EDID HANDLING WITH
ANALOG SIGNAL DISTRIBUTION

By Chris Kopin – CTS, ISF-C, Tom Kopin – CTS, ISF-C, Matt Kopin – CTS

April 2010

KRAMER WHITE PAPER
WWW.KRAMERELECTRONICS.COM

TABLE OF CONTENTSINTRODUCTION . . .................................................................................................1
MAKE-UP OF A COMPUTER RESOLUTION. . ...........................................................2
THE STANDARDIZATION PROBLEM .....................................................................3
PROBLEM AND SOLUTION...................................................................................4
ABOUT THE AUTHORS.........................................................................................5In an Industry Full of Acronyms, There are a Few That
Stand Out Over The Rest
In today’s world of widescreen displays with fixed resolutions, this one is certainly at the top of the list:
EDID - which stands for Extended Display Identification Data. EDID is a data structure indicating thecapabilities of a display that is communicated to a computer graphics source.
It seems many articles are written about the necessity of EDID when using digital HDMI and DVI signals;
however, usually these articles tend to leave out the critical role it plays when using analog signals in asystem. The goal of this article is to show the importance of EDID, describe the symptoms when EDID is notavailable, and provide insights into corrective measures to restore it. So if you’ve got a comfortable seat…
let’s begin!
To understand what the EDID is and how it works, let’s start by looking at the history of analog computer
graphics video. Computer graphics video signals originally used a crude set of ID bits on pins 4, 11, and
12 to sense whether the display was color or B&W and higher or lower than XGA (1024x768) resolution.
Before long, a more sophisticated means of one-way serial communication was adopted called the Data
Display Channel or DDC, which allowed the monitor to communicate its needs to the computer graphics
video source. Later improvements lead to the current EDID standard. EDID data, which can be thought ofas “the train,” travels on the DDC channel (pins 12 & 15 of the HD-15 connector), which can be thoughtof as the “the tracks.” This new standard uses a bi-directional I²C bus for identifying 128 bytes of data thatdescribe different parameters of the display device, including information such as the model number, serial
number, manufacture date, native resolution, timing, color space, audio capabilities, and more.
So you might say…that’s nice, but so what?
Well, the all important “so what” with EDID is that widescreen monitors today do not follow any type of timing
standard, and they are all now fixed resolution displays with specific, and often differing native resolutions.
Gone are the good old days of variable resolution CRTs, a time when the most common resolution, XGA,
always seemed to work just fine on any display.
Let take a closer look….

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EDID HANDLING WITH ANALOG SIGNAL DISTRIBUTION

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Today, when most people mention a computer graphics video resolution, they describe it in viewable or activepixels, such as 1920x1080. However, there is more going on in a computer signal than the number ofactive pixels being sent (See Figure 1). In the past, active pixels alone were enough to describe a resolution,
because they referred to an existing industry standard. Resolutions like 1024x768 and 1600x1200 were inthis category. Every monitor that supported 1024x768(XGA) supported the exact same pre-defined standard
version of XGA. When you chose 1024x768 as your output resolution, every computer outputted thesame standardized version of XGA. This made all devices automatically compatible with each other. Today’s
rapid acceleration toward High Definition and widescreen display devices has pushed aside standardization.
Reliance on communication between the display and source, the “handshake,” has become vital.

Total Pixels
Back Porch

Front Porch

ACTIVE VIDEO

Figure 1: Make-Up of a Computer Resolution (Horizontal Only)

Figure 1 illustrates that a resolution is made up of much more than just the active pixels. Every resolution
has a large set of parameters that must all be met in order to achieve a perfect image. Through its EDID, adisplay communicates all of these parameters to the source. The EDID can be displayed in what is called amode line. The following is an example of one possible mode line for a 1024x768 signal:
Detailed timing #1............. 1024x768p at 60Hz (4:3)
Mode line.............. “1024x768” 65.000 1024 1048 1184 1344 768 771 777 806 -hsync –vsync

Beginning with 65.000 (the pixel clock for this resolution), the first four numbers describe the horizontal
timing, the next four numbers describe the vertical timing, and finally the polarity of the sync pulses areindicated. Remember, if displaying XGA is your goal, this EDID mode line is overkill because every display andevery computer support the same standard XGA mode line.

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EDID HANDLING WITH ANALOG SIGNAL DISTRIBUTION

Knowing that the standardization problem exists mostly in widescreen resolutions, let us look at an example
using 1920x1080. These mode lines came from the EDIDs of the computer graphics video inputs ofdifferent 1080p monitors:

Monitor 1 – LG LCD•

Detailed timing #1....... 1920x1080p at 60Hz (16:9)•

Mode line............... “1920x1080” 148.500 1920 2008 2052 2200 1080 1084 1089 1125 +hsync +vsync

Monitor 2 – Westinghouse LCD•

Native/preferred timing…. 1920x1080p at 60Hz (16:9)•

Mode line............... “1920x1080” 138.500 1920 1968 2000 2080 1080 1082 1087 1111 +hsync -vsync

Monitor 3 – Samsung LCD•

Native/preferred timing.. 1920x1080p at 60Hz (16:9)•Mode line............... “1920x1080” 138.500 1920 1968 2000 2080 1080 1083 1088 1111 +hsync -vsync

Monitor 4 – Panasonic Plasma•

Detailed timing #1....... 1920x1080p at 60Hz•

Mode line............... “1920x1080” 148.500 1920 2008 2052 2200 1080 1084 1089 1125 -hsync –vsync

Monitor 5 – Vizio LCD•

Native/preferred timing.. 1920x1080p at 60Hz•

Mode line............... “1920x1080” 136.500 1920 1952 1984 2048 1080 1081 1084 1111 +hsync +vsync

You can clearly see that while all these monitors claim to be 1920x1080 monitors, they each support adifferent version of 1920x1080. It is also clear that describing active pixels alone is not specific enoughto define a resolution. Display manufacturers have taken to producing non-standard displays, because thespecific timings are completely described in the EDID of the monitors they build. When a computer isconnected directly to any of these monitors, the mode line is read by the PC and automatically the PC
modifies its output to comply properly with that display.
What happens if that EDID is not present?
When the EDID of the display is not available to the PC, the computer graphics card decides on its own
what to output to the monitor. Since no standards exist in monitors, no standards exist for computers
either. Most often, the computer outputs a version of 1920x1080 that does not match the mode line of thespecific monitor attached. This results in a picture that is not sized correctly on the display with the mostcommon complaints being, “I can’t see my toolbar” or “My picture is severely shifted to the left or right.”
Other results include no picture at all, or error messages that display, “Mode Not Defined” or “Invalid Mode.”
When no EDID is available, some computers only provide a short list of output resolutions for the customer tochoose from. This short list is comprised of standardized resolutions such as XGA and may not even include
your desired native resolution. With no EDID, the computer becomes the master, the display becomes
the slave, and incompatibilities result because the display cannot adapt to every mode line it may receive.
However, with EDID, the display becomes the master and the computer changes its output to match themode line of the display, thus creating a perfect plug-and-play environment.

KRAMER ELECTRONICS, LtdEDID HANDLING WITH ANALOG SIGNAL DISTRIBUTION3In the ProAV industry, DDC pins 12 and 15 (which carry the EDID) are not passed in many applications, suchwhen RGBHV 5-wire cable is used. Touted as the best way to send a computer signal over any distance, it
has the fatal flaw of not passing the DDC pins and therefore the EDID information. While it is still the best
“pipeline” over which to send a signal, 5-wire cable does not ensure that the correct signal is being sent.
A few applications that do not consider EDID include; twisted pair transmitters and receivers, distribution
amplifiers, switchers and matrix switchers. Due to the critical importance of EDID, manufacturers likeKramer are beginning to build computer graphics video products with EDID support built-in, as well as
standalone products to help manage EDID in systems that employ these other signal management tools.
Here are some examples…
The Kramer VA-1VGAN is an EDID emulator that is designed to capture EDID information from the monitor
and make it readily available for the computer no matter what type of cable or other challenges are presentin the installation. The VA-1VGAN will capture, in non-volatile memory, the resolution, the mode line, and all
detailed timing parameters, in turn making an exact copy of the EDID information stored in the monitor. Inapplications, such as those involving the use of RGBHV matrix switchers, where EDID information will nottravel from the display to the source, the VA-1VGAN can be used to emulate the handshake necessary forthe computer to output the correct resolution and timing parameters.
Many twisted pair transmitters and receivers are guilty of causing the same issues. By not passing EDID
information from the display back to the source, your computer will be left guessing what resolution it isexpected to output. See the example below…

The Problem

VideoPc

VideoVideoVideoTx

EDID

Rx

Display

Our solution comes in the form of our VA-1VGAN. Using the VA-1VGAN we store the EDID information fromthe display device to the input of our VA-1VGAN. The VA-1VGAN is then placed directly after the PC so thatwhen the computer is connected it sees the EDID just as it would if it was connected directly to the display.
Now that our computer graphics card has the appropriate EDID information and timing parameters, it cancreate the exact native resolution of our display ensuring a perfect image despite still travelling through ourtwisted pair solution. This solution can be adapted to any application where EDID may be lost due to cable
type or the lack of EDID compliance in specific ProAV products.

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EDID HANDLING WITH ANALOG SIGNAL DISTRIBUTION

The Solution

Video
Video

EDID

VideoTx

VA-1VGA

Pc

VideoVideo

EDID

Rx

Display

EDID emulators are a much desired solution when there are cables or devices present that do not have any
type of EDID handling. As the industry recognizes the importance of EDID, products are now being developedthat take into account how EDID is handled. For example, new distribution amplifiers (DAs) allow EDID
information to be passed back from one of the outputs whereas older DAs cut the DDC pins before entering
the DA. See the following example. The latest Kramer, 1:2 computer graphics video DA, the VP-200K, now
allows EDID information to be passed back through the DA from the display device connected to output 1.
OUTPUT 2
Video
EDID

Display
OUTPUT 1
VideoVideoEDID

EDIDPcDA

Display

In a solution like this one, we do not have issues with the lack of EDID and getting a correct picture does not
require the use of an EDID emulator. Though, in an application like this one, monitor selection for output oneis very important. A computer graphics card only has the ability to output one resolution at a time. Therefore,
if EDID information is only being passed back from output 1, we need to connect a monitor whose EDID isacceptable to all of the monitors connected to the DA.
In an ideal world, all the display devices connected to the DA would be the exact same make and model, use
all the same EDID information, and any one of the monitors could be connected to output 1. In the real world,
it is not uncommon to have any number of different monitors connected to the same DA. In this case, wemust choose the display we connect to output 1 carefully.

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Let us explain further. In its simplest form, let’s say we have four monitors connected to a DA with thefollowing native resolutions: 480P, 720P, 1080i, and 1080P. When using a DA similar to the one abovethat passes EDID information from output 1 we must choose the 480P monitor for output 1. Choosing
any one of the other displays would feed back EDID information to the computer that would cause it tooutput a resolution that would not be understood by the 480P monitor. If this monitor is removed from theapplication, the 720P monitor could be connected to output 1 and still satisfy all the displays connected.
In conclusion, providing the correct EDID information to a graphics card is critical to successful systemdesign and operation. Whether that is accomplished via a standalone product like Kramer’s VA-1VGAN or via
a product with this circuitry built in, it is imperative to have the proper EDID handshake in every application.

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