To compensate for CRT artifacts, an image frame has been traditionally broken up into two fields that contain even and odd scan lines. Each field thus contains half of the vertical resolution of a frame. Most standard definition video cameras record in this field manner (in NTSC they record 59.94 fields per second) which looks great in a CRT display.
To reproduce the image in its original resolution on a “progressive” type display –LCD, Plasma, etc.- both fields can be combined. However, because each field is recorded at slightly different time (1/59.94 = 16.68 ms) some artifacts may show up in the resulting picture. These artifacts, particularly visible in scenes where there is fast motion, appear as a kind of line crawling over the picture.
Most video original material is interlaced, but some original film material that has been converted to video (Telecine) might also be interlaced. For that reason, when transcoding video for playback in computer systems or on “progressive” type displays, always check if the video source is interlaced and if so, apply de-interlacing. If the source is a Telecine of a film, use an inverse telecine setting.
The link below gives some great examples of interlacing. Check it out.
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