Hi. How do I get Dolby digital from a movie recorded in 2.1 format

[rsj.auto]

Rxv-659 yamaha. I need to know how to decode movies recorded in 2.1 format to play in pro logic.5.1. The movies recorded in 5.1 format plays the surround but 2.1 only plays the fronts and centre.

 

[americanaudiophile]

You will need to select a surround mode for a stereo input.
https/www.manualslib.com/manual/197545/Yamaha-Rx-V659.html?page=53#manual