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Over the years of using HDMI monitors, I always had some trouble with the RGB color range, full (0-255) vs limited (16-235).

The HDMI Spec v1.3a says, in section 6.6 (page 95, PDF page 111):

Black and white levels for video components shall be either “Full Range” or “Limited Range.” YCBCR components shall always be Limited Range while RGB components may be either Full Range or Limited Range. While using RGB, Limited Range shall be used for all video formats defined in CEA-861-D, with the exception of VGA (640x480) format, which requires Full Range.

This basically says, "for all TV formats (except for VGA), use limited-range RGB. Other formats are used by computer monitors which we don't care about." In practice this obviously won't work and didn't work because there are too many 1080p computer monitors that expect full-range RGB. And it gets worse in the 4K era.

Eventually I reached the conclusion that I'll never buy another HDMI monitor, but laptop vendors just won't switch to DisplayPort...

That said, at the same time, I was repeatedly surprised by how well those HDMI-DVI adapters (cables) works. The HDMI source (and GPU drivers) seems to be detecting those adapters and notice the sink is actually a monitor and not a TV, and then switch to full-range RGB instead.

How is this implemented in practice (e.g. by AMD and Intel GPU hardware and drivers)?

Related:

How to make a HDMI signal be identified as HDMI instead of DVI?

VESA DisplayPort Interoperability Guideline

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    \$\begingroup\$ "laptop vendors just won't switch to DisplayPort" - they are, but through an Alternate Mode on a USB-C port. \$\endgroup\$
    – Finbarr
    Sep 12, 2022 at 12:38
  • \$\begingroup\$ There are tons of laptops with (mini) DisplayPort. Though I’m not sure it changes much, I wouldn’t be surprised if the video data was exactly the same. \$\endgroup\$
    – jcaron
    Sep 13, 2022 at 12:24
  • \$\begingroup\$ I have a docking station connecting over USB-C to my laptop which has (among other things) 2 DisplayPorts and a HDMI connection. I got another dongle that adds a USB-A and HDMI connector to the other USB-C port. There are plenty of possibilities on laptops, so it's not as bad as you think. Considering size is a factor on laptops, it is common they don't have as much ports as a PC has by default. This does not mean you can't add them. \$\endgroup\$
    – Mast
    Sep 13, 2022 at 19:37
  • \$\begingroup\$ "laptop vendors just won't switch to DisplayPort" – my HP laptop from 2018 has only DP and VGA, but no HDMI; it seems a lot of HP products are DP-only. \$\endgroup\$
    – user1686
    Sep 14, 2022 at 5:12

2 Answers 2

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More on Justme's answer:

HDMI 1.3a specs:

8.3.3 DVI/HDMI Device Discrimination

In order to determine if a sink is an HDMI device, an HDMI Source shall check the E-EDID for the presence of an HDMI Vendor Specific Data Block within the first CEA Extension. Any device with an HDMI VSDB of any valid length, containing the IEEE Registration Identifier of 0x000C03, shall be treated as an HDMI device. Any device with an E-EDID that does not contain a CEA Extension or does not contain an HDMI VSDB of any valid length shall be treated by the Source as a DVI device (see Appendix C).

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The adapter does nothing, it just a piece of cable which has HDMI connector on one end and DVI-D on the other end.

It's the monitor that says it's not a HDMI monitor but a DVI monitor.

The PC will read monitor EDID data over the DDC bus, and if the EDID does not indicate that it's a HDMI compatible monitor then it is a DVI monitor. Exact details how it is determined is defined in the HDMI standard and can be read from there as other answers have already quoted.

And in case of being connected to a DVI sink device, the HDMI source device must (as per specs) switch to DVI protocol compatible output with no guard bands and data islands, as HDMI must (as per specs) only be sent to a device which says it supports HDMI, and HDMI protocol requires guard bands and data islands to determine that it's HDMI protocol instead of being DVI.

And the requirement for consumer TV resolutions to be transmitted as limited range RGB (16-240) from a HDMI output was from a time when HDMI was a new standard for consumer video while DVI for computer use already had gained momentum.

Computer graphics used their own established resolutions and consumer/professional video devices used their own established resolutions.

Computers also typically used full range 8-bit RGB (0-255) for graphics while professional video typically used 10-bit limited range for digital component (YCbCr) video, commonly truncated to 8-bit limited range for consumer use.

Early HDMI versions did not support signaling to indicate whether the data sent is limited or full range.

Therefore, it made sense to assume that computer graphics resolutions are sent as full range like before on DVI computer interface, and TV video resolutions are sent as limited range like they were previously sent over digital interfaces such as SDI.

Which also means that a 1080p monitor likely expects full range data from a DVI connector, while it expects limited range data by default from HDMI connector. Modern HDMI monitors may additionally support full range data over HDMI, which PC can detect and optionally use it if display supports.

Also another thing is which standard is used to send the 1080p video. If sent with CEA video timing, that's a limited range consumer format for TVs, but if sent with VESA video timing such as Reduced Blanking, it's no longer a CEA format so full range computer graphics data can be assumed.

So this may not be easy to digest as a consumer - you need to think which cables and connectors to use while connecting equipment, and additionally allow or force full range output on PC, maybe even set the video timing standard.

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    \$\begingroup\$ Whether the adapter "does nothing" depends on the adapter. If you look at the second reference given in the question, you'll note that there are two kinds of adapters: Type 1 which is indeed passive, but is rated only to 165 MHz, and Type 2 which "[provides] a register indicating the adapter type.... The Intel Linux GPU driver will check for this and prevent using frequencies higher than 165 MHz if a Type 1 is detected. The Intel Apple and Windows GPU drivers do not check for this, so it may or may not work." \$\endgroup\$
    – cjs
    Sep 13, 2022 at 7:43
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    \$\begingroup\$ @cjs This is about connecting HDMI output to DVI monitor. You talk about DisplayPort to HDMI adapters which are unrelated. Even if you used one to connect to DVI monitor, there will be a single-link DVI connection, and single-link connections are up to 165 MHz, so there is no problem. HDMI output can go up to 340 MHz using HDMI protocol and up to 600 MHz using HDMI 2.0 protocol, and up to 40 Gbps using HDMI 2.1 protocol. \$\endgroup\$
    – Justme
    Sep 13, 2022 at 8:12
  • \$\begingroup\$ You can get adapters with HDMI male & HDMI female connectors, which pass through the video signals directly, but insert a different (often programmable) EDID. This can be useful, for example, to tell the computer "This device is really DVI not HDMI" (or vice versa) when it ends up doing the wrong thing by default. \$\endgroup\$
    – psmears
    Sep 13, 2022 at 9:18
  • \$\begingroup\$ @psmears That does not help. Since the PC has HDMI output, it is mandated by HDMI rules, which means TV resolutions must be output as limited range video levels to a DVI display. Unless forced by user via drivers to output full range regardless. Of course HDMI displays can advertize support for full range and PC can automatically send full range. And you can't expect a DVI monitor to work properly when HDMI protocol is sent to it, and many don't. \$\endgroup\$
    – Justme
    Sep 13, 2022 at 10:05
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    \$\begingroup\$ Yes there are more even more nuances to this (eg life might be easier if all devices kept to the specs, or behaved consistently and sensibly, but in practice they don't!). My claim was that it can be useful to insert a different EDID, for example lying about the HDMI vs DVI status or indeed perhaps something else. This is certainly true because I've done it (both ways), on a number of occasions, and it was indeed very useful :) Your experience may be different (there is huge variety in support among devices), but saying that "this is never useful" is too strong a claim. \$\endgroup\$
    – psmears
    Sep 13, 2022 at 13:58

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