Parabola generators with auxiliary reset function

Abstract

A parabola generator for geometric correction of a television raster, comprises: a resettable circuit for generating a repetitive ramp signal and a resettable integrator for generating a repetitive parabola signal from said repetitive ramp signal. A first resetting circuit initializes the ramp circuit at a frequency related to a scanning frequency. An auxiliary resetting circuit initializes the integrator prior to integrating each repetitive ramp. The auxiliary resetting circuit minimizes timing errors from variations of the ramp signal due to transient conditions. In one embodiment, the scanning period is a horizontal scanning period and the ramp and parabola circuits are reset at the end of each horizontal scanning interval. In this case, the first and auxiliary resetting circuits are responsive to horizontal retrace pulses, for example the leading edges of the horizontal retrace pulses. A further circuit modulates the periodically reset parabolic signal at a vertical scanning rate. In another embodiment, the scanning period is a vertical scanning period and the ramp and parabola circuits are reset at the end of each vertical scanning interval. In this case, the first and auxiliary resetting circuits are responsive to vertical retrace pulses, for example the leading edges of the vertical retrace pulses. A further circuit modulates the periodically reset parabolic signal at a horizontal scanning rate. The parabolic waveform is also reset responsive to a timing signal, rather than by allowing the mimimum level of the waveform to be defined only by uncontrolled operation of an integrator.