The ratio of television picture width to picture height.
A standard television display conventionally has an aspect ratio of 4:3 while a widescreen display is 16:9.
The physical aspect ratio of a given display device is fixed but the content presented may have a different aspect ratio.
Consequently a display may be either standard or widescreen and the material presented may be either 4:3, 16:9 or some other aspect ratio.
In DVB a widescreen image is transmitted by compressing it in the horizontal dimension. This is possible as horizontal resolution is less important to visual perception. The resulting anamorphic image would therefore appear distorted if presented without compensation on a standard display.
A DVB signal provides an Active Format Descriptor flag that is used to indicate the original aspect ratio of the material as set by the broadcaster. This can be used to provide dynamic switching of the aspect ratio.
The set-top box may provide an option to set the aspect ratio of the display as either 4:3 or 16:9. If this is not set correctly the image may be distorted when viewing certain types of material.
The set-top box may also provide settings to allow the user to establish preferences for how to present material that does not match the aspect ratio of the display. These may include letterbox, to allow widescreen material to be accommodated within horizontal borders on a standard display, or centre cut-out which displays only the central portion of a widescreen image.
If these options are not set correctly the image may be distorted when viewing certain types of material.
For interactive services, the display plane used to present text and graphics is generally logically independent of the video plane. This can present issues when dealing with different combinations of video and display aspect ratio.
Generally, the output of the graphics display plane is simply stretched to width of the physical display. Therefore a specific co-ordinate position on the graphics plane may not correspond to a specific pixel co-ordinate of the video as presented by the combination of set-top box and physical display. Another way of considering this is that the graphics plane is drawing on the glass of the display device rather than the video picture.
A further complication is the pixel aspect ratio of the video system. A PAL video image has a nominal resolution of 720×576 pixels, whereas the corresponding resolution in a square pixel computer system is 768×576 pixels. This must be taken into account when mapping from one domain to the other to avoid spatial distortion of images.