LG OLED42C2PSA 4K OLED TV & M1 MacBook Pro Setup Issues

TLDR; don’t buy an LG OLED42C2PSA 4K OLED TV to use as a monitor with an M1-based Mac. The HDMI port doesn’t support HDMI 2.1 and the USB-C/TB4 ports don’t have enough bandwidth to provide the full 4K, 120Hz, 10-bit colour at 4:4:4 chroma via an RGB signal.

The perfect monitor does not exist - 8K, 42", 10-bit colour depth, 99%+ DCI-P3 colour gamut and a 120Hz refresh rate - but the LG OLED42C2PSA 4K OLED TV is pretty damn close, supporting all that except for the 8K bit. When it appeared for only $1,356 at The Good Guys in a 24-hour sale earlier this week, I whipped out the AMEX and did a classic impulse buy. An hour later it was on my desk.

Native res vs 200% HiDPI (aka boomer’s iPhone mode)

Like most impulse buys, it did not go well and I have buyers remorse.

I’ll start with the good news. This display is gorgeous. I’ve never used or had an OLED TV in my home before (only seen them on handheld devices or at shops) and wow, they aren’t kidding about those deep blacks. I’m usually anti-dark mode for my day to day UI, but in the brief 72 hours I owned the OLED42C2PSA, I used it in dark mode just to enjoy that infinite black.

Once you’ve tweaked the settings properly as per this video to suit monitor use, as well as enable HiDPI mode via BetterDisplay so macOS renders text as if it was on a HiDPI display (i.e: semi properly), it’s great as a huge desktop monitor with loads of screen real estate for all sorts of activities.

Unfortunately, this where the good news ends. Connecting this TV to use as a monitor on my 2021 MacBook Pro (M1 Pro) was a total pain in the arse. No matter what I tried I could not get 4K, 120Hz, 10-bit colour in an RGB signal. 4K 60Hz? Sure, but lots of monitors do that. I wanted 120Hz and I wanted it in 10-bit colour.

The highest quality I could achieve after 3 days of fiddling with EDID settings, plist files and multiple $60-$70 cables.

First of all, the HDMI port on the 2021 MBPs doesn’t support HDMI 2.1, necessary to get 4K, 120Hz. That’s a known issue. But we’ve got DisplayPort and USB-C, right? That’ll do it, yeah? Well, no. There’s just not enough bandwidth! According to Extron’s video bandwidth calculator we need 40.1GBps of bandwidth.

The maximum bandwidth of Thunderbolt 4 is 40Gbps, a bee’s dick shy of enough, so a USB-C to HDMI 2.1 cable will not work as there’s just not enough space for all those 1’s and 0’s. Even after following this tutorial on the Mac Rumours forum to hack away at the EDID profile and force macOS to output at 120Hz, 10-bit, 4:4:4, the cable craps out and the TV shows No Signal as that last 100mbit of data can’t get through.

Good, but not good enough.

DisplayPort 1.4 can support this using a feature called Display Stream Compression (DSC), which uses lossless compression to squish the data down from 40Gbps+ to something more manageable (around 13-14Gbps). This only works when going from DisplayPort to DisplayPort - not DisplayPort to HDMI and the only inputs on the OLED42C2PSA are HDMI.

This bloke on the MacRumours forum managed to get a 48" C1 LG OLED going at 4K, 120Hz, RGB 10b using this “Cable Matters 48Gbps USB C to HDMI Adapter Supporting 4K 120Hz and 8K HDR - Thunderbolt 3 and Thunderbolt 4 Port Compatible - Maximum Supported Resolution on Any Mac via This Adapter is 4K@60Hz” cable from Amazon, plus some EDID hacking.

I doubt it is in full 4:4:4 chroma as theoretically there isn’t enough bandwidth to support all that, but who knows. Maybe this adapter is enabling DSC somehow and passing it on via HDMI? Pretty fancy for a $35 adapter. Either way, I couldn’t be fucked spending a 4th day of messing around with this monitor to find out.

PCs with fancy Nvidia and AMD graphics cards and the latest Apple M2 Pro and Max SoCs support HDMI 2.1 don’t have to go through any of this nonsense as HDMI 2.1 has 48Gbps of bandwidth. Get a good HDMI cable and you’re set you lucky bastard!

In the meantime, I passed the OLED42C2PSA on to a friend who will use it as a TV and I will persist with my lesser quality monitor and wait for The Perfect Mointor to manifest itself.