The Digital Fact Book - Quantel

AAliasingUndesirable ‘beating’ effects caused by sampling frequencies being too low faithfullyto reproduce image detail. Examples are:1) Temporal aliasing – e.g. wagon wheel spokes apparently reversing, also movementjudder seen in the output of standards converters with insufficient temporal filtering.2) Raster scan aliasing – twinkling effects on sharp boundaries such as horizontallines. Due to insufficient filtering this vertical aliasing and its horizontal equivalentare often seen in lower quality DVEs as detailed images are compressed.The ‘steppiness’ or ‘jaggies’ of poorly filtered lines presented at an angle to the TV rasteris also referred to as aliasing.See also: Anti-aliasing, Interpolation (temporal & spatial), Into digits (Tutorial 1)AliensA familiar term for alias effects, such as ringing, contouring and jaggy edges caused by lackof resolution in a raster image. Some can be avoided by careful filtering or dynamic rounding.Alpha channelAnother name for key channel – a channel to carry a key signal.See also: Keying, 4:4:4:4Anaglyph (Stereoscopic)A type of stereoscopy in which the left eye and right eye images are separated by colorfiltering and then superimposed as a single image rather than two separate images.Each eye sees only the required image through the use of complementary colored filters(e.g. red and green or red and cyan). Anaglyph glasses have been popular over the yearsfor viewing 3D comics and some 3D films (particularly on VHS and DVD).See also: 3DAnamorphicGenerally refers to the use of 16:9 aspect ratio pictures in a 4:3 system. For example,anamorphic supplementary lenses are used to change the proportions of an image to 16:9on the surface of a 4:3 sensor by either extending the horizontal axis or compressingthe vertical. Signals from 16:9 cameras and telecines produce an ‘anamorphic’ signalwhich is electrically the same as with 4:3 images but will appear horizontally squashedif displayed at 4:3.22