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Comment by nuker | Hacker News Reader

MacOS broke the Display Stream Compression (DSC) in Big Sur, and has not fixed it since. I can have 4k@144Hz in Catalina but only 4k@60Hz in Big Sur - Ventura. On the same hardware, applies to all Intel based macbooks.

Tried support, geniuses, no luck.

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I have a LG 5k monitor that stopped reliably working at the correct resolution with Catalina and then, I think, got worse with Big Sur. MacOS just refused to even let me choose the proper resolution.

Forcing the DisplayPort version from 1.4 to 1.2 in the monitors settings (which would have the side effects of disabling DSC) allowed it to work correctly over Thunderbolt.

I eventually found a firmware update for the monitor that made it work with Macs in DisplayPort 1.4 mode again.

It worked fine with Windows all along, except that DisplayPort 1.2 meant I couldn't get full resolution at 60hz over the actual DisplayPort connection, only the TB3.

The whole saga is documented on https://apple.stackexchange.com/questions/388951/macbook-pro...

Unfortunate, DSC works fine on apple silicon. I did notice displayport-related regressions on my old 2015 mac way before big sur though, Apple's track record is not great.

Yea, the Apple Engineering Department asked me to run a ton of diagnostics, both on Catalina and Ventura, then came back with “Would you ask Dell to confirm their monitor is compatible with MacOS Ventura specifically”. Lol

I believe they broke it for the Pro Display XDR. I had the same. 4K 144Hz HDR10.

When it came out people were wondering how they were driving it at 6K HDR10.

If you can dial your monitor back to DSC 1.2 things get slightly better.... I could do 4K 120 SDR, or 75 HDR.

Also applies to Mac Pro - my 2019 cheesegrater was affected, too.

> they broke it for the Pro Display XDR

What do you mean, they had to kill DSC for Pro Display XDR to work?

They had to kill it for Pro Display XDR to be a compelling upgrade.

At the time of the release, people were wondering how the Pro Display had sufficient bandwidth to drive it. Big Sur came out at the same time.

Everyone using monitors with DSC 1.4 to the extent of their monitors capabilities saw those capabilities crippled and remain crippled ever since.

Those devices continued to work fine under Catalina.

Downgrading DSC from 1.4 to 1.2 on those monitors gave "better" performance (i.e. higher than without DSC, and higher than they were with DSC 1.4 with Big Sur+).

Hundreds or more bug reports were filed. This affected users of different Macs, different GPUs, different monitors.

The only common thread was DSC 1.4 and Big Sur (Ventura, Monterey... this still hasn't been 'fixed').

Given that the Pro Display and Big Sur came out 'together', it's very hard not to draw the conclusion that Apple found "sufficient bandwidth to drive the Pro Display XDR" by making changes to their "DSC 1.4" implementation that only the Pro Display understands.

FWIW, I ran `/S/L/E/AppleGraphicsControl.kext/Contents/MacOS/AGDCDiagnose -a` on Catalina and Ventura and diffed it.

MacOS Ventura seem to properly sense that monitor supports DSC: "DSC Support: 1, DSC Algorithm revision: 33" , but then simply disables it anyway: "DSC Enable: 0x0".

Interesting. And with maybe a little sense of shame, while I still have those monitors (27GN950-Bs), I actually have a Pro Display XDR on my M2 Studio (and the 2019 Mac Pro before it).

Torn between not wanting to support such behavior, and being one of the minority that could actually justify the Pro Display (photographer who works in at times challenging light environments).

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