CS275825B6 - Circuit for video signal noise rejection - Google Patents

Abstract

An FM demodulator for a video signal reproducing apparatus, which is responsive to an input FM signal includes an FM demodulation circuit 11 for changing the input FM signal to a baseband video signal and a noise removing circuit 13, 14 for substantially removing noise from the variable amplitude video signal. The noise removing circuit 13, 14 is comprised of a clipping circuit 13 for removing portions of the video signal carrying the noise and an amplitude expansion circuit 14 for increasing the amplitude of the video signal at the location of the removed portions to a predetermined level. Alternatively, the level expansion circuit may precede the clipping circuit (Figure 6). <IMAGE>

Description

The invention relates to a circuit for suppressing the noise of a video signal. for example, a luminance signal of a reproduction video device whose input terminal of a frequency demodulated signal is connected to an input of a low pass filter, the output of which is connected to the input of a deemphase circuit, the output of which is connected to the input of the noise suppression circuit.

In the past, many improvements have been made in the art of video signal reproducing devices, such as television receivers or video tape recorders, to increase the number of images on the screen of such devices. Oak is well known, sharpness and signal / noise ratio are particularly important factors for enhancing image quality.

The sharpness of the image is verified by the frequency characteristics, i.e. the response of the reproduction circuits in the television or video tape recorders to the shape of the signal. For example, if said characteristics are insufficient for the leading and closing edges of the pulse signals, then the resulting image on the screen has poor sharpness. Oak is known, the luminance signal is contained in a band of frequency modulated composite video signals along with other signals, such as a color signal. The response to the signal shape of the reproduction video circuit is determined by the transmission characteristic of the circuit. Thus, in order to obtain a good response to the shape of the signal, it is necessary to extend the frequency band of the reproduction video circuit transmission. In particular, it is desired to extend this band to as high a frequency as possible.

Many attempts have been made to improve the impulse transmission characteristics of reproduction video circuits. However, it was difficult to further improve the pulse transmission characteristics because the transmission frequency band of the reproduction video circuits was expanded to a relatively wide range as a result of previous advances in circuit design. In particular, improving the quality of videotapes by improving pulse transmission characteristics has become difficult. This is due to the fact that the transmission frequency band of reproducing video circuits in video cassettes is limited to a smaller width than the range of such circuits in television receivers.

Accordingly, attempts to improve image quality have been made in terms of the signal / noise ratio of the screen on the screen. However, an increase in the signal-to-noise ratio is accompanied by a narrowing of the transmission frequency band, i.e., a deterioration of the pulse transmission characteristic for image reproduction. For example, if we try to improve the signal-to-noise ratio, especially in video tape recorders, the pulse transmission characteristics deteriorate such that considerable noise occurs at the leading or reverse edges of the pulse signals, for example luminance signals in the frequency modulated composite video signal band. Thus, it is important to increase the signal-to-noise ratio while keeping the pulse transmission characteristics at a prescribed level.

The following three methods are known for increasing the signal / noise ratio of VCR images:

- increasing the preemphase height gain in the recording device, i.e. before the reproduction of the image;

- increasing the noise suppression of the noise suppression circuit in the reproducing video device;

- increasing the component of the relatively high noise / noise ratio in the frequency modulated signal, in other words, the low frequency signal component that is lower than the carrier signal, to increase the signal / noise ratio of the baseband signal after demodulating the frequency modulated signal,

If we try to increase the high frequency enhancement rate according to the first method, some frequency components of the signal pass when the white level and black level are cut off, so that the pulse transmission characteristic deteriorates.

In the second method, the noise suppression circuit excludes the high frequency component from the video signal, reverses the phase of the excluded signals after limiting the amplitude of the high frequency component by the limiter, and then adds the excluded signals to the original video signal. Thus, low-level and high-frequency noise is suppressed in the original video signal. If we attempt to increase the suppression rate, the signal-to-noise ratio will improve on the flat portions of the signal. However, noise is not eliminated in steep portions of the signal where the signal varies by a high amplitude jump and thus with a high frequency component. This can happen, for example, in the part where the signal changes from black to white. In addition, the modulation of such noise increases. Thus, the impulse transmission characteristics will deteriorate and noise in the steep portion of the signal will become more pronounced.

In the third method, when the component of the low frequency signal, which is lower than the carrier signal, increases, the image signal inversion between the black level and the white level becomes much easier, while at the same time deteriorating the image quality in the part where the black level changes to white. More specifically, the portion of the signal when it changes from a black level to a white level is that portion where the carrier signal of the frequency modulated signal moves at the highest frequency. As a result, the carrier / noise ratio of the signal deteriorates in the variable portion of the signal.

Therefore, in a third method that does not use a low carrier / noise component, such as the above-mentioned methods, although the signal / noise ratio is improved in the flat portion of the signal, the variable portion of the signal deteriorates where the signal changes from black to white. level. Seated from the causes of signal deterioration is the fact that the carrier frequency, which is equivalent to the frequency in the variable part of the signal. It is located at the upper end of the frequency modulated signal transmission band. Therefore, the use of low carrier / noise components should be avoided. That is, in the third method, both the amplitude and phase of the frequency modulated signals Jevi acrone to distort in the transmission path. As a result, the variable portion of the signal deteriorates from black to white, so that the noise in this variable portion of the signal becomes more pronounced.

□ as explained above, if we try to improve the signal-to-noise ratio of the luminous signal of the previous videotape recorders, the impulse transmission characteristics will deteriorate and the noise in the steep portion of the signal will increase substantially. The signal / noise ratio can thus be set at a compromise value. As a result, existing videotapes have the drawback that the signal / noise ratio deteriorates in the part of the signal where the change from black to white occurs.

These drawbacks eliminate the wiring for suppressing the sum of the video signal, the input of which the frequency-deactivated signal terminal is connected to the input of the low-pass filter, the output of which is connected to the input of the deemphase circuit. The low-pass filter output is connected to the deemphase circuit input through the video peak restoration circuit and through the clipping circuit of noise-containing video peaks.

The effect of the circuit according to the invention is that the noise generated at the leading edge of the luminance signal is removed by trimming in the white-level trimming circuit and a portion removed by trimming is compensated by the extension in the refresh circuit. Therefore, by the wiring according to the invention, the signal / noise ratio can be improved without deteriorating the transmission characteristics, and thus a high quality noise-free image can be obtained in the area where the luminance signal changes from black to white.

The invention is illustrated in the drawing, wherein Fig. 1 is a block circuit according to the invention, Fig. 2 shows a diagram of the shape of signals processed according to the invention, Fig. 3 shows a principle circuit according to the invention, and Fig.

In Fig. 1, the input terminal S1 of the frequency modulated signal is connected to the input of the frequency demodulation circuit 11. The output of the demodulation circuit 11 is connected via a low-pass filter 12 to the input of a level extension restoration 14 whose output S4 is connected to a white level trimming circuit 13. whose output is connected to the input S3 of the deemphase circuit.

The waveform diagrams of the wiring of Fig. 1 are shown in Fig. 2. The luminance signal S2, as shown in Fig. (A) in Fig. 2, is first extended at the amplitude level in the amplification level recovery circuit 14 &apos; 2. The extension signal N at the leading edge of the luminance signal S2 is also extended, as shown in Figure (b) in Figure 2.

Giant. Fig. 3 illustrates the embodiment of a renewal circuit 14 for extending the level and a white level trimming circuit 13 of Fig. 1. 3, a recovery circuit 14 for expansion

It consists of three PNP transistors £1, 22 (33, ,4) / diode 02 and inductance 121 and capacitor C24. The fourth ĚNP transistor 54 forms together with diode 03 a trimming circuit 13 white.

EN 275825 86

-r levels. In the recovery circuit 14, the first transistor Q1 forms. the input isolation amplifier BA3 together with the base surge resistance R22 and the eraitor load resistor R23. The base of the first transistor Q1 is connected to the ground terminal G via a base bias resistor R22. The collector of the first transistor Q1 is connected directly to the ground terminal G. The emitter of the first transistor Q1 is connected to the PS terminal of the voltage source Vcc. Further, the emitter of the first transistor Q1 is connected via a series coupling of the coupling capacitors C21 and the resistor R24 connected to the base of the second transistor Q2.

The second transistor Q2 and the third transistor Q3 form a high gain, non-inverse-type HGA amplifier with feedback resistor R24, emitter resistors R25 and R26, collector load resistor R26, base bias resistors R29 and R30 and capacitor C22. The emitters of the second transistor Q2 and the third transistor Q3 are coupled to each other via a resistor R27 and connected to the emitter resistors R25 and R26 and to the PS terminal of the power supply. The collector of the second transistor Q2 is connected directly to the ground terminal G. The collector of the third transistor Q3 is connected to the ground terminal G via the collector load resistor R28. The base of the third transistor Q3 is connected to the ground terminal G via parallel resistor R30. the base of the transistor Q3 is coupled via a feedback resistor R24 to the base of the second transistor Q2. The collector of the third transistor Q3 is connected via the coupling capacitor C23 to the base 8 of the fourth transistor Q4. The base of the fourth transistor g4 is coupled to the ground terminal G via a base bias resistor R32. Further, the base of the fourth transistor Q4 is coupled via a further bias resistor R31 to the terminal PS of the voltage source. Emitter of fourth transistor Q4 is connected via 9 přes ground terminal G through resistor R34. of the capacitor C24 forms a centering circuit for the leading edge of the luminance signal, as will be explained below. Furthermore, the anode of the diode D2 is connected via a resistor R35 and its cathode. The collector of the fourth transistor Q4 is connected to the voltage source PS terminal via the collector load resistor R33. Further, the collector of the fourth transistor Q4 is coupled to the base of the third transistor Q3 in the refresh circuit 14 via a signal feedback diode D3. The collector of the fourth transistor Q4 is further coupled to the OUT terminal of the circuit.

3. The baseband timing signal S2 extending from the low-pass filter 12 is applied to a high gain, non-inverse-type HGA amplifier via the input isolation amplifier BA3 and the coupling capacitor C21. The polarity of the Brightness signal S2 at terminal P21 between coupling capacitor C21 and the second transistor Q2 is negative, as shown in graph (a) in Fig. 4. In the HGA amplifier, the amplified Brightness signal S2 * seen in graph (b) in Fig. 4 at terminal P22 between coupling capacitor C23 and the base of fourth transistor 84. Amplified Brightness Signal S2 * Is applied to the fourth transistor Q4. The diode 02 becomes conductive when a high amplitude level of the leading edge of the luminance signal 52 'is applied thereto. Thus, the leading edge of the luminance signal S2 ' is effected by a series connection of the inductance L21 and the capacitors C24, i.e. the series resonant circuit PE. When the resonant frequency of the series resonant circuit PE is set to about 1 MHz, which is the most central component of the luminance leading edge S2 ', the amplitude level of the luminance leading edge S2' is extended. As a result, the signal S4 shown in graph (c) in Fig. 4 is obtained. The polarity of the expanded luminance signal S4 is granular on the collector of the fourth transistor Q4, as shown in graph (c) in Fig. 4. of the signal S2 ', the collector potential of the fourth transistor Q4 is as close as possible to the voltage Vcc of the source. Thus, the leading edge of the luminance signal S4 is cut off at a prescribed level close to the Vcc source voltage. As a result, the output signal S3 shown in graph (d) in FIG. 4 is obtained at the output terminal OUT. Oak can be seen from the course of the cut-off Brightness signal S3 shown by the graph (d) in Fig. 4. Noise is removed; the signal N appearing at the top of the leading edge of the expanded luminance signal S4 and the leading edge of the trimmed luminance signal S3 has a sufficient amplitude p-grow noise N.

It will be apparent to one of ordinary skill in the art that various changes and variations may be made and equivalent elements may be made within the disclosed embodiment, as an example of an embodiment of the invention, without departing from the spirit of the invention.

Claims (1)

A video noise suppression circuit having an input of a frequency demodulated signal coupled to an input of a low-pass filter, the output of which is connected to an input of a deemphase circuit, the output of which is connected to the input of a noise suppression circuit by subtraction. a filter (12) is connected to the input (S3) of the deemphase circuit through the video peaks restoration circuit (14) and through the trimming circuit (13) of the noise containing video peaks.