Got a tip for us?
Let us know
Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
How many monitors do M1 Pro/Max support without HDMI?
- Thread starter sunny5
- Start date
- Sort by reaction score
Apple outlines it in their Tech Specs:
So M1 Pro supports two external displays (up to 6K) over two of the three USB/TB ports (likely one per side) and M1 MAX supports three external displays (up to 6K) over each of the three USB/TB ports.
So M1 Pro supports two external displays (up to 6K) over two of the three USB/TB ports (likely one per side) and M1 MAX supports three external displays (up to 6K) over each of the three USB/TB ports.
Exactly 2 on M1 Pro. And that also includes HDMI.
So if I'm connected to a monitor already via HDMI, I can only connect to 1 more.
M1 Pro and M1 Max have Thunderbolt 4 ports so each port can connect two displays.
M1 Pro is limited to two displays total (two to Thunderbolt, or one to Thunderbolt and one to HDMI)
M1 Max is limited to four displays total (HDMI counts as a display if used)
Each Thunderbolt port is a separate bus so in the case of the M1 Max, I guess you can connect max four displays to the left side or max three displays to the right side.
If M1 Pro/Max supports DSC, then you should be able to connect two Apple Pro Display XDR displays to a single Thunderbolt 4 port and you can get USB 3.0 write speed (5Gbps) to the XDR Usb Hubs. You should be able to connect 4 XDR displays, but maybe there's a total pixel per second limit so one of them will have to run at 4K if it is detected at all? No one has tested this.
If M1 Pro/Max does not support DSC, then you can only connect one XDR per port and you can only write to the XDR's USB hub at less than 1 Gbps.
I'm not sure there's a way to know if DSC is being used for the XDR (by examining ioreg), so testing the USB write speed is a good method.
M1 Pro is limited to two displays total (two to Thunderbolt, or one to Thunderbolt and one to HDMI)
M1 Max is limited to four displays total (HDMI counts as a display if used)
Each Thunderbolt port is a separate bus so in the case of the M1 Max, I guess you can connect max four displays to the left side or max three displays to the right side.
If M1 Pro/Max supports DSC, then you should be able to connect two Apple Pro Display XDR displays to a single Thunderbolt 4 port and you can get USB 3.0 write speed (5Gbps) to the XDR Usb Hubs. You should be able to connect 4 XDR displays, but maybe there's a total pixel per second limit so one of them will have to run at 4K if it is detected at all? No one has tested this.
If M1 Pro/Max does not support DSC, then you can only connect one XDR per port and you can only write to the XDR's USB hub at less than 1 Gbps.
I'm not sure there's a way to know if DSC is being used for the XDR (by examining ioreg), so testing the USB write speed is a good method.
With 3 x TB4 and 1 x HDMI 2.0, a maximum of two external displays on the M1 Pro seems like a surprising limitation.
Heck, my mid-2014 MBP, with 2 x TB2 and 1 x HDMI 1.4, can drive three external displays. I.e., it can drive displays from each of its video-capable ports.
Heck, my mid-2014 MBP, with 2 x TB2 and 1 x HDMI 1.4, can drive three external displays. I.e., it can drive displays from each of its video-capable ports.
Well as has been stated, but I feel like waffling, officially the M1 Pro will support up to two external 6K displays. M1 Max allows for three external displays up to 6K and one 4K.
However, there’s always the option of using DisplayLink to add additional monitors beyond the officially supported numbers. There is naturally a performance impact with DisplayLink, but it’s pretty negligible.
However, there’s always the option of using DisplayLink to add additional monitors beyond the officially supported numbers. There is naturally a performance impact with DisplayLink, but it’s pretty negligible.
I don’t know how the specific design works, but the issue is that the system needs to have a frame buffer or two for each screen, plus many offscreen buffers, and the chipset has a 2D/3D GPU which drives data into those frame buffers. So that’s the limit, not the number of connectors.
The supported resolutions are very high, so I don’t know why you can’t have, say, three 4K screens instead of two 6K screens. Then again, two external 4K monitors is already a massive amount of screen real estate.
One thing I do know is the HDMI has a max of 4K60, so if you somehow want 6K resolution on two screens, you’ll need to use Thunderbolt (DisplayPort).
The actual DisplayPort 1.4 to HDMI 2.0b chip used by the M1 Pro/Max Macs can support higher resolutions by lowering the refresh rate. You just need to ensure the bandwidth (multiply pixel clock by bits per pixel) doesn't exceed 14.4 Gbps. With proper driver and display support, chroma sub sampling can be used to reduce effective bpp down to 12 (similar to the 12 bpp DSC mode that Apple uses for the Apple Pro Display XDR display except DSC is much smarter about the info it throws away so it looks much better than 4:2:0).
6K could work from HDMI 2.0 using 4:2:0 8bpc 55Hz (or 10bpc 45Hz for HDR) but as far as I know, the Apple Pro Display XDR doesn't support 4:2:0 (it does support 6bpc but macOS does not). You would need a super fancy adapter to convert HDMI 2.0 4:2:0 to DisplayPort 1.4 RGB with DSC (the XDR is limited to HBR2 when using a single DisplayPort connection - I haven't seen it do HBR3 except in the Thunderbolt non-DSC case). All HDMI 2.0 to DisplayPort adapters that I know are limited to 4096 width. HDMI 2.1 to DisplayPort 1.4 adapters may exist in the future that could do > 4096 width (maybe available next year) but I don't know if they'll include DSC encoding.
https://insights.club-3d.com/thread/hdmi-2-1-to-usb-c-displayport-1-4-alt-mode/
Of course, M1 Macs don't have an easy method to create display timings so this will not be easy to test until that's solved.
Macs also don't have a method to enable/disable chroma sub sampling. But maybe it can be altered by modifying the display preferences (at least for M1 Macs as discussed at https://gist.github.com/adaugherity/7435890#gistcomment-3761736 ).
bandwidth limits in the display engine I expect. HDMI 2.1 is up to 48 Gbps, while HDMI 2.0 is only up to 18Gbps
What determines the bandwidth limits? Do TB/HDMI interface directly with the SoC, or do they interface via, e.g., PCIe 4.0 lanes? If the latter, it would be the number of PCIe lanes allocated to each port that would determine the bandwidth.
Not being able to use all the capabilities of a port has never been an excuse to not use a port.
PCIe has nothing to do with display output limits.
The limit probably has to do with the GPU outputting only DisplayPort 1.4. An adapter is used to convert to HDMI. DisplayPort 1.4 can only do 25.92 Gbps but HDMI 2.1 can do 42.67 Gbps.
DisplayPort 1.4 can do 8K60 with DSC @ 8bpp. HDMI 2.1 can do 8K120 with DSC @ 8bpp.
I've only see Apple use DSC @ 12 bpp. DisplayPort 1.4 can do 8K60 with DSC @ 12bpp using a reduced blanking timing (not HDMI timing).
4K120 requires only half the bandwidth of 8K60 so it's easily doable by DisplayPort 1.4.
So even though HDMI 2.1 output from a DisplayPort 1.4 GPU doesn't allow all the capabilities of HDMI 2.1, it would still be useful.
Maybe Apple used the HDMI 2.0 adapter because they've used it before and didn't want to try something new. Maybe they want to make their own HDMI 2.1 adapter (to give them more control over the output options).
The Apple TV has an HDMI 2.1 output but it's limited to 4K60.
HDMI 2.1 is substantially different that HDMI 2.0. It’s not simply faster. My guess is that either the silicon wasn’t available or it added too much cost.
For the key feature of attaching a display for conference room, presentations, etc., the HDMI 2.0 is completely sufficient.
For a 4K desktop daily monitor, though, DisplayPort is a better choice on this laptop, because DP can deliver 4K60 with 10-bit 444, and the HDMI connector stops at 4K60 8-bit 444
The HDMI port is certainly not on this machine for the purposes of running a daily external monitor.
Has anyone had any luck running three monitors on an M1 Max off a dock with a single connection to the laptop
I have proved I can run three displays with three separate TB to HDMI cables, but the dock I tried so far only supported two downstream monitors, even though it had 3 downstream TB4 ports
I have proved I can run three displays with three separate TB to HDMI cables, but the dock I tried so far only supported two downstream monitors, even though it had 3 downstream TB4 ports
I don't really see why not - it looks perfectly capable of outputing to a 4K monitor @60Hz, or am I mistaken?
Considering I (and I'll bet many users) only have FHD or QHD (2560x1440) monitors, I don't see the HDMI 2.0 port as having any limitation. I have *never* seen any 4K monitors in my customers' corporate offices - mostly they are cheap 23-27" screens. I plug into these all day long when working on-site, and go home and plug in to similar monitors (albeit slightly higher quality ones)
Thunderbolt port can only have two displays unless you use DisplayLink USB adapter for a display.
That's not what I meant. The MacBook Pro has three Thunderbolt 4 ports which are the be-all-end-all for display connectivity. They are there for that, among other things.
The HDMI port is there for convenient connecting to infrastructure. If you (and by you I mean anyone) are determined to use this port for primary monitor output, don't complain or question its veracity. That's not why the port is there.
I'm not arguing that the TB4 ports aren't more capable...just that they don't appear to offer any advantage over the HDMI port with many displays.
I have an M1 Mac Mini and connect to one monitor with the HDMI and to the second with one of the TB3 ports - there is no qualitative difference between these two with my monitors. Why can't I use HDMI for my primary display? People have used these happily for years....
If you have an HDMI port for a display, why would you need to use one of the multi-prupose TB4 ports instead? I have other peripherals that need these (external disks, audio interfaces, vide capture device).
I'm not sure what your point is. Are there some magical features of the TB4 ports for video that I'm unaware of for use with monitors 4K and lower resolution? I'm genuinely curious!
I have an M1 Pro which I hooked up to two monitors via a dock with two DisplayPort outputs. I opened the MacBook Pro to confirm that display would also work and then, just for the hell of it, I connected to my iPad with Sidecar.
All four displays seemed to work fine and I was able to drag windows between all four displays with no problem.
I was slightly surprised because I expected the two monitors plus built in display to work but I wasn't sure the iPad wouldn't count as another monitor in the two external monitor restriction.
(For the record, the external monitors are both 2560 x 1440).
I was using DisplayPort but I don't see why it would be different if HDMI were involved.
I might see how many monitors I can connect if they aren't 6k on my M1 Pro...
Suspect if the resolution isn't as high you can connect more; and I don't have a single 6k display, let a lone 2-3 of them to worry about
Suspect if the resolution isn't as high you can connect more; and I don't have a single 6k display, let a lone 2-3 of them to worry about
Convenience.
I can plug into the monitor with a single cable and get charged from it, use the built-in USB ports it has for peripherals I leave at the desk (e.g., network adapter), time machine backup drive, etc.
Good answer...for those that have monitors that can supply power via USB-C/DisplayPort!
Assuming you don't have such nice monitors (last time I checked, USB-C DisplayPort & power delivery was only on the more expensive models), is there any inherent advantage of using the TB4 ports if you have an FHD, QHD or 4K monitor with only HDMI, full-size DisplayPort or DVI, that has no USB-hub?
I can see power delivery and USB hub features as beeing useful, but without these, are they any reasons NOT to use the HDMI port?
the HDMI port on the current MacBook Pros is only HDMI2 (because Apple ran out of bandwidth on the SOC and HDMI is less important) and not HDMI 2.1, which is lower bandwidth than the Thunderbolt 4 ports can handle - which means lower refresh rate than you can get out of a Thunderbolt 4 port at very high resolutions (above 4k).
I think it tops out at 4k/60 (or something like that) and the TB4 can go higher (e.g., 4k/120, 8k/60 or 8k/120 not sure).
If you've got a normal person monitor then sure, the HDMI 2.0 port is fine. And yeah, use it (I'm using mine right now) - HDMI cables/ports on monitors are far more common than type C USB, and it's one less dongle if your. monitor doesn't have USB-C directly.
Last edited: