Splitting VGA signals and resolution restraints on portable systems

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Splitting VGA signals and resolution restraints on portable systems

You may encounter situations where there is a need to use a Video Graphics Array (VGA) splitter with a portable system cloning output to more than 2 displays, the displays may not be able to set optimal resolution.

If the displays are attached one at a time to the splitter while in the operating system, optimal resolution may be achieved, but once a power state change occurs the resolution on all 3 displays is reduced.

This can occur from the VGA port on the system or the port on a docking station, regardless of whether the system LCD panel is used or not, (Lid closed).

This phenomenon is caused by a combination of Windows Plug and Play Manager operation, analog signal dynamics, and load balancing. It may appear that the signal is being split only once at the external adapter.  In reality, the signal is also split at the video card/LVDS junction for the internal LCD and the external VGA port. For analog VGA signals, the Plug and Play Manager only handles one additional VGA display device, and in clone mode only.

The LCD panel is loaded during boot, and once external VGA display is detected, the Windows Plug and Play Manager has to load the monitor driver, link the monitor to the system and alert the display adapter to send the properly formatted cloned signal. As long as the 2nd and 3rd VGA displays are energized individually this configuration works fine.

When three displays, (system LCD panel and 2 external VGA monitors), are connected, and the displays are energized simultaneously, the Plug and Play Manager and the display adapter fail to refresh all 3 displays at once. As a result, the display signal is throttled (low or basic resolution) due to load balancing across the three displays. When the external displays are connected one at a time, the first external VGA monitor is loaded by the Plug and Play Manager to proper resolution. When the 2nd VGA monitor attached, it is "linked" to the others with the same resolution, and no Plug and Play notification occurs. (This is similar to daisy chaining monitors). As long as a reboot or sleep state does not take place, this configuration may hold at optimal resolution.

Solution

Due to the limitations of the VGA analog signal, only two displays can be properly cloned and maintain optimal resolution through power state changes. If you require more than 2 displays, it is best to use the digital signal ports, such as Digital Video Interface (DVI), DisplayPort (DP), or High Definition Multimedia Interface (HDMI) display connections. If VGA splitting to more than two displays must be used, the workaround is to set the optimal resolution on the system’s LCD panel, then attach each monitor to the VGA splitter one at a time.

The optimal resolution will be lost on reboot or sleep states, so the process would have to be repeated each time there is a power state change. A high quality VGA splitter should also be used, such as a Monoprice adapter.

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