Aliasing is a problem associated with interlaced video sources and affects sharp lines and edges during camera movement. Interlaced images are made up of two fields which make up each frame. The first field contains the odd numbered scan lines (1,3,5,7,9 etc.), the second field contains the even lines. In a panning shot where a given object is moving on the screen, it will have moved a small distance between field one and field two in effect meaning that the field one "half" object will be slightly out of line with the field two "half" object which causes the object to appear to shimmer. As soon as motion stops, the problem disappears. Depending on specific symptoms this is sometime known as combing or twitter.