JPS5939107A - Fm signal demodulating system - Google Patents

Abstract

PURPOSE:To improve the quality of a demodulated picture, by connecting a fixed reference BPF having a specified band characteristic and a variable BPF in cascade, and controlling the band width of the variable BPF so as to improve the ratio of the power of a received carrier wave to noise power. CONSTITUTION:An input signal Ci from an input terminal 6 passes through the reference BPF8 having the specified band width. A part of this signal is detected at a signal level detector 1 and an output corresponding to the ratio of the power of an input carrier wave to noise power (C/N) is obtained. Further, the signal Ci passes through a circuit 5 and enters the variable BPF2. The BPF2 is controlled so that the pass band width is narrowered as the signal level is decreased from the vicinity of the threshold value. The output of the BPF2 enters a frequency discriminator 3 through the circuit 4 and the frequency demodulation is attained. Thus, the equivalent C/N entering the discriminator 3 is improved to improve the S/N of the demodulation signal. When the receiving C/N is considerably low, spike noise specific to this operating region is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a frequency demodulation method that improves the noise characteristics of a demodulated signal of an FM modulated wave with a simple configuration.

As the simplest method for demodulating conventionally frequency-modulated signals, a frequency demodulation method using an LC circuit or a delay line is known and often used.

In this case, S (signal to noise ratio) of the FM demodulated (detected) demodulated signal to C (carrier power to noise power ratio) of the FM modulated input signal is - (C/l'J)
It is expressed as F I (F I is a constant), and the demodulation S/N
is proportional to the C/N of the input signal.

On the other hand, C and C limit the noise and signal bandwidth, so
It is determined by the passband width of the bandpass characteristic device used on the input side of the demodulator. Normally, the above-mentioned relationship is maintained up to approximately C/NklOdB, and below that, the relationship deteriorates rapidly. This point is called the threshold point.

Generally, in communications for transmitting TV signals, such as satellite communications, an FM modulation method is used to transmit the signals. In this case, the operating point of the communication line during reception is often set near the threshold hold due to limitations on satellite transmission power, stability of the satellite communication propagation path, and economical efficiency of ground reception equipment. As a result, sometimes the reception input decreases due to changes in the environmental situation, the reception point becomes below the threshold hold, the demodulated image is significantly disturbed by the spike noise peculiar to TV transmission, and even the demodulated image cannot be obtained. reach the state. Therefore, improving the spike noise using a simple method during satellite reception 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. In this field, a method for improving noise (improving demodulated image quality) using a simple configuration is considered to be an extremely important issue.

The present invention provides one solution to this problem,
A cascade connection is formed between a reference bandpass transducer having a fixed bandwidth and a specific variable bandpass transducer that changes the passband width of the signal, and the reference bandpass transducer has a maximum modulation frequency of the TV signal. fh1 color subcarrier frequency f, maximum frequency deviation Δf, modulation index of color subcarrier component m
Bandwidth B for an FM input signal with t=lf/f. 2 (Δf + fh), the variable bandpass filter has a signal power transfer characteristic T (1), and a bandwidth B. The modulated signal power P after passing through the reference bandpass r-wave generator. The ratio of the noise power No to

The noise type (just [, "fr < Bo/2, T (0) = 1) for the normalized modulated signal power PT in FM
As the signal level decreases to near or below the threshold level, the output of the signal level detector controls the passband width of the variable bandpass transducer, and the transmission bandwidth of the signal transmission path is adjusted. Narrowly, in the relationship between the FM modulation index of a specific modulation frequency component (color subcarrier component) and a bandpass characteristic device having a specific bandpass characteristic, characterized in that demodulation is performed by the frequency discriminator. , improve the equivalent C/N entering the discriminator, and improve the S/N of the demodulated signal. Especially when the reception C/N in TV signal reception is low, it is necessary to improve the noise caused by the threshold hold, which is most noticeable in the TV image quality on the monitor, and to obtain good image quality that is visually easy to see. This method is used.

Embodiments will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram showing one embodiment of the present invention. 1
is a signal level detector or C/N detector, 2 is a variable bandpass filter whose passband width is variable, 3 is a frequency discriminator consisting of an LC circuit or a delay line, and 4 and 5 are as required. 6 is an FM signal input terminal, 7 is an FM signal demodulation (detection) output terminal, and 8 is a fixed bandwidth B. A standard bandpass filter with . Various methods can be considered for detecting the C/N of the input signal, but the details thereof will be omitted since they are not the essence of the present invention.

In FIG. 1, the input signal Ci at the input terminal 6 has a bandwidth B. passes through a standard bandpass p-wave filter 8. At this time, the signal and noise bandwidths are determined thereby. A part of the signal is detected by the signal level detector 1, and an output corresponding to the magnitude of the signal level, in other words, the magnitude of the input φ is obtained.

On the other hand, the input signal C1 passes through a circuit 5 such as an amplifier and enters the variable bandpass p-wave device 2. This bandpass characteristic device 2 specifies the FM modulation index of a specific modulation frequency component of the input signal and this bandpass characteristic as the signal level becomes smaller near the sysledge hold according to the output of the signal level detector 1. In this relationship, the passband width is controlled to become narrower continuously or stepwise. This determines the signal and noise bandwidth for pFM demodulation. The output of the bandpass filter 2 is frequency demodulated into a frequency discriminator 3 via a circuit 4 such as an amplifier or an amplitude limiter.

Generally, when demodulating an FM signal, the passband increase (demodulated signal bandwidth) Bo of a bandpass characteristic device is used to sufficiently pass the modulated signal energy and maintain good waveform distortion of the demodulated signal. If the frequency deviation (peak value) is Δf and the highest modulation frequency of the modulation signal is fh, then Bo=2(Δ
(However, in an actual demodulation system, taking into account fluctuations in the signal carrier frequency, the bandwidth may be set to B.).

At this time, the C/N reaching the discriminator 3 is Cs/
(kBo) (k is a constant).

In FIG. 1, the variable bandpass ν wave filter 2 is removed, or the bandwidth of the variable bandpass filter 2 is changed to the standard bandpass F wave filter 8 in the previous stage.
Bandwidth of B. If it is wide enough, it operates like a normal FM demodulator.

The S/N of the signal demodulated by the discriminator with respect to the input signal level is shown in FIG. 2. As the input signal Ci becomes smaller, the S/N deteriorates and becomes 10
Threshold point (C1=Ct) at about dB
In the following conditions, it deteriorates rapidly.

Normally, in satellite reception of TV signals using a conventional demodulator, the reception operating point is a threshold point Ct and a number d
It is set in the P region of FIG. 2, which is about B higher, and the value at this time is I.

However, as described above, due to changes in environmental conditions, the reception operating point may become lower than the Ct point. .

The degree of this problem increases as the receiving equipment becomes more economical and simple.

By the way, when the signal input is higher than the threshold point, the quality of the image on the monitor of the TV signal transmitted and demodulated by the FM method is mainly due to image quality deterioration due to thermal noise etc. entering the demodulator and the transmission path. Image quality degradation is determined by waveform distortion due to nonlinearity of phase and amplitude characteristics, and is greatly influenced by the demodulation bandwidth B of the signal. In general, narrowing this bandwidth is advantageous in terms of noise such as thermal noise, but it increases the deterioration of image quality due to waveform distortion.

Now, the bandwidth is B as mentioned above. When looking at the demodulated image quality of the TV signal in the vicinity of the threshold bold in the conventional demodulation system selected for E2 (Δf + fh), it is visually apparent that the characteristic image quality is affected by threshold-hold noise (spike noise) based on thermal noise. The deterioration is extremely noticeable, and the deterioration in image quality due to waveform distortion due to nonlinearity of the transmission path is masked. When the signal level is below the threshold, the image quality deteriorates significantly as the level decreases, and the image is suddenly swamped by this spike noise, making it impossible to identify the received image. Therefore, if the image quality deteriorates in this area and the spy noise is suppressed, the image quality can be significantly improved.

The principle of the present invention will be explained with reference to the configuration shown in FIG. Focusing only on the highest modulation frequency fh' of the modulated signal, consider the case where FM modulation is performed with the maximum frequency deviation Δf. (In the later specific example, f' is assumed to be a color subcarrier component under certain conditions.) At this time, the modulation index m, = Δf/fh'. The bandwidth of the standard bandpass F-wave device is B. = 2 (Δf + fh'), and its characteristics are as shown in Figure 3 (■).
. It is assumed that the attenuation characteristic is flat and has a steep attenuation characteristic outside the band. Input signal power P that is FM modulated at modulation frequency fh'. becomes (1).

The output of a standard bandpass wave transducer has a bandwidth of B. Therefore, in the above equation (1), 0 nfh'<.

Only the components of are transmitted. Let this output signal power be PO2.

On the other hand, the noise power of the standard bandpass transducer 8 is N.

Then, No=kBo (2) (k is a constant) is given as, and the output C of the standard bandpass wave transducer 8 is P
,'/No.

Next, the effect of the variable bandpass filter 2 will be explained.

Let the power transfer characteristic of the variable bandpass filter 2 be TCf),
Assume that it has a symmetrical transmission characteristic with respect to the center frequency, as shown by ■ in FIG. Further, f indicates a detuned frequency from the center frequency of the Oki wave device, and when f=o, T(o)=1.

Let PT be the signal output power of the variable bandpass transducer 2.

Then, P is T PT=Σ,r,,'(m,)・'r (pfh')p”
=-n =Jo2(mf)+2Jt2(mf)T(fhz)+2
J22(mf)T(2fhriten""""' 2Jn2(
n+f)T(nfh') (3). Therefore, the output C/N of the variable bandpass filter 2 is the (
The ratio between equation 3) and equation (4) is p, /N.

For high input signal levels, the bandwidth B of the standard bandpass waver 8. By limiting the demodulation bandwidth, as the input signal level decreases, the output C/N of the standard bandpass p-wave converter 8:
po/ha. When T(f) reaches near the threshold, the variable bandpass p-wave transmitter 2 with a specific transmission characteristic T(f) is activated, and the variable bandpass is adjusted so that the relationship P, C, ) po'C0(5) holds. Controls the bandpass wave transducer 2. At this time, only the threshold point is improved.

As a result, the threshold point C in Figure 2, from Ct to C
There is a change in t' and a significant improvement in ΔS. This effect is Ct
The following S/N deterioration characteristics are rapid and large, but on the other hand, the noise on the TV monitor is significantly reduced, resulting in a much better image quality.

Further, a more detailed explanation will be given using an actual satellite broadcasting system as an example.

Now, we will set up a system that transmits a color video signal having a maximum signal component of 4.2 MHz with a maximum frequency deviation of 5 MHz. It is also assumed that the emphasis of CCIR, REC, and 405-1 is applied.

On the transmitting side, the video signal to which the pre-emphasis circuit is applied is given power weighting of about -10 dB to low frequency components and about +3 dB to high frequency components. As a result, the frequency change (deviation) is the greatest as an FM modulated wave.
The color subcarrier component (3
, 58MHz).

Now, let's take as an example the state where the color subcarrier component is the highest among the typical color bar signals as a standard video pattern, and find the state after pre-emphasis is applied.゛〒1′M
Hz (7) Often takes the maximum frequency deviation. usually,
In general video, the deviation due to this frequency component is much smaller. At this time, the maximum modulation index for the color subcarrier is m7 = 6/3.58 = 1.67
becomes.

On the other hand, the bandwidth B of the standard bandpass filter. is as usual,
Set Bo=2(6+4.2)=20.4MHz.

The signal component P passed through the standard bandpass filter. l is P(1
”” Jo2(mf) + 2 Jf2(mf) +
2 J22(mf)=0.172+0.663+0.1
50=0.985 (7). The first term is F
The center frequency (carrier) component of the M modulated wave, the second term indicates the power component of the first sideband 3.58 MHz away from the center, and the third term indicates the power component of the second sideband 7.16 MHz away from the center. .

On the other hand, the output noise power N of the regular filter. If the Zomaro power density is normalized to 1/MHz, No=Bo=20.4 (8) Pa'
0.985 Therefore, -=-(9) No 20.4.

This spectrum distribution is shown in FIG.

Next, regarding the variable bandpass door door, first assume a case where the bandwidth has already been sharply narrowed, as shown in the characteristic (3) in FIG.

At this time, the relationship between the signal power P and the noise-to-noise power NT at the output of the variable bandpass filter, and the degree of improvement η in both are as follows. (Unit omitted) i) When 14.32〈b〈20.4 ii) 7.
16<b<+ 4.321.20(η(2,41 (0,8dB) (3,8dB) m) b, <7.1, all modulation components of the FM modulated wave are removed. ,
Demodulation is not possible.

Next, a case will be explained in which a single resonant system as shown in FIG. 5, which is easy to implement, is used as a variable bandpass filter.

In Figure 5, the wave generator has a fixed inductance and a capacitance C1.
It is composed of a resistor r and a variable resistor R.

When the input signal p67Ng is large, the variable resistor R is controlled to a low resistance value near the variable resistor threshold hold, and the passband width is changed.

When the variable resistance R is set appropriately, the transfer characteristic T(f) of this single resonance system can be approximated as follows.

Figure 4 (■) shows the pass characteristics of the 3-dB band, where bo is 3 dB bandwidth.

When , the output signal power of the variable bandpass waveform generator is Qη becomes.

On the other hand, the output noise power N of the variable bandpass p-wave device becomes.

Now, bandwidth B. is 20.4 MHz, so N,
= b6tan-1""'J's'O Therefore, the degree of improvement η of C by the variable bandpass filter is as follows.

3 dB bandwidth. , if we calculate the degree of improvement, we get the 6th
It becomes a figure. In either case, an improvement in C/N can be obtained with a simple band-pass filter using a single resonance system as shown in FIG. In a normal video signal, the amplitude of the color subcarrier component is often small, and in that case, the power of the modulation component of the FM modulated wave is concentrated in the carrier wave power and the first sideband wave in Fig. 4, so the main Significant improvements are obtained with the inventive scheme. Figure 7 shows the bandwidth of the variable band pass mud wave device. 7
Modulation index m of color subcarrier when MHz
, which shows the degree of improvement η for .

By the way, as an example of other satellite systems, the maximum frequency deviation 1
0.75 MHz, maximum modulation frequency 4.2 MHz
Consider the case of

As in the previous example, we focus on the color subcarrier component and use a single-resonant system as the variable bandpass P-wave device.

In this case, the modulation index of the color subcarrier component is B o
= 2 (10,75+4.2)=29.9 MJ(
It is z. 3dB bandwidth of variable bandpass p-wave filter. When the degree of improvement η of C/H is planned for the following figure, it becomes as shown in FIG. From this, in the system of this embodiment, the C/N cannot be improved by the method using this type of variable bandpass offshore transducer, and the C/N tends to deteriorate as the band becomes narrower. . That is, the usefulness of this method changes depending on the relationship between the FM modulation index of the transmitted video signal, especially the color subcarrier component, and the transmission characteristics of the variable bandpass filter.

In other words, among the transmission signal/grammeters of the target system, the modulation index mf (color subcarrier frequency fh') by the color subcarrier, the transmission characteristic T(f) of the variable bandpass filter, and the reference band Bandwidth B of the pass filter. By increasing T (f) such that 0 nfh'<-, the C/H improvement effect is produced.

Broadcasting satellite systems (usually Kuban) as shown in the previous example
(d use), even if a simple variable bandpass p-wave device as described above is used, a great effect can be obtained.

FIG. 9 shows the state of control of the variable bandpass p-wave device 20 passband width. Curve ■ is the bandwidth B of the standard bandpass filter. Threshold point Ct determined by
When the passband is controlled in steps in the vicinity,
Curve ■ is the case where gentle control is applied as the signal level decreases.

The higher the input signal level, the higher the demodulation S/N.
Slight distortion of the demodulated waveform due to sudden changes in the passband width may affect the demodulated image quality, and in that case,
The gentle control of the variable band pass offshore wave device is effective as shown by curve ①.

The rate of transmission characteristic control of the variable bandpass transducer with respect to changes in the input signal level is based on the transmission characteristics of the variable bandpass transducer and the signal level (or c/N) from the viewpoint of the desired input signal level and demodulated image quality. It is selected appropriately depending on the characteristics of the detector.

It is also possible to use the control voltage of the AGC amplifier as this signal level detector.

As explained above, the feature of the present invention is that the desired demodulation bandwidth can be continuously varied by cascading a fixed standard band-pass mud wave device having standard band characteristics and a town-variant band-pass mud wave device. The input signal thread is determined by the bandwidth of the former standard bandpass P-wave filter, and around the 7° hold point, in the region where the input signal is large, the bandwidth of the variable bandpass filter is determined by the standard bandpass filter. is sufficiently larger than that of the input FM, and as the input signal becomes lower than the threshold point, the input FM
Specific modulation frequency component of the signal (color subcarrier component)
In a specific relationship between the modulation index and the pass characteristic of the variable band pass P wave device, the pass characteristic of the variable band pass P wave device is changed continuously or stepwise, and on the output side of the variable band pass P wave device. Improve C/N. As a result, significant improvements have been made to the G19 S7'N, and visually, the typical TV (video) signal characteristic of a one-size-fits-all in this operating area has been improved.
The deterioration of image quality due to noise and noise is greatly improved, and good image quality can be obtained.

The present invention improves the reception C/N in a specific signal region and improves the demodulated image quality with a simple configuration.

Therefore, it can be used extremely effectively for simple satellite reception i, etc., which fits TV (No. 9) at a low reception input level.

[Brief explanation of the drawing]

Figure 1 is a professional diagram showing an embodiment of the present invention, and Figure 2 is an S diagram.
/N-Human power signal diagram, Figure 3 is a characteristic diagram of the 411 zone-pass offshore transducer, Figure 4 is a diagram of wave tram distribution and band-pass characteristics,
Figure 5 is a circuit example of a variable bandpass p-wave filter, Figures 6, 7, and 8 are diagrams showing the degree of improvement, and Figure 9 is a diagram showing the control status of the variable bandpass filter. be. J...Level detector, 2...Variable bandpass offshore wave device, 3
... Discriminator, 8... Standard bandpass filter. Patent applicant Oki Electric Industry Co., Ltd. Japan Broadcasting Corporation

Claims (1)

[Claims] A signal level detector that detects the magnitude of the input signal or the carrier power-to-noise power ratio (C/N) of the input signal, and a fixed passband that determines the signal and noise bandwidth for demodulation. a standard bandpass filter having a width, a variable bandpass filter and a frequency discriminator for making the passband width variable from the fixed passband width to a narrower passband width; Bandpass F wave device. A variable bandpass filter, a disc, and a delimiter are connected in series to the transmission path of the received input signal, and the standard bandpass filter has the maximum modulation frequency fh of the TV signal, the color subcarrier frequency fy + maximum frequency deviation Δf. . Modulation index m of color subcarrier component, = Δf/f v
For an FM input signal with a bandwidth of B. ~2(Δf+f
h), the variable bandpass FFE device has a signal power transfer characteristic T (f), and a bandwidth B. The noise power N with respect to the modulated signal power Po after passing through the reference band-passed waveformer
0 (where nfl, <Bo/2, Jn(m,) is the Bessel function) and the bandwidth B at the output of the variable bandpass p-wave device. The noise power N for the normalized modulated signal power PT in
, the ratio Jo2(mf)+2Jt2(mf)T(fu)+2J2
"(mf)T(2f,) As the signal level decreases to near or below the FM threshold level, the passband width of the variable bandpass transducer is controlled according to the output of the signal level detector, An FM signal demodulation method characterized in that the transmission bandwidth of the signal transmission path is narrowed and demodulation is performed by the frequency discriminator.