Editing Digital television (section)

=== Compression artifacts, picture quality monitoring and allocated bandwidth ===
DTV images have some picture defects that are not present on analog television or motion picture cinema, because of present-day limitations of bit rate and compression algorithms such as [[MPEG-2]]. This defect is sometimes referred to as [[mosquito noise]].<ref>{{cite web
 | author1=Le Dinh, Phuc-Tue
 | author2=Patry, Jacques
 | url=http://www.videsignline.com/howto/180207350
 | title=Video compression artifacts and MPEG noise reduction
 | work=Video Imaging DesignLine
 | date=February 24, 2006
 | access-date=April 30, 2010
 | archive-date=March 14, 2006
 | archive-url=https://web.archive.org/web/20060314201405/http://www.videsignline.com/howto/180207350
 | url-status=dead
 }}</ref>

Because of the way the human visual system works, defects in an image that are localized to particular features of the image or that come and go are more perceptible than defects that are uniform and constant. However, the DTV system is designed to take advantage of other limitations of the human visual system to help mask these flaws, e.g., by allowing more [[compression artifacts]] during fast motion where the eye cannot track and resolve them as easily and, conversely, minimizing artifacts in still backgrounds that, because time allows, may be closely examined in a scene.

Broadcast, cable, satellite and Internet DTV operators control the picture quality of television signal encoders using sophisticated, neuroscience-based algorithms, such as the [[structural similarity index measure]] (SSIM) video quality measurement tool. Another tool called [[visual information fidelity]] (VIF), is used in the [[Netflix]] [[VMAF]] video quality monitoring system.

Quantising effects can create contours—rather than smooth gradations—on areas with small graduations in amplitude. Typically, a very ''flat'' scene, such as a cloudless sky, will exhibit visible ''steps'' across its expanse, often appearing as concentric circles or ellipses. This is known as [[color banding]]. Similar effects can be seen in very dark scenes, where true black backgrounds are overlaid by dark gray areas. These transitions may be smooth, or may show a ''scattering'' effect as the digital processing dithers and is unable to consistently allocate a value of either absolute black or the next step up the greyscale.