DUAL MODE PROGRESSIVE SCAN SYSTEM WITH AUTOMATIC MODE SWITCHING BY IMAGE ANALYSIS

The line rate of a video input signal is doubled for display in progressive scan fashion by dual mode processor which generates added lines for the display by interpolation when displaying normal television pictures and by replication (repeating lines) when displaying characters. The processor mode is automatically controlled by an image analyzer which identifies fields of text by counting "major" transitions of the video input signal occurring within a field and comparing the count with a threshold value. Errors in identification are minimized by a further counter which integrates the field count and a hysteresis detector which controls the processor operating mode in accordance with high and low limits of the integrated field count.