A device which generates horizontal and vertical sync signals of a composite signal to create a soft picture on a display monitor by preventing a free running effect of a composite signal which is caused by a mode conversion between recorded and unrecorded areas of the picture when a composite signal generated from an alternative source such as a VCR instead of an external input signal is applied to the display monitor. The device includes a microcomputer for generating a sync select signal based on a display mode, and a composite signal analyzing and horizontal/vertical oscillating circuit for generating horizontal and vertical oscillating signals based on a luminance signal separated from a composite signal input. A horizontal sync signal generator processes the horizontal oscillating signal, to generate a composite horizontal sync signal. A vertical sync signal generator processes the vertical oscillating signal, to generate a composite vertical sync signal. A sync signal switching circuit selectively outputs one pair among the composite horizontal and vertical sync signals respectively generated from the horizontal and vertical sync signal generator and horizontal and vertical sync signals transmitted from a host computer in response to the select signal generated by the microcomputer.