This term describes a process which guarantees that the original aspect ratio os maintained (= OAR), even if the output device or video standard demonstrates a different aspect ratio (both generally 4:3 (1.33:1) or 16:9 (1.78:1), whilst film is often created with an aspect ratio of 2.35:1 or 2.40:1). For this purpose, horizontal black bands add to the image until the required aspect ratio is achieved.
The process is becoming particularly important in the case of anamorphic storage on DVD. In this case, the video with an aspect ratio of 16:9 (may contain coded bands if the OAR was > 1.78:1 has been compressed to the SAR (Storage Aspect Ratio) used. In the PAL area of application, the SAR for DVDs is 720:576 (the pixels are therefore strongly rectangular, whilst they are only slightly rectangular for non-anamorphic saving but never look square).
A connected widescreen television will stretch the image accordingly. For playback on a device with an aspect ratio of 4:3, however, strong distortions appear. For this reason, each DVD player should offer “Letterboxed” output. If it activated, the electronics compress the signal being played back and fill up the free areas with black bands until an aspect ratio of 4:3 is achieved at an SAR of 720:576. Although this means that vertical resolution is lost, it allows a correct display to be achieved.