Electric lamp and discharge devices: systems – Cathode ray tube circuits – Cathode-ray deflections circuits
Reexamination Certificate
1999-06-04
2001-04-17
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Cathode-ray deflections circuits
C315S371000
Reexamination Certificate
active
06218791
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a deflection circuit with a deflection correction circuit. The invention further relates to a display apparatus with such a deflection circuit. Such a deflection circuit is particularly suitable for computer monitors or other display apparatuses which have to display information with substantially different line frequencies.
2. Description of the Related Art
In a display apparatus with a cathode ray tube in which an electron beam is deflected, the angle between the electron beam and the screen of the tube varies during scanning. Therefore, the scanning rate has to be corrected as a function of the position of the point where the beam impinges on the screen. This correction is known as “S-correction”, because, when the electron beam is deflected by the electro-magnetical field generated by a deflection coil, the sawtooth-shaped deflection current flowing through the deflection coil has to be corrected to become S-shaped. The S-correction is obtained by an S-capacitor which is arranged in series with the deflection coil.
In a known line deflection circuit, a power supply voltage is supplied to a series arrangement of a choke and a periodically switched electronic switch. A diode, a flyback capacitor, and a series arrangement of the line deflection coil and the S-capacitor are arranged in parallel with the switch. The diode has a polarity so as to be conductive during at least part of a period during which the switch is non-conductive. After the switch has been switched off, the line deflection coil resonates with the flyback capacitor: during the flyback period a high flyback voltage occurs across the flyback capacitor and the current through the line deflection coil reverses sign. At the end of the flyback period, which is the start of the scan period, the diode becomes conductive and the average voltage built up across the S-capacitor occurs across the line deflection coil to cause the S-corrected sawtooth current to run through the line deflection coil. Before the middle of the scan period, where the deflection current changes sign, the switch has to be closed again to maintain the S-capacitor voltage across the line deflection coil. A line period is defined as the sum of the flyback period and the scan period.
In a display apparatus which has to display display signals with substantially different line periods, it is known to adapt the value of the S-capacitor in dependence on the line period. A number of branches is connected in parallel to the S-capacitor, each branch comprising a switch arranged in series with a capacitor. The correct S-capacitance value is approximated by determining, in response to the detected line frequency, which switches should be opened and which switches should be closed. The switches are opened or closed continuously during the period in time that display signals with a certain line frequency are received.
European Patent Application EP-A-823812 discloses that an adjustable S-correction in a line deflection can be obtained by arranging only one switch in series with an extra capacitor arranged in parallel with the S-capacitor. The amount of S-correction is adapted by controlling the on-time of the switch during a line period. In this way it is possible to gradually adjust, in dependence on the line frequency, the S-correction, the line amplitude, and the inner pincushion correction. The inner pincushion distortion is corrected by varying the amount of S-correction as a function of the vertical position.
It is a drawback of the prior art that the shape of the S-correction waveform is a superposition of two or more different parabola functions. It is thus impossible to generate any given shape of the correction waveform within a scan period without using a large amount of parallel branches. A great flexibility in generating the shape of the correction waveform is required to fit a vast variety of cathode ray tubes, each with their specific requirements with respect to the correction waveforms.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a deflection circuit with a deflection correction circuit which, for a large range of deflection frequencies, is able to modulate the current through the deflection coil with any given shape in a scan period. In this way it is possible to correct, with a very high accuracy and within a large range of deflection amplitudes and deflection frequencies, the inner pincushion distortion (and, hence, the S-distortion) and also the linearity.
To this end, a first aspect of the invention provides a deflection circuit with a deflection correction circuit. A second aspect of the invention provides a display apparatus with such a deflection circuit. Advantageous embodiments of the invention are defined in the dependent claims.
The deflection circuit according to the invention comprises a deflection correction circuit with a controllable active voltage source arranged in the loop formed by the deflection coil, the S-capacitor, and the flyback capacitor. The voltage source receives a further power supply voltage and a modulating signal to supply a modulating voltage varying in response to the modulating signal. The active voltage source is able to generate any desired waveform within a deflection period. During the scan period of the deflection period, the voltage across the deflection coil is determined by a voltage which is the sum of the modulating voltage and the voltage across the S-capacitor. Thus, since the modulating voltage has any given shape within a scan period, the voltage across the deflection coil varies according to the same shape. Consequently, an appropriate modulation of the current through the deflection coil is obtained. The active voltage source comprises a switching element which is switched on and off with a frequency which is substantially higher than the deflection frequency to obtain waveforms of the correction signal with a frequency content which is substantially higher than the deflection frequency, so that any desired waveform can be generated within the scan period. The deflection correction circuit, according to the invention, enables the generation of a gradually adjustable waveform instead of discrete correction steps as is the case if a desired number of S-capacitors are switched in parallel. A user is able to adjust the waveform to obtain a perfect correction without problems.
European Patent Application EP-B-0490265, corresponding to U.S. Pat. No. 5,043,637, EP-B-0490265 discloses an embodiment of the generally used diode modulator circuit. The diode modulator circuit shown comprises a second resonant circuit arranged in series with the first resonant circuit composed of the deflection coil, the S-capacitor, the flyback capacitor and the diode. The second resonant circuit comprises a series arrangement of a modulator coil and a modulator capacitor arranged in parallel with the modulator flyback capacitor and the modulator diode. The operation of the diode modulator circuit is based on the fact that the average voltage across the S-capacitor is varied by modulating the average voltage across the modulator capacitor via a coupling coil connected to the junction of the modulator coil and the modulator capacitor. Due to the essential tuning of the circuit to a frequency, related to the line frequency, it is impossible to generate any given correction waveform in a line period. It should be noted that during the scan period, either the switch or the diode arranged in parallel with the switch are conducting. Consequently, the series arrangement of the deflection coil and the S-capacitor is short-circuited; there is no active voltage source generating a correction current containing frequencies which are much higher than the deflection-current frequencies. Only the S-capacitor is present in series with the deflection coil. The S-capacitor is not an active voltage source, and is only able to generate a low-frequency S-correction current.
As is known, the diode modulator is primarily intended to co
Kennis Roger P. H.
Misdom Johannes A. C.
Teuling Dirk J. A.
Vaneerdewegh Franky M. M. E.
Verhees Johannes L. M.
Goodman Edward W.
Nguyen Hoang
U.S. Philips Corporation
Wong Don
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