What is aspect ratio conversion (ARC)?

In many standards conversion applications, we need to modify the aspect ratio of the picture.  The problem of converting SD 4:3 material to HD 16:9 remains for stored assets, and complex aspect ratio conversions continue to be required when repurposing movie material for TV and mobile distribution.

Movies intended for cinematic distribution may be shot at 1.85:1 or 2.35:1 thus requiring aspect ratio conversion for distribution to viewers typically using 16:9 display screens.  Preservation of the entire width of the production may require “letterbox” presentation using black bands above and below the image.  Display avoiding the black bands implies cropping the content horizontally, then scaling the picture to occupy the full display height and width.

Some audiences may find letterboxing distracting, especially on small screens as found on aircraft seat backs or mobile phones, so centre-cutting followed by zoom may seem quite appealing.  However, the director’s artistic intent may be lost, especially if important action takes place away from the centre of the screen.  Manual or automated pan and scan can be beneficial as the editor can choose the most important scene areas, but nevertheless, some of the content will inevitably be lost.