US3914545A
Expired / Lapsed
US
⏱️ 0.0 years remaining
Automatic contrast control utilizing three control signals
Assignee: Zenith Electronics LLC
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Filing: 1974-09-09
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Granted: 1975-10-21
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Expires: 1994-09-09
Abstract
Abstract An automatic contrast control circuit in a color television receiver for stabilizing the average DC level of the luminance information at a desired level and preventing focus blooming. The control circuitry, which is suitable for fabrication as a monolithic integrated circuit, contemplates the provision of a gain-controlled luminance amplifier stage for driving an image reproducer with luminance information having a stabilized black level. An average detector coupled to the amplifier stage output develops a control signal representative of the average DC level of the luminance information and applies it to the amplifier stage, varying its gain inversely with changes in the average luminance level. A peak limiter circuit is also provided for modifying the control signal to reduce the amplifier stage''s gain whenever an AC brightness component comprising the luminance information exceeds a defined threshold level, regardless of the average DC level of the luminance information.
Inventors
- 👤 Christopher M Engel
Bibliographic Details
Application Number
—
Jurisdiction
US
Data Source
google_patents
Last Synchronized
2026-10-25 03:27
CPC Classifications
H; H04; H04N; H04N5/00; H04N5/44; H04N5/57
Claims Analysis
Total Claims: 12
Claim 1
Independent Claim
227 words
In a television receiver having a luminance processing channel for translating instantaneous luminance signals derived from received broadcast transmissions to an image reproducer, said luminance signals including black level reference information, an automatic contrast control circuit comprising in combination: a gain-controlled luminance amplifier stage having an input, and an output coupled to said image reproducer; clamping means coupled to the output of said stage for stabilizing the black level of said luminance signals at a reference level; automatic brightness means including average detector means having an input coupled to said amplifier stage and to said clamping means for receiving said luminance signals and developing therefrom control signals representative of the average DC level of said luminance sigNals, and an output for feeding back said control signals to said amplifier stage to vary its gain inversely with changes in the average DC level; beam current limiter means coupled to said average detector means for monitoring and limiting the beam current developed in said image reproducer; and peak detector means having an input coupled to said clamping means for receiving said luminance signals and an output coupled to said average detector means for modifying said control signals whenever the brightness components of said luminance signals exceed a threshold level to effect overriding inverse gain variations for said amplifier stage irrespective of the average DC level of said luminance signals.
Claim 2
Dependent Claim
227 words
In a television receiver having a luminance processing channel for translating instantaneous luminance signals derived from received broadcast transmissions to an image reproducer, said luminance signals including black level reference information, an automatic contrast control circuit comprising in combination: a gain-controlled luminance amplifier stage having an input, and an output coupled to said image reproducer; clamping means coupled to the output of said stage for stabilizing the black level of said luminance signals at a reference level; automatic brightness means including average detector means having an input coupled to said amplifier stage and to said clamping means for receiving said luminance signals and developing therefrom control signals representative of the average DC level of said luminance sigNals, and an output for feeding back said control signals to said amplifier stage to vary its gain inversely with changes in the average DC level; beam current limiter means coupled to said average detector means for monitoring and limiting the beam current developed in said image reproducer; and peak detector means having an input coupled to said clamping means for receiving said luminance signals and an output coupled to said average detector means for modifying said control signals whenever the brightness components of said luminance signals exceed a threshold level to effect overriding inverse gain variations for said amplifier stage irrespective of the average DC level of said luminance signals.
Claim 3
Dependent Claim
46 words
An automatic contrast control circuit in accordance with claim 1, wherein adjustable level shifting means are interposed between said amplifier stage and said average detector means, said adjustable level shifting means providing a contrast control for manually varying the average DC level of said luminance signals.
Claim 4
Dependent Claim
63 words
An automatic contrast control circuit in accordance with claim 1, wherein said average detector means includes a capacitor having an output terminal coupled to said amplifier stage and a second terminal coupled to a plane of reference potential, said capacitor being charged by luminance signals from said amplifier stage and developing control signals representative of the average DC level of said luminance signals.
Claim 5
Dependent Claim
79 words
An automatic contrast control circuit in accordance with claim 3, wherein said control signals with respect to a plane of reference potential are equal to the potential at which black level is stabilized minus the potential drop between black level and the average DC level of said luminance information, said control signal increasing with respect to said plane of reference potential responsive to decreasing average DC levels of said luminance signals and decreasing responsive to increasing average DC levels.
Claim 6
Dependent Claim
100 words
An automatic contrast control circuit in accordance with claim 3, wherein said peak detector means includes a semi-conductor arrangement for providing said capacitor with a low impedance discharge path whenever said brightness components exceed a predetermined threshold level, the impedance of said discharge path being dependent on the amplitude of said brightness components and the discharge interval of said semiconductor arrangement being the time period during which said brightness components exceed said threshold level, said semiconductor arrangement further decreasing said control signals with respect to said plane of reference potential irrespective of the average DC level of said luminance signals.
Claim 7
Dependent Claim
117 words
An automatic contrast control circuit in accordance with claim 5, wherein said semiconductor arrangement comprises first and second transistors, said luminance signals from said amplifier stage being coupled to the input base electrode of said first transistor, said first transistor further having an emitter electrode coupled to said capacitor output terminal and a collector electrode coupled to the base electrode of said second transistor, said second transistor having a collector electrode coupled to said capacitor output terminal and an emitter electrode coupled to said plane of reference potential, said semiconductor arrangement being conductive to provide said capacitor with a low impedance discharge path whenever said brightness components exceed the base-emitter junction breakdown voltage of said first transistor.
Claim 8
Dependent Claim
44 words
An automatic contrast control circuit in accordance with claim 6, wherein said gain-controlled luminance amplifier stage includes a pair of transistors arranged in a differential amplifier configuration, the gain of which is dependent on the bias applied to the base electrodes of said transistors.
Claim 9
Dependent Claim
75 words
An automatic contrast control circuit in accordance with claim 7, wherein inverter means invert and couple said control signals to said base electrodes in said amplifier stage, the inverted control signals increasing the gain of said amplifier stage whenever the average DC level of said luminance signals decreases and decreasing the gain of said amplifier stage whenever the average DC level of said luminance information increases or whenever said brightness components exceed said threshold level.
Claim 10
Dependent Claim
33 words
An automatic contrast control circuit in accordance with claim 3, wherein said beam current limiter means provide a low impedance discharge path for said capacitor whenever the beam current exceeds a predetermined level.
Claim 11
Dependent Claim
39 words
An automatic contrast control system in accordance with claim 9, wherein said beam current limiter means monitors pulses from a voltage multiplier high-voltage system, said pulses being proportional to the beam current generated during the previous horizontal scan line.
Claim 12
Dependent Claim
68 words
An automatic contrast control circuit in accordance with claim 10, wherein said beam current limiter means comprises a transistor having a base electrode coupled to said voltage multiplier high-voltage system, an emitter electrode coupled to a plane of reference potential and a collector electrode coupled to said capacitor, said transistor providing a low impedance discharge path whenever said pulses exceed the base-emitter junction breakdown voltage of said transistor.
References Cited (Backward Citations)
USPTO Annuity & Maintenance Windows
| Annuity Stage | Window Opens | Payment Deadline | Surcharge Period Until | Estimated Fee | Status |
|---|---|---|---|---|---|
| 3.5 Years (Year 4) | 1978-10-20 | 1979-04-21 | 1979-10-21 | $2000.00 | Window Closed / Lapsed |
| 7.5 Years (Year 8) | 1982-10-20 | 1983-04-21 | 1983-10-21 | $3760.00 | Window Closed / Lapsed |
| 11.5 Years (Year 12) | 1986-10-20 | 1987-04-21 | 1987-10-21 | $7700.00 | Window Closed / Lapsed |