Television – Image signal processing circuitry specific to television – Noise or undesired signal reduction
Patent
1995-10-06
1997-12-02
Hsia, Sherrie
Television
Image signal processing circuitry specific to television
Noise or undesired signal reduction
348616, 348620, H04N 521
Patent
active
056941773
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to video signal processing and more particularly to video signal filtering.
In one example, filtering of video signals is undertaken to reduce noise.
The use of a median filter, which selects the middle value from a number of inputs, is an established noise reduction technique in video signal processing. A typical median filter for this application has inputs corresponding to the centre sample, to horizontally adjacent samples on the same scan line and to vertically adjacent samples in the same horizontal position but on adjacent scan lines.
To achieve a higher level of noise reduction, particularly for low frequency noise, it has been proposed to add temporally adjacent samples. Thus, inputs for the median filter are taken from the same horizontal and vertical position in the immediately preceding and the immediately succeeding frames of the video signal. Inclusion of these points has the added benefit of reducing aliasing on still images, which is a known defect of two dimensional median filters. Motion between frames does not introduce blur since a median filter involves no feedback. Motion will usually result in the median filter selecting the centre sample or one of the spatially adjacent samples.
Whilst the extension of the median filter from two to three dimensions--that is to say the introduction of temporally spaced samples offers important advantages, it carries a penalty in terms of delay. To achieve the temporally spaced sample points, it is necessary to delay the "main" signal line containing the centre sample point and the spatially adjacent sample points, by one frame or (in 25 frames per second system) by 40 ms.
This delay can be a major drawback of median filter noise reduction techniques, since compensation will be required in the audio channels--and in cases where there is parallel processing of the video signal--in the other video channels. In contrast, recursive noise reduction techniques do not suffer from this delay, although they of course require efficient motion detection if motion blur is to be avoided.
Another example of video signal filtering is the decoding of composite television signals to separate luminance and chrominance information. A wide range of decoder circuits have been proposed to suit the differing demands posed by the numerous applications for decoders. These demands differ not only in the cost to performance relationship, but also in the behaviour of the filter; the design of a decoder is generally regarded as a compromise between competing criteria, the priority of which will differ from application to application.
The treatment of motion is an important aspect of the design of decoders, as with other video filters. Decoders have been produced which are motion adaptive in that they switch between modes of operation depending upon whether or not motion is detected. Non-adaptive decoders have also been produced in which the filter coefficients are selected such that reasonable performance is normally achieved irrespective of the presence of motion. A good example of a decoder in this category is disclosed in GB-B-2 173 971. This describes a PAL decoder using a total of nine contributions taken from five consecutive fields of the video signal. The performance of this decoder is generally regarded as very good and comparable with some alternatives which carry the additional costs of motion adaption. The described decoder does, however, require four field delays. This has the disadvantages described above in relation to noise reduction circuits.
Many more examples exist of video filters which offer good performance, compared for example with recursive or motion adaptive alternatives, but which rely on information being taken from the fields on either side of the reference point or, indeed two or more fields on either side. The difficulties that the presence of the necessary delays introduces in parallel audio or other video channels has hitherto been accepted as an inevitable consequence of achieving the desired fi
REFERENCES:
patent: 4737850 (1988-04-01), Lu
patent: 4789893 (1988-12-01), Weston
patent: 5404178 (1995-04-01), Kondo
Auty Simon
Flannaghan Barry
Weston Martin
Hsia Sherrie
Snell & Wilcox Limited
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