Dual Link capture

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  • Posts: 2
  • Joined: Wed May 16, 2018 2:59 pm
  • Real Name: Robin Carlisle

Dual Link capture

PostWed May 16, 2018 3:06 pm


I am playing out 2160p60 from a Decklink 4K Extreme using 2x 6G outputs, using Media Express. The control panel is set to dual-link mode.

On a different computer I have a Decklink 4K Extreme 12G. I have run both cables to that and using Media Express I can see the video coming through, perfectly. So far, so good.

The issue comes when I use the C++ API. In the VideoInputFrameArrived() function I only get half the pixels, the odd lines. The other lines are black.

It looks like the dual-link mode interleaves the data and I am only reading the data from on the inputs. How do I get all the pixel data?

Hope someone can help.

Thanks and best,


Cameron Nichols

Blackmagic Design

  • Posts: 74
  • Joined: Mon Sep 04, 2017 4:05 am

Re: Dual Link capture

PostFri May 18, 2018 5:09 am

Hi Robin,

It is likely that physically there is issue with one of your dual-link SDI connections.

The Capture preview provided by Media Express is similar to IDeckLinkGLScreenPreviewHelper where the video frame is scaled to 1/4 height for 4K frames by dropping rows. So even with CH_B unplugged in dual-link setup, the capture preview looks OK in Media Express. With this setup, you can then grab still frame (Ctrl-G) and open in a TGA viewer to see dropped lines and poor chrominance



  • Posts: 2
  • Joined: Wed May 16, 2018 2:59 pm
  • Real Name: Robin Carlisle

Re: Dual Link capture

PostFri May 18, 2018 8:46 am

Hi, Thanks for your reply.

I did think that, because Media Express does drop the lines as you say. However, I have done some grabs in Media Express and I get the following results :-

ch_A only - every other line is green (which would be black if converted to RGB, I believe)
ch_B only - the entire picture is black
ch_A and ch_B - the picture is complete and seems to be perfect

So, it seems that Media Express knows how to deal with the dual-link better than my program written with the API.

Pictures attached. Had to crop 2 of the images to 1920x1080 because of file size limitations on this forum


chA_and_chB.jpg (459.39 KiB) Viewed 84 times
chB.jpg (250.02 KiB) Viewed 84 times
chA.jpg (892.56 KiB) Viewed 84 times

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