Vertical synchronization system

In a vertical synchronization system for a television receiver receiving vertical sync pulses and having a local source of clock pulses whose frequency is a multiple of the nominal horizontal line frequency, a counter for counting the clock pulses, and means for enabling a vertical scan generator in response to the counter reaching a count representative of a nominal line rate/frame rate ratio coincidentally with receipt of a vertical sync pulse, the improvement comprising:

• a first sync window generator, including means responsive to the counter reaching a first count representative of a line rate/frame rate ratio less than nominal for generating a first window-open signal, and means responsive to the counter reaching a second count representative of a line/rate frame rate ratio greater than nominal for generating a first window-close signal, the interval between said first window-open signal and said first window-close signal defining a wide sync window;
• a second sync window generator, including means responsive to the counter reaching a third count representative of a line rate/frame rate ratio less than nominal but closer to nominal than the first count for generating a second window-open signal, and means responsive to the counter reaching a fourth count representative of a line rate/frame rate ratio greater than nominal but closer to nominal than said second count for generating a second window-close signal, the interval between said second window-open signal and said second window-close signal defining a narrow sync window;
• gating means adapted to receive vertical sync pulses and selected window-open and window-close signals for coupling, to the vertical scan generator and to the counter, vertical sync pulses received during a sync window defined by the received window-open and window-close signals, for thereby initiating vertical scan and resetting the counter to an initial count, and responsive to a received window-close signal for triggering the vertical scan generator and resetting the counter when no vertical sync pulse is received during a sync window;
• means responsive to the counter reaching at least one count approximately equal to nominal line rate/frame rate ratio for generating a control signal;
• a detector responsive to time-coincidence between the control signal and a vertical sync pulse for generating a coincidence signal, and responsive to a lack of time-coincidence between the control signal and vertical sync pulses for generating a non-coincidence signal;
• and
• means responsive to said non-coincidence signal for coupling said first window-open signal and said first window-close signal to said gating means, and responsive to said coincidence signal for coupling said second window-open and window-close signals to said gating means,
• whereby vertical scan is initiated and the counter is reset by the first window-close signal until an incoming vertical sync pulse is received during the wide sync window, whereupon vertical scan is initiated and the counter is reset by the incoming vertical sync pulse, and when a vertical sync pulse is received in time coincidence with the generation of the control signal, a narrow sync window mode of operation is effected during which the received vertical sync pulse continues to initiate vertical scan and reset the counter, and noise received outside the narrow sync window is decoupled from the vertical scan generator and the counter.