Abstract:
Disclosed is an automatic noise removal device which detects an amplitude of noise contained in a high frequency component of a picture signal during a flyback period of the picture signal, produces a threshold level depending on the detected signal, and removes from a video signal a high frequency component of the picture signal contained in a picture signal at a lower level than the threshold level.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an automatic noise removal device for removal of high frequency components from a picture signal only when the S/N ratio is bad in a television receiver. 
     A noise component contained in a television signal generally exists in high frequencies of the television signal. Therefore, a mere removal of the noise may be attained by attenuating the high frequencies of the television signal. 
     The attenuation of the high frequencies, however, brings about a loss of the information representing the contour of the picture being reproduced. Therefore, the resolution of the reproduced picture is deteriorated, resulting in a significant reduction in the picture quality. For this reason, it is required that the high frequencies with higher amplitudes than that of the noise be left in the picture signal to the extent possible. 
     A technique to satisfy this requirement is disclosed in U.S. Pat. No. 4,031,547. The prior art technique adjusts the amount of attenuation of the high frequencies in accordance with the level of the picture signal, thereby to successfully remove only the noise with relatively small deterioration of the resolution. In a picture reproduced by a television receiver, a deterioration of the resolution in a dark portion of the reproduced picture contributes little to a deterioration of the picture quality, but this portion of the picture is greatly influenced by noise. Based on this fact, when the picture is dark, that is, when the level of the picture signal is lower as seen from a black level, the high frequencies are attenuated relatively greatly. When the picture becomes bright, that is, when the level of the picture signal becomes high, the high frequencies are transmitted and not attenuated. In this way, only the noise component may be effectively removed while deteriorating the picture quality only to a relatively small degree. 
     According to this method, however, the high frequency component is always lost in the dark portion. Therefore, even in a noise-free condition with a good S/N ratio, the resolution must always be reduced in some degree. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide an automatic noise removal device for a video signal which ensures an effective gain of the high frequency component in a good S/N condition so as to have a satisfactory resolution even in a dark picture, but which operates so as to reproduce a picture with undistinguishable noise in a poor S/N condition. 
     In accordance with the present invention, a high frequency noise component of a television picture signal is detected during a retrace period. The threshold level used for removal of the high frequency component in a noise removal circuit is controlled in accordance with the amplitude of the detected signal so that a high frequency noise component which is smaller than a certain amplitude does not appear in the picture. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The single FIGURE is a circuit diagram of an embodiment of an automatic noise removal device for a video signal according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of an automatic noise removal device for a picture signal according to the present invention will be described with reference to the accompanying drawing. In the FIGURE, reference numeral 1 designates a picture signal input line, 2 to 4 transistors, 5 to 9 resistors, 10 to 12 inductors, 13, 14 capacitors, 15, 16 diodes, and 17 a picture signal output line. 
     In the circuit, the transistor 2, the resistors 5 to 7 and inductors 10 and 11 make up a well known secondary derivative high frequency compensating circuit which emphasizes the high frequency component of a picture signal supplied from the input line 1 and supplies it to the output line 17, in order to obtain a sharp picture. A frequency characteristic G(ω) of a transmission system ranging from the input line 1 to the output line 17 in the circuit is represented as follows: ##EQU1## where ω: 2π f 
     L 10 , L 11  : Inductances of the inductors 10 and 11 
     R 5  to R 7  : Resistances of the resistors 5 to 7. 
     Because of the presence of the second term in the numerator of the above equation, the circuit is called a second derivative circuit. 
     The transistor 2 serves as a common collector type amplifier for the low frequency component of the video signal, while serves as a common emitter type amplifier for the high frequency component thereof. The inductor 11, and the resistors 6 and 7 make up a low pass filter, through which the low frequency component is transmitted to the output line 17 from the emitter of transistor 2. The inductor 10 serves as a low impedance element for the low frequency components. Accordingly, the high frequency component only appears at the collector of transistor 2 to allow the high frequency component to pass through the resistor 6 to the output line 17. A DC voltage source B+ biases the diodes 15 and 16, so that when the diode 15 or 16 is cut off, the second derivative circuit operates as a high frequency emphasis circuit to emphasize the high frequency component of the picture signal for clearly delineating a reproduced image to improve the resolution of the image and applies it to the output line 17. 
     A circuit enclosed by a one-dot-chain line controls the operation of the high frequency emphasis circuit in accordance with the S/N of the received picture signal. In the circuitry, the transistor 3 is provided for switching, and for this purpose the emitter is supplied with a horizontal sync pulse or a synchronizing signal P to turn on the transistor 3 during a part of or the entire horizontal retrace. The collector of the transistor 3 is connected to a parallel resonance circuit including an inductor 12, a capacitor 13, and a damping resistor 8, to provide a filter circuit responsive mainly to a signal of 1 to 2 MHz. 
     Accordingly, a signal of approximately 1 to 2 MHz will appear at the collector of the transistor 2 during the horizontal retrace period of the picture signal, when this signal appears at the collector of the transistor 3. Since the high frequency component of 1 to 2 MHz appearing during the retrace period can be considered as only a noise component, only the noise component contained in the picture signal supplied to the input line 1 is detected at the collector of the transistor 3, and is supplied to the base of the transistor 4. The nominal picture signal amplitude supplied to the input line 1 is kept substantially constant by an AGC circuit of the conventional type in the receiver. Therefore, the amplitude of the noise appearing at the collector of the transistor 3 is proportional to a reciprocal of the S/N ratio of the video signal, and therefore, can be regarded as a measure thereof. Therefore, a signal relating to the S/N ratio is detected by the transistor 3. 
     The transistor 4 is provided for noise removal control, and for this purpose amplifies the noise component inputted to the base thereof and filter this signal to smooth it by a combination of the capacitor 14 and the resistor 9 to control the average conduction current flowing through the diodes 15 and 16. Accordingly, during a total period including a picture display interval, the average conduction currents flowing through the diodes 15 and 16 change in direct proportion to the amplitude of the noise appearing at the collector of the transistor 3 during the retrace interval. Those conduction currents are substantially zero in a low noise reception condition, while they increase as the noise level increases. 
     Since a series circuit including the diodes 15 and 16 is connected in parallel with the inductor 10, when these diodes conduct, the inductor 10 is shorted. At this time, the second derivative term in the equation for G(ω) becomes zero, so that no high frequency compensation is carried out and the noise appearing at the output line 17 is removed. 
     As a result, according to the embodiment of the present invention, the average conduction currents of the diodes 15 and 16 are controlled by the S/N ratio of the received picture signal. Accordingly, when the S/N ratio is high, the average conduction currents of the diodes 15 and 16 become substantially zero. The diodes 15 and 16 are left cut off at all levels of the picture signal supplied from the input line 1 under this condition, so that the picture signal of which the high frequency component is emphasized over a range from dark to bright in the image, appears at the output line 17. As the S/N ratio reduces, the average conduction currents flowing through the diodes 15 and 16 increase in amplitude. 
     More specifically, under a certain S/N condition, the current flowing through the resistor 9 is kept at a substantially fixed DC value and flows substantially equally through the diodes 15 and 16, which forms a diode bias current. Under this condition, when the high frequency component current starts to flow through the collector of the transistor 2, the current of the diode 16 is decreased by the increment of the collector current. When the increment of the current reaches the initial current value of the diode 16, the diode 16 is cut off. Incidentally, the current of the diode 15 is increased by the amount of the decrease of the current of the diode 16, since resistor 9 acts as a constant current source. Until the current signal amplitude of the high frequency component signal appearing at the collector of the transistor 2 reaches the value of the bias current of the diode 16, the diodes 15 and 16 shortcircuit the inductor 10 to stop the high frequency emphasizing operation and permit the noise removal operation to occur. When the current signal level of the high frequency component signal appearing at the collector of the transistor 2 exceeds the bias current, the diode 16 is cut off to release the shortcircuited state of the inductor 10, thereby to once again enable the high frequency emphasizing operation. 
     It can be seen from the foregoing description that the diodes 15 and 16 form a switching circuit for the high pass filter of which the coil 10 is a part, and this switching circuit is controlled by the control signal supplied thereto from the collector of transistor 4 via the resistor 9. More particularly, when the control signal is low, which occurs for a low noise condition where the S/N ratio is high, the diode 16 will be cut off at a low level of the high frequency component appearing at the collector of the transistor 2, so that the high freguency emphasizing operation will be enabled. On the other hand, when the control signal is high, which occurs for a high nose condition where the S/N ratio is low, the diode 16 will conduct for higher levels of the high frequency component, so that the coil 10 will be shorted and the high frequency emphasizing operation will be prohibited. Thus, the level of applied control current at which the diode 16 is cut off represents a &#34;threshold level&#34; above which the high frequency emphasizing operation occurs. 
     Therefore, according to the embodiment of the present invention, when the S/N ratio of the picture signal is good, a satisfactory high frequency emphasis is carried out over a range from bright to dark portions of the picture signal, so that a picture image with a high resolution is reproduced. With reduction of the S/N ratio of the input signal, the noise removal operation becomes gradually more active. Therefore, it is possible to reproduce a picture image with little noise, but with high resolution. 
     For this reason, the automatic noise removal device according to the present invention is very effective for suppressing not only the random noise in receiving a weak electric field signal but also the beat interference by the radio interference, for example. 
     As described above, according to the present invention, the noise component with small amplitude and the beat interference component are effectively suppressed, while the contour signal with a large amplitude for constituting a major contour portion in a small picture image is emphasized. As a result of sharpness of the picture is never deteriorated. 
     As described above, since the threshold level for the noise removal is controlled in accordance with the S/N of the received signal, a small noise can effectively be removed without damaging the resolution. Therefore, the present invention provide an automatic noise removal device which successfully overcomes the disadvantages of the prior art and can reproduce a picture image with low noise and with high resolution.