Image retention in Target Display Mode (solved)

Hi,

I have a 2009 27" iMac which I use as an external monitor in target display mode. This has worked for many years but suddenly today I have a problem, which is reproduced by the following steps:

1) Start the iMac & login

2) Start the external computer (windows PC with DisplayPort output) and select the iMac display as the monitor

3) Press CMD+F2 to switch on target display mode. Desktop of the PC appears

4) Move a window on the PC desktop about on the iMac screen. A strong after-image appears in the previous windows position (this is known as Image Retention)

5) Press CMD+F2 to switch off target display mode. Desktop of the iMac appears, but flashing at about 25Hz, interlaced with a strong after-image of the PC desktop

6) Wait a few minutes. The flashing stops and the after-image of the PC desktop fades.

7) Move a window on the iMac desktop about on the iMac screen. No image retention is visible, system behaves normally.

Interestingly, the horrible flashing and image retention seen at step 5 can be seen even after removal of the displayport cable and a reboot of the iMac! I truly thought the system was fried when I first discovered this, but waiting ~5 minutes or so and the flashing gradually fades.

After much searching on the internet I discovered some other people with similar-ish experience. One person curiously reported that the problem only appeared when they had a second monitor connected to the external video source as well as the iMac. Since my external PC is a laptop I thought I'd experiment with this too, and instead of just using an external monitor I enabled the "extend desktop" feature so that the laptop display and iMac in target display mode were both active. Lo and behold, THE PROBLEM WENT AWAY!

Now, although this is the opposite of what the other guy was reporting, it started me thinking about what might be different between the two scenarios, and by trial and error I eventually tracked it down.

The Solution

=========

The image retention bug reported in step 4 & step 5 above is triggered by the PC graphics card monitor refresh rate being set to 50Hz. Switch it to 60Hz, and all is fine!

Try these steps if you want to experience the bug for yourself:

A) Start the iMac & login

B) Start the external computer (windows PC with DisplayPort output) and select the iMac display as the monitor

C) Press CMD+F2 to switch on target display mode. Desktop of the PC appears

D) Bring up the PC's "Screen Resolution" dialog and select "Advanced Settings". On the Monitor tab, change the "screen refresh rate" to 50Hz and press apply. The display will flash once as the graphics adaptor in the PC is reconfigured.

E) Move a window on the PC desktop about on the iMac screen. Enjoy the hideous image retention that results

F) On the Monitor tab of the PC dialog, change the screen refresh rate back to 60Hz and press apply. The display will flash once as the graphics adaptor in the PC is reconfigured, and then the fun will begin. Your PC desktop will now start to flash horribly.

G) DON'T PANIC. Just wait a bit and the flashing will subside. It takes a few minutes but eventually everything is now back to normal. No flashing, no image retention. You are now good to go.

Sounds like this is a bug to me. The weirdest bit is the way the flashing & image retention survive a full hard reset and power cycle of the iMac - must be a hardware issue with the graphics card in the iMac coupled with some hefty internal capacitors. Never seen anything like it!