JPS62242480A - Fm demodulating device - Google Patents

Abstract

PURPOSE:To compensate signal lack due to the clipping processing of a clipping means by extending the amplitude level of a changing part from the black level to the white level of a demodulated output. CONSTITUTION:A luminance signal S1 is demodulated by an FM demodulating circuit 11. A noise N is mounted on a leading part from the black level to the white level of an output signal S2 of an LPF 12. The noise is removed by passing the signal S2 through a white clipping circuit 13. The amplitude level of the leading part of a signal S3 is extended by an extending circuit 14. Consequently, a signal component lacked by the clipping processing can be compensated. Thereby, a luminance signal S5 having excellent waveform characteristics and removing the noise N can be obtained by the deemphasis processing or the like of the signal S4.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) This invention is applicable to, for example, a video tape recorder (hereinafter referred to as V
The present invention relates to an FM demodulation device suitable for FM demodulating a reproduced luminance signal in the TR.

(Prior Art) In recent years, the image quality of home-use VTRs has been increasing, and it has become desirable to have a good S/N ratio and so-called clear images.

Here, a clear image means that the waveform characteristics are good, that is, the rising and falling characteristics of the waveform are faithful to the original signal.

In order to obtain good waveform characteristics, it is necessary to widen the signal transmission band. However, in home VTRs, the transmission band has to be narrowed. Therefore, it is difficult to improve the above-mentioned waveform characteristics in a home VTR.

From the above, in order to achieve high image quality, it is important how to improve the S/N ratio while maintaining the current waveform characteristics. However, when trying to improve the S/N ratio in home VTRs, the waveform characteristics deteriorate and edge noise visually increases, making it difficult to achieve both. In other words, home VTR
As a method for improving the S/N ratio, there are, for example, the following methods.

■ Increase emphasis I in the emphasis circuit.

■ Increase the amount of cancellation in the noise canceller circuit during playback.

■ In particular, in order to improve the baseband S/N ratio after FM demodulation, components with a good C/N ratio (carrier-to-noise ratio), that is, lower sideband components lower than the carrier, are Increase usage rate.

However, if the emphasis is increased using the method (2), more signal components will be lost during the white clip and dark clip processing during recording, and the waveform characteristics will deteriorate. In addition, in method (■), the noise canceller circuit extracts high frequency components from the video signal, limits the amplitude of this with a limiter, inverts the phase, and adds it to the original video signal to cancel out the noise, which is a low level high frequency component. Therefore, increasing the canceling salt improves the S/N ratio in flat parts of the waveform, but it also improves the S/N ratio in flat parts of the waveform, but in parts where there is a sudden change in signal level that becomes a high-level high frequency component, for example, in black. In the transition portion from the level to the white level, noise is not removed and remains for a long period of time, resulting in poor waveform characteristics and noise in the transition portion becoming noticeable. Roughly,
Increasing the utilization rate of the lower sideband component, which is method (2), makes it more likely that inversion will occur, and at the same time, the change from black level to white level will deteriorate. That is, the transition part from the black level to the white level is where the carrier in the FM band goes to the highest frequency. Therefore, in this part, C/
The N ratio is the worst. Therefore, as described above, in a method that does not use components with a poor C/N ratio, the S/N ratio in the flat part of the waveform is improved, but the transition part from the black level to the white level is deteriorated. Note that this is also due to the fact that the carrier frequency corresponding to the changing part is located at the full edge of the FM transmission band in order to avoid using components with a poor C/N ratio.

That is, if this is done, the amplitude and phase characteristics in the transmission path tend to be distorted, and as a result, the change characteristics from the black level to the white level deteriorate, and the noise at this change portion becomes noticeable.

As mentioned above, in conventional home VTRs, when trying to improve the S/N ratio of the luminance signal, the waveform characteristics deteriorate and the noise in the changing parts visually increases, so the S/N ratio increases.
The N ratio could only be set as a compromise between the two. As a result, there was a problem that the S/N ratio was poor at the transition portion from the black level to the white level.

FIG. 9 shows this situation. FIG. 4(a) shows the luminance signal waveform immediately after being returned to the baseband by demodulation, and FIG. 2(b) is an enlarged view of the portion A.

It can be seen from FIG. 6B that there is a lot of noise at the leading edge of the rising portion (the transition portion from the black level to the white level). By passing this signal through a de-emphasis circuit, a noise canceller circuit, etc., a waveform as shown in FIG. 4(C) is obtained. The figure (d) is an enlarged view of the part A. As is clear from this figure (d), the noise at the rising edge cannot be removed by the de-emphasis circuit or the noise canceller circuit. As a result, the boundary between light and darkness is affected by noise, leading to a reduction in image quality.

(Problems to be Solved by the Invention) As stated above, in conventional home VTRs, the S
If the S/N ratio is improved, the waveform characteristics deteriorate and the noise at the transition portion from the black level to the white level increases, so there is a problem that the S/N ratio can only be set at a compromise between the two.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an FMI adjustment device that can remove noise present at the transition portion from black level to white level without deteriorating the waveform characteristics, for example, in a home VTR. do.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention provides means for demodulating a luminance signal converted into an FM signal, as well as a means for demodulating a demodulated output to a predetermined level on the white level side. In order to compensate for signal loss caused by the clipping process, the apparatus is configured to include means for extending the amplitude level of the transition portion from the black level to the white level.

(Function) In the above configuration, the noise □ present in the transition portion from the black level to the white level can be removed by the clipping process. The decompression process can compensate for signal loss at the changing section due to the clipping process. Therefore, according to the present invention, it is possible to satisfy both the S/N ratio and the waveform characteristics in the changing section.

(Implementation VA) Hereinafter, a solid line example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing the configuration of an embodiment of the present invention.

In FIG. 1, the luminance signal S1 converted into an FM signal is F
The M demodulation circuit 11 demodulates the signal. A baseband luminance signal S2 is obtained from this demodulated output by a low pass filter (hereinafter referred to as LPF) 12. This signal S2 is applied to a white clipping circuit 13, where it is clipped using a predetermined level on the white level side as a threshold. This clip output S3 is given to the expansion circuit 14. This expansion circuit 14 responds only to the rising edge of the signal S3 and expands the amplitude level of this portion. This expanded output S4 is given to a de-emphasis circuit and a noise canceller circuit (not shown), and is subjected to predetermined processing.

The operation of the above configuration will be explained with reference to FIG. FIG. 2 shows the portion A of the ninth factor in the luminance signal. Furthermore, in this case, the brightness signal is of positive polarity.

The output signal S2 of the LPF 12 includes noise N at the rising edge from the black level to the white level, as shown in FIG. 2(a). However, this noise N is removed by passing the signal S2 through the white clip circuit 13, as shown in FIG. 2(b).

The amplitude level of the rising portion of the signal S3 from which the noise N has been removed is expanded by the expansion circuit 14. As a result, the signal components lost due to the clipping process are transferred to the second
Compensation is made as shown in Figure (C). Therefore, by subjecting this signal S4 to de-emphasis processing or the like, a luminance signal S5 without noise N and having excellent waveform characteristics can be obtained, as shown in FIG. 2(d).

FIG. 3 is a circuit diagram showing the specific configuration of FIG. 1, centering on the white clip circuit 13 and expansion circuit 14. In addition, in the third, Qi is a transistor, R1 is a resistor, and C
1 is a capacitor, Ll is a coil (i=1.2...
・). This also applies to FIG. 7, which will be described later.

In the figure, the baseband luminance signal S2 is connected to the buffer transistor Q1. The signal is input to an inverting operational amplifier consisting of transistors Q2 and C3 via a coupling capacitor C1. Now, the polarity of the signal S2 at the terminal P1 is shown in Fig. 4(a).
As shown in , it is negative.

The output of the above operational amplifier is capacitor C3 and resistor R13.
It is grounded by. Therefore, terminal P
The polarity at 2 is positive as shown in FIG. 4(b).

As a result, at the tip of the signal S3 on the white level side, the potential of the emitter of the transistor Q4 serving as a buffer approaches the power supply voltage V ccl, :rl. As a result, the current for driving loads such as capacitor C3 and resistor R13 becomes minimum, making it impossible to drive them.

As a result, a signal S3 whose white level side is clipped is obtained at the terminal P2. At this time, since the signal S3 is fed back via the resistor R5, it works to compensate for this clipping. This compensation operation causes the clipped waveform to rise slightly.

Furthermore, the output of transistor Q4 is provided to a grounded base amplifier consisting of transistor Q5.

At this time, in the portion where the signal S3 rises from the black level to the white level, the diode D1 is turned on, and peaking is performed by the coil L1 and the capacitor C4. This peaking frequency is the most central component of the rising part, 1M-
If the setting is set to around 100, it is possible to obtain a signal $4 in which signal loss due to clipping is almost compensated for, as shown in FIG. 4(C).

Note that the gains of the transistors 02 to 04 are set to a high gain so that the diode D1 is turned on at the rising edge of the brightness signal S2. The coupling capacitor C1 is an FM carrier during recording.

It has a function of preventing deviations in clipping characteristics due to deviations in frequency shift and variations in the output potential of the F M (I tone circuit 11).

As described in detail above, in this embodiment, the noise at the rising edge is removed by the clipping process, and the signal component missing due to the clipping process is compensated for by the expansion process. Therefore, in this embodiment, the S/N ratio can be improved without deteriorating the waveform characteristics. As a result, the image at the transition portion from the black level to the white level is not affected by noise, and it is possible to obtain the best image quality.

Incidentally, in the previous embodiment, a case has been described in which the decompression process for compensating for signal loss due to the clip process is performed after the clip process, but it goes without saying that the decompression process may be performed before the clip process. FIG. 5 shows this embodiment. According to FIG. 5, the order of the white clip circuit 13 and the expansion circuit 14 is switched. As a result, the brightness signal S2 has its rising portion expanded and then clipped at a high level. The waveforms of each of the signals 82 to S5 in this case are shown in FIG. Also, LPFl 2° white clip circuit 13. An example of a specific configuration of the expansion circuit 14 is shown in the seventh section.
In the figure, the waveforms of each part are shown in FIG.

In FIG. 7, transistors Q2 and Q3 are.

Configure a high gain non-inverting amplifier. Furthermore, an extension circuit 14 is configured at the emitter of the transistor Q4 by a diode DI, a resistor R15, a coil Ll, and a capacitor C4. This expansion circuit 14 converts the brightness signal S2 (see Figure 8 (1)) (negative polarity) amplified output 82' (see Figure 8 (C)) applied to the base of the transistor Q4 from the black level to the white level. Figure 8 (
An expanded output S4 as shown in C) is obtained. Then, by clipping this with diode D2, as shown in FIG.
), the noise N at the falling edge can be removed.

Although two embodiments of this invention have been described above, it goes without saying that this invention can be modified in various other ways without departing from the gist of the invention.

[Effects of the Invention] As described above, according to the present invention, it is possible to provide an FMtll device capable of removing noise in the transition portion from black level to white level without deteriorating waveform characteristics. can.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing the configuration of an embodiment of the present invention, and FIG.
The figure is a signal waveform diagram 1. for explaining the operation of Figure 1. Third
1 is a circuit diagram showing an example of the specific configuration of FIG. 1, FIG. 4 is a signal waveform diagram for explaining the operation of FIG. 3, and FIG. 5 is a circuit diagram showing a configuration of another embodiment of the present invention. Figure 6 is the 5th
7 is a circuit diagram showing an example of the specific configuration of FIG. 5. FIG. 8 is a signal waveform diagram for explaining the operation of FIG. 7. The figure is a signal waveform diagram for explaining the conventional problem. 11...FM demodulation circuit, 12...PF, 13...
- White clip circuit, 14... extension circuit. (a) (c) Article 9 (b) (d)

Claims (1)

[Claims] Demodulating means for demodulating a luminance signal converted into an FM signal; clipping means for clipping the demodulated output of the demodulating means at a predetermined level on the white level side; 1. An FM demodulator comprising: expansion means for expanding the amplitude level of the transition portion of the demodulated output from the black level to the white level in order to compensate.