silver imac displaying collage photos

LG B9 and MacBook Pro 2021, H10 HDR, Chroma 4:4:4 and 4K@120Hz (someday)


OK, this is so embarrassing, I've had my LG B9 (I thought for years it was a C9, but it is about the same thing) panel for two years now (got as the Pandemic was happening in 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 2x2 set of chroma for a 4x2 set of pixels. This works because humans can't see colors that well. And 4:2:0 means that for every 4x2 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, but I cannot figure it out.

MacBook Pro 2021 limited to 4K@60 in hardware for HDMI but works at 4K@120 on DisplayPort

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. However, there are ways to get beyond this but it requires a DisplayPort input which is not common for television.

MacBook Pro M1 2021 does 4K@60 as HDR10 and Rec.2020 but only Chroma 4:2:2 with 8 bpc or Chroma 4:2:0 at 10 bpc

OK, I'm confused, this should be in full 4:4:4 with the right cable and into HDMI 2.1 port, but it still only shows although the B9 says that it is HDMI 2.1 on all ports, so there is something wrong there.

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 to DisplayPort 1.4 DSC or DisplayPort 2.0 cable to HDMI 2.1 for 4K@60 10-bit 4:4:4 for the Mac

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. And it is what is needed, but there are no convertors that use this.
  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 10bit 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 Display 1.4 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. With this one installed, I actually go 4K@60 8bpc 4:2:2 not 4:4:4, so the question is what is generating this. However (see below) the documentation specifically says that 4K@120 and 10bpc are not supported on from the Mac.

How to support HDR 10-bit color on LG B9 or C9? Argh

This is complicated but you can use SwitchResX but the B9 (and the C9 for that matter) supports 10 bits per color (10 bpc), the normal mode is 8 bpc to give you as Apple says Millions of colors. For HDR, you'd rather have the fine gradation of 12 bpc even though the native panel is 10 bpc but getting 8 bpc is all I can get out of right now.

Normally with an Intel MacBook, if you go to About This Mac > System Report > Graphics/Displays you would get a report saying how bpc you have, but with the M1 MacBook, you only get resolution. You can tell if you have HDR by going to System Preferences > Displays and then right clicking on the LG panel and you will see a High Dynamic Range check box and also what refresh rate it handles. In this case only 60 Hertz (as this is a not a DisplayPort device). You can verify this by starting the Apple TV application and you should see a new category in Library called 4K HDR. Or you can go to YouTube and look for an HDR movie and click in the options for 4K HDR movie and you should see it is very bright. And you can see what it is output by right clicking not he video and selecting Stats for Nerds. On my panel it says Color is smpte2084(PQ) /bt20202 which is great.

You can also download a video click that shows a 10-bit gradient pattern to see if it is completely smooth from AVS Forum try the spinning quantization (the second video).

All of this confirmed that I'm only getting 8bpc but it is not clear why, the Mac can do it, the LG B9 panel says it can do it as well, so that leaves the Razer Thunderbolt or the adapter which says it supports 10 bpc. Sigh.

No, could it be the Razer Thunderbolt 4 Dock? No

One confusing thing that I see is that the Razer Thunderbolt 4 Dock specification says it can support o 1 x 4K 60Hz display for the M1 Mac. I'm not clear if this is for non-Pro versions. So I just pulled the cable out of the Razer Thunderbolt and plugged it directly into the MacBook Pro and no change so I get from the LG menu it is running at HDR-10, 8 bits per color, chroma sampling 4:2:2

Could it be the Cable Matters 48Gbps USBC Adapter 201388-GRY

The specification sheet on this says 8K video requires Nvidia RTX 20x0 or RX 600 or higher displays and that it pushes 48 Gbps on HDMI 2.1. It does say that M1 Macs support only a 4K@60 Hz maximum which confirms what the other reports say. You need DisplayPort to get 120 Hertz. It also says VRR/G-Sync variable refresh isn't supported. And it says 10-bit color depth HDR is not supported for Mac computers.

The more detailed troubleshooting guide does show the 4K@120 Hertz only works with X series and C9 but not the B9, I'm not sure if that is a typo or not. And, yes there is even a Windows utility where you can check the firmware of the adapter.

The Ultimate Limit Apple needs FRL support

OK, so to get to it, this is actually a software problem. You can get a Bootcamp version to run with 120 Hertz support, but the problem is a software limit on the pixel clock. Thee deep problem is that if you have an external HDMI 2.1 display and connect via Thunderbolt2 which is using DisplayPort 1.4 protocol and then converting this to HDMI 2.1 (whew was that confusing). But the basical problem is the Realtek DisplayPort-to-HDMI 2.1 protocol convert (called the Realtek RTD2173) is limited to 600 MHz. And this is what is inside the Cable Matters and the Club 3D chips.

The deeper diagnostic information, but the basic problem is that to get higher resolutions and chroma, you need more data. So TM means 18GBps which is HDMI 2.0 spec and 4L10 means 4 lanes of 10GB or 40GBps. You can apparently modify the

And they specifically say that on an LG CX, there is no way to get 4K HDR Chroma 4:4:4 at 60 hertz because in the Mac, if the monitor is HDMI, it is limited to HDMI 2.0 and so the maximum due to the 18 Gbps limit is 4K HDR 4:2:2 8 bpc.

There is actually a Perl script that let's you dump the actual timing available inside the Mac. The deep problem is that there is an HDMI 2.1 Fixed Rate Link (FRL) which provides the specific timing needs. So when the Mac sees an EDID from a monitor (this is the monitor capabilities), if there is an FRL field, it just ignores them and instead of using 10LM, it defaults to the TM or the old HDMI 2.0 18Gbps limit. Specifically it means that TMDS in the EDID which is limited to 6bps or 600 Mchars/sec. That means 4K UHD at 60Hz can either be 4:4:4 at SDR or YCbcr 4:2:2 10 bpc (HDR) at most but even here 10bpc is not supported.

Since Apple GPUs do not support HDMI natively, so you need a DisplayPort to HDMI 2.1 protocol adapters which come from Synatpics or Realtek. There is a Parade unit as well that they work with so maybe look for adapters that have the PS195 and PS196 and hope that Apple adds FRL support. This thread on Macrumors by the way is 17 pages long!

If you really want to get deep into it here is what is happening. On the Mac side, the dongle has a chip which on Display Port 1.4 has these speeds (so its really hard to get a Display Port 1.4 to HDMI 2.1 to work without compression either DSC or sub chroma sampling:

  • 12.96 Gbps with 2 lanes of HBR3
  • 25.92 Gbps with 4 HBR3 lanes

On the HDMI 2.1 side you have:

  • 8.16 Gbps for TMDS at 3Gbps/lane and 3 lanes
  • 14.40 Gbps for TMDS at 6Gbps/lane and 3 lanes
  • 21.33 Gbps with FRL at 6 Gbps/lane for 4 lanes
  • 28.44 Gbps with FRL at 8 Gbps/lane for 4 lanes
  • 35.56 Gbps with FRL at 10 Gbps/lane at 4 lanes
  • 42.67 Gbps with FRL at 12 Gbps per lane, 4 lanes

You can put Datastream Compression DSC on to reduct the need from 30 bits per pixel for 10-bit HDR to 8. The other tricks are to do chroma subsampling to reduce the number of bit per pixel

What display work with M1 Macs at 4K 144 Hertz (all have DisplayPort 2)

Tonsky has been keeping track of this and some monitors with DisplayPort (and without these pesky EDID problems and FRL problems. But the bad news is that no LG CX monitors seem to work at 4K120 let alone HDR 10-bit 4:4:4 although plenty of monitors with DisplayPort like the Gigabyte M32U doe (and on a Mac Studio there is a report that dual Gigabyte M32U at 144 Hertz works so maybe that's the ticket)

As a final aside, Tonsky points out that you probably want to get the best screen quality by choosing exactly half resolution. The default on the MacBook Pro M1 is ¾ which leads to all kinds of artifacts. I don't do that on a 14" Mac because I need the additional bits and the rasterization seems fine.

And no, it doesn't seem you can spoof it with say USB C -> Display Port -> HDMI sadly, which is a little strange to me, but that's what they say. Basically, It is probably because there protocols are all intelligent and when the monitor reports its capabilities with an EDID, then the trouble starts because the Mac doesn't know how to deal with FRL

If you just want ti to work, then you can get 2.5K at 144 Hertz over HDMSI 2.0 and it will run at Chroma 4:2:0 in HDR. Sad rally.

Intel MacBooks 2018 or later do support 4K@120

And final aside is that the 2019 MacBook Pro will support 4K@120Hz so its a M1 MacOS limitation. So with an LG CX and the Cable Matters 201388 and a 2019 Intel MacBook Pro, it will run at 4K@120. It is using HDM 2.1 FRL Signaling with 4L6 that is 4 lanes at L6 or 6Gbps. Note that older Mac like the 2017 MacBook Pro with RX560 cannot do this because they do not have DSC support. You need to have a 2018 MacBook Pro or later

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