Television – Basic receiver with additional function – For display of additional information
Reexamination Certificate
1998-08-04
2001-07-24
Hsia, Sherrie (Department: 2614)
Television
Basic receiver with additional function
For display of additional information
C348S707000
Reexamination Certificate
active
06266099
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an amplifier circuit for video display. In particular, the present invention relates to an amplifier circuit for providing on screen a signal for a second image overlaying the video image.
2. Background of the Invention
On-screen Display (OSD) is a technique by which a signal for a second image is substituted for a video signal, so as to overlay the second image over the original video image. To provide the OSD image, the OSD signal is typically injected into an amplifier circuit for the video signal while, simultaneously, the video input signal to the amplifier circuit is attenuated or “blanked”. To prevent the video image from showing through the OSD image, while the OSD signal is active, coupling of the video signal to the amplifier's output signal by feed through and crosstalk must be suppressed.
In the prior art, to suppress feed through and crosstalk, a complex electronic circuit which requires a large silicon area is used. Alternatively, a dedicated pin in a video driver circuit is provided for receiving a control signal which directs whether video or OSD signal is output from the video driver circuit. These methods are unsatisfactory because of the requirements of a larger die and a dedicated pin. In addition, these methods tend to degrade bandwidth, and are particularly to difficult to implment with an 8-volt power supply.
SUMMARY OF THE INVENTION
The present invention provides, in a video amplifier circuit, an efficient on-screen display capability, which can be implemented with very few components (hence, without occupying a large silicon area), without affecting bandwidth of the video amplifier, and without affecting the switching times between an on-screen image and a video image. The present invention avoids feed through and cross-talk of the video signal into the on-screen display image by adjusting the gains of both stages of a 2-stage video amplifier.
In one embodiment, the video amplifier includes: (a) an input stage receiving a video signal and providing on an output terminal an amplified video signal; (b) an output stage, receiving the amplified video signal and an on-screen display signal, and providing (i) an output video signal at an output terminal, based on the amplified video signal, when the on-screen display signal is inactive, (ii) an output on-screen display signal, when the on-screen display signal is active, and (iii) a control signal indicating whether or not the on-screen display signal is active; and (c) an attenuation circuit receiving the control signal. The attenuation circuit of that embodiment attenuates the amplified video signal of the input stage, when the control signal indicates that the on-screen display signal is active.
In one embodiment, the attenuation circuit of the video amplifier circuit includes a transistor having an output terminal coupled to the output terminal of the input stage, and a gate terminal coupled to receive the control signal. The control signal can be generated in the output stage by a comparator coupled to receive the on-screen display signal and a reference voltage. In one embodiment, the amplified video signal of the input stage of the video amplifier is adjusted by a drive control signal and a contrast control signal.
According to another aspect of the present invention, the output stage of the video amplifier also includes an attenuation circuit which attenuates the output video signal of the output stage, when the on-screen display signal is active.
The present invention is better understood upon consideration of the detailed description below and the accompanying drawings.
REFERENCES:
patent: 4795949 (1989-01-01), Wilber
patent: 11-103398 (1999-04-01), None
patent: 11-272255 (1999-10-01), None
Hsia Sherrie
Kwok, Esq. Edward C.
National Semiconductor Corporation
Skjerven Morrill & MacPherson LLP
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