Handling EDID with Analog Signal Distribution
Anagrams and abbreviations are thrown around the audio-visual industry constantly but one that appears to be popping up regularly of late regarding displays is EDID. We explain what EDID actually is and how it can affect your AV system. There is a collection of products slowly growing that bear EDID in mind and decrease issues when connecting to different equipment.
So, for those of you that don’t know, EDID stands for Extended Display Information Data and describes the data structure that indicates the capabilities of a display that is communicating with a computer graphics source.
All widescreen monitors now have fixed resolution displays with specific and often differing native resolutions and do not follow any type of timing standard. XGA resolution was most common and used to work fine on any display but not anymore. Although many monitors on the market claim to be 1920×1080 monitors, they each support a different version of 1920×1080, due to a difference in active pixels.
The EDID can be displayed in what is called a mode line. Below is an example of one possible mode line for a 1024×768 (XGA) signal:
Detailed timing #1……. 1024x768p at 60Hz (4:3)
Mode line…………… “1024×768” 65.000 1024 1048 1184 1344 768 771 777 806 -hsync –vsync
Pixel clock > horizontal timing > vertical timing> polarity of sync pulses
Here are two examples of different displays with the same resolution, showing different mode lines:
Monitor 1 – LG LCD
Detailed timing #1……. 1920x1080p at 60Hz (16:9)
Mode line…………… “1920×1080” 148.500 1920 2008 2052 2200 1080 1084 1089 1125 +hsync+vsync
Monitor 2 – Samsung LCD
Native/preferred timing.. 1920x1080p at 60Hz (16:9)
Mode line…………… “1920×1080” 138.500 1920 1968 2000 2080 1080 1083 1088 1111 +hsync -vsync
If EDID is not present on the PC the graphics card will decide what to output to the monitor. Normally the computer outputs a version of 1920x 1080 that doesn’t match the mode line of monitor. This causes the image to be sized incorrectly or the picture being shifted too far across. It causes problems such as the toolbar not being visible or the error message displaying due to an “invalid mode”. No EDID causes on a short list of output resolution choices and only standardized ones at that, so it may not include your preferred size.
In the AV industry it is DDC pins 12 and 15 that carry the EDID. Although something such as RGBHV 5-wire cable is often claimed to be the best way to send a computer signal over any distance, it does not pass the DDC pins and therefore the EDID information is lost.
Other applications that do not consider EDID include:
- Twisted pair transmitters/receivers
- Distribution amplifiers
- Matrix switchers
To avoid this, manufacturers are starting to create computer graphics video products with EDID support built in. There are also new standalone products that help manage EDID in systems that employ these other signal management tools.
An example is the Kramer VA-1VGAN, an EDID emulator that captures EDID information from the monitor and makes it available to the PC regardless of cable types or other problems present in the installation. The VA-1VGAN captures the resolution, mode line and all detailed timing parameters, making an exact copy of the EDID info in the monitor. For example, if you are using RGBHV matrix switchers the VA-1VGAN will emulate the handshake necessary for the computer to output the correct resolution and timing parameters. Twisted pair transmitters and receivers often also have this problem. By not passing the EDID info from the display back to the source, the PC will not know what resolution is expected.
An EDID emulator stores the EDID info from the display to its [the emulator’s] input. Then placed directly after the PC, the emulator sees the EDID just as it would if connected directly to a display. Now that the computer graphics card has the correct information and timing parameters it can create native resolution ensuring a perfect image despite travelling through a twisted pair transmitter.
Now that the industry is realising the importance of EDID, more products are being developed to accommodate handling the data. New distribution amplifiers allow EDED information to be passed back from one of the outputs unlike older DAs that would cut the DDC pins before entering the DA. The diagram shows this:
In an ideal world, all the display devices connected to the DA would be the exact same make and model, they would all use 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, we must choose the display we connect to output 1 carefully.
Let us explain further. In its simplest form, let’s say we have four monitors connected to a DA with the following native resolutions: 480P, 720P, 1080P, 1080P. When using a DA similar to the one above that passes EDID information from output one we must choose the 480P monitor for output one. Choosing any one of the other displays would feed back EDID information to the computer that would cause it to output a resolution that would not be understood by the 480P monitor. If this monitor is removed from the application the 720P monitor could be connected to output one and still satisfy all the displays connected.
If your system is currently lacking the EDID hand shake, necessary for a successful system design, feel free to contact us for more information on products available.