DIY 5k Monitor - success :-)

[KayPee]

Welcome to the forum KayPee.

Your iMac display panel is not 10-bit, unfortunately. The late 2015 and later are 10-bit panels.
PaulD-UK can correct me, if I am mistaken.

@Regulus67 Thank you so much for the warm welcome and clarification!

That makes things much clearer then. I wasn't aware that the late-2014 wasn't 10-bit. I'm happy with the display quality as it is currently on the iMac, so if that's 8-bit then all good!

However, I did read on this thread that although it's not a true 8-bit panel, the iMac does some magic to make it "appear" like 10-bit. Do you know if there's any truth in this? If so, would the set-up with the R1811 and one Thunderbolt 3 to DP1.4 cable support this? (Sorry if this is a silly question!)

Basically I want to maintain the same level of display quality that my iMac has currently, whatever that may be.

Also, a couple of additional questions if you don't mind:

• Do you think the R1811 is still the best choice considering I don't need the two DP1.4 ports? Am I better off saving some money and going with the older R9A18? Cost is a factor but I don't mind spending a little extra for better quality and future-proofing.
• How bad is the low brightness issue? I've seen posts about people installing additional boards to pump the brightness up. Do you think this is necessary? Currently, my iMac brightness is set to max during the day.
• Do you have any recommendations for installing a FaceTime camera? i.e. where this would plug in?

Thanks so much

 

[PaulD-UK]

@KayPee
@Regulus67 is right. 10 bit-capable iMac displays were introduced in later generations of iMac.
For 10 bit colour it should be 1.07 billion of colours.
For 8 bit colour you can resolve only 16.7 millions of colours.

For the late 2014 Retina iMac Apple quotes:
iMac (Retina 5K, 27-inch, Late 2014) - Technical Specifications
Display
27-inch (diagonal) Retina display with IPS technology;
5120‑by‑2880 resolution with support for millions of colors

And a screen panel database website says: Color Depth16.7M 100% sRGB
sRGB colour space is by definition only 8 bit.

Late 2014 iMac 5K panels are mostly LG LM270QQ1 SD(A2), but may also be A1 or A3.

Interestingly for the late 2015 iMac, Apple's Tech specs are the same:
iMac (Retina 5K, 27-inch, Late 2015) - Technical Specifications
Display
27-inch (diagonal) Retina 5K display with IPS technology
5120‑by‑2880 resolution with support for millions of colors*

Only in 2017 does this change:
iMac (Retina 5K, 27-inch, 2017) - Technical Specifications
Display
27-inch (diagonal) Retina 5K display
5120‑by‑2880 resolution with support for one billion colors
500 nits brightness
Wide color (P3)

Late 2015 iMac 5K panels are LG LM270QQ1 SD(B1)
and late 2017 iMac 5K panels are LG LM270QQ1 SD(C1)
Later panels are D1-F1.

*The 2015 panel is actually capable of 10 bit P3 colour.
This Apple presentation confirms this, and explains why it matters:

Get Started with Display P3 - WWDC17 - Videos - Apple Developer

Wide color displays allow your app to display richer, more vibrant and lifelike colors than ever before. Get a primer on color...

developer.apple.com

The basic fact is that ALL the iMac 27" 5K panels are natively only 8 bit.
However after 2015 they are capable of showing 10 bits by using FRC, which use a temporal dithering method that combines successive colours in the same pixel to simulate the desired shade. a process of rapidly switching two 8 bit colours so the screen looks like 10 bits.

"I'm just not sure whether a R1811 board would allow me to achieve this over one cable. Some posts say that DP1.4 isn't sufficient to support 5K @ 60 with 10-bit over one cable. You need to use two ports, buy an expensive splitter and reflash the firmware. If this is the case, I'm better off saving the money and getting the older R9A18 board and getting 2 DP1.2 cables.
Other posts say that you can achieve 5K @ 60 with 10-bit over one Thunderbolt 3 to DP1.4 cable, which would be great."

DP 1.4, which is capable of @joevt's 6) HBR2 x4 with DSC at 12bpp can do 5K at 60Hz 10bpc.
So all the available boards in my list of 10 bit ones do 5K60 10bpc.
Except, as you say, the R9A19 V1.1, which only does 5K60 8bpc over a single cable.

For colour accuracy I personally think the R1811 V4 board is probably the best, because it has the most fully featured firmware, from the Taiwanese chipset manufacturer. It tells you the current bit depth it is working at.
Other boards appear to use firmware developed by the retail channel in China, and their boards are repurposed 4K 144Hz monitor boards.
However, they all seem to use the same Realtek RTD2718Q video controller chip.
So performance is dependant on firmware supplied, and only the R1811 and R9A18 have easily available firmware support.

One further factor. The LM270QQ1 A series screens can suffer from image retention issues.
There is an example from a a few posts back in this thread.
Later screens are far less likely to suffer from this, although the 2015 B1 screens can develop pink edge coloration.

Edit: "How bad is the low brightness issue?"
I think this is a non-issue.
It appears that 'low brightness' screens are re-glassed broken screens available from China, which have had their LEDs replaced or the wiring damaged in some way.
I don't know if this is the whole of the problem, but if you start out with a good screen, and are careful in the build process, these boards all give a good result.

The other problem is that non-retina iMac screen driver conversion boards available are much older designs, and have backlight CC boards incapable of driving iMac screens at full brightness.

The retina 5K iMac boards cost about 10 times as much, and don't have this problem.

Facetime cameras have to be USB. Several people in this thread have built conversions using various cameras, either from older 2009-2011 iMac (or Apple Thunderbolt Display) cameras.
Or from AliExpress 1080p or 4K ones.
This is an example.