Digital Video Standards
There are three digital video standards in common use in PCs today: DVI, HDMI and Display Port (DP). What is less commonly realised is how many versions of each of these three standards exist. Unfortunately many devices do not make it clear which version they support, which can be important if one wishes 4K at 60Hz, or even 2560x1440 at 60Hz.
|Standard||Introduced||Max res @60Hz||Audio?|
(Where 2560x1600 is supported, the rarer 2880x1620 might also be.)
So "4K" at 60Hz requires a minimum of DP 1.2 or HDMI 2.0.
To add to the confusion, many devices claiming to be HDMI 1.3 or 1.4 do not actually support the higher clock rates of those standards. They retain the maximum 165MHz clock of the older 1.2 standard, and thus their maximum resolution at 60Hz is still 1920x1200. This is the case for many ATI/AMD Radeon HD5xxx and HD6xxx graphics cards.
Support for more than 8 bits of depth per channel exists in DVI-DL (optional, rarely present, and never in -SL), HDMI from 1.3, and all versions of Display Port. Using more than 8 bits per channel reduces the maximum resolution at any given refresh rate, so, for instance, HDMI 2.1 is needed for 10 bits per channel and 4K at 60Hz.
The analogue nature of the old "VGA" standard makes its maximum resolution less well defined (just as the picture too becomes fuzzier at higher resolutions!). The original standard was 640x480, but by 1999 many video cards could produce an analogue 2048x1152 or even 2048x1536 signal, and their RAMDACs had a maximum clock rate of 400MHz.
This uses an annoyingly large plug, fails to carry audio, and the standard has not changed since 1999. One reason that the plug is large is that there are effectively up to three video signals present. One is an optional analogue "VGA" signal, about which no more need be said, save that, if present, a DVI to VGA adapter is trivial, with no electronics required. One is the DVI-SL signal. The final is the optional extra link to make "dual link" DVI, DVI-DL.
The picture shows DVI-SL (top) and DVI-DL (bottom) plugs. The absence of six central pins in the -SL version is very clear, so it is easy to tell if a lead will support more than 1920x1200. The six extra pins are used as three pairs, one pair for each of R, G and B.Not all video cards support the second link, not all DVI leads have the extra pins and wires, and not all monitors support it.
Unlike HDMI and DP, there is no standardised "mini" or "micro" connector.
HDMI was originally designed for TVs, not computers, hence the inclusion of audio and the poor resolution. Audio data are transmitted in the blanking intervals of the video signal.
The video signal of HDMI up to 1.2 is highly compatible with DVI-SL, and converters need no electronics. But for resolutions beyond 1920x1200 HDMI raises its transmission clock beyond the 165MHz maximum of DVI, and becomes incompatible with DVI.
Unfortunately the ports and cables look identical for all current versions of HDMI, yet, to achieve higher resolutions than 1920x1200, sender, receiver and cable must all support the higher clock rates.
HDMI cables may be marked as "standard" (1.0-1.2), "high speed" (1.3-1.4b), "premium high speed" (2.0-2.0b) and "ultra high speed" (2.1).
DisplayPort is the newest standard, and has always supported 2560x1600@60Hz. Like HDMI, it comes in multiple versions, and it can be hard to discover which a particular video card, cable and display really support.
DisplayPort would not be compatible with HDMI or DVI, save that many (but not all) DisplayPort devices are designed to switch to the HDMI standard if an HDMI/DVI device is detected. Such ports may be marked "DP++", and the protocol works for transmitters (video cards) but not receivers (monitors). Passive adapters require that the monitor accept a 3.3V signal when a 5.0V one would have been expected, else active adapters which boost the voltage may be used.
DisplayPort, from version 1.2, can support multiple monitors daisy-chained on a single output. Whereas DVI and HDMI produce a continuous stream of pixel data, using separate links for each colour, DP transmits data in packets. Just as with PCIe, the number of lanes for data tranmission can vary. Typically four, sometimes two or eight, and unrelated to the number of colour components in the signal.
A Thunderbolt connection may, and generally does, include DisplayPort 1.1 or 1.2. But it might also be data-only.
USB-C might support DisplayPort 1.2, 1.4 or 2.0. More rarely it supports HDMI 1.4b. These are available via its "alternate modes", but do not have to be present -- the connection might be data-only. If present, video may be able to co-exist with USB 2.0 traffic.
In the Real World
Far too many devices state HDMI, DP or DVI without specifying which version they support, and surprisingly many do not support standards released years before they were first manufactured.
I am writing this using a Iiyama ProLite XB3070WQS monitor. It was manufactured in 2014, and has a native resolution of 2560x1600 and DVI, HDMI and DP digital inputs. However, only the DVI and DP inputs support 2560x1600. The HDMI one is HDMI 1.2 only, and, worse, the modes the monitor advertises stop at 1920x1080, which has a 16:9 aspect ratio, and do not include 1920x1200, which has the 16:10 aspect ratio of the screen. A 1920x1200 HDMI signal is accepted, but nothing will select it automatically.
I have an XFX R7 240 graphics card (which I don't use). It offers HDMI 1.4a, but only DVI-SL and no DP. Like the monitor, it dates from about 2014, and whilst it supports 2560x1600@60Hz, it does not support it in any form compatible with the monitor.
I also own a Radeon RX550. Whilst it does support DVI-DL, the AMDGPU driver for Linux fails to recognise this, and treats it as single link. Fortunately it has DP too, so can drive my monitor at 2560x1600 directly. Its sockets are in the above photograph, which gives no hint that the HDMI output is HDMI 2.0.
But in practice I tend to use a Radeon HD6570 from 2011, which supports 2560x1600 fine over its DVI-DL interface.
Even Intel CPUs with embedded graphics lag behind the standards. A desire to run a 3840x2160 monitor at 60Hz is not unreasonable, and needs HDMI 2.0 as standardised in 2013. However, an Intel Core i7-8086K, launched in 2018 and the top of its range at the time, supports HDMI 1.4, not 2.0, and will drive 4K monitors at 60Hz over DisplayPort only. I believe that the first desktop Intel CPU to support HDMI 2.0 was the `Rocket Lake' generation launched in 2021. However, AMD has supported it since at least 2018 (Ryzen 3 2200 G).