silver imac displaying collage photos

LG C9 and MacBook Pro 2021, H10 HDR, Chroma 4:4:4 and 4K@120Hz (someday)


OK, this is so embarrassing, I’ve had my LG C9 panel for two years now (got as the Pandemic was happening December 2019) and I’ve actually never had it optimized for output. The instructions should be similar for the LG CX and C1, but here are all the crazy steps you have to go through:

  1. LG PC Mode.This is actually quite tricky to setup. But with the LG C9, the icons that appear in the HDMI input menus means something (as it does on Samsung). When you plug your laptop in, you want to change the icon to a “PC” because that invisibly sets it to set the black level. To do this, you go hit the Home button and then click on Home Dashboard. Then at the upper right of the screen is an icon that looks like a pencil. Click on it and then you can use the mouse pointer to click on the icon for your inputs and change them all to PC for your MacBook Pro 2021 connection.
  2. What this does is change the “crushing” of color inputs. With traditional television, they only accept a limited range of 16-235 because old TVs could handle really bright and dark images, but with modern monitors, you want it to accept the entire range of 0-255. It also minimizes the input lag, so when you are gaming, the pixels are being drawn really fast.
  3. By default the HDMI inputs are set to accept two color depths. That is how many bits of color per channel. The default is 8 bits of color or 10 bits of color. And then to get that across an HDMI 2.0 connection, it has to compress the color information. this is a little complicated, but even 4K displays will do compression because there is so much data with 8-bits of color across four channels. So they will typically take 4 bits and do Chroma Subsampling. Normally, the color selection is uncompressed, so with three color channels that a 8-bit color (that is 256 bits for the three color channels called a color space Y’CbCr where Y is the luminence or how bright a pixel is that then Cr is the differences to get you to red and Cb is the difference to get you. So uncompressed, what 4:4:4 means is that for a 4 pixel wide block, the next set are the number of samples taken horizontally. That is you get all the detail but with 4:2:2 you compress the next two channels, so you only get 2×2 set of chroma for a 4×2 set of pixels. This works because humans can’t see colors that well. And 4:2:0 means that for every 4×2 block, you only get 2 x 1 sets of colors.
  4. LG Deep Color (aka HDR). For the HDMI port, you need to make sure Deep Color is selected, this basically means that the full levels are being shown. This is set by switching to your MacBook input and then hitting the Settings button and then All Settings > Picture > Advanced Settings > HDMI Ultra Deep Color and then making sure this is turned on. What happens is the “SDR” default mode which is 8-bits of color and 4:2:0 chroma compression or 10-bits of color at the same 4:2:0 at 4K can now go to 8-bits with 4:4:4 or 4:2:1 which is nicer.
  5. Since LG supports HDMI 2.1 on all HDMI ports, this is great because at 48Gbps. The translation here is that it allows 12-bit color (wow!) at 4K with 120Hz refresh rates on their C9. The CX is actually a little slower supporting 40Gbps, so they can only do 10-bit color although this canadian article implies that 12-bit is supported now in the C1 at 4:4:4 60Hertz and 4:2:2 at 120 Hertz. This HDMI 2.1 allows HDR10+ (not supported on the C9) which means it can do per frame compression of the high definition color and variable refresh rates on the panel too. Also internally these panels are really doing 4:2:2

These settings enable the LG C9 and later to accept up to full Black with 4 K at 120 Hertz at uncompressed 4:4:4 chroma with . Woohoo! YOu can dive in deep and see what the television is actually displaying by going to the diagnostic menu:

  1. Choose settings on the remote and then All Settings > Channel.
  2. Now here is the tricky part hover the mouse over the Channel Tuning so it highlight but don’t click on it.
  3. Now type 11111 on the remote and you will enter a diagnostic menu
  4. Right arrow to the HDMI Mode and it will highlight and press the clickwheel

You will now see the diagnostic menu which will tell you:

  1. The resolution hopefully 3840 x 2160 which is 4K
  2. The chroma sampling which should say 4:4:4
  3. The number of bits which hopefully is as high as possible 8, 10 or 12
  4. And the color space which should be REC.2020 for HDR

MacBook Pro 2012 HDMI 2.0 limit to 4K@60 4:2:2

Well, if you plug in a cable (I actually had a bad one) that is HDMI 2.1 certified so it can handle the full 48Gbps, you are going to be unhappy because Apple in their infinite wisdom supplied an HDMI 2.0 port. This matters because HDMI 2.0 is only 18Gbps.

What that means practically is that with a standard HDMI cable connected to the LG, the most you should be able to see is 3840 x 2160 at 60 Hertz and Chroma sampling of 4:2:2. And in fact, if you plug a MacBook Pro in, you should see the panel light up. Go to System Preferences > Displays and then right-click on the icon which is the LG and you should see Refresh Rate of 60Hz and then if you have done this right there should be a checkbox for High Dynamic Range. Click that and you get 4K@60 4:2:2 and you can verify it at the diagnostic menu. YOu should also the letters HDR popup for a second at the upper right of the screen

Kind of disappointing when the display can handle 4K@120 4:4:4

MacBook Pro 2021 limited to 4K@60 in hardware

Ok, the first thing to note in the specifications is that it says that M1 Max can support 4 displays at 60 hertz, so there seems to be a hard limit here that is not due to any specifications since Thunderbolt 4 (same as Thunderbolt 3) can support 40Gbps.

This is kind of sad and is probably because there is a limit the bit rate clock or something like that. It is really too bad because the internal display can run at 120 Hertz, so why can’t we do that for a bigger monitor

The confusion of Thunderbolt 4, USB 4 and DisplayPort 2

As an aside technically what is happening is that the USB C connector in the Macs supports Thunderbolt 4. This means that they have pins that supply power up to 100 watts using USB Power Delivery (PD) and then they have data pairs as well. There are four pairs and each is 10Gbps so that’s how you get to 40Gbps. One small note is that because it is so fast, Thunderbolt 3/4 cables only go 40Gbps at 0.5 meter and drop to 20Gbps with > 0.5-meter cables or you need an “active” cable which is more expensive.

Now Thunderbolt 3 has the same maximum bandwidth of 40Gbps, but Thunderbolt 4 lets you divide it better. Specifically, you can multiple downstream TB4 devices while TB3 is only a daisy chain. Also, TB4 allows two 4K displays or a single 8K display. And if you using PCI Express (TB4 is really PCI express protocol over longer wires), you get 16Gbps PCIEx for TB3 and double that for TB4. And TB4 has more security so you can’t get hacked as easily by evil Thunderbolt devices.

What is going on here is even more confusing technically. What Thunderbolt 4 really is, is the ability to send over a twisted pair one of three protocols, USB 4 for data, 4xPCI Express 3.0 for connection to eGPUs and DisplayPort for video, and also other lines that can send just power. For Thunderbolt 3, it supports 4x PCI Express 3.0, DisplayPort 1.2 x 2 streams and USB 3.1 Gen 2.

So here is the terminology that is correct across eight high-speed pins, you get a total for four data channels that can be split up. There is also a set of power pins and then a low-speed pair for control.

  1. USB4 Portocols and data rates are supported, so it speaks USB 4 40Gbps and 20Gbps
  2. PCI Express 3.0 at 10Gbps per lane
  3. Display Port Alt Mode 2.0. This actually runs DisplayPort 2.0 which supports 8K@60Hz and uses compression in 8 bit and 8K@60 in 10 bit. Because monitors are not synchronous, it is set so that it can use all 8 lanes at once. just like DisplayPort 1.4.). DisplayPort 1.4 could do this but with lossy compression of the color channels.

The nightmare of finding a Thunderbolt 4 cable to HDMI 2.1 for 4K@60 10-bit 4:4:4

So the big question now is what’s the difference between Thunderbolt 4 DisplayPort 2.0 Alternative mode and the HDMI 2.1 that the LG accepts. Well first of all DisplayPort 1.4 has been out there a long time and support 26Gbps allows 4K@10 with 10-bit HDR. And HDMI 2.1 as we’ve covered is even better supporting 48Gbps with static HDR and also dynamic HDR encoding with 8K!

DisplayPort can use passive or active cables to do this translation to HDMI since it natively supports 16K@60 10bit color at 4:4:4 with DSC (compression) which uses all four pairs in a dedicated DisplayPort cable.

When in USB Alt Mode, it gets two pairs and can run at 8K@30 10bpc 4:4:4 or two 4K@120 10bpc 4:4:4, so this works

The trick is finding the right cable, even if we can’t get to 120 Hertz, can we get to 10-bit color and chroma 4:4:4?. Given the decoder ring above, we can a cable that ideally speaks DisplayPort 2.0 through USB C on one end and on the other end speaks HDMI 2.1:

First there are cables that have a DisplayPort connector on one end and an HDMI 2.1 connector on the other:

  1. Parade PS195 DisplayPort 2.0 to HDMI 2.1 convertor. This basically takes a DisplayPort 2.0 signal (remember this thing has 80Gbps) and converts it to HDMI 2.1 (48Gbps). These are the chips that then go into active cables
  2. Club3D DisplayPort 1.4 to HDMI 2.1. This is for use with nVidia graphics cards and it supports DSC 1.2 comporession so you can get 4K@120 10b-it 4:4:4
  3. UPTab Display 1.4 to HDMI 2.1 4K@240 HDR 4:4:4.

Then there are cables with USB C connectors on one end and HDMI 2.1 outputs. I’ve tried the first and confirm the 4:2:0 limitation so onto trying the second. It is more expensive at $60, but should do better color:

  1. Uni USB C to HDMI Cable 4K@60 but 4:2:0. As an example, I bought a Uni cable which is supposed to run 4K@60, but it doesn’t support the HDR mode (I’m not sure why, the active electronics). So when I plug this into a Razer Thunderbolt 4 or directly into the MacBook Pro 2021, it does give 4K@60Hz but is limited to 4:2:2 and does not support HDR at all. Sigh.
  2. Cable Matters USB C to HDMI 2.1 4K@60 10bpc 4:4:4. This is a USB C adapter that supports DisplayPort Alt Mode. It doesn’t support VRR/G-Sync or FreeSync refresh, but does claim to support 8K@30 10bpc RGB 4:4:4

Related Posts

This site uses Akismet to reduce spam. Learn how your comment data is processed.