How were the video screens in Chaplin's Modern Times filmed?

The factory scenes in Modern Times feature huge futuristic screens used to make video calls. They obviously didn’t use real television screens, as technology was not advanced enough to do it. The screens are huge, and the image is very clear. Many movies made several decades later don’t achieve the same level of quality in their screen-within-a-screen scenes.

Was it projected, while taking care to not have flickering due to slightly mismatched frame rates, and not to illuminate the surroundings with the projection, with a lot of manual editing? Some other techniques? How did they achieve such amazing level of quality?