Video Aspect Ratio
Video comes in several aspect ratios. The two main ones are widescreen (16:9) and standard (4:3). The aspect ratio of an image describes the proportional relationship between its width and its height. It is commonly expressed as two numbers separated by a colon, i.e., 16:9. Preferred aspect ratio is 16:9 (formatted for HD screens). For vertical video, the recommended aspect ratio is 9:16 (most phones), 3:4 (iPad), and 10:16 & 2:3 (other phones/tablets).
For more information please refer to the Digital Video In-Stream Ad Format Guidelines doc section 2.2.1. This document provides useful information on frame rate, audio data, etc. We also refer to HLS and recommend using these files to create the adaptive bitrate file fragments. In addition, section 2.2.2 focuses on SSAI through a “Mezzanine file” that can easily be used to transcode on the fly. 
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