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Synchronization system for overlay of an internal video signal upon an external video signal

  • US 4,639,765 A
  • Filed: 02/28/1985
  • Issued: 01/27/1987
  • Est. Priority Date: 02/28/1985
  • Status: Expired due to Fees
First Claim
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1. A video signal mixing system comprising:

  • an input means for receiving an external composite video signal;

    a horizontal synchronization pulse detecting means connected to said input means for detecting the horizontal synchronization pulses of said first composite video signal;

    an external signal loss detector connected to said input means for detecting and indicting the loss for longer than a first predetermined period of time greater than the period of said horizontal synchronization pulses the loss of said external composite video signal;

    a video clock frequency standard for determining a frequency;

    a video clock signal generating means connected to said horizontal synchronization pulse detecting means and said external signal loss detector for generating a video clock signal, said video clock signal generating means havinga horizontal phase lock loop means connected to said horizontal synchronization pulse detecting means including a horizontal controlled oscillator for generating internal horizontal synchronization pulses in phase synchronization with said detected horizontal synchronization pulses, anda video clock phase lock loop means connected to said horizontal phase lock loop means and said video clock frequency standard including a video clock controlled oscillator for generating said video clock signal having a frequency corresponding to and corrected by the frequency of said internal horizontal synchronization pulses when said external signal loss detector does not indicate the loss of said external composite video signal and having a frequency corresponding to said frequency determined by said video frequency standard when said external signal loss detector indicates loss of said external composite video signal, whereby momentary loss of said external composite video signal for a time less than said predetermined period of time causes said frequency of said video clock signal to be controlled by said horizontal controlled oscillator;

    a video signal generating means connected to said video clock signal generating means for generating an internal composite video signal in synchronization with said video clock signal thereby generating said internal composite video signal having horizontal lines in synchronism with said detected horizontal synchronization pulses; and

    a video switching means connected to said input means, said external signal loss detector and said video signal generating means having an output means for selectively connecting either said external composite video signal or said internal composite video signal to said output means, said video switching means selecting said internal composite video signal when said external signal loss detector indicates loss of said external composite video signal, whereby said output means receives a combined video signal having synchronized horizontal lines.

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