JPS5836095A - High sensitivity fm demodulation system - Google Patents

Abstract

PURPOSE:To improve noise characteristics in the modulation of a low level FM video signal by providing a phase detector for phase-detecting an input signal to a narow-band variable filter and a filter for extracting a color subcarrier component from a phase-detected signal. CONSTITUTION:An FM video signal from an input terminal 13, after passing through a variable band-pass filter 7 which varies in passing center frequency and a limiter circuit 11, is demodulated by a discriminator 12. An input and an output signal to and from the variable band-pass filter 7 are supplied to a phase detecting circuit 8 for phase comparison, and a color subcarrier band-pass filter 9 extracts a color subcarrier component from the detection output. The center frequency of the filter 7 is controlled by the color subcarrier component. Thus, the C/N of the filter 7 is made better than the input C/N to obtain a demodulated signal having improved noise characteristics.

Description

[Detailed Description of the Invention] The present invention has a simple configuration and provides wideband
The present invention relates to a high-sensitivity demodulation method that improves the threshold level of an I″V (video)-FM modulated wave and improves the noise characteristics of a demodulated signal.

Conventionally, as the simplest method for demodulating a frequency-modulated signal, a frequency demodulation method using a discriminator using an L, C, circuit or a delay line has been used. In this case, the S/N (signal to external sound ratio) of the 1i'M demodulated (detected) signal to the C/N (carrier power to noise power ratio) of the FM modulated input signal is S/N=C /N
, FI (Fl two constants), and the demodulation S/N is proportional to the C/N of the input signal.

On the other hand, this C/N is determined by the passband width of a bandpass Kerr single waver used on the input side of the demodulator in order to limit noise and signal bandwidth. Normally, according to this method, C/
The above-mentioned relationship is maintained up to approximately 1 dB, and at C/H below that, the S/N+ deteriorates rapidly. This point is called the slenderness hold point.

In general, in communications for transmitting TV (video) signals, such as satellite communications, an FM modulation method is often used to transmit the signals. In this case, the operating point of the communication line during reception is often set near the threshold due to limitations on satellite transmission power, safety of the satellite communication propagation path, and economical efficiency of ground reception equipment. Therefore, sometimes due to changes in environmental conditions, the receiving input decreases, the receiving level falls below the threshold, the demodulator on the TV monitor is significantly disturbed by the impulse noise characteristic of TV transmission, and even the demodulator's output is affected. 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 the TV demodulated image quality and, ultimately, the economic efficiency of receiving equipment.
Particularly in satellite receiving apparatuses for broadcasting satellite communications and the like, a method for improving noise (improving demodulated image quality) with a simple configuration is an extremely important issue.

Incidentally, there have been various methods for improving the threshold. As an example, a dynamic tracking filter [-M demodulation method] which is somewhat related to the present invention is shown in FIG. In the figure, 1 is a narrow band variable band pass r-wave device whose center frequency changes, 2 is an IJ emitter circuit, 3 is a frequency discriminator circuit, 4 is a low-pass r-wave device,
5 is a signal input terminal, and 6 is an I'M signal detection output terminal. The I''M signal wave that enters from the input terminal 5 passes through the center frequency variable bandpass single-wave unit 1, passes through the limiter circuit 2, and is demodulated by the discriminator 3.The demodulated signal has a low frequency i11
After passing through the r-wave generator 4 and removing high-frequency noise other than the modulated signal frequency, the center frequency of the variable r-wave generator 1 is controlled. At this time 5
The center frequency of the variable fj wave generator l is controlled so as to completely follow the instantaneous frequency change of the input signal. 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 r wave generator.

By the way, as is well known, a color video signal is composed of a luminance signal and a color subcarrier component, and has a baseband bandwidth of 4.2 MtIzKf, which is an extremely wide band signal. 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 density and saturation of the image.

Due to the unique properties of color video signals such as , the signal component of the discriminator 3 is stably and faithfully K in terms of phase and amplitude through a feedback circuit consisting of an amplifier, a low-frequency r-wave generator, and an attached circuit of the internal circuitry of the variable f-wave generator 1. "matching the frequency change": It is a very difficult problem to apply an electric current to one center frequency variable element. If this condition cannot be set, the li'M of the input signal for the correctly fed back modulation frequency component
The changing component is effectively passed through a variable narrow band P-wave generator, and the lI! 1M
However, there are cases where it acts in the opposite direction for modulated frequency components.As a result, for example, if modulated frequency components with a large frequency deviation are not fed back correctly, a large FM deviation may occur. This component with a moving power is removed according to the pass characteristics of the variable 41-wavelength filter with a narrow bandwidth, and then the C/C which enters the emitter circuit and the discriminator circuit.
This may have the opposite effect, such as degrading N and dropping it to a state below the threshold.

Furthermore, even if these feedback circuits are set correctly, the variable bandpass p-wave generator is controlled by the wideband noise accompanying the input signal passing through this wideband feedback circuit, and the noise modulation component is also controlled by the narrowband variable p-wavelength filter. Pass through the wave device effectively. As a result, when accompanied by large noises, the demodulated image is distorted, and particularly 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 image is low (and the color sub-gear component is small), broadband noise is effectively demodulated and becomes very noticeable as color noise on a monitor.

The present invention aims to solve these problems,
To provide 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, the input and output signals of the variable e-wave generator are phase-detected using a narrow-band variable 4" single wave generator whose center frequency changes, a phase detection circuit, a color subgear control circuit, etc. The color subcarrier component is extracted from the subcarrier detected, and the center frequency of the variable r wave generator is varied in response to the input frequency change.Furthermore, the input/output ratio for the subcarrier is adjusted to the input level. On the other hand, a variable r-wave generator is controlled using a non-linear circuit that is non-linear.

Thereby, the output C/N of the variable e-wave device 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 a six-cased embodiment of the present invention, in which 7 is a variable bandpass P waveform whose passing center frequency changes depending on an external signal, 8 is a phase detection circuit, and 9 is a color subcarrier band through which color subcarrier frequency components are passed. 10 is a color subcarrier adjustment circuit that can vary the amplitude and phase of the color subcarrier component; 11 is a limiter circuit; 12 is a frequency discriminator circuit; 13 is an F'M
a signal input terminal; 14 is a Ti'M detection signal output terminal;
15 is a phase shifter used as necessary.

The signal from the input terminal 13 passes through the variable r wave generator 7 and further passes through the limiter circuit 11, and then passes through the discriminator 1.
It is demodulated by 2. Then, the demodulated (detected) signal is taken out from the output terminal /I. The input and output signals of the variable N=' wave detector are phase-compared by a phase detection circuit, and among the detected output signals, the video color subcarrier components (3,
58MfJz) is extracted and amplified by the r wave generator 9, and then controls the passing center frequency of the variable wave generator 70. In this configuration, the center frequency of the variable P-wave device 7 is demodulated by the phase detection circuit 8 as described later, and the instantaneous frequency due to the color subcarrier modulation component of the input 1i'M signal is determined by the extracted color subcarrier frequency component. controlled to vary in accordance with the deviation. At this time, the adjustment circuit 10 and phase shifter 15 may be used to satisfy this condition, if necessary. As a result, the lJi force C'/N of the variable r-wave device 7 is
This is improved compared to the human 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 set to sufficiently pass the energy of the modulated signal and demodulate the waveform well.
If the FM frequency deviation (peak value) of the input signal is Δf and the highest frequency of the modulation signal is 9, it is determined as Bota2 (Δf19).

Under certain conditions, the narrower the demodulation bandwidth is, the worse the waveform characteristics of the demodulated signal will be. However, since demodulation reduces noise, the threshold level can be lowered. However, if the frequency band is casually narrowed, the signal component with a large frequency shift will pass through the narrow band; As a result, on the output side of the narrowband single-wave device, the signal power decreases even more than the noise removal effect of the r-wave device, causing a 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 band-pass single-wave device, the method outside the modulation that causes the largest frequency shift, that is, the color video signal to which pre-emphasis is applied on the modulation side, is used. Among them, we focused on the color sub-gearia component that is likely to give the largest frequency deviation, and in response to the instantaneous frequency change caused by this component, we
A method is adopted in which the center frequencies of the TI wave generators are matched. 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 4.2 MITz. Among them, the luminance signal mainly has a horizontal scanning frequency (15
, 75 KHz), and color components are concentrated near 3.58' Mrlz.

This kind of video signal is, for example, CC1l+...It...(...
When the pre-emphasis circuit determined by C.405-1 is applied, the low frequency part of the signal is approximately -10 (Tano), and the high frequency component is approximately -1-3 (I + (I)). ?Ii
The weight of force can be injured.

Looking at a typical standard color bar signal with the highest degree of saturation as a video signal, the signal's l l large amplitude + 4 (l l H,
The maximum amplitude of the luminance signal is 771 It, 1
8, 3.58 Ml 17゜0 The amplitude of the laser component is 8811tl. The amplitude is 12
7 I n, +>, the maximum amplitude of original Shingetsu], 4. O
The amplitude is close to ITl,E. 1 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 as the passband width becomes narrower, it becomes a problem regarding C/N change. This color may be related to the rear component. For these reasons, the center frequency of the narrowband variable '6-1 waveform generator is changed to follow the input frequency deviation using the 3.58 MT-Tz acid component, and the signal power is effectively transmitted to the frequency discriminator. However, the C/N ratio is improved by removing much noise. In addition, the color subcarrier control system uses frequency components close to a single signal (3,58 MI (z
), 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, FM demodulation of the input signal is performed by comparing the phases of the input and output signals of the variable band pass wave generator 7 shown in FIG.
Although it has been mentioned that an 8 MI Tz acid component is obtained, this operation will be explained.

Now, for the sake of simplicity, it is assumed that the variable bandpass r-wave device is composed of a single-resonant system as shown in Figure 3, and a small portion of the input/output signal of the fi-wave device 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 i1'' single wave detector are not affected by the phase detection circuit.

At this time, the input and output voltages of the bandpass and single wave filters are:
, 1°C6, the following relationship holds true.

1].

Here, Δf is the detuning frequency from the center frequency of the r-wave generator, b
o corresponds to 3 (circular bandwidth) of the P wave detector. Phase detection circuit 8
A signal equivalent to the input/output voltage of the r wave generator 70 is supplied.

If the input voltage Li is taken as J'J-Ele'', the detection output e of the phase detection circuit is as follows.

Here K is a constant. If the fixed phase Φ is set to 0 and the normalization characteristics are determined, the result will be as shown in FIG.

FIG. 4 shows the frequency change of the input signal of the P-wave device 7.
The output e of the phase detection circuit 8 exhibits frequency discriminator characteristics, and the FM signal is demodulated.

Therefore, Color Zabucarrier 3.58 ME (z component also,
This demodulates the signal and the signal passes through the color subcarrier (Ff wave)? After passing through i, the center frequency of the variable frequency j wave generator is controlled. , in this case: lt! The direction of I control is related to the input signal frequency change. It must be determined that the polarity of the output voltage of the phase detection circuit detected by the detection voltage coincides with the change in the center frequency of the variable frequency converter due to the detection voltage.

In addition, if the frequency detection (demodulation) sensitivity of this variable -'i-wave device is set to z1, and the center frequency fluctuation sensitivity of the variable 11-wave device based on the color sub-gear amplitude obtained from the detection output is made sufficiently large, then The center frequency of the e-wave generator is input 1”
It should be controlled to match the frequency change due to the color sub-gear component of the M signal.

Next, in the control using color subcarrier components performed in this way, the C/
A summary of the N improvement effect is shown below.

Fig. 5 shows the instantaneous frequency change of the F'M signal from the input terminal 3 and the instantaneous change of the center frequency of the variable band pass r wave generator 7. )l indicates the former, and broken line (C) indicates the latter.几 is variable tide 1 wave device 7
is the center frequency when is not controlled by the color subcarrier component. For simplicity, the signal instantaneous frequency change is
It consists of frequency changes (al and frequency changes (1) due to the luminance signal) due to the color subgearia (3,58 MIIz), and the frequency difference is large, Δf and Δ, respectively.
It is assumed that the frequency shift of J is applied. On the other hand, the center frequency (broken line (C)) of the variable W-1 waveform follows the signal instantaneous frequency change (a), and its deviation is assumed to be equal to Δf.

FIG. 6 shows the state of the Shingetsu power passing through the variable band pass I/I wave device 7, where the horizontal axis f represents the frequency, and the vertical axis P represents the power passed through the f wave device. 1) on the solid line.

(b) and the broken line (C1 is the same as in Fig. 5, and (rl
l, (ds.

(d") shows the pass characteristic of the variable p-wave device.

Now, the instantaneous frequency of the signal is 1. When it is at (-10Δ/,), the center frequency of the variable f-wave generator is at 8, and when the instantaneous frequency changes to 12 (-90Δf1+Δfc), the frequency shifts to 10Δfc. Therefore, a frequency difference of Δfi occurs between the center of the variable single wave generator and the instantaneous signal frequency. Therefore, the signal power passing through the r-wave device is reduced by L.

Now, if we consider a variable bandpass waveform constructed from a single resonator as described above, its power passing characteristic can be normalized and expressed as '1-'. Here, Δf is the detuning frequency from the center of the r-wave device, 1). is the 3dB bandwidth of the p-wave generator. In Fig. 2, the input signals to the variable r 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, it is assumed that the bandwidth is limited by the normal bandwidth BO as described above.

At the input of the variable dj waver 70, if the signal power is 1 and the noise power density is 1/Hz, the bandwidth BO (Irz)
The noise power at is B. So, the input is C, /N.

becomes i/r3n. By the way, on the output side of the p-wave device 7,
Since the signal frequency is Δf1 away from the center of the t''1 wave device, the passing signal output is I).

becomes.

As for the noise power passing through the r-wave device, as shown in FIG. 6, the center frequency of the r-wave device has a frequency width B. within (rll, (d'),
3, when the center frequency is at the center of B., that is, f.
= The noise power passing through the j-wave device is the highest, and at that time, the output C/N of the l''1-wave device 7 is the lowest. Therefore, the worst condition for noise is the noise power in this state. Therefore, the output C/N (-C6/No) Hadozo, Jinro.

Now, taking an example of an actual satellite system, 4.2M1l
The video signal with the band z has a maximum frequency deviation of 10.75M.
1 rz). Also, CCII (,405-]
Emphasis 11. ! 1′ property applies. At this time, the bandwidth 1(.) of the normal demodulator is selected as 3('1 MIIz. 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 middle, I) is the 3 (■bandwidth) of the variable bandpass p-wavelength converter, Δf1 is the frequency deviation due to the outside of the brightness modulation after applying pre-emphasis on the video signal modulation side, and the degree of improvement η is the same as described above. The smaller Δf1 is, the greater the improvement effect can be obtained.

Although various types of variable bandpass f1 wave generator 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. The input/output terminal of the wave detector, 18 is a variable capacitance element (for example, a varactor), and 19 is a 3.58 M signal extracted from the phase detection circuit 8.
This is an input terminal for the l-Tz color subgear signal component. Note that 19a is a filter for preventing the high frequency of the terminal 16 or 17 from flowing back to the control signal input terminal]9;
191) is 3.58 MIT7. In the resonant Zuro filter circuit, the control signal at terminal 190 is 1. This prevents short-circuiting. -1- The existence of the series resonant circuit and the parallel resonant circuit is also one of the characteristics of the circuit component according to this method. Lo and Co (DC component of variable capacitance) are the center frequency f of the input FM signal. K resonance is selected. In this state, when a signal is applied from the input terminal 16 of the color subgear signal, the variable capacitor 18 changes accordingly, and the signal becomes 1. + Vary the resonance frequency of Co.

As a result, the center frequency of the bandpass δ1-wave device changes, resulting in a desired variable bandpass P-wave device.

As described above, the present invention has shown a simple and easy-to-implement threshold improvement method based on the characteristics of a color video signal to which pre-emphasis is applied. 1st
In this embodiment, variable r
The phase of the input and output signals of the wave generator is compared by a phase detection circuit, and the color subcarrier component obtained thereby is used to control the passing center frequency of the variable lj wave generator through a control circuit. By handling narrowband signals in such a circuit configuration, a stable control circuit can be easily constructed, and arbitrary control conditions can be easily satisfied. In addition, because the control is based only on this specific narrowband signal, the variable r-wave generator does not respond to the AI sound frequency components that are input to the variable r-wave generator, and only responds to those that accompany the input modulation signal. Noise having the same frequency component as , is removed according to the pass characteristics of the li wave filter.

In addition, the C/N improvement circuit using the variable bandpass A11-1 waveformer can be configured independently of the frequency discriminator for signal demodulation, making it convenient to use and applicable to ordinary FM demodulators. It also has excellent versatility. While having these many features, the C/N ratio of the input signal is improved, that is, the threshold level is improved, and 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 color subgear signal control circuit has a non-linear structure in which the input/output ratio (So/S + ) of the signal level changes depending on the magnitude of the input signal S. 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 nonlinear j] (resistors, etc., but the first
It has the Okawa force characteristic (a) or (1)) as shown in diagram O. That is, in a region where the input signal Si is small, the input/output ratio S is the same. /S,
and S in the region where Sl is large. /S, is made large.

In the example shown in FIG. 2, the color sub-gear component demodulated by the phase detection circuit and extracted by the f1 wave generator 9 is accompanied by noise within the passband of the r wave generator 9. Therefore, when the color sub-gear component of the video signal becomes small, that is, when the color saturation is low, this noise component is amplified by an amplifier system such as 10 in the control system and acts on the variable f-wave unit 7. Therefore 13
The noise of the same frequency component accompanying the input signal is effectively passed through the variable p-wave generator 7 and demodulated.

As a result, these power 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 in the color subgear control system, and in the case of a modulation signal with a small color subgear component, the input/output nonlinear circuit 20 is inserted into the variable: I-1 wave generator 7. By reducing the control signal, the variable r wave generator
By suppressing changes caused by the chromaticity, deterioration of the demodulated image quality due to these single chromatic sounds is prevented.

The present invention can be easily realized by using a variable bandpass J' waver, a phase detection circuit, a color subgear signal control circuit, a frequency discriminator circuit, etc.
Improving C/N, that is, improving the threshold level - Improving image quality at low reception inputs 3. Therefore,
It can be used very effectively in satellite receivers that receive video-Ii''M signals at low reception input levels.

4 Simple drawing? Figure 1 shows an example of the configuration of the conventional do-M demodulation system, Figure 2 shows an example of the configuration of the first embodiment of the I-M demodulation system according to the present invention, and Figures 3 and 3 4, FIG. 5, FIG. 6, and FIG. 7 show a second embodiment of the demodulation method, and FIG. 10 shows an example of the characteristics of the nonlinear circuit in FIG. 9.

1...Variable band pass 11 wave generator 2...Limiter circuit 3...Frequency discriminator circuit 4...
...Low pass 1" wave generator 5...Signal input terminal 6...Ii"M signal detection output terminal 7...
・Variable bandpass r wave generator 8...Phase detection circuit 9...Color subgear bandpass δ1 wave generator 1
0...Adjustment circuit 1]...Limiter circuit 12...Frequency discriminator circuit 13.
... Signal input terminal 14 ... Do-M detection signal output terminal] 5 ...
・Phase shifter + (i...Variable j1 waver input terminal 17,...
,,,Variable power “i-wave device output terminal +8 ,,,,,,Variable capacitance element +9 ,,,,,,Control signal input terminal 2 (1,,,,,,,Nonlinear circuit qj1°Applicant) Jin Kogyo Co., Ltd. Special ♂ 1; Patent attorney Megumi Yamamoto - Beauty 1 Figure) Breast 2 Figure OiL (MH7I Endf/Qa

Claims (1)

[Claims] An input terminal to which m'MtQ is applied, a variable band pass l' wave device connected to the terminal and whose center frequency of the pass band characteristic can be controlled, and a limiter connected to its output. a phase detection circuit connected to the circuit and the frequency discriminator circuit power and output; a narrowband color subcarrier bandpass ti waver connected to the output of the phase detection circuit and passing a subgear component of the video signal; The center frequency of the band-pass f-wave generator is set to 1.''M of the input signal. [High sensitivity characterized in that the signal is adjusted by the adjustment circuit so as to match the frequency change due to the color sub-gear component and is demodulated (detected) by 1.mm by the frequency discriminator and is output from the detection output terminal] 'i'M demodulation method. (2) An input ψ1111 to which an FM signal is applied, a variable band pass r wave generator whose center frequency of the pass band characteristic can be controlled, connected to the terminal, and a a limiter circuit and 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-1 waveform generator; a narrow-band color subcarrier band-pass p-wave device connected to the output of the circuit and passing the subgear component of the video signal; a color subcarrier adjustment circuit for adjusting the amplitude and phase of the r-wave device; a non-linear circuit connected to the circuit whose input/output level ratio varies non-linearly with respect to the input level; and means connected to the output of the non-linear circuit for controlling the center frequency of the variable bandpass r-wave generator; Adjusting the center frequency of the variable r-wave device by the adjustment circuit so as to match it by a frequency change due to the color subcarrier modulation component of the input FM signal,
A high-sensitivity Ti''M demodulation method characterized in that a signal FM demodulated (detected) by the frequency discriminator circuit is extracted from a detection output terminal.