The term is often encountered in the context of HDMI EDID . But what is that actually?
The idea behind EDID (Extended display I identification D ata) is a user-friendly plug & play concept: An HDMI device should output the best possible format that can be processed by the end device - but not a format that overwhelms the end device. This is negotiated via EDID data. For example, no 4K TV picture should be output on a Full HD television - the television can neither process nor display it. In such a case, without EDID, there would no longer be any menu displays, just a black screen. This makes it difficult to set a different resolution manually. So it makes sense if the HDMI source knows which image formats are supported based on the EDID data. In this way, it can optimally adjust the number of pixels, color space and frame rate.
The sound is handled the same way. Therefore, a Dolby Digital 5.1 sound is only output if the remote station supports it. Video projectors generally have no or only stereo speakers and do not process surround sound. Therefore, a player directly on the projector delivers at most stereo sound. Projectors with audio digital outputs are an exception. These also accept 5.1 sound if the HDMI source can deliver it. The video projector itself then remains silent in many cases. And forwards the sound to an AV receiver or surround bar.
The HDMI end device (i.e. television, projector or AV receiver) uses EDID to signal which capabilities and formats are supported. For example, this looks like this:
In the example above you can see that the TV has a native resolution of 3840×2160 pixels. So it's a 4K TV. The possible resolutions that can be accepted via HDMI are listed below. The sound formats that the television can process are also listed. In the example, these are DTS and AC3 (Dolby Digital) up to 5.1 sound and stereo.
A connected HDMI source always delivers the best possible format that is contained in the medium and supported by the end device. In the example above, a UHD Blu-ray with 5.1 sound can be played on this television. The output of 7.1 sound is not possible because the television does not support it. Even if 7.1 sound is saved, it is not output.
Audio
The following audio formats are defined and negotiated via EDID:
| format | Max Channels | transmission |
| LPCM | 2 | HDMI, SPDIF |
| LPCM | 8th | HDMI |
| Dolby Digital | 6 | HDMI, SPDIF |
| DTS | 6 | HDMI, SPDIF |
| Dolby Digital Plus | 8th | HDMI |
| Dolby TrueHD | 8th | HDMI |
| DTS HD | 8th | HDMI |
These are the technical formats. Dolby Atmos can, for example, be transmitted via Dolby Digital Plus (compressed) or via Dolby TrueHD (uncompressed) and contain objects for up to 128 audio tracks. However, Dolby Atmos (and also DTS-X) have a 7.1 format and therefore use 8 channels.
HDMI splitter and EDID mode
It gets complicated when not just one source and one television set the format. For example, if you connect a second TV that has different capabilities via an HDMI splitter. Therefore, better splitters have EDID management. So an indirect setting option for the output of the HDMI source. With a EDID copy function an HDMI splitter of the HDMI source pretends that 2 identical end devices are connected. The priority depends on the port. What happens when you connect a 4K TV and a Full HD TV to one source? It will output 4K when copy EDID function for 4K display is on. The Full HD television then has neither picture nor sound. Because it cannot process the signal. (However, if HDCP 2.2 copy protection is active, only Full HD is output to both devices. Full HD devices do not support HDCP 2.2.)
A setting in one Mixed or bypass mode mixes the capabilities of the terminals. This ensures that the HDMI signal is always processed. But maybe not as desired. When connecting the projector and TV to an HDMI splitter, you would like to deliver surround sound to the TV (which is connected to a surround system), which the projector does not support. The fact that the projector remains silent would certainly be acceptable. So with such a configuration, you need an HDMI splitter that prefers the TV.
The strict, automatic negotiation via EDID softens some newer devices. It is very flexible, for example, if the new Fire TV Stick 4K allows the sound format to be set manually. But not every user knows that with DD+ sound, the television remains silent and only sounds via an AV receiver with a corresponding decoder.
It should also be mentioned that there are separate HDMI EDID emulators. This can be used to deliver specific EDID information to the HDMI source in a very targeted manner. Supported image resolution, frame rate, color space, sound format and more can be manipulated in a targeted manner.
EDID issues
If an HDMI source is no longer directly connected to the HDMI sink, i.e. there is an HDMI switch or splitter in between, problems can arise. It may be that one of the desired audio or playback formats is no longer playable. This is often due to conflicts when assembling the EDID information. For example, HDMI audio extractors usually add an EDID entry that supports 5.1 audio formats Dolby Digital and DTS. However, the entire EDID has only 256 characters. For example, if a TV puts an entry for Dolby Vision support in its EDID in an unexpected area, this can be overwritten by the Audio Extractor. As a result, Dolby Vision can no longer be selected on the HDMI source. Typically, Dolby Vision still works when a passthrough EDID mode is selected on the Audio Extractor. Then the original EDID of the TV gets to the HDMI source.
Such a problem can be diagnosed simply by comparing the two EDID data. So reads the data with an HDMI device in between and without it. To do this, you connect a computer to a television or HDMI device as the HDMI source and read out the data. This works very well with Linux and Windows, tools can be found at Wikipedia or at EnTech . Graphics card drivers also sometimes have such a function, e.g Nvidia . For home cinema applications, the enhanced EDID (E-EDID) must be read out. This is 256 bytes in size, in contrast to the EDID for monitors with only 128 bytes. Either an EDID emulator can be used to solve this, or the firmware can be adapted.