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variety.com

To Deliver ‘The Simpsons’ in 4:3 Aspect Ratio, Disney Plus Had to Rearchitect Its Content-Delivery System
To deliver "The Simpsons" episodes in 4:3 format, Disney Plus had to reconfigure its content-delivery engine.

So what took so long?
Basically, the Disney Plus team had to reconfigure its content-delivery engine and build a brand-new feature to accommodate a concept it hadn’t previously anticipated: Giving viewers access to the same content but with different underlying video attributes, according to Joe Rice, VP of media product at Disney Streaming Services. And, it had to do that without breaking any of the existing features of Disney Plus.
Some background: Disney Streaming Services uses the MovieLabs Digital Distribution Framework (MDDF) industry standard, which defines the methods for representing content assets and associated metadata. The issue was that the existing content model assumed only one video edit (while multiple audio or subtitle components could be associated with that video asset) for each Entertainment Identifier Registry (EIDR) ID tag.
So the DSS team revamped its content model to introduce the concept of multiple media “facets,” or multiple combinations of audio, video, and subtitle components. Now, components delivered under the same EIDR ID can be grouped in multiple combinations, laying the groundwork to support user-selected aspect ratio preference while maintaining existing content interaction features.