Dual Link capture

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robsterooni

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

Dual Link capture

PostWed May 16, 2018 3:06 pm

Hi,

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,

Rob
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Cameron Nichols

Blackmagic Design

  • Posts: 114
  • 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

Regards
Cameron
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robsterooni

  • 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

Thanks,

Rob
Attachments
chA_and_chB.jpg
chA_and_chB.jpg (459.39 KiB) Viewed 468 times
chB.jpg
chB.jpg (250.02 KiB) Viewed 468 times
chA.jpg
chA.jpg (892.56 KiB) Viewed 468 times
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Cameron Nichols

Blackmagic Design

  • Posts: 114
  • Joined: Mon Sep 04, 2017 4:05 am

Re: Dual Link capture

PostTue May 29, 2018 1:06 am

Hi Rob,

The captured images are expected, the dual-link SDI capture setting is determined by the received payload ID in the SDI frame. When you have CH_A only, the capture card is expecting CH_B, so only half the pixel data is received correctly.

When you have CH_B, the card cannot detect the appropriate format based on the received payload ID, so you only capture black.

But the dual-link/payload is processed by the DeckLink card itself. The received video frame in the VideoInputFrameArrived callback in DeckLink SDK will be composite frame and similarly as captured by Media Express.

You should also check the dual-link setting for output from the DeckLink 4K Extreme. The dual-link output setting is configured by either Desktop Video Setup or with bmdDeckLinkConfigSDIOutputLinkConfiguration Configuration ID.

Regards
Cameron

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