JPS6027208A - Demodulating method of frequency-modulated signal - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention provides a wideband TV at the receiving input with a simple configuration.
This invention relates to a highly sensitive FM signal demodulation method that completely improves the noise characteristics of a demodulated signal of an FM (television frequency modulation) modulated wave.

BACKGROUND ART Conventionally, as the simplest method to demodulate a frequency modulated signal, a method using a discriminator using an LC circuit or a delay line is often used.In the case of 0 and (7), the C/N ( The S/N (signal-to-noise ratio) of the FM demodulated (detected) signal with respect to the carrier 1R power-to-noise power ratio is: S/N = C/N・K
(K: constant), and S/N is proportional to C/N.

The C/N is determined by the bandwidth B of the bandpass p-wave generator that determines the signal and noise bandwidth of the FM wave, and the bandwidth is usually calculated according to the Carson rule B''; 2 (Δf + fh) (where Δf is the FM modulation The frequency shift width of the wave (fh is the highest modulation frequency of the modulated wave).

The above relationship between C/N and S/H changes linearly when the C/N is about 10 db1, but as the C/N increases below that, the S/N becomes impulse noise peculiar to FM demodulation. This results in rapid deterioration. The point is called the threshold point.

In general, in communications that transmit all TV (video) signals, particularly in satellite communications, an FM modulation method is often used for the transmission. In satellite communication lines, the power margin for reception is limited as much as possible and the reception operating point is set near the threshold due to limitations on satellite transmission power, stability of the satellite communication propagation path, and economics of ground reception equipment. There are many cases.

Therefore, due to changes in environmental conditions and various deteriorations in receiving equipment, sometimes the receiving point falls below the threshold,
The demodulated image quality on the TV monitor may deteriorate significantly.

This problem is particularly important for broadcasting satellite receivers that use small antennas, and it would be extremely effective if the threshold characteristics could be improved in a simple way. Generally, a method is used in which the bandwidth In of the demodulating band-pass p-wave device is made as narrow as possible and the C/Ni is made high.

However, due to the spread of the spectrum of the FM modulated wave, that is, the degree of modulation, it is not possible to simply narrow the demodulation bandwidth Bi, which may lead to adverse effects. Especially F.M.
When the modulation index is large, the spectrum of the modulated signal is broadened, and narrowing the demodulation band 'Klj reduces the modulated signal component more than the noise component, resulting in 1l-1:
This also results in deterioration of C/N'c and deterioration of threshold characteristics.

Furthermore, when the frequency deviation of the TV (video) FM signal is large (image quality), a truncation noise is generated, resulting in an undesirable delay.

Purpose The present invention aims to provide a highly sensitive FM signal demodulation system that improves the C/N ratio and the threshold characteristics.

Summary of the Invention In the present invention, a demodulation operation mechanism is devised by focusing on the characteristics of a signal that is FM-modulated with a commonly used color video signal or the like to which pre-emphasis is applied. In other words, a variable phase shifter applies inverse modulation to the modulated component by the color subcarrier that gives the largest frequency shift, compresses the frequency shift of this component, and creates a narrow band pass p-wave filter narrower than the Carson bandwidth. By completely passing through the signal, a method is realized in which the carrier power to noise power ratio (C/N) k is improved and the threshold characteristics are completely improved.

In particular, by designing a circuit that focuses on specific frequency components and applying a passive variable phase shifter, a demodulation circuit that is stable and easy to design can be easily realized.

Now, let's consider a video signal to which emphasis is applied. Usually, a color video signal consists of a luminance signal and a color signal, and in the case of the NTSC system, it contains frequency components up to about 4.2 MHz. Of these, the luminance signal is mainly concentrated in a low frequency region that is a multiple of the horizontal scanning frequency (15,75 KHz), and the color component is concentrated in the vicinity of 3.58 MHz. This kind of video signal is, for example, CCIR, REC.
When the pre-emphasis circuit determined by 405-If is applied, the low frequency portion of the signal is given a power weighting of approximately +3 dB relative to the high frequency component of approximately -tociB.

Looking at a typical standard color par signal with the highest degree of saturation as a video signal, the maximum amplitude of the signal is 140 IRE, while the maximum amplitude of the luminance signal is 77 IRE.

The amplitude of the 3.58 MHz color subcarrier component is 8
There are 8IRE. Therefore, when all of this signal is weighted by the pre-emphasis circuit described above, the amplitude of the color subcarrier component becomes 127 IRE, which is close to the maximum amplitude of the source signal, 140 IRE. Therefore, the instantaneous frequency change (shift) of the FM signal modulated by the video signal to which pre-emphasis is applied is the largest, and this is the problem with regard to the C/N change due to the narrowing of the passband. It may be considered that this is due to color subcarrier components.

Example FIG. 1 shows an example of the basic configuration of an embodiment of the present invention.

The FM signal input terminal 1 is connected to a variable phase shifter 2 controlled by an external electric signal, a narrow band pass transducer 3, a frequency discriminator 4 consisting of a limiter, an amplifier, etc.
M signal demodulation (detection) output terminal 5, bandpass F wave generator 6 that passes all components near the color subcarrier (3°58 MHz in the case of NTSC system), amplifier 7 for this frequency component, phase adjuster 8 for this component Consists of etc. Here, the FM signal enters input terminal 1, variable phase shifter 2,
After passing through the narrowband pass P-wave device 3, the discriminator 4
The frequency is demodulated by Demodulated baseband (video)
A part of the signal passes through a bandpass P-wave device 6 and an amplifier 7, etc., which pass all specific frequency components (components near the color subcarrier), and after being phase-adjusted by a phase modulator 8, it is sent to a variable phase shifter 2. Added. According to this control signal, variable phase shifter 2
The phase of the FM signal input from the input terminal 1 is changed. In this state, demodulation (detection) of the FM signal
The signal is taken out from the demodulation output terminal 5. Note that the configuration in which the subcarrier component feedback circuit consisting of the bandpass P-wave unit 6, the amplifier 7, and the phase adjuster 8 and the variable phase shifter 2 are removed in FIG.
This is exactly the same as the M i=1 device.

Next, the operation of FIG. 1 will be explained by focusing only on the color subcarrier component that gives a large frequency shift as described above among the FM modulated TV (video) signal.

Now, paying attention to the phase of the input FM signal, 5i=Asin(c++t+asinpt) -fil
It is assumed that the output signal sound 5o of the variable phase shifter 0=Asin(a+t+bsinpt)-121. ω is the central angular frequency of the FM signal, and p is the modulation angular frequency (corresponding to the color subcarrier component in this case). The angular frequency deviation of the input Si is ΔΩ=a p , and the angular frequency deviation of the variable phase shifter output So is Δω=l)p.

Sensitivity q KO of the discriminator, gain f K 1 of the color subcarrier passing P-wave generator and amplifier, and the phase of the phase adjuster θ, the control voltage eOil″j for the variable phase shifter:, eo-Ko- 1・Δa+ cos (p t−θ)−
13). On the other hand, if the relationship/system between the phase amount 0 of the variable phase shifter and the control voltage eO is Φ= e o K z, then
Φ=KO-Kl・Kl・Δωcos (p t-θ)・
...(4).

Now, adjust the phase value of the phase adjuster 8 for the color subcarrier component (for example, θ-/2) to give I) =KO-Kl-2 ・Δωsin p t=
Δωs in p t −(5)Ko −Kl
- to 2 = K - (5a). At this time, phase shifter Φ
The phase relationship of the input signal % S 1 tSO is b 5inpt = a 5inpt − (1) − (
6) b s in pt = (a −K ψΔa+
) sinpt −(7)b = a −K・Δω ・・
・(8) becomes. Therefore, the angular frequency deviation ΔΩ of the input Si
= ap (7) The angular frequency deviation of the variable phase shifter output SO is -pΔω (9) where Δω=ΔΩ−.

That is, under such conditions, the angular frequency deviation of the output signal SO of the variable phase shifter is compressed to 1/(1+-p) compared to that of the input signal. The threshold characteristic of the signal at 4 is the passing 1? due to demodulation. It is determined by the bandwidth of the VJ wave transmitter. In the case of a normal demodulator that does not have a feedback circuit for color subcarrier components, the bandwidth Bo of the passband p-wave generator required for demodulation RLil is assumed to use the Carson bandwidth, and Bo = 2 (p + ΔΩ) ... ( On the other hand, in the present invention where the return is made through the I hill, the improvement degree η of the threshold level according to the present invention is as follows. By increasing the coercivity Ki, η>1 and 9 , the threnshold characteristic is improved.

What is important in the present invention is to focus on the color subcarrier signal component with the widest frequency shift and the highest modulation frequency for the modulation of the video FM signal, and to use a variable phase shifter and a feedback circuit. By compressing the frequency shift caused by this component and narrowing the aftertone bandwidth, the threshold characteristics are improved.

However, the video baseband signal (0 to 4.2 MH
z), the frequency shift of the F'M signal is not compressed for signal components other than those near the color subcarrier signal. Therefore, the frequency characteristics of the baseband signal obtained by demodulating the FM signal have different characteristics compared to the normal method.In other words, the demodulated output of the color subcarrier frequency component with compressed frequency deviation is It decreases in comparison.

FIG. 2 shows the frequency characteristics of the baseband signal demodulated by the method shown in FIG. If the phase amount of the phase adjuster 8 is not appropriate, the frequency deviation width of the output FM signal of the variable phase shifter 2 will increase compared to that of the human input signal, and the threshold characteristic may be completely degraded. In this case, unlike FIG. 2, the frequency characteristics of the demodulated baseband signal may have a higher amplitude near the feedback frequency than at other frequencies.

The variable phase shifter 2, which plays an important role in FIG. 1, can have various forms, all examples of which are shown in FIG. This is a circuit well known as the bridge method, where n,
n' is a signal input terminal, and rn and m are signal output terminals. r is a resistance, R is a variable resistance, and C is a variable capacitance.

Here, the output signal phase changes by changing the variable resistor R or the variable capacitor 1Ct. R or C is realized by a variable resistor or variable capacitance diode all around.

FIG. 4 shows a 144 configuration example using a series resonant system as the variable phase shifter 2. In FIG. n, n' are signal input terminals, m,
m' is the signal force 11. C is a variable capacitance, L is an inductance, and R is a load resistance.

Now, in Fig. 4, the input terminal e in e in = E sin to t --- (+4+
Then, the output voltage e out is -R e out = -s in (ω is one ψ) -
(+5)ZI becomes.

That is, by changing the variable capacitor C, the phase of the output voltage changes.

The relationship between the variable capacitance C and the amount of phase shift of the variable phase shifter 2 is shown in FIG.

Therefore, by the phase control as described above, the frequency iii+i shift of the FM wave due to the color subcarrier component is compressed, and the above effect can be obtained.

However, in this case, according to Equation 15, a change in the amplitude (amplitude modulation) occurs in the output signal due to a change in the variable capacitance, that is, a change in the phase. Therefore, the limiter used after the phase shifter needs to have a sufficient amplitude suppression effect.

On the other hand, in this circuit system, it is possible to effectively utilize its own resonance characteristics (narrow band pass characteristics), and by giving it a pass band characteristic narrower than the Carson bandwidth, the narrow band pass Pe, device 3 in Fig. 1 can be used. It can be omitted and is effective in simplifying circuit construction. Although an example using a series resonant system is shown here, a similar variable phase shifter can also be easily realized using a parallel resonant system.

What is noteworthy is that the FM signal is controlled by a variable phase shifter! 1! The frequency deviation of the r-modulated frequency component is completely compressed, the narrow band pass characteristic is improved by C,/N-2, and the threshold characteristic is completely improved.

In addition, the use of a passive variable phase shifter and the feedback of a specific frequency component provide a stable and easy-to-implement circuit system.

However, due to the compression of the specific modulation frequency shift, the amplitude of the compressed frequency component decreases due to the frequency characteristics of the FM detection signal. Therefore, if better characteristics are desired, it is necessary to perform all characteristic compensation for that frequency component.

Figure 6 shows an example of the characteristic compensation circuit 9, in which a parallel resonant system consisting of L t Ctr is fully connected to the output of the frequency discriminator, and the load impedance at a specific frequency is completely increased and the amplitude characteristic is increased for compensation. do.

The resonant frequency of the resonant system is set near the specific frequency.

Effects As described above, the present invention provides a high-sensitivity FM signal demodulation system that is easy to implement with a simple configuration.

Therefore, this method provides an extremely effective means, especially in systems such as broadcasting satellite receivers, which have a small threshold margin and where simplicity, simplicity, and low cost are all important.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the basic configuration of an embodiment of the present invention, FIG. 2 is a diagram showing the frequency characteristics of the baseband signal demodulated by the method shown in FIG. 1, and FIG. 3 is a diagram showing the variable phase shifter 2. An example of an all-oil electric circuit diagram; Figure 4 is an electric circuit diagram using a series resonant wire as the variable phase shifter 2; Figure 5 is a diagram showing the relationship between the variable capacitance C and the amount of phase shift of the variable phase shifter 2. , FIG. 6 is an electrical circuit diagram showing an example of the characteristic compensation circuit 9. In FIG. 1... FM signal manual terminal, 2... Variable phase shifter, 3
...Narrow band pass P-wave device, 4...Frequency discriminator, 5...Demodulation output terminal, 6...Band pass I1
5 wave generator, 7...amplifier,...phase adjuster, 9...
Characteristics Auxiliary Circuit Agent Patent Attorney Nishikyoyojin Part 1st Figure 2 Figure 4

Claims (1)

[Claims] A variable phase shifter to which an input FM signal is applied and a narrow band pass p having a bandwidth narrower than the width of the Carson band of the FM signal.
An input FM signal comprising a wave generator, a frequency discriminator, a wave generator that passes a specific frequency component of the signal FM detected by the frequency discriminator, and a phase adjuster for the frequency component. A variable phase shifter and a narrow band pass F-wave filter were passed through, and a frequency discriminator was used to make FM @ waves, and a part of the detected output signal was passed through a phase adjuster and a phase adjuster that passed all of the above-mentioned specific frequency components. The variable phase shifter is controlled by the posterior vaginal signal, and the phase is controlled by the variable phase shifter in a direction that completely compresses the frequency deviation due to a specific modulation frequency component of the FIVI signal, and the detected signal from the frequency discriminator is extracted. A demodulation method for frequency modulated signals characterized by: