Television – Image signal processing circuitry specific to television – Noise or undesired signal reduction
Reexamination Certificate
1999-07-01
2001-02-27
Peng, John K. (Department: 2714)
Television
Image signal processing circuitry specific to television
Noise or undesired signal reduction
C348S622000, C382S262000
Reexamination Certificate
active
06195132
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a noise reduction signal processing apparatus for reducing noise components included in television video signals; and, more particularly, the invention relates to a noise reduction signal processing apparatus for reducing noises using a median filter.
Conventional techniques used in noise reducing apparatus for reducing noises in video signals using a median filter are disclosed, for example, in “Proposal of a Direct-Current Addition Type Noise Reducer and Development of a MUSE Decoder Dynamic Picture Image Processing Noise Reducer” (Izumi and four others, academic circle magazine of television, Vol. 48, No. 12, pp 1553-1564 (1994); and in the official gazette of Unexamined Published Japanese Patent Application No. 9-163185. This apparatus is referred to as a DC-shifting type NR (Noise Reducer).
Hereunder, a description will be provided concerning the operation and characteristics of the median filter used for such a noise reducing apparatus. This median filter receives input digital video signal data sampled at a desired time and digital video signal data sampled at two points, which are separated from each other in the direction of time according to the desired lengthwise symmetrical or asymmetrical sampling counts. The median filter then outputs video signal data detected at those three different sampling times.
Since the median filter can smooth small amplitude signals more effectively than low-path filters, noises in these output signals can be reduced effectively if small amplitude noise waves are included in those large amplitude video signals. The median filter can therefor pass those large amplitude signals detected at precipitous rising and falling portions without losing such precipitous shapes and timings. Consequently, the signals output from this median filter can be assumed to be ideal original video signals including no noise, whereby those signals are referred to as reference signals.
This noise reducing apparatus compares input signals with the voltage levels of respective reference signals and judges the difference between the voltage level of each input original signal and the voltage level of each reference level, so as to get the noise included input signals close to the reference signals. And, as a result of such judgment, if the voltage level of the input signal is larger than the voltage level of the reference signal, a DC value equivalent to a noise reduction value to be described later is subtracted from the original signal value, thereby obtaining a signal, which becomes a noise-reduced video signal. On the contrary, if the voltage level of the input original signal is smaller than the voltage level of the reference signal, a DC value equivalent to a noise reduction value to be described later is added to the original signal, thereby obtaining a signal, which becomes a noise-reduced video signal. If the voltage level of the original signal is equal to the voltage level of the reference signal, the original signal is output as it is.
The DC value described above is a noise reduction value corresponding to an effective voltage, which is an average level of a noise. This is obtained by detecting a noise component in the synchronizing signal, etc., where there is no video signal, such as in the vertical blanking period, in the input video signals which are original signals, and then integrating the component.
SUMMARY OF THE INVENTION
In such a noise reducing apparatus formed by the conventional techniques described above, the median filter has been used for selecting and controlling a computing method for determining whether to add or subtract the DC value for reducing noises to or from noise included input video signals or to output original signals without performing either of addition and subtraction with respect to the original signals. The DC value equivalent to a noise reduction value is obtained by integrating the voltage level of a noise detected from the synchronizing signal portion at every fixed time whose cycle is sometimes very long with respect to the noise generation cycle and a constant for which the user sets a specific value manually from the outside as the optimal value for reducing noises effectively. In the case of the conventional techniques, a noise reduction value is thus computed for every original signal in the same way.
Consequently, a fixed noise reduction value is always found regardless of the signal level difference between the original and reference signals. For Example, if a noise reduction level is set and computed for a large signal level difference between original and reference signals, an excessive value is added/subtracted to/from the original signal at a less signal level difference section. This will thus separate the object signal farther from the original signal. And, this results in the addition of a new distortion signal to the original signal. In this case, a new noise waveform is added to the original signal, causing the picture quality to be degraded.
Addition of such a new distortion signal to the original signal, however, can be avoided if the noise reduction value is set at about one LSB, which is the minimum unit of sampled video digital data. In this case, since the noise is reduced only by one LSB at a section whose level is over one LSB, the noise reduction effect becomes very small.
In the case of the noise reducing apparatus which uses conventional techniques as described above, therefore, the noise reduction value is a DC value (fixed value) and the actual noise level of the video data is not uniform. This is why distortion is added to the original signal through computation of this DC value, which is equivalent to this noise reduction value even when a median filter is used for selecting such an arithmetic operation method as addition, subtraction, etc. The conventional techniques have thus been plagued with a problem in that accurate noise reduction is impossible because the picture quality is degraded, left-over noise remains even after noise reduction processing, and, according, the effect of the noise reduction becomes low.
The present invention has been formulated in consideration of such problems of the conventional techniques. The present invention provides a noise reduction signal processing circuit and display apparatus which can reduce various voltage level noises included in video signals accurately while preventing the addition of a new distortion signal and not allowing any noise to remain in those video signals after noise reduction processing is complete.
The features of the present invention are embodied in the following structure. That is, a noise reduction signal processing circuit is used for a display apparatus for displaying a video signal, comprising: a median filter which receives the video signal and which executes a filter processing on the inputted video signal and outputs a reference signal; a subtracter which is connected with the median filter, and which outputs a difference signal that indicates the difference between reference signal outputted from the median filter and the video signal; a minimum value detection circuit which outputs the difference signal from the subtracter or a limitation value, whichever is smaller, as a minimum value signal; and an adder which adds a noise reduction signal on the basis of the minimum value signal output from the minimum value detection circuit and the video signal.
In addition, the following structure also embodies the features of the present invention. That is, a display apparatus has a noise reduction signal processing circuit, the noise reduction signal processing circuit comprising: an A/D converter which converts an input video signal into a digital video signal; a delay circuit which is connected with the A/D converter, and which delays the digital video signal from the A/D converter; a median filter which is connected with the A/D converter, and which generates a reference signal on the basis of the digital video signal from the A/D converter; a subtracter wh
Kimura Katsunobu
Matono Takaaki
Nagata Tatsuo
Sudo Koichi
Sugiyama Masato
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Peng John K.
Tran Trang U.
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