These devices may introduce signal losses on their own if they are not equipped to equalize and restore incoming signals. Cascading such devices along a signal path may lead to accumulated losses that could ultimately impact image display reliability. Whenever possible, use of these devices should be minimized to keep the HDMI signal path as simple as possible.
Device Compatibility
HDMI and other digital video formats utilize EDID (Extended Display Identification Data) communication, originally developed for use with analog VGA ports. EDID communication is a two-way data exchange that allows a display to convey its operational characteristics, such as its native resolution and refresh rate, to the source device, which then generates the necessary video characteristics to match the needs of the display.
This automates and optimizes compatibility between the source and display, without requiring the user to configure them manually. In pro A/V applications where computers are the most common source devices, EDID communication can save significant time and effort in system setup.
The EDID for consumer displays includes special extension data, known as CEA-861, intended to optimize video and audio compatibility between HDMI-equipped devices. This includes HD resolutions and the various multi-channel audio formats for Blu-ray disc and HDTV. The primary content within the EDID pertains to computer-video resolutions and does not include audio.
Despite the potential for EDID communication to simplify integration, it is often unsuccessful. Issues associated with EDID are widely known throughout the commercial A/V industry and have many causes. Sometimes the display has no EDID, or fails to convey its EDID to the source. Or, the source may misread or ignore the incoming EDID information. In both cases, the source may impose a default resolution, or fail to output anything at all.
Pro A/V systems typically include several source devices and displays, often at different native resolutions. Making them work together requires switching, splitting, and routing of A/V signals, all of which have implications for EDID exchange. EDID communication is initiated when the source senses a connection from the display through Hot Plug Detect (HPD). When a switcher changes to a new source, the previous HPD is cut off and a new HPD is established. Reliable switching depends on the switcher’s ability to send the HPD signal to the new source following a signal switch, so that the source can receive EDID from the display.
EDID was intended for a single connection between one source and one display. The situation becomes considerably more complicated when a signal needs to be split or routed. Distributing a signal to multiple displays may not be a problem if they are identical, but what if they are different, at various native resolutions? An integrator may select one display to establish EDID communication with the source, and then roll the dice on the others.
Technologies have been developed by some manufacturers to manage EDID when signals are switched or distributed. Some devices enable their own EDID communication with sources connected to the inputs by sending pre-stored, emulated EDID to them. They may also be able to capture EDID information from the display, store it, and then communicate it to the sources.
With either approach, the switching or distribution device always maintains EDID communication with all connected sources, even with a signal switch or split. An HDMI matrix switcher may include more sophisticated EDID management, due to the fact that separate EDID communication is required for each input / output tie.
Content Protection
As with residential A/V installations, commercial A/V system designs with HDMI usually need to include consideration for HDCP for playback of copy-protected content such as Blu-ray. When a commercial A/V system will include playback of protected content, there are three important considerations.
