E.4 Clean aperture
The bandwidth limitation of an analog signal (pre- and post-filtering) can introduce transient ringing effects which intrude into the active picture area. Also, multiple digital blanking operations in an analog-digital-analog environment can increase transient ringing effects.
Furthermore, cascaded spatial filtering and/or techniques for handling the horizontal and vertical edges of the picture (associated with complex digital processing in post-production) can introduce transient disturbances at the picture boundaries, both horizontally and vertically. It is not possible to impose any bounds on the number of cascaded digital processes which might be encountered in the practical post-production system. Hence, recognizing the reality of those picture edge transient effects, the definition of a system design guideline is introduced in the form of a subjectively artifact-free area, called clean aperture.
The clean aperture defines an area within which picture information is subjectively uncontaminated by all edge transient distortions.
The clean aperture should be as wide as is needed to accommodate cascaded digital manipulations of the picture. Computer simulations have shown that a transient effect area defined by 16 samples on each side and 9 lines at both top and bottom within the digital production aperture, would represent an acceptable (and practical) worst-case level of protection in allowing two-dimensional transient ringing to settle below a subjectively acceptable level.
This gives rise to a clean aperture of 1888 horizontal active pixels by 1062 active lines whose quality is guaranteed for final release.