In a typical CCTV Monitor, the interlaced display draws only half of the lines on the screen for each frame, alternately drawing the odd and even lines for each frame. This reduces flicker by taking advantage of the persistence of vision effect, producing a higher refresh rate.
When displaying video on a display that can support a high enough refresh rate such that flicker isn’t perceivable, interlaced video can be de-interlaced for better viewing. All current displays (e.g., LCD) require de-interlacing. De-interlacing requires the display to buffer one or more fields and recombine them into a single frame. In theory this would be as simple as capturing one field and combining it with the next field to be received, producing a single frame. However, the originally recorded signal was produced as a series of fields, and any motion of the subjects during the short period between the fields is encoded into the display. When combined into a single frame, the slight differences between the two fields due to this motion results in a “combing” effect where alternate lines are slightly displaced from each other. Therefore, modern de-interlacing systems buffer several fields and use techniques like 3D de-interlacing to reducing the tearing effect.