decklink HDMI out RGB, not YCbCr

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Vincenzo Ferreri

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decklink HDMI out RGB, not YCbCr

PostSun Aug 19, 2018 6:41 pm

I have connected a Decklink Studio 4k through HDMI to an OLED tv, using 10bit Full range settings on Resolve 15. On the TV I put accordingly "Full", but I have 2 options here: HDMI RGB range and HDMI YCbCr.
The one working changing to Full levels was the YCbCr (i checked with low pluge bars), instead I want to make a connection with HDMI RGB out, that is more precise color output. The tv can connect to UHD RGB 4:4:4@60Hz. With these initial tests I used only FHD, so also a lot less bandwidth.

How should I set the decklink to get this RGB out with HDMI?

BTW, I tested with a direct pc HDMI out with RGB 8bit Full and it works...
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Andrew Kolakowski

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Re: decklink HDMI out RGB, not YCbCr

PostSun Aug 19, 2018 7:10 pm

Your card is based on HDMI 1.4 chip.
There is no support for RGB mode on UHD at any fps:

"HDMI Color Precision
10-bit RGB HD
10-bit YUV in HD and 4K"

Even HDMI 2.0 has not enough bandwidth for UHD 10bit RGB at 60Hz (only 50Hz is doable). Most hardware provides support for UHD 50/60Hz 10bit using 4:2:2 sampling.
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Vincenzo Ferreri

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Re: decklink HDMI out RGB, not YCbCr

PostSun Aug 19, 2018 7:57 pm

Yes, it is 1.4, but that does not mean I can't connect with RGB 444 10bit with FullHD. I don't need 60Hz too, it was only a test just for saying the tv can display that, it hasn't a bandwidth limit.
So, let's say I want a FHD 10bit RGB 444 25p on HDMI, that is completely compatible with 1.4. Instead now I have YCrCb and in Resolve I see RGB444 only under SDI tab...
I read a BM answer saying: "Ive just checked this for you and the specification for 2K video on the DeckLink Studio 4K, is 10bit RGB 444."

How can I reach this?
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Andrew Kolakowski

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Re: decklink HDMI out RGB, not YCbCr

PostSun Aug 19, 2018 9:09 pm

I thought you can get HD RGB, just not UHD RGB.
RGB should work for HD.
Try this:
- set monitoring to HD 23.976p (don't use 50p/60p as this is going to be 4:2:2)
- set in Resolve monitoring settings: "Use 4:4:4 SDI"

Do you get RGB connection to TV?
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Vincenzo Ferreri

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Re: decklink HDMI out RGB, not YCbCr

PostMon Aug 20, 2018 6:08 am

Thank you, that did it! But I suggest calling that a generic "444 output"... I've not tried it before thinking it was only for SDI...

Anyway, it works also for RGB 1080p 10bit 60Hz. At UHD only YCbCr.
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Andrew Kolakowski

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Re: decklink HDMI out RGB, not YCbCr

PostMon Aug 20, 2018 2:53 pm

HDMI on BM cards is like a "aside output". Main processing and settings are done for SDI.

Vincenzo Ferreri wrote:Anyway, it works also for RGB 1080p 10bit 60Hz. At UHD only YCbCr.


Oh yes, forgotten this is SDI 6G card. On standard 3G one it would be most likely but limited to 4:2:2 on HDMI (as 3G SDI can't be 60Hz and 4:4:4 at the same time)

Just make sure it's real 4:4:4 for 50/60Hz, not 4:2:2 converted for HDMI output to 4:4:4. Bars will have pin sharp edges if the are 4:4:4. Red square on black background will also show it well.
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Vincenzo Ferreri

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Re: decklink HDMI out RGB, not YCbCr

PostTue Aug 21, 2018 9:00 am

Yes they are solid reds, real RGB.
After all, these are inside the HDMI 1.4 bandwidth spec.
I read on an article:
HDMI Bandwidth is rated in Gigabits per second.
HDMI 1.0->1.2: ~4 Gb
HDMI 1.3->1.4: ~8 Gb
HDMI 2.0: ~14 Gb (NEW)
[...]
A second of 1080p 8bit 60FPS data requires ~3 Gb of bandwidth, well within the range supported by the original HDMI 1.0 spec (though why we even had to deal with 1080i is a good question, probably due to the inability to even meet the HDMI spec).

To compare, a second of 1080p 12bit 60FPS data requires ~4.5 Gb of bandwidth. Even 1080p 16bit 60FPS needed only ~6 Gb, well within the range supported by HDMI 1.3 (where Deep Color was introduced). Plenty of headroom still. Only when we push 2560×1440 12bit 60FPS (~8 Gb) do we hit the limits of HDMI 1.3.

So, 10bit 1080p @60Hz is way below the top limit bandwidth of 1.4.
No way on UHD instead...
I could try with 6G SDI with a SDI-HDMI 2.0 adapter, but it's a lot pricey, still limiting to UHD@30Hz. At that point it's better to buy an Extreme 12G...
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Andrew Kolakowski

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Re: decklink HDMI out RGB, not YCbCr

PostTue Aug 21, 2018 10:06 am

Yes, they are easily within HDMI bandwidth, but you have to understand how card works. It's processing in main engine which dictates real precession 4:2:2 vs 4:4:4. This can be later "adjusted" for specific output. It would be bit unfair and bad practice to process with 4:2:2 and then present this signal on HDMI as 4:4:4, but it can happen, so always better to verify these things.
For example if you connect 3G SDI device then your HD 60p will be 4:2:2 on SDI (so main RGB signal needs to be converted to YUV), but 4:4:4 on HDMI. Quite often SDI dictates precision, so this why it's better to check. Depending on design card works internally either in RGB or YUV. Looks like 4K Studio has RGB processing for HD and YUV for 4K.
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Vincenzo Ferreri

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Re: decklink HDMI out RGB, not YCbCr

PostTue Aug 21, 2018 10:48 am

Ok.
I was wrong on the 6G: I read a BM answer
Please note that the SDI output of the DeckLink Studio 4K is a maximum of 6G.
This means that any 4K formats will also use YUV 4:2:2 color sampling as there is not enough bandwidth over 6G to use RGB 4:4:4.
You would need to use a DeckLink 4K Extreme 12G in order to output 4K RGB 4:4:4 however this card supports 8, 10, 12-bit RGB 4:4:4 in all modes up to UHD 30p, and 8, 10-bit YUV 4:2:2 in all modes above this.
This means that any 4Kp50/60 content will always be sampled as YUV 4:2:2


So, yes, SDI UHD only YUV... and Extreme has RGB 444 UHD only upto 30p, above this again YUV.
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Andrew Kolakowski

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Re: decklink HDMI out RGB, not YCbCr

PostTue Aug 21, 2018 12:13 pm

Yes, BM answer actually points you to fact that even if HDMI/SDI has enough bandwidth for RGB output you may be still restricted by internal processing capabilities.

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