External HDMI monitor on iMac problems

Feb 14, 2022
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Hi, We had an Intel iMac for years as a photography desktop using a Vizio 54' monitor to mirror the display for students to view. It worked perfectly every time.

Adobe no longer supports the Intel iMacs with their newest version of Lightroom, our main tool.

Our first upgrade attempt was a Mac Mini M1. We had troubles with both the Dell monitor purchased for that Mini and the above Vizio. Eventually, Dell told us they did not support the monitor in an M1 environment - so we returned both the Dell monitor and the Mac Mini M1.

Our second attempt is our present 24" iMac M1. That took two tries because we didn't understand (and neither did the Apple rep) that the iMac required a special adapter to use with a VESA mount - already installed for the Intel iMac. The new iMac was loaded via a cable transfer from the retiring mac - so all the apps and files should be identical. We also upgraded our family of photo products from Adobe and others to the current M1 version. We have since updated MacOS several times while looking for our problem

We attached the Vizio to the iMac M1 initially with a displayport to HDMI adapter purchased for the upgrade. This gave the new iMac an HDMI port and an SD card.

When using the Vizio as an extended monitor, the setup performs perfectly.

When configured as a mirrored monitor, after a few minutes the Vizio "becomes" the main monitor - that is mirrored on the internal iMac monitor. Two issues develop at this unpredictable time:

1. In the display monitors, the brightness control normally associated with the iMac monitor is replaced with the "overscan" slider associated with HDMI monitors.
2. The HDMI overscan monitor is replaced by a non-functional brightness slider - since HDMI brightness is controlled by the monitor not the host.

At this point, the Vizio and iMac color profiles are interchanged, making photo display chancy.

During the process with Applecare, they requested we eliminate the display HDMI adapter and replace it with a HDMI<>USB-C cable plugged directly into the iMac. We did this trying all 4 iMac ports - same problem.

The iMac is not usable in a photo display environment with this problem.

As a guess, we purchased a Mac Air M2 - the monitor works as designed.

We also tried an older MacBook Air intel, the monitor works as designed.

We are guessing this problem is related to the M1. Any idea how to proceed?

Thank you for reading this far.

Cory Cooper

May 19, 2004
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Hello and welcome.

Sounds like all the troubleshooting you did yourselves, as well as with AppleCare pretty much exhaust any other suggestions. I second the use of an Apple USB-C Digital AV Multiport Adapter.

It may very well be an M1 issue. I have only used an M1 mini with a single display connected via the HDMI port, but I don't have any experience with the M1 iMacs and multiple displays.


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