Vertical position-jitter elimination

Vertical position-jitter eliminating circuit for eliminating jitter in a vertical position of a second information displayed on a display device on which a first information is displayed, the first information determining an addressing of the display device, the circuit comprising:

• a first processing circuit coupled to receive the first information, for generating vertical and horizontal start pulses to control vertical start positions of fields of lines, and horizontal start positions of the lines, respectively, of the first information on the display device;
• timing means for supplying a vertical position signal, defining a vertical starting position on the display of the second information, by counting a predetermined number of the horizontal start pulses;
• and
• a second processor circuit coupled to receive said vertical position signal for inserting a display of said second information on said display device,
• characterized in that the vertical position-jitter eliminating circuit further comprises:
• shifting means coupled to receive a vertical start pulse and a shift period, for supplying, in a first field of the first information, a first vertical trigger pulse having an active edge corresponding to an active edge of a first vertical start pulse shifted in time over a first shift period,
• wherein the timing means are coupled to receive a vertical trigger pulse for initiating, in each field of the first information, the counting of the predetermined number of the horizontal start pulses, whereby an active edge of the vertical trigger pulse is an active edge of a corresponding vertical start pulse shifted over a current shift period,
• wherein the shifting means are arranged for supplying, in each field of the first information, a next vertical trigger pulse having an active edge corresponding to an active edge of a next vertical start pulse shifted in time over the current shift period,
• and in that the circuit further comprises:
• time detection means coupled to receive a vertical start pulse, a vertical trigger pulse, and the horizontal start pulses, for supplying, in predetermined fields of the first information, a current time difference signal being a measure of a time difference in occurrence between an active edge of a current vertical trigger pulse, and an active edge of a current horizontal start pulse nearest to the active edge of the current vertical trigger pulse,
• wherein the shifting means are further coupled to receive the current time difference signal, and a predetermined value, for checking, in said predetermined fields of the first information, whether the current time difference signal is smaller than a current predetermined value, and
• wherein the shifting means generates, in a next field, a next shift period unequal to the current shift period, if the current time difference is smaller than the current predetermined value, whereby said next shift period is selected such that an absolute value of a time difference between an active edge of a hypothetical vertical trigger pulse derived from the current vertical start pulse by shifting over said next shift period, and the active edge of the current horizontal start pulse nearest to said hypothetical vertical trigger pulse, is not smaller than the current predetermined value, said next shift period forming the shift period for a next field of the first information.

Method of eliminating jitter in a vertical position of a second information displayed on a display device on which a first information is displayed, the first information determining an addressing of the display device, the method comprising the steps of:

• generating vertical start pulses and horizontal start pulses to control vertical start positions of fields of the first information and horizontal start positions of lines of the first information, respectively, on the display device, characterized in that the method comprises the further steps of:
• generating, in a first field of the first information, in response to active edges of first horizontal start pulses, first horizontal count pulses, at least one of the first horizontal count pulses having an active edge which is the active edge of a corresponding horizontal start pulse shifted in time over a predetermined first shift period, the at least one of the first horizontal count pulses being nearest to an active edge of a corresponding vertical start pulse;
• counting, in each field of the first information, a predetermined number of horizontal count pulses in response to an active edge of a corresponding vertical start pulse, to define a vertical start position for a display of the second information on the display device, whereby an active edge of at least one of said horizontal count pulses is an active edge of a corresponding horizontal start pulse shifted over a current shift period, the at least one of said horizontal count pulses being nearest to an active edge of the corresponding vertical start pulse;
• generating, in each field of the first information, in response to active edges of next horizontal start pulses, next horizontal count pulses, at least one of said next horizontal count pulses having an active edge which is shifted with respect to the active edge of a corresponding one of the next horizontal start pulses over said current shift period, the at least one of said next horizontal count pulses being nearest to an active edge of a corresponding next vertical start pulse;
• and
• generating, in predetermined fields of the first information, in response to active edges of corresponding vertical start pulses, adjusted shift periods by the steps of:
• determining a current time difference between an active edge of a current vertical start pulse, and an active edge of a current horizontal count pulse nearest to the active edge of the current vertical start pulse;
• checking whether the current time difference is smaller than a current predetermined value;
• and
• generating a next shift period unequal to the current shift period, if the current time difference is smaller than the current predetermined value, whereby said next shift period is selected such that an absolute value of a time difference between an active edge of a hypothetical horizontal count pulse nearest to the active edge of the current vertical start pulse, which hypothetical horizontal count pulse is derived from a corresponding current horizontal start pulse by shifting over said next shift period, and the active edge of the current vertical start pulse, is not smaller than the current predetermined value, said next shift period forming the current shift period for a next field of the first information.