PANASONIC TX-28XD3C QUINTRIX CHASSIS EURO-2M CRT TUBE PANASONIC A66ECF50X32.

PANASONIC TX-28XD3C QUINTRIX  CHASSIS EURO-2M  beam scan velocity modulation (SVM) system for a television receiver,

a video signal is applied to a differentiator followed by a limiting differential amplifier. A driver amplifier coupled to the limiting amplifier drives an output stage that supplies current to an SVM coil. Certain video signals with large high frequency content may tend to produce excessive dissipation in the devices of the output stage. To prevent this, a current source for the differential amplifier is controlled by a voltage which is a measure of the average current through the output stage. The magnitude of the current source is varied to thereby vary the peak-to-peak signal output from the limiting amplifier to prevent overdissipation of the output devices. The presence of random noise in the video signal can produce unwanted SVM operation which can impair the viewed image. The unwanted noise component in the video signal can be reduced in amplitude by coring. The coring is unaffected by the variable limiting.

Beam scan velocity modulation (SVM) apparatus with a svm disabling circuit employed for picture sharpness enhancement is disclosed. The beam SVM includes a picture display device, a source of a first video signal having a picture information displayed on the device when the source is selected, an OSD/TELETEXT display generator having on-screen display information or teletext display information displayed on the device when the generator is selected, a scan velocity modulating circuit coupled to the source for modulating information displayed on the device in accordance with the video content of the first video signal, and a svm disabling circuit responsive to the pulses and the dc voltage and coupled to the scan velocity modulating circuit for modifying operation of the scan velocity modulating circuit when the OSD/TELETEXT display generator is selected. The generator produces pulses, on a line by line basis, and a certain level of dc voltage, indicative of insertion of the picture information and of a full-screen teletext display information. With the beam scan velocity modulation apparatus with svm disabling circuit, an OSD display over a certain display size or a full-screen teletext display is obtained on the screen without any ghost image caused by luminance signal.



1. Field of the Invention
This invention relates generally to beam scan velocity modulation (SVM) systems employed for picture sharpness enhancement and more particularly to an output current limiting, apparatus employed in an SVM system.
2. Description of the Related Art
It is well known that an improvement in apparent picture resolution can be achieved by modulating the beam scan velocity in accordance with the derivative of the video signal which controls the beam intensity. This video signal is referred to as the luminance signal and the derivative of the luminance signal is employed for such control. An advantage of this method over a peaking approach to picture sharpness enhancement is the avoidance of blooming of peaked white picture elements.
It is known in the prior art to apply a differentiated video signal to the input of a double ended limiter incorporating a pair of threshold circuits. The limiter consists of two separate differential amplifiers, where each amplifier is separately biased to provide double ended limiting as well as to provide coring. The limiter arrangement develops a doubly clipped signal output which does not respond to excursions of the differentiated signal which lie below selected threshold magnitudes. Thus the gain of the limiter is such as to provide sharpness enhancement for slow transients while precluding excessive supplemental beam deflection with fast transients. The coring capability of the limiter arrangement significantly lessens the likelihood of noise visibility.
It may be desirable, however, to use a single differential amplifier stage, followed by another stage which will provide the coring function. In such an arrangement, it may be easier to design cost effective circuitry that still meets the requirements of a flat group delay response.
As indicated above, in order to provide beam scan velocity modulation, one differentiates the video signal. A differentiator has an increasing output with increasing frequency. Thus, if the input video signal has higher than normal high frequency components, then a linear system would deliver higher than normal output current and dissipate higher than normal power in the output stage. In such a prior art system, it is possible to overdissipate the output stages of the beam scan velocity modulation system by responding to a particular video signals with much high frequency content.
Circuits are known in the prior art which, in addition to providing signal limiting, reduce power dissipation in the output stages. In such circuits, the current flowing in the output power amplifier is detected to provide a control signal used to control the gain of a preamplifier in a preceding stage. This action suppresses the increase of power dissipation in the output power amplifier when a video signal of a certain frequency characteristic is received. No coring of the differentiated signal is provided, and hence there is exhibited inferior operation in the presence of noise. Furthermore, since the feedback reduces the signal gain as a function of output power, overall SVM operation is reduced, tending to produce a less pleasing visual effect.
Still other circuits are known which operate in a different manner to limit power dissipated in the SVM output stages. In these circuits parallel resistor capacitor combinations with long time constants are provided. These RC combinations are in series with emitter electrodes of transistors which are employed in the output power amplifiers of the SVM system. The transistors operate in a Class B mode with the top transistor conducting on one half cycle of its input waveform and with the bottom device conducting on the other half cycle.
Using this scheme, the bias of the base emitter junction becomes a function of the average amount of high frequency detail in the television image and thereby undesirably introduces more or less output stage coring of the signal depending upon the scene information. Furthermore, this approach requires relatively large magnitude, high voltage capacitors which are expensive and bulky.
As an example, the capacitors used may be 47 μf in value and the resistors 20 ohms in value. The voltage requirements on the capacitors may be in excess of 150 volts. Hence, these capacitors are quite large, bulky and expensive.
SUMMARY OF THE INVENTION
In accordance with an inventive arrangement, a first amplifier is responsive to an input video signal and provides peak-to-peak limiting. A driver amplifier receives the limited signal via a buffer amplifier and provides noise coring subsequent to limiting. An output amplifier coupled to the driver amplifier energizes a scan velocity modulation circuit in accordance with the limited and noise cored video signal.
In accordance with another inventive arrangement, a scan velocity modulation circuit includes means for monitoring the current in the output stage of the SVM circuit and controlling the operation of a preceding stage differential amplifier in accordance with the monitored current. Advantageously, this can prevent overdissipation in the output stage.


1. Field of the Invention
The present invention relates to a beam scan velocity modulation (SVM) apparatus, and more particularly to a beam scan velocity modulation apparatus with an SVM disabling circuit employed for picture sharpness enhancement.
2. Description of the Prior Art
It is well known that an improvement in apparent picture resolution can be achieved by the use of modulation of the beam scan velocity in accordance with the derivative of a video signal which controls the beam intensity. This video signal is known as the luminance signal and the derivative of the luminance signal is employed for the beam SVM. The beam SVM will improve the picture sharpness in a color television system employing a color kinescope.
Many modern color television receivers also employ alternate video sources. An example of such an alternate video source is commonly referred to as an on screen display (OSD) generator. The function of the OSD generator is to provide additional display informations to a viewer while viewing a typical television program. Thus, OSD generator provides for the display on the television screen of time, day, channel number and other various control informations.
In implementing OSD display, the OSD informations are presented as graphical data together with the normal pictures.
Techniques for generating this type of graphical data which is superimposed upon the television picture are well known in the art. Such techniques include OSD generators which count television scan lines and insert at the correct pixel locations the proper graphics to thereby display the time of day, channel number and words such as "CONTRAST", "COLOR", "MUTE" and so on. The use of an on screen display and an associated OSD generator requires the substitution of a different video signal or kinescope drive for the normal video signal which is being processed by the television receiver. In this manner, the pertinent information can be superimposed upon the viewed image.
A scan modulation circuit modulates the picture displayed on a display device in accordance with the video content of a first video signal. An alternate video signal possesses its picture informations displayed on the display device when the alternate signal is selected. The operation of the scan modulation circuit is varied in accordance with this selection. A problem may occur in regard to scan velocity modulation in television receivers which also include an alternate video signal source such as on screen display generator. As is known, the SVM apparatus operates to modulate the horizontal beam scan velocity in response to differentiated luminance information from the main video source. This modulation may occur prior to OSD deletion of the main luminance signal and insertion of the character signal.
U.S. Pat. No. 5,072,300 (issued to Mark R. Anderson) discloses a beam scan velocity modultion apparatus, which controls the current in a scan velocity modulation (SVM) coil by a blanking pulse in order to eliminate the effect of SVM artefact generation during the operation of an OSD generator in a television receiver. In this arrangement, a ghost image representative of the deleted portions of the main luminance signal may appear on the television screen near or behind the inserted OSD character information since the current flowing in the SVM coil is not completely controlled. The ghost image behind the characters generated by the OSD display appears as an outline of the picture contained in the deleted portions of the main luminance signal.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a beam scan velocity modulation (SVM) apparatus with an SVM disabling circuit capable of eliminating the above-mentioned picture interference when OSD display information and TELETEXT information are displayed over a certain display size on the screen.
In order to achieve the above object, the beam scan velocity modulation (SVM) apparatus with an SVM disabling circuit according to the present invention comprises:
a picture display device;
a source of a first video signal having a picture information displayed on the picture display device when the source is selected;
an OSD/TELETEXT display generator having on-screen display information or teletext display information displayed on the picture display device when the OSD/TELETEXT display generator is selected, the OSD/TELETEXT display generator producing pulses on a line-by-line basis and a certain level of a dc voltage indicative of insertion of the picture information and of a full-screen teletext display information;
a scan velocity modulating circuit coupled to the source for modulating information displayed on the device in accordance with the video content of the first video signal; and
an SVM disabling circuit responsive to the pulses and the dc voltage and coupled to the scan velocity modulating circuit for modifying operation of the scan velocity modulating circuit when the OSD/TELETEXT display generator is selected.
With the beam scan velocity modulation apparatus with SVM disabling circuit, an OSD display over a certain display size or a full-screen teletext display is obtained on the screen without any ghost image caused by a luminance signal.

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