Television – Video display – Cathode-ray tube
Reexamination Certificate
1999-09-29
2001-09-18
Eisenzopf, Reinhard J. (Department: 2614)
Television
Video display
Cathode-ray tube
C348S745000, C348S746000, C315S368110, C315S368130
Reexamination Certificate
active
06292235
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the field of raster distortion correction systems with switchable digital interpolating filters.
Raster distortion correction, for example convergence and pincushion correction, is an important aspect of television performance, particularly for large direct view and projection television receivers and monitors. Projection television receivers can present a very difficult challenge to distortion correction systems due to the off-axis orientation of two of the three projection tubes and due to the need to provide separate distortion control systems for each projection tube. In projection television receivers, the green tube is usually in a central orientation. After the raster of the green tube is corrected, the rasters generated by the red and blue tubes must be corrected and converged to match the raster generated by the green tube.
Generally, distortion correction data can be stored in a digital memory, read out of memory, processed by an interpolator to provide additional correction data, converted to analog form, analog low-pass filtered and amplified for use as a convergence correction deflection signal. Each distortion correction circuit must be optimized not only for each projection tube's internal geometry and mounting orientation, as well as the screen size and screen orientation, but for the horizontal scanning frequency of the input video signal as well. The analog low-pass filter, which can form an input for a preamplifier, is a part of the circuit that can be most sensitive to differences in horizontal scanning frequency. Moreover, many receivers presenting significant distortion correction challenges are already adapted to operate at the standard horizontal scanning frequency (1fH) and twice the standard horizontal scanning frequency (2fH). In fact, such receivers will also need to process video signals having a horizontal scanning frequency three times (3fH) the standard frequency.
If the distortion correction system operates with high horizontal frequency (for example 2fH or 3fH, or more) and very short retrace times, the frequency of the digital filter, and thus also the frequency of the D/A converters, is very high during horizontal retrace. This can lead to exceeding the permissible maximum frequency of the D/A converters. It is desirable to prevent exceeding the permissible maximum frequency.
Digital filters, particularly finite impulse response (FIR) filters, are sensitive to noise and signal transients that occur during horizontal retrace because FIR filters rely on earlier samples to generate output values. Even if the maximum frequency of the D/A converters is not exceeded, operation of the D/A converters at high scanning frequencies can cause large transient signals that can disturb proper operation of the digital filter at the beginning of horizontal trace. It is desirable to prevent the digital filters from disturbance due to transient signals occurring during horizontal retrace.
SUMMARY OF THE INVENTION
A distortion correction system in accordance with inventive arrangements comprises: a switchable digital filter for interpolating distortion correction values, said distortion correction values together with interpolated distortion correction values being supplied as an output, proper operation of said digital filter being disturbed by transient signals occurring near the beginning of horizontal trace; a digital to analog converter for converting said supplied output to an analog convergence correction signal, said digital to analog converter having an operating frequency and generating a transient signal during horizontal retrace as said operating frequency increases due to an increased horizontal scanning rate; a low pass filter coupled for receiving said analog convergence correction signal and generating an analog deflection signal; an auxiliary deflection coil driven by said analog deflection signal; and, means for switching said digital filter off during said horizontal retrace, whereby said digital filter is protected from disturbance by said transient signal and said digital to analog converter is protected from exceeding a maximum operating frequency.
A method for distortion correction in accordance with inventive arrangements comprises the steps of: digitally filtering distortion correction values for generating interpolated distortion correction values, proper operation of said digital filter being disturbed by transient signals occurring near the beginning of horizontal trace; supplying said distortion correction values together with interpolated distortion correction values as an output; converting said supplied output to an analog convergence correction signal at an operating frequency, said converting step generating a transient signal during horizontal retrace as said operating frequency increases due to an increased horizontal scanning rate; low pass filtering said analog convergence correction signal and generating an analog deflection signal; driving an auxiliary deflection coil with said analog deflection signal; and, inhibiting said supplying step during said horizontal retrace, whereby said digitally filtering step is protected from disturbance by said transient signal and said converting step is protected from exceeding a maximum operating frequency.
In accordance with inventive arrangements, the digital filter can be advantageously switched off during horizontal retrace. Switching off the filters during the horizontal retrace advantageously prevents the transient signal from occurring and can advantageously prevent exceeding the permissible maximum frequency of the D/A converters.
Calculation of the filter values can, in fact, continue during horizontal retrace so that when the digital filter is switched on again no disturbing filter build-up occurs.
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patent: 5353026 (1994-10-01), Wilson
patent: 5506481 (1996-04-01), Wada et al.
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patent: 5734233 (1998-03-01), Masumoto et al.
patent: 5751122 (1998-05-01), Park
patent: 6002454 (1999-12-01), Kajiwara et al.
patent: 6014168 (2000-01-01), Webb et al.
George John Barrett
Gleim Gunter
Heizmann Friedrich
Runtze Albert
Eisenzopf Reinhard J.
Fried Harvey D.
Laks Joseph J.
Thomson Licensing S.A.
Tran Trang U.
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