1080p from iMac via BMD to monitor - Works for 10 seconds

I'm having some trouble gettinng a monitor to work with my iMac, via SDI using BMD Microconvertors.

The setup
The iMac uses a Thuderbolt-to-HDMI adapter for the second monitor.
Connecting this directly into the monitor works without any issues. Has been like this for over a year.

The requirment
I would like to move my monitor some distance away (about 15 meters).

The setup
The iMac is set to output 1080p/60
[HDMI] -> [ BMD HDMI to SDI 3G] -> [BMD SDI to HDMI 3G] -> Monitor
The SDI cable is a known-good cable, 15 meters long, tested and working pergfectly in another setup.
The Microconvertor are both powered with BMD wallwart PSUs.

The issue
The monitor gets a good, stable, very clear image for about 10 seconds then goes black.
The monitor does not complain of no-signal. It seems to be getting a valid, but black, signal.
The iMac does not complain - it still thinks it is outputing the image.
Both green lights are on, on both convertors.

Both convertors have the latest firmware.

I have tried replacing each convertor in turn, same problem every time.

Any idea why the image goes black after a few seconds?

Sounds like an HDCP issue - Macs love to turn it on even when not needed (no protected content being displayed) and no BMD gear can decrypt such protected signals.

When this is an issue, you often get a few seconds of clean video before the Mac decides to encrypt it as the HDMI handshake completes, which sounds exactly like your issue.

In your case, since you're just using this as an HDMI extension, you're probably better off with an active or optical HDMI cable which should just work.

Sounds like an HDCP issue - Macs love to turn it on even when not needed (no protected content being displayed) and no BMD gear can decrypt such protected signals.

Oooh - yes that would make sense! Thank you

Interesting, because I have another iMac at another venue, a newer model iMac but with the same setup, and it works. In fact I was playing Amazon Prime videos on it last night - via BMD Microconvertors! Now I'm wondering how on earth that even worked? Ahh well...

I will go an investigate HDCP on iMacs and see what I can find.
Thanks again for your help!