Television – Image signal processing circuitry specific to television – Chrominance-luminance signal separation
Patent
1996-05-16
1998-11-24
Kostak, Victor R.
Television
Image signal processing circuitry specific to television
Chrominance-luminance signal separation
348571, 348708, H04N 977
Patent
active
058414851
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the processing of analog or digital signals and is concerned in the most important example with filtering techniques.
A well known filtering technique involves calculation of the linear sum of several copies of the input signal shifted in time or space. These copies are then summed with appropriate weights using appropriate coefficient multipliers and adders. The frequency characteristic and pulse response of the resultant filter are linked by the well known Fourier transform. It is a fundamental limitation that, all other things being equal, narrowing the frequency bandwidth of the filter lengthens the pulse response, and vice versa. It is not generally possible to achieve simultaneously a narrow frequency bandwidth and a short pulse response.
In a variety of applications, particularly in video signal processing, it is often desirable to suppress or to extract a narrow band of the incoming signal frequency range without any significant signal delay and without the appearance of overshoots or ripples on the pulse response.
It is, therefore, an object of one aspect of the present invention to provide an improved method of filtering capable of combining both narrow band filtering and short, ripple-free pulse response.
It is another object of one aspect of the present invention to provide an improved filter which offers steep frequency response without placing unduly high demands on the complexity of the filter circuitry and without introducing untoward delay in the signal path.
Accordingly, the present invention consists in one aspect in a method of filtering an input signal, comprising the steps of detecting the frequency of the input signal to provide a control signal and modifying the input signal in dependence upon the control signal.
Advantageously, the control signal is provided by conducting first and second filtering operations on the input signal to derive respective first and second intermediate signals and comparing said intermediate signals to derive the control signal.
Preferably, a ratio of the first and second intermediate signals is taken to derive the control signal, more preferably, in a manner which is level independent.
In another aspect, the present invention consists in a signal filter comprising a signal modification channel containing frequency response regulator means and a parallel channel containing frequency detection means serving to detect the frequency of an incoming signal and to control the response of the frequency response regulator means in dependence thereon.
Advantageously, the frequency detection means comprises first and second, different filters and means for deriving a control signal as a function of the respective filter outputs.
Preferably, the means for deriving a control signal serves to derive a ratio between the respective first and second filter responses.
Suitably, the control signal is a non-linear function of said ratio, possibly independent of sign.
The separation, according to aspects of this invention, of the two functions of detecting frequency and modifying the amplitude or phase of an input signal (which functions are combined in conventional filters) introduces considerable flexibility in filter design.
The manner in which a control signal is derived--according to a preferred form of this invention--by calculating the ratio of the output signals of two different conventional filters is particularly important. The resulting control signal may be a linear or non-linear function of the ratio and the complete filter may be termed "logometric". Since a logometric filter involves division rather than the addition operation of corresponding filters, it is able to combine narrow bandwidth or steep frequency response (or both) with short and accurate pulse response.
It should further be noted that a logometric filter takes into account not only the level of the suppressed or extracted component of signal spectrum, but also the relationship with other spectrum components. In contrast, a conventional filter supply ignores those parts o
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Kostak Victor R.
Snell & Wilcox Limited
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