Apple TV stream issues on some channels
Apple TV, Gen 1. Some TV channels won't stream at full screen when mirrored from Apple MacBook Air
MacBook Pro 13″, macOS 10.14
Question marked as
Apple TV, Gen 1.
The first generation Apple TV box from 2007 isn’t supported anymore in any meaningful way. Your model probably isn’t 1st generation, but instead a later model. The 1st generation never did support AirPlay streaming.
Identify your Apple TV model - Apple Support
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Some TV channels won't stream at full screen when mirrored from Apple MacBook Air
AirPlay Screen Mirroring captures the screen and sends that as a live re-encoded movie to the Apple TV box. Going from 4:3 screen to 16:9 TV will have some undefined space, filled as pillarbox. When the content itself is letterboxed as well, then it may display with a black border all around.
If your app supports it (iTunes, Apple TV app, QuickTime Player, Safari web video, Prime Video app), then use in-app AirPlay controls instead. Then only the content stream is send to the TV, retaining 16:9 as 16:9, or keeping wider formats (e.g. 2.35:1) at screen width.