JPH01199366A - System for demodulating fm - Google Patents

Abstract

PURPOSE:To improve the recording density of a video signal recorder and to improve a reproducing a signal picture quality by band-restricting a reproduc ing signal by means of a filter, thereafter, 2-multiplying it by means of a 2-multi plying circuit, further, band-restricting an FM signal to be 2-multiplied by means of the filter, next, compensating a frequency characteristic and thereafter, demodulating the signal by means of a 2-multiplying demodulator. CONSTITUTION:A reproducing FM signal is band-restricted by a BPF (band-pass filter) or an LPF (low-pass filter), next, it is 2-multiplied by the 2-multiplying circuit composed of a phase shifter and a multiplier, further, an unnecessary wave generated at the 2-multiplying circuit is removed by the BPF or an HPF (high-pass filter), next, the frequency characteristic is compensated by an equalizer, and thereafter, the signal is demodulated by a 2-multiplying demodulating circuit. Thus, a beat disturbance (moire) generated by the 2-multiplying demodulating circuit can be reduced, and by the improvement of an S/N due to the reduction of a carrier frequency and the enlargement of a deviation (frequency deviation) in a recording signal, the recording density of the video signal recorder can be improved, and the reproducing signal picture quality can be improved.

Description

[Detailed Description of the Invention] (Industrial Application Field) This invention relates to an FM signal demodulation method, and in particular to a low carrier wave PM recording method for video signals used in VTRs (video tape recorders), VDs (video discs), etc. This is related to demodulation.

(Summary of the Invention) The present invention relates to the demodulation of a video signal using a low carrier FM recording method used in a VTR, etc., in which the reproduced FM signal is band-limited using a BPF (band pass filter) or an LPF (low pass filter). Then, it is multiplied by 2 using a 2-fold multiplier circuit composed of a phase shifter and a multiplier, and then this 2-fold multiplier is
Unwanted waves generated in the multiplier circuit are removed by 8PF or +Ipp.
(high-pass filter), and then the frequency characteristics are compensated by an equalizer, and then demodulated by a 2/4' demodulation circuit.

This results in beat disturbance (
S
By improving the N ratio, the recording density of the video signal recording device and the image quality of the reproduced signal are improved.

(Prior Art) Currently, the double demodulation method is mainly used for FM demodulation in low carrier PM recording methods used in conventional VTRs and the like. A schematic diagram of a conventional example of the double demodulation method is shown in FIGS. 2 and 3.

FIG. 2 shows what is called a pulse count type FM demodulator. The input FM signal is converted into a rectangular wave by the amplitude limiting circuit 7, and then converted into a narrow pulse having both positive and negative polarities by the differentiating circuit 8. Furthermore, a pulse train of only positive or negative polarity is generated by the double-wave rectifier circuit 9. This operation is a doubling operation, as can be seen from the fact that the period of the pulse train of the same polarity is twice that of the original pulse train. This pulse train drives the monostable multi-by-break 10 to create a pulse train with a constant width and a repetition period twice that of the input FM signal.

Further, by removing twice the FM acid component using the LPF 11, a demodulated signal component is obtained.

However, the lower sideband wave of twice the FM acid component mixed into the video band is not removed by the LPF 11 and becomes a beat disturbance called moiré. Demodulated signal in case of double FM demodulation method (
The relationship between D) and moire (U, broken line) is shown in FIG. 4. In the case of a single wave input, the ratio of moire and demodulated output is approximately given by the following equation.

D βω.

UJ, (2β)・ω where β is the modulation index, ω1 is the demodulated signal frequency, and ω is the frequency of the sideband mixed into the demodulated signal band (for example, Ryo Takahashi supervised "VTR Technology" Broadcasting Technology Book 5, Japan Edited by Broadcast Publishing Association, 1st edition published October 20, 1980, 68
(see page).

In addition, in order to keep the moiré amount below 40 dB for general video signals, it is necessary to set the carrier frequency to 1.5 times or more the highest frequency of the video signal.
Recording Carrier Frequency and Video Bandwidth” 1982, IEICE University 1
96).

Figure 3 is a schematic diagram of what is called a delay line type PM demodulator, where the part that generates the doubled pulse train is delayed by a certain time τ (approximately 1 for carrier 20 M II z).
0n 5ec) The difference is that the exclusive OR is performed with the signal obtained by the above circuit, but the effect is the same.

Furthermore, a 4-fold FM demodulation method using a 906 phase shift circuit configured with a Hi network has been developed to solve the drawbacks of the 2-fold FM demodulator. FIG. 5 shows a schematic diagram of this system. In this FM demodulation method, the input FM acid component R
The signal is inputted to a 90@phase shifter 16 configured with a C network, where two outputs with phase shifts of 90° different from each other in a fixed frequency range are obtained. These FM
After each acid component is amplitude controlled by amplitude limiting circuits 17 and 19, it is subjected to double FM demodulation by double FM demodulators 18 and 20. If the demodulated outputs of these two systems are logically added in the OR circuit 21, a quadrupled demodulated output is equivalently obtained (Yamamitsu et al., ``1-inch High Definition VTRJ National Technic
al Rep,, Vo131. No, 6. De
c.

1985).

(Problem to be Solved by the Invention) When the reproduced FM signal of a VTR is demodulated using the double FM demodulation method, the original FM acid component disappears, but a sideband wave of twice the f'M component is mixed into the demodulated video signal. This results in a beat disturbance called Moiré. In order to suppress the amount of moiré to less than -40 dB for general video signals, the FM carrier frequency must be set at least 1.5 times the upper limit of the video signal band, and the disadvantage is that the deviation cannot be increased. was there. Therefore, in FM recording systems such as VTRs that use the double demodulation method, the setting of FM allocation (frequency allocation of carrier waves) is restricted due to moire generated from the demodulation mechanism.

In addition, in the quadruple demodulation method using a 90° phase shift circuit, which was developed to solve the drawbacks of the conventional double demodulation method, the band of the 90" phase shift circuit used is 1 day guard, e.g.
It has the disadvantage of being narrow at 10MHz.

Furthermore, two demodulators with the same characteristics are required, and adjustment is complicated.

Therefore, an object of the present invention is to eliminate the above-mentioned drawbacks, and to provide an FM system that has a relatively simple configuration, does not cause beat interference due to moiré, can reduce the FM carrier frequency compared to the conventional one, and can expand the frequency deviation.
However, we do not intend to provide a demodulation method.

(Means for Solving the Problems) That is, the FM demodulation method of the present invention demodulates a low carrier FM modulation signal recorded on an information recording carrier, and after band-limiting the reproduced signal with a filter, n= 0.
1, 2, ..., the signal is multiplied by a doubling circuit using a phase shifter and a multiplier that gives a phase delay of nπ/2 regardless of the frequency, and then the doubled FM signal is multiplied by two. The FM signal multiplied by two weeks is band-limited by a filter so as to remove unnecessary components caused by calculation errors, and then the frequency characteristics are compensated by an equalizer, and then demodulated by a double demodulator. It is something.

(Examples) The present invention will be described in detail below by way of examples with reference to the accompanying drawings. FIG. 1 shows a schematic block diagram of an FM demodulation circuit for explaining the operation of the system of the present invention in principle. In FIG. 1, the input FM signal is subjected to, for example, BPF 1, in which unnecessary signals outside the transmission band and noise components outside the band are removed. Next, the signal is divided into two parts, one of which remains unchanged, and the other by phase shifter 2, which changes the phase of the signal by nπ/2 (n=o,
Only the phase is changed by 1, 2, 3 degrees, . . .) and is input to the multiplier 3. The output of the multiplier 3 is a signal obtained by doubling the input FM signal and unnecessary components generated due to calculation errors and the like. A BPF 4 removes this unnecessary component, and an equalizer 5 compensates for the frequency characteristics. Therefore, only the doubled FM signal is input to the doubled FM demodulator 6. As a result, the demodulated component and the FM acid component multiplied by 4 are output at the demodulator frequency.

Here, if there are few unnecessary components in the reproduced signal, BPF1 is LPF, and if there are few unnecessary components in the doubled output, BPF is
4 is replaced by HPF. In addition, the double demodulator is a pulse counter type FM demodulator, a delay line type FM demodulator (
(see FIGS. 2 and 3), etc. can be used. Further, the equalizer does not need to be used when compensation of frequency characteristics is flat.

Due to moiré caused by the conventional double FM demodulation system, there were limitations on carrier frequency and deviation settings. The FM carrier frequency is 1 for the highest frequency of the video band.
．． The deviation is set to 5 times or more, and the deviation is set to a value that keeps the amount of moiré below a certain value. Further, the quadrupling demodulation method using a 90'' phase shift circuit configured with an RC network has a problem in that the usable band is narrow and the circuit is complicated.

The output spectrum of the 4x FMIF modulator in the method of the present invention is as shown by the solid line in Figure 4, and it can be seen that moire is significantly reduced compared to the output spectrum of the 2x FM demodulator as shown by the dotted line in Figure 4. The ratio between the demodulated output (D) and the moiré (U) in the quadrupling demodulation method is approximately calculated by the following equation.

D 2βω.

UJ, (4β)・ωU (Refer to Japanese Patent Application No. 62-88899) Demodulation output of 4x PM demodulation system and 2x FM demodulation system (
FIG. 6 shows the calculation results of the relationship between D) and moiré (U).

As described above, the amount of moiré is significantly improved in the quadruple demodulation method (solid line) of the present invention compared to the conventional double demodulation method (broken line). Therefore, since the moiré generated from the demodulation mechanism is not a constraint in setting the FM allocation, it is possible to reduce the FM carrier frequency and expand the deviation.

Therefore, it has become possible to improve the playback image quality by improving the playback S/N and reducing moire.

In addition, compared to the conventional quadruple demodulation method that uses a 90° phase shift circuit using an RC network, the usable band is O~50.
It has the advantage of having a wide band of around MHz (the upper limit is determined by the band of the double demodulator) and a simple circuit.

(Effects of the Invention) As described above, if the FM demodulation method of the present invention is used, beat disturbance (moiré) generated by the double demodulation mechanism can be reduced with a relatively simple configuration, and the carrier wave frequency in the recorded signal can be reduced. By increasing the S/N ratio by increasing the frequency shift, it is possible to improve the recording density of the video signal recording device and the image quality of the reproduced signal.

[Brief explanation of the drawing]

FIG. 1 shows a schematic block diagram of the basic configuration of the method of the present invention, FIGS. 2 and 3 show block diagrams of FM demodulators of pulse count method and delay line method, respectively, and FIG.
Figure 5 shows the output spectrum of the M demodulator.
A block diagram of a quadrupling FM demodulator using a 90@phase shift circuit configured with a C network is shown, and FIG. 6 shows the amount of moire interference due to the demodulation mechanism. 1.4...BPP 2...Phase shifter 3.
... Multiplier 5 ... Equalizer 6, 18
．． 20...2x FM demodulator 7, 12.17.19.
...Amplitude limiting circuit 8...Differentiating circuit 9...
・Double-wave rectifier circuit 10... Monostable multi-bi break 11, 15.22... LPF 13... Delay circuit 14... Exclusive OR circuit 16...90
``Phase shift circuit 21...OR circuit

Claims (1)

[Claims] 1. In an FM demodulation method for demodulating a low carrier FM modulation signal recorded on an information recording carrier, after band-limiting the reproduced signal with a filter, n=0, 1, 2,... A doubling circuit that uses a phase shifter and a multiplier that provides a phase delay of nπ/2 regardless of the frequency is used to double the FM signal, and then the unnecessary components generated due to the doubling calculation error are extracted from the doubled FM signal. An FM demodulation system characterized in that the frequency characteristic of the doubled FM signal is limited by a filter so as to be removed by a filter, the frequency characteristics are compensated for by an equalizer, and then demodulated by a double demodulator.