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July 21, 2022

EDID Really Helps HDMI Connections – Usually!

These days, most of us expect our display devices to simply ‘plug & play’, but ensuring that connections make the most of both the source device and the display is not quite as easy as it should be. That’s the view of the Consumer Technology Association (CTA), which has published a White Paper on the topic particularly related to EDID (Extended Display Identification Data), a technique to help with plug and play over HDMI connections. It’s free to download here. We’ll have a look at what that White Paper says, but first some background.

If you already know about EDID and handshaking and are not interested in ‘how we got to here,’ you can jump ahead to here.

PC Makers & Microsoft Got Things Started

The PC was the first volume electronics product usable at different resolutions and formats. TVs were simply color or monochrome with a fixed resolution and were configured for standardized resolution signals appropriate to the region of the world where they were sold. IBM, however, sold PCs that could be configured with different graphic cards to support different resolutions and graphic modes in color and monochrome. A host of board makers developed various graphics boards with their own configurations.

Initially, the right monitor had to be used with the right board as monitors only supported single frequency operation. An EGA (640 x 340 color) card had to be used with an EGA monitor. A CGA (640 x 200 color), MDA (720 x 350 mono), or VGA (640 x 480) card had to be used with their respective monitors, and software had to be written to support a particular mode.

After You, No, After You!

A Paradise Autoswitch EGA card from the 80s. Note the DIP switches to force the card to work in a particular way.

However, board makers quickly started to develop ‘autoswitch’ cards that could emulate any of the different boards and map the signal to the monitor used so that any software could be used with any display. Then monitor makers made multi-scanning monitors that could connect to almost any board and act like the kind of monitor expected by the board. (The author, in the 1980s, was one of the first to try connecting them – with the result that neither seemed to be able to decide what to pretend to be! The graphics board makers added a driver feature to allow the user to force the board to emulate a particular board to break the stalemate!)

Anyway, as the PC world moved to support SuperVGA (800 x 600) and XGA (1024 x 768), things started to get complicated when matching boards and monitors. So VESA, the organisation formed by PC companies and monitor/board makers to standardise graphics and displays, came up with the VDIF file format. This file format allowed a monitor maker to supply data in a file that the graphics card driver could use to optimise its output for the monitor. Unfortunately, it was not widely adopted, but, more importantly, Microsoft decided that for Windows 95, it would adopt ‘Plug and Play’ technology.

Plug & Play Arrives

The idea of Plug & Play was that anything simply connected to a Windows 95 PC should be able to be either recognized and identified by the system or have information about itself communicated automatically to the system. Before Windows 95, the user had to manually configure Windows whenever something connected or disconnected. At one time, a change in display resolution entailed a complete re-installation of Windows! There’s an original advert for Windows 95 Plug & Play on YouTube.

Microsoft Introduced Plug & Play with Windows 95

So VESA got to work and came up with ‘Display Data Channel’ or DDC in 1994. That standard defined a way for the monitor and the host system to communicate data using one of the pins on a VGA connector. There were multiple levels of DDC, which also allowed control by the system of aspects of the display, but at the simplest level, DDC1 allowed the host PC to read a file from a ROM in the monitor. The file format was standardised and was known as the Extended Display Identification Data or EDID.

The data format allowed information such as the size, resolution, gamma, color performance, and supported timings to be stored in a standard way. It also allowed vendors to add customized and extra data if desired when an extended version (E-EDID) was introduced in 1999. E-EDID v1.3, introduced in February 2000, was adopted as the format for displaying data in HDMI for consumer and TV use. HDMI arrived in 2002, so it has always included EDID support.

EDID is also Used in Pro AV

EDID is essential not only in PCs but in professional AV, where installers may need to monitor EDID to see what is going on and may even need to ‘spoof EDID’ to force a range of different devices to work together. Extron has a helpful page on its site digging into some of these issues. Several makers can supply EDID analysers and controllers for sophisticated configurations.

Being Established Doesn’t Mean it Always Works Correctly!

So EDID is well established. However, that doesn’t mean it can’t go wrong. If EDID does go wrong, there is a danger that devices that send the signals (sources) and devices that display the signals (sinks) do not work optimally because, for some reason, the two devices do not align to work at their optimum settings. The White Paper that the CTA produced highlights three sets of circumstances that can cause problems.

  1. As technology advances (as with the development of 8K displays or new generation audio), new sink devices have capabilities that are not recognized by the source device. In the worst case, this can lead to no image or audio.
  2. The source and sink device may be happy to work at the highest quality level (such as 8K), but something between the two devices is incapable. That could be a cable or a receiver or splitter/repeater. Again that can mean no image.
  3. Sink devices with high capability also tend to include legacy EDIDs to ensure that some older devices (as in case 1) can still work, but that can sometimes cause the system to drop back to a lower quality than is possible, and the user may not realise they do not have the best result. (For example, at one time, although TV viewers reported that they were ‘watching TV in HD’, they didn’t have the broadcast content or devices to provide HD, even though they were watching on HD TVs).

The provision of legacy EDIDs for older resolutions is the usual way for the first two problems to be solved but does not solve the third problem, and the CTA guide addresses this.

Ask The Viewer

The CTA recognises that one way to solve problem 3 is to ‘ask the viewer’. That is to say that the system can try higher video formats until there is a failure, getting the user to confirm when the image is correct and backing off when the progression has gone too far. User confirmation is handy (as it is, for example, when switching resolution in Microsoft Windows).

The CTA defined a number of standard timings that go from 640 x 480 VGA to 10K resolution in CTA-861 (now in version -H and available to freely download, although a heavy read at 280 pages). There is a list of 153 on the Wikipedia page about EDID. HDMI EDID supports those timings.

HDMI allows vendors to send their own data via EDID using ‘Vendor Specific Data Block’ or VSDBs. These blocks can contain the data that brands want to make available about their sink devices. However, it also has “Sink Capability Data Blocks’ or SCDBs. These can contain the standard timings and resolutions that HDMI lists. For newer source devices, SCDBs provided as the default EDID by the sink device, ensure that there will be a mode that can be used to optimize settings further. This can be done by using a wizard, for example, to get feedback on the correct operation by the user.

Unfortunately, older devices may not be able to recognize these SDCBs and may ignore them. Older HDMI 2.0 devices will ignore the SDCB, so the CTA has advised vendors to manually or automatically switch to VSDBs that advertize capabilities that will work with older devices to get things started.

Again, for older devices with HDMI 1.4b, the SDCB will be ignored, but in this case, the output will be limited to just the reduced capabilities in the EDID.

If you are having trouble getting devices to start correctly, there is some helpful troubleshooting advice on the Extron page we mentioned earlier.

CTA highlighted that HDMI might continue to try to develop solutions to these problems caused by the continued improvement and development of display devices.

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