H - Electricity – 04 – N
Patent
H - Electricity
04
N
350/26, 350/49
H04N 5/18 (2006.01) H04N 7/171 (2006.01)
Patent
CA 1133115
ABSTRACT A high performance video clamp and self-balancing inverter circuit for providing the DC coupling of a video signal together with substantially perfect inversion of any portion of a composite television signal. Under the control of synchronously timed pulses, two switches are alternately turned on and off to selectively apply the video signal to either the inverting or non-inverting input terminal of a video amplifier, thus separating signal portions selected to be inver- ted from those portions selected to remain non-inverted. To ensure perfect balance between inverted and non-inverted output signals, prior to processing, at least two reference pedestals are added to each field of the composite video signal to raise selected blanking level portions thereof to a level equal to 50% of the reference white level. When received, each pair of reference pedestals is separated via activation of the switches so that, at the output of the video amplifier, the first pedestal appears as a non-inverted signal and the second pedestal appears as an inverted signal. A selectively switched, negative feedback control system samples these respective output signals and derives an error signal of such polarity that the DC levels of both reference pedestals are balanced at precisely 50% of the reference white level. Selective activation of a keyed clamp, which completes the feedback loop, clamps the video signal to this 50% reference potential, thus assuring constant balance between inverted and non-inverted portions of the output signal, regardless of input video signal changes.
359564
Meredith & Finlayson
Pay Television Corporation
LandOfFree
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