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Can someone explain to me re the external 4K monitor resolution

danb1979

danb1979

macrumors 65816
Original poster
Feb 5, 2015
1,386
2,288
Preston, Lancs - UK
My M1 MBA is connected to an LG 24" 4k monitor, via HDMI using a USB C hub

I tried a USB C to DP cable but the screen was very dim, the HDMI connection drastically improved that

However when I look at the info in System Report re the display; this is what I get:


Apple M1:





Chipset Model: Apple M1


Type: GPU


Bus: Built-In


Total Number of Cores: 7


Vendor: Apple (0x106b)


Metal Family: Supported, Metal GPUFamily Apple 7


Displays:


Colour LCD:


Resolution: 2880 x 1800


UI Looks like: 1440 x 900 @ 60.00Hz


Main Display: Yes


Mirror: Off


Online: Yes


Automatically Adjust Brightness: No


Connection Type: Internal


LG Ultra HD:


Resolution: 3840x2160 (2160p/4K UHD 1 - Ultra High Definition)


UI Looks like: 1920 x 1080 @ 30.00Hz


Mirror: Off


Online: Yes


Rotation: Supported


Automatically Adjust Brightness: Yes





Is this correct? I'm a novice when it comes to external monitors bar a previous 2k monitor I had with a previous 2018 Mac Mini; so just want to make sure that I've done what I can to get the best experience

Cheers :)
 
You might want to check under /System Preferences/Displays

I know from experience that my default settings are 1920*1080 upscaled to size your 4k pannel, try and play around with scaled resolutions and find the sweet spot for you.

Screenshot 2021-07-27 at 13.47.42.png
 
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Thanks; I have tried that but if I move the scales, the text becomes so small its laughable...

Seems the 1920x1080 is the sweet spot
 
danb1979 said:
Is this correct?
Click to expand...
No. According to System Profiler, your monitor is only running at 30 Hz which is terrible.

Let me guess - your USB-C hub provides USB 3 ports - which will restrict it to 30 Hz at 4K resolution.

You'll need a hub that has USB 2.0 ports - that will allow 60 Hz at 4K.

This has to do with how many lanes are available for a video signal via USB-C.
 
Last edited:
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The screen shouldn't be any dimmer over USB C to DP than HDMI. You can probably fix that by adjusting your monitors brightness. Your choice of HDMI adapter is also restricting you to 4K@30Hz. This is half of what the monitor should be at and may be noticeable from just moving your mouse.
 
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Amethyst1 said:
No. According to System Profiler, your monitor is only running at 30 Hz which is terrible.

Let me guess - your USB-C hub provides USB 3 ports - which will restrict it to 30 Hz at 4K resolution.

You'll need a hub that has USB 2.0 ports - that will allow 60 Hz at 4K.

This has to do with how many lanes are available for a video signal via USB-C.
Click to expand...
That's not true anymore. You can get hubs that do 4K@60hz over HDMI and USB 3.0. The new version of Apple's USB C Digital AV adapter for example:

About the Apple USB-C Digital AV Multiport Adapter - Apple Support

Connect a Mac that has a Thunderbolt 4, Thunderbolt 3, or USB-C port to an HDMI display, while also charging your Mac and connecting to a USB-A device. iPad models that have a Thunderbolt or USB-C port can also use this adapter.
support.apple.com support.apple.com
 
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salamanderjuice said:
That's not true anymore. You can get hubs that do 4K@60hz over HDMI and USB 3.0. The new version of Apple's USB C Digital AV adapter for example:

About the Apple USB-C Digital AV Multiport Adapter - Apple Support

Connect a Mac that has a Thunderbolt 4, Thunderbolt 3, or USB-C port to an HDMI display, while also charging your Mac and connecting to a USB-A device. iPad models that have a Thunderbolt or USB-C port can also use this adapter.
support.apple.com support.apple.com
Click to expand...

Thanks for that

Looks like this UGreen one does what's needed: https://www.amazon.co.uk/UGREEN-USB...sprefix=usb+c+hdmi+hub+4k+60hz,aps,288&sr=8-3

I'm trying to not pay £70 for the Apple one!
 
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salamanderjuice said:
The screen shouldn't be any dimmer over USB C to DP than HDMI. You can probably fix that by adjusting your monitors brightness. Your choice of HDMI adapter is also restricting you to 4K@30Hz. This is half of what the monitor should be at and may be noticeable from just moving your mouse.
Click to expand...

Thanks

I did try to alter the brightness etc; it made little difference and the settings of the monitor when connected via DP are different to HDMI

I'll look into the HDMI hub connection first
 
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I run a shop of 2018 Mac Minis, each with a pair of 24" LG 4K monitors and use the hub linked below to run the second monitor on each machine. All monitors are calibrated and appear to look the same.

www.amazon.com

HooToo USB C Hub, 6 in 1 USB C Adapter, USB C Dongle with 4K to HDMI, 100W PD Charging Port, 3 USB 3.0 Ports, SD Card Reader, USB-C Hub for MacBook Pro, iPad Pro, XPS More Type C Devices

HooToo USB C Hub, 6 in 1 USB C Adapter, USB C Dongle with 4K to HDMI, 100W PD Charging Port, 3 USB 3.0 Ports, SD Card Reader, USB-C Hub for MacBook Pro, iPad Pro, XPS More Type C Devices
www.amazon.com
 
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jakebrosy said:
I run a shop of 2018 Mac Minis, each with a pair of 24" LG 4K monitors and use the hub linked below to run the second monitor on each machine. All monitors are calibrated and appear to look the same.

www.amazon.com

HooToo USB C Hub, 6 in 1 USB C Adapter, USB C Dongle with 4K to HDMI, 100W PD Charging Port, 3 USB 3.0 Ports, SD Card Reader, USB-C Hub for MacBook Pro, iPad Pro, XPS More Type C Devices

HooToo USB C Hub, 6 in 1 USB C Adapter, USB C Dongle with 4K to HDMI, 100W PD Charging Port, 3 USB 3.0 Ports, SD Card Reader, USB-C Hub for MacBook Pro, iPad Pro, XPS More Type C Devices
www.amazon.com
Click to expand...
That hub is limited to 4K at 30 Hz. Not good.
 
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Amethyst1 said:
Did you ensure it's the new model? The older model of the Apple adapter is limited to 4K at 30 Hz.
Click to expand...
The new model is better than the old model for sure but it can only do 4K 60Hz 8bpc if you use one of the following:

1) 533 MHz timing - HDMI usually uses 594 MHz and there's no way to make custom timings on M1 Macs.
2) DSC - does Apple support Display Stream Compression on the M1 Mac mini with this adapter? DSC works in Catalina but not Big Sur on Intel Macs but M1 Macs can only use Big Sur or later. DSC can only work from certain GPUs (RTX, RDNA, Ice Lake, Tiger Lake, M1)
3) 4:2:2 or 4:2:0 - chroma sub sampling reduces color quality (such as red text on blue background - should be ok with greyscale text?) macOS doesn't let you change the color depth or color format and it doesn't show you what its using for the color depth or color format.

I would definitely go with DisplayPort if possible. It has more bandwidth than HDMI, so it can do 4K 60Hz 10bpc or HDR RGB without DisplayPort 1.4 or DSC or chroma sub sampling.

Which LG 4K display are you using?
 
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joevt said:
The new model is better than the old model for sure but it can only do 4K 60Hz 8bpc if you use one of the following:

1) 533 MHz timing - HDMI usually uses 594 MHz and there's no way to make custom timings on M1 Macs.
2) DSC - does Apple support Display Stream Compression on the M1 Mac mini with this adapter? DSC works in Catalina but not Big Sur on Intel Macs but M1 Macs can only use Big Sur or later. DSC can only work from certain GPUs (RTX, RDNA, Ice Lake, Tiger Lake, M1)
3) 4:2:2 or 4:2:0 - chroma sub sampling reduces color quality (such as red text on blue background - should be ok with greyscale text?) macOS doesn't let you change the color depth or color format and it doesn't show you what its using for the color depth or color format.

I would definitely go with DisplayPort if possible. It has more bandwidth than HDMI, so it can do 4K 60Hz 10bpc or HDR RGB without DisplayPort 1.4 or DSC or chroma sub sampling.

Which LG 4K display are you using?
Click to expand...

All that means nothing to me sorry

But it's this LG monitor I'm using:

www.amazon.co.uk

LG Electronics UHD 4K Gaming Monitor 24UD58-B, 24 inch, 4K, 60Hz, 5 ms, IPS Display, FreeSync, HDMI, Displayport, Anti Glare, Tilt Adjustable

LG Electronics UHD 4K Gaming Monitor 24UD58-B, 24 inch, 4K, 60Hz, 5 ms, IPS Display, FreeSync, HDMI, Displayport, Anti Glare, Tilt Adjustable
www.amazon.co.uk
 
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Apple adapter received; it changed it to 1920x1080 60hz; so cheers :)

But; that's been spugged away in my drawer and the monitor is going back 😂

It's still a very dull, grey monitor. It doesn't get bright like my MBA screen and TBH it's not an easy screen to look at in my office which is really bright at times, due to the direction of the sun and that's with blinds closed!

So; back to the drawing board!
 
danb1979 said:
Apple adapter received; it changed it to 1920x1080 60hz; so cheers :)

But; that's been spugged away in my drawer and the monitor is going back 😂

It's still a very dull, grey monitor. It doesn't get bright like my MBA screen and TBH it's not an easy screen to look at in my office which is really bright at times, due to the direction of the sun and that's with blinds closed!

So; back to the drawing board!
Click to expand...
The next step is to fiddle with your monitor settings, I have never head of the connection causing this kind of issues.
 
danb1979 said:
Apple adapter received; it changed it to 1920x1080 60hz; so cheers :)

But; that's been spugged away in my drawer and the monitor is going back 😂

It's still a very dull, grey monitor. It doesn't get bright like my MBA screen and TBH it's not an easy screen to look at in my office which is really bright at times, due to the direction of the sun and that's with blinds closed!

So; back to the drawing board!
Click to expand...
Did you try turning off "Automatically Adjust Brightness"?
 
Out of interest, does the Apple AV adaptor have the model number printed on it? I'm just wondering how you check which model it is without the box?
 
salamanderjuice said:
Did you try turning off "Automatically Adjust Brightness"?
Click to expand...

Yes; that was off. Monitor's brightness was to 100%...

It's nothing to do with my M1 MBA or cable; it's the monitor itself

I've done more reading of feedback on Amazon too and it's not just me that's found these issues recently; there are a few similar comments that the screen isn't very bright but also the DP connection is duller than the HDMI connection.

I've no idea why; but it's clearly a known 'niggle'

Anyway; it's gone, I'll do without for now :)
 
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