I got my HDHomerun Prime going today and some of my channels are HD and others are SD. The HD channels look fine, but the SD channels are surrounded by a black border, both on the left and right, and the top and bottom. Is there a way to change the output to stretch the content to fill the screen?
Go to the settings tab of the app and select the Stretch to Fit option.
That does stretch the content the width, but not the height. The content still has black bars on top and bottom.