Mirroring forces wrong aspect ratio

Hi, I'm using mirroring on OS 10.9.5 (Mac Mini 6GB RAM) to go out to my TV. It's been working fine for years, with the computer monitor still having the correct aspect ratio for that display, and I assume the TV may have had bars right and left, but I never noticed any. I don't know whether the the TV was fitting or the computer monitor was overscanning, but both looked fine. Now today, I started the computer, and the monitor has black bars top and bottom to force true 16:9m whereas previously, the computer monitor display has been full with no black bars. I opened the display panel and saw resolution had gone to the TVs resolution, so I changed it back to the monitor's native resolution, but changing that setting has no effect on the display. It stays at 16:9 instead of filling the 1600x1200 area, as it has been doing previously. Anyway, does anyone know what may have caused this change, and/or any suggestions as to how to fix it? I already checked the monitor's built-in settings for any vertical expansion options, but I guess that went out with the CRTs. Thank you for your help!