Compatible non linear pre-emphasis for composite video signals
AssigneeNational Transcommunications Limited
InventorBrian H. Beech
Life of this patent
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Abstract
When a video signal including a component modulated onto a subcarrier (such as a PAL signal) is subjected to a non-linear pre-emphasis known for MAC signals, with the object of reducing the effects of noise encountered in transmitting the composite signal through an f.m. channel, problems of noise and differential gain remain. These problems are overcome by subjecting the composite signal to a filtering step, before application of the non-linear processing, so as to filter out the subcarrier, the chrominance subcarrier in a PAL signal, and to compand the chrominance sidebands. Preferably a notch filter is used which has a notch shaped as a Gaussian curve.
Claims15
A method for pre-emphasising a video signal comprising a first component and a second component, the second component being modulated onto a subcarrier, the method comprising the steps of:
- a) filtering the high frequency portion of the video signal to attenuate the signal over a range of frequencies including the subcarrier frequency, the filtering attentuating the signal at the subcarrier frequency substantially to zero;
- b) applying a non-linear processing to the filtered high frequency portion of the video signal to produce a first pre-emphasised signal;
- c) applying a non-linear de-emphasis to the first pre-emphasised signal to produce a reconstructed signal, the transfer function of the applied de-emphasis being inverse to that of the desired pre-emphasis;
- d) comparing the reconstructed signal with the input video signal to produce a first error signal;
- and
- e) subtracting the first error signal from the pre-emphasised signal to produce a closer approximation to the desired pre-emphasised signal.
Apparatus for pre-emphasising a video signal comprising a first component and a second component, the second component being modulated onto a subcarrier, the apparatus comprising:
- a notch filter for filtering the high frequency portion of the video signal, the filter attenuating the signal at the subcarrier frequency substantially to zero;
- means for applying a non-linear processing to the filtered high frequency portion of the video portion of the video signal to produce a first pre-emphasised signal;
- means for applying a non-linear de-emphasis to the pre-emphasised signal to produce a reconstructed signal, the transfer function of the applied de-emphasis being inverse to that of the desired pre-emphasis,
- means for comparing the reconstructed signal with the input video signal to produce a first error signal, and
- means for subtracting the first error signal from the pre-emphasised signal to produce a closer approximation to the desired pre-emphasised signal.
Dependencies are inferred from the claim text.