Tuesday, March 18, 2008

Black Frame Insertion

BFI / MPA: what is really going on
This system might be surprising at first if you have never heard of it. This configuration obviously has nothing or not much in common with the BFI described by BenQ in their data sheets. It is however very close to Samsung’s MPA technology.

The Korean manufacturer described it at the last CeBIT and presented a 24" monitor based on a PVA panel. The principle relies on the introduction of an artificial screening by successively shutting down neons located behind the panel. The similarities are striking, aren't they? So what should we think about this VA monitor, so similar to the one developed by Samsung? There are several possibilities. People at the origin of the technical documents could have misinterpreted the information, AU Optronics could have surprisingly changed their opinion, or the third possibility is that this would be an adaptation of Samsung technology at the expense of the one developed by AU, which might have been delayed.

Either way we aren’t talking about a complete MPA, and it includes a second part that is absent from the current BFI (can we still use this name?). Samsung monitors are clocked at 120 Hz. The images received at 60 Hz are duplicated twice for a refreshing frequency of 120 Hz instead of 60 Hz. Another subtle difference is that BenQ uses the time when a segment is black to begin making part of the next image. The objective is that the user won't see more afterglow from the faster rate. Area switched on: image 1. One of the 16 segments is switched off. During this period of 1/60 Hz /16 neons = 1 millisecond, the monitor begins to draw image 2 and erase the previous one. The backlighting is switched on and if reaction time of the liquid crystal was high enough, image 2 would have been drawn and the previous one erased. In fact, this response time of 1 ms is a little bit short regarding the liquid crystals currently implemented in the panel. It can slightly diminish the afterglow sensation due to the higher rate, but it can't erase it completely yet.

Here now is the functioning as we have understood it (we have to say here that no one at BenQ confirmed these points, their official version still involves black frames):

The monitor is divided into 16 horizontal areas of 75 lines, with a CCFL tube behind each area. T is the time measured.

At T=0, the processor shuts down the first area, the highest one, and leaves the 15 others with the image displayed.

At T=1, area 1 is still dark and the processor searches in the monitor’s memory for the piece of the next image, sending the display command to the liquid crystals.

At T=2, the crystals display the image or are finishing the second image, the processor switches on the backlight in area 1, and turns off area 2.

The loop consists of drawing a piece of the image on a dark monitor, drawing the next slice in the dark, turning on, shutting down the area below, and so on. Once the area 16 is drawn, the area 1 is switched on and the next image is drawn.

This is a progressive type of display. Each time 15 of the 16 areas are switched on while one is in the dark. As we have seen above, each area is in the dark for approximately 1ms.

For now, the fraction of second is often too short for the drawing of the next image to be made in the dark. However, it can already minimise afterglow due to the change from one color to another and in intermediary color scales.

Let's take the case of a monitor with a maximum response time of 16ms, with a 8ms rise and 8ms fall time. If we wish to change one area from red to blue, and if the rise and fall were linear, the dot displayed would show the following colors:

In fact the curb followed by each color isn't linear. It looks rather like this:

If we follow the time transition, colors change like this:

Before the dotted line are the colors displayed during the first millisecond. The black square shows a monitor with a rise and fall time of 8 ms, whose colors transitions will happen in the dark. In our example, rotation no longer starts with a pure 100% red, but rather with a 25% red and 25% blue. Afterglow should be reduced.


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