Why did my Mac Mini display res change when I changed my Apple TV monitor?

I have an old Sharp Aquos LCD TV (LC-26GA4UM) that I’ve been using since 2004. In 2010, I bought an Apple Mac Mini (A1347) and have been using the Sharp TV as it’s display. For reasons I don’t recall now, I connected the two via the DVI port on the TV and the HDMI port on the Mac using a DVI-to-HDMI adaptor. Whenever the display connection was established, the TV displayed that it was using 1366x768@60Hz. I had this arrangement upstairs in my home office for many years. At some point, I bought an Apple TV (1st gen) and used an old Philips LCD TV connecting both of them via each of their HDMI ports using an HDMI cable. The Philips TV and the Apple TV were displaying 1080p. I had that arrangement downstairs in my family room for many years. Occasionally, I had the need to use AirPlay to display Mac Mini video output to the Phillips/AppleTV setup, and it would switch displays and resolutions just fine when I enabled then disabled AirPlay. And all was working okay in HD resolution land.

Recently though to bring myself up-to-date, I bought a new 4K TV and a new 4K Apple TV, so in preparation to setup that up, I decided to relocate the Phillips TV to my bedroom, and downgraded the monitor for the original Apple TV with an even older SD LCD TV that doesn’t support HDMI - It’s a (ahem) “TruTech” TV that only has component connections up to 720p. So I connected the Apple TV with an HDMI adaptor that outputs component video. And that worked okay.

Over the weekend, I had the need to enable AirPlay as I’ve done before, and the Mac Mini switched displays and resolutions just fine, but when I disabled AirPlay, the Mac Mini didn’t switch back to HD 1366x768 output for the Sharp TV, keeping the display at 720p or 1080x1024 output. Viewable but annoying as a computer monitor with small text. When I went to change the display resolution in Preferences, the menu choices that I was accustomed to seeing for the Sharp TV were changed - drastically reduced to only a few lower res options, with one exception - 1080i. When I selected that option, the display went black with the message from Sharp “Signal Incompatible With Input” (or something along those lines). I noted that I could go back to 720p by hitting return, and sure enough it did. So I tried 1080i again, and again it was not supported, but unfortunately i hit another key instead of return (possibly the up-arrow key), and from then on, I was locked out of changes to the display and couldn’t see anything. I had no choice but to hard-reset the Mac Mini using the power button with many applications running, and I could see the gray boot screens and Apple logo (while the Sharp TV displayed the 1366x768 DVI subwindow), but when the boot was finished and the desktop was started, the screen went black, the “incompatible signal” appeared, and the Sharp TV displayed dashes for it’s resolution. Multiple times I hard-booted the Mac Mini and saw this same behavior each time.

Using an iPad, I started a chat with Apple and they informed me to reset the NVRAM/PRAM using Option-Command-P-R at power-on but that had no effect, then boot up to “safe mood” which allowed me to see the login screen (again with the Sharp TV displaying 1366x768) but when I logged in and the desktop was started, the Mac Mini would switch resolutions back to something unsupported (I think it’s remembering 1080i). Again this was tried twice with the same behavior.

After giving up for the night, I returned the next morning to try and solve this, or at least improve upon it. Pulling the Sharp TV out, I realized the HDMI port was not being used, so I installed one, unplugging the DVI-HDMI adaptor from the Mac Mini, and switched the input on the Sharp TV. Viola! A display. But I realized it was 720p, and when I went to the Display Preferences, again only a couple of low res choices were available, along with 1080i which I wasn’t going to touch. So I switched the cables and input back to DVI, and again the “incompatible signal” message was presented. Switching the cables and input again back to HDMI, the 720p display came back. On a whim, I enabled AirPlay and mirroring, and now I could see two Display Preference screens, one each for the Apple TV and the Sharp TV on the Sharp monitor. Doing this, a 1080p option was available as a resolution and when I selected it, the display changed to “high res” but it clearly was not optimal for use for the text was hard to see, almost like it was “high-res screen scaled down to a low-res display”. When I disabled AirPlay, the Sharp TV switched back to 720p. With the Sharp TV still on HDMI input, I restarted the Mac Mini, and now the boot screens were not visible until the desktop was started, which was still 720p. What’s up with the boot screens disappearing now?

Pulling the Mac Mini out, I realized I wasn’t using the Thunderbolt 2 port so I went to the Apple Store and found a Mini DisplayPort to HDMI adaptor by Belkin. Back at home, I now had the HDMI cable plugged into the Thunderbolt adaptor and the HDMI port on the TV, while at the same time had the DVI cable plugged into the HDMI adaptor and port on the Mac and the DVI port on the TV. Upon boot, the desktop stayed at 720p but two Display Preference screens were visible one each for the Sharp DVI and Sharp HDMI connections. However now I noticed there was 1 1280x1024 resolution available as well as a 1280x728 resolution. Playing with these two resolution settings and mirroring changes, I finally was able to see the screen at the high res setting without it being streched or squashed, and the Sharp TV display 1366x768, as I have been trying to get it to do since the day prior. So I tested the restart and again the boot screens could not been seen but the high-res display returned when the desktop was started. So I unplugged the HDMI port cables leaving only the DVI and the screen remained unchanged. When I went to restart the Mac, there just happened to be a software update (High Sierra 10.13.6) so I opted to upgrade noticing that support for AirPlay 2 was added. During the install, the screen went black, but I could see a white Apple logo and the Sharp TV still displayed the 1366x768 subwindow. Then it restarted, and voila! I could see the gray boot screens again while the install completed. When done, the desktop started and the high res screen was still working okay with the TV still displaying 1366x768 input.

So finally, after almost 24 if switching resolutions, cables, rebooting, rebooting with options, and installing, I finally got the Mac Mini and Sharp TV looking like it did before enabling AirPlay. I’m afraid now to try AirPlay for fear of getting back into this display ****, and so I’ll just go without for a few months since I don’t need to use it.

So after all of that, this is what I think and this is my question. With this older model Sharp TV, it doesn’t have the ability to set or reset resolutions at the TV itself - It is dependent upon the signal received to set/change the resolution. I think that the Apple TV somehow downgraded the Mac Mini’s display resolution, and for some reason resetting it’s NVRAM would not clear the stored settings for the Sharp TV. If I’m right, why oh why, would the Apple TV change the NVRAM configuration for the Mac Mini?

I would like to know how this all works before I setup my new 4K TV and 4K Apple TV. Will installing them impact the Apple TV and low-res TruTech TV config? Or impact the Mac Mini and Sharp TV config? If so, why would that be? If these Apple TVs much change the configs for the other TVs to match displays, shouldn’t they also return them back to their previous settings/displays when AirPlay is turned off? I would hope Apple would not allow one Apple TV to change the NVRAM settings of another Apple TV or another Mac PC so that when these devices are restarted, they default to their already golden display settings. I hope. And I hope someone can explain what caused me to delve into this 24-hour display **** just be enabling and disabling Apple TV. Thank you :-)