 7, and the FM signal is demodulated. Therefore, the color subcarrier 3.58MHz component is also demodulated by this, and after passing through the color subcarrier pass waver, the center frequency of the variable waver is controlled. In this case, the direction of control must be determined so that the polarity of the output voltage of the phase detection circuit obtained coincides with the change in the center frequency of the variable waver due to the detected voltage with respect to the change in input signal frequency. In addition, if the sensitivity of the center frequency fluctuation of the variable wave generator due to the color subcarrier amplitude obtained from the detection output is made sufficiently large compared to the frequency detection (demodulation) sensitivity of the variable wave generator, the center frequency of the wave generator will always be broken. The number is controlled to match the frequency changes due to the color subcarrier components of the input FM signal. Next, a summary will be given of the C/N improvement effect of the input/output signal by the variable wave device in the control using the color subcarrier component performed in this manner. FIG. 5 shows the instantaneous frequency change of the FM signal from the input terminal 13 and the instantaneous change of the center frequency of the variable band pass wave generator 7, with solid lines a and b.
indicates the former, and broken line c indicates the latter. ₀ is the center frequency when the variable wave generator 7 is not controlled by the color subcarrier component. For simplicity, the signal instantaneous frequency change is expressed as a color subcarrier (3.58M
Hz) and a frequency change b due to the luminance signal, the difference in frequency between them is large, and it is assumed that they are subjected to frequency shifts of Δ _c and Δ _i , respectively. On the other hand, the center frequency of the variable wave generator (dashed line c) follows the signal instantaneous frequency change a,
Let the deviation be equal to Δ _c . FIG. 6 shows the state of signal power passing through the variable bandpass transducer 7, where the horizontal axis indicates frequency and the vertical axis P indicates the passing power of the transducer. Solid line a,
b and broken line c are the same as in Fig. 5, and d, d',
d'' indicates the pass characteristic of the variable wave transmitter. Now, when the instantaneous frequency of the signal is at t ₁ (= ₀ + Δ _i ), the center frequency of the variable wave transmitter is at ₀ , and the instantaneous frequency is at t ₂ (= ₀ +Δ _i +Δ _c ), the frequency shifts to ₀ +Δ _c .
Therefore, a frequency difference of Δ _i occurs between the center of the variable wave generator and the instantaneous signal frequency. Therefore, the signal power passing through the transducer decreases by L. now,
If we consider a variable bandpass transducer composed of a single resonator as described above, its power passing characteristics can be standardized as It can be expressed as Here, Δ is the detuning frequency from the center of the wave device, and b ₀ is the 3 dB bandwidth of the wave device. In FIG. 2, the input signals to the variable wave generator 7 are accompanied by noise, but these are subject to some form of band limitation. Here, in order to compare with the conventional demodulation method, as mentioned above, the normal bandwidth B ₀
Assume that it is limited by At the input of the variable wave generator 7, the power of the signal is reduced to 1
If the noise power per unit frequency, that is, the noise power density, is 1/Hz, then the noise power in the bandwidth B ₀ (Hz) is B ₀ and the input C _i /N _i is 1/B ₀ . By the way, on the output side of the wave generator 7, since the signal frequency is Δ _i away from the center of the wave generator, the passing signal output is

[Formula] becomes. The noise power passing through the wave generator is as shown in Figure 6.
The center frequency of the transducer is d, d′, within the frequency width B ₀
d", so its amount is not uniform. When the center frequency is at the center of B ₀ , that is, ₀ , the wave transmitter passing noise power is the largest, and at that time the output C/N of the wave generator 7 is Therefore, if we calculate the noise power in this state as the worst condition for noise, we get: becomes. Therefore, the output C/N (=C ₀ /N ₀ ) is C ₀ /N ₀ = [{1+(2Δ _i /b ₀ ) ² }b ₀ · tan ^-
1 B(10)/b ₀ ] ^-1 , and the C/N improvement degree η (=
C ₀ /N ₀ /C _i /N _i ) is expressed as η=[{1+(2Δ _i /b ₀ ) ² }b ₀ /B ₀・tan ^-
1 B ₀ /b ₀ ] ^-1 (5) Now, if we take an actual satellite system as an example,
The video signal with a band of 4.2MHz has the highest frequency deviation.
Transmitted at 10.75MHz. Additionally, the emphasis characteristics of CCIR405-1 are applied. At this time, the normal demodulator bandwidth B ₀ is chosen to be 30MHz. When the present invention is applied to this and the improvement effect of equation (5) is determined, the result is as shown in FIG. 7. In the figure, b ₀ is the 3 dB bandwidth of the variable bandpass transducer, and Δ _i is the frequency shift due to the brightness modulation after pre-emphasis is applied on the video signal modulation side. The degree of improvement η gives the minimum value as described above, and the smaller Δ _i is, the greater the improvement effect can be obtained. Although various types of variable bandpass transducer 7 can be used in addition to the series resonant system shown in FIG. 3, FIG. 8 shows an example of a configuration using a parallel resonant system. 16 and 17 are input/output terminals of the variable waveformer, 18 is a variable capacitance element (for example, a varactor), and 19 is a phase detection circuit 8.
This is the input terminal for the extracted 3.58MHz color subcarrier signal component. In addition, 19a is a filter for preventing the high frequency of the terminal 16 or 17 from flowing back into the control signal input terminal 19, and 19b is a filter.
A filter circuit that resonates at 3.58MHz, terminal 1
9 control signals are prevented from being shorted by L ₀ . The existence of the series resonant circuit and the parallel resonant circuit is one of the features of the circuit configuration of this system. L ₀ and C ₀ (DC components of the variable capacitance) are selected so as to resonate at the center frequency ₀ of the input FM signal.
In this state, when a signal is applied from the input terminal 16 of the color subcarrier signal, the C of the variable capacitor 18
changes accordingly, causing the resonant frequency consisting of L ₀ and C ₀ to vary. As a result, the center frequency of the bandpass transducer changes, resulting in a desired variable bandpass transducer. As described above, in the present invention, a simple and easy-to-implement threshold improvement method is presented based on the characteristics of a color video signal to which pre-emphasis is applied. In the first embodiment, a variable band pass waveform whose center frequency changes depending on an external signal and a phase detection circuit are used, and the input and output signals of the variable waveform are compared in phase by the phase detection circuit, and the color obtained by this is compared. The subcarrier component controls the passing center frequency of the variable wave generator through a control circuit. By handling narrowband signals in such a circuit configuration, a stable control circuit can be easily updated, and arbitrary control conditions can be easily satisfied. Also, because the control is based only on this specific narrowband signal, the variable wave generator does not respond to noise frequency components that are not input to the variable wave generator, and the noise of the same frequency components that accompany the input modulation signal is ignored. , are removed according to the transmission characteristics of the wave transmitter. In addition, the C/N improvement circuit using a variable band pass waver can be constructed independently of the frequency discriminator for signal demodulation, so it can be applied to ordinary FM demodulators, making it convenient and versatile. It is also excellent in terms of While having these many features, the C/N ratio of the input signal is improved, that is, the threshold level is improved, and noise in the demodulated image quality is significantly improved. FIG. 9 shows another embodiment of the present invention, which further improves the demodulation characteristics of the previous example. This is because in the first embodiment (Fig. 2), the control circuit for the color subcarrier signal has a nonlinear input/output ratio (S ₀ /S _i ) of the signal level that changes depending on the magnitude of the input signal S _i . The circuit 20 is inserted, and the other circuits are exactly the same as the previous example. This circuit can be realized by a combination of non-linear resistors, etc., and has input/output characteristics a or b as shown in FIG. That is, in a region where the input signal S _i is small, the input/output ratio S ₀ /S _i is made small,
In a region where S _i is large, S ₀ /S _i is set large. In the example shown in FIG. 2, the color subcarrier component demodulated by the phase detection circuit and extracted by the wave generator 9 is accompanied by noise within the passband of the wave generator 9. Therefore, when the color subcarrier component of the video signal becomes small, that is, when the color saturation is low, this noise component is amplified by the amplifier system such as 10 in the control system.
It acts on the variable wave generator 7. Therefore, the noise of the same frequency component accompanying the input signal from 13 effectively passes through variable wave generator 7 and is demodulated. As a result, these noise components act as color noise, especially in demodulated image quality, resulting in deterioration of image quality. Therefore, in the embodiment shown in FIG. 9, an input/output nonlinear circuit 20 is inserted into the color subcarrier control system, and in the case of a modulation signal with a small color subcarrier component, the control signal to the variable wave generator 7 is reduced. By suppressing changes in the variable waveform due to noise, deterioration of demodulated image quality due to these color noises is prevented. The present invention can be easily realized by using a variable band pass waver, a phase detection circuit, a color subcarrier signal control circuit, a frequency discriminator circuit, etc., and improves the C/N, that is, the threshold level. to improve image quality at low reception inputs. Therefore, at low reception input level, video-FM
It is used extremely effectively in satellite receivers that receive signals.

[Brief explanation of the drawing]

FIG. 1 is a configuration example of a conventional FM demodulation system, FIG. 2 is a configuration example of a first embodiment of the FM demodulation system according to the present invention, and FIGS. 3, 4, 5, 6, and Fig. 7 is a diagram for explaining the operation of the present invention, Fig. 8 is a configuration example of a variable band pass waver, and Fig. 9 is a diagram for explaining the operation of the present invention.
A second embodiment of the FM demodulation method, FIG. 10, is an example of the characteristics of the nonlinear circuit shown in FIG. 9. 1... variable band pass wave generator, 2... limiter circuit, 3... frequency discriminator circuit, 4...
...Low pass wave generator, 5...Signal input terminal, 6...
FM signal detection output terminal, 7... variable band pass wave generator, 8... phase detection circuit, 9... color subcarrier component band pass wave generator, 10... adjustment circuit, 1
1... Limiter circuit, 12... Frequency discriminator circuit, 13... Signal input terminal, 14...
FM detection signal output terminal, 15... Phase shifter, 16...
...Variable wave device input terminal, 17...Variable wave device output terminal, 18...Variable capacitance element, 19...Control signal input terminal, 20...Non-linear circuit.

Claims (1)

[Claims] 1. An input terminal to which an FM signal modulated with an emphasis-applied color television signal is applied, and a variable bandpass transducer connected to the terminal and capable of controlling the center frequency of the passband characteristic. , a limiter circuit and a frequency discriminator circuit connected to the output thereof, a demodulation output terminal connected to the output of the frequency discriminator circuit, and a phase detection circuit connected to the input and output of the variable bandpass transducer. and a narrow-band color subcarrier bandpass waveform connected to the output of the phase detection circuit and passing the subcarrier component of the color television signal.
a color subcarrier adjustment circuit that controls the variable bandpass waver by adjusting the amplitude and phase of the output of the waver, and the center frequency of the bandpass waver is set to the color subcarrier component of the FM input signal. The high-sensitivity FM is characterized in that the adjustment circuit adjusts the frequency according to the frequency change caused by the frequency discriminator, and the signal that is FM demodulated (detected) by the frequency discriminator is outputted from the demodulation output terminal.
Demodulation method. 2. An input terminal to which an FM signal modulated by a color television signal to which emphasis is applied is applied, a variable band pass waver connected to the terminal and capable of controlling the center frequency of the pass band characteristic, and connected to its output. a frequency discriminator circuit, a demodulation output terminal connected to the output of the frequency discriminator circuit, a phase detection circuit connected to the input and output of the variable bandpass waveform, and the phase detection circuit. a narrowband color subcarrier bandpass transducer connected to the output of the color television signal for passing the subcarrier component of the color television signal;
a color subcarrier adjustment circuit that adjusts the amplitude and phase of the output of the wave generator; and a color subcarrier adjustment circuit that is connected to the output of the adjustment circuit and that adjusts the output level to approximately 0 when the output level is below a predetermined value. and a non-linear circuit that controls the center frequency of the variable bandpass waver by varying the center frequency of the variable bandpass waver, and the center frequency of the variable bandpass waver is varied in accordance with a frequency change due to a color subcarrier modulation component of the input FM signal. A high-sensitivity FM demodulation method characterized in that a signal adjusted by the adjustment circuit and FM demodulated (detected) by the frequency discriminator circuit is extracted from the demodulation output.