How Edid Works - ATEN Video DynaSync Executive Overview

Video DynaSync™

How EDID Works

EDID is now used by many computer graphic cards and monitors. EDID is defi ned by the international video
standards group VESA (Video Electronics Standards Association). EDID involves a ROM chip in the video dis-
play which stores pertinent video data about that monitor in such a way that it can be "read" by a connected
computer's graphics card via a DDC conductor in the video cable. This scheme works quite well with a single
video display and a single computer.
Why Legacy KVMs Have A Problem With EDID EDID does not generally help when a conventional KVM switch
is inserted between the computer and the monitor.
The DDC (Display Data Channel) is a digital connection between a monitor and a graphics card. DDC allows
the display to communicate its EDID specifi cations to the graphics card. When you connect a computer whose
graphics card supports EDID to an EDID-equipped monitor, the computer looks at the monitor's EDID memory
(via the DDC channel of the video cable) and it automatically sets its video output to the optimum resolution for
that monitor. This resolution handshake is invisible to the end user, and it's very helpful with direct-connector
monitors, but it doesn't automatically benefi t a KVM switch solution because legacy KVM switches typically have
no mechanism whereby the monitor's actual EDID information can reach switched computers as they boot up,
so their graphics cards are typically set to inappropriate output resolutions.
With simple KVM switching, a single monitor is able to connect to either of two (or more) computers as com-
manded by the user. When the various computers wired to the KVM switch are booted up, they may or may not
be able to detect the monitor's EDID; it depends on whether the switch has connected a given computer to the
user's monitor, and whether the DDC conveys the monitor EDID to the computer.
Some KVM switches offer a default EDID to the computers, but this default is just an arbitrary monitor resolution.
While an arbitrary EDID may allow switched computers to boot up as though they had read the actual monitor
EDID, the chances are that they'll boot with a less-than-optimum resolution.
Remember that modern LCD monitors have fi xed pixel locations and don't do a good job with other-than-that-
optimum pixel mapping; they are not like old multi-sync CRT monitors that could re-cast electron beams to light
up varying size dots comprised of very small phosphors on the screen surface. In some cases the modern moni-
tor may be unable to display anything when the user actually switches to them using an incorrect default EDID
value, or the computer may take a while to re-sync to the correct monitor resolution. In rare cases (with too high
a default EDID resolution) monitor damage can occur. None of these situations is desirable.
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