X-bit labs

Contemporary LCD Monitor Parameters: Objective and Subjective Analysis (page 4)

Category: Other

by Oleg Artamonov

[ 01/23/2007 | 02:21 PM ]

BenQ has already implemented this idea in its Black Frame Insertion (BFI) technology. A BFI-enabled monitor is supposed to insert additional black frames into the output image, thus emulating the operation of an ordinary CRT.

At first, the extra frames were supposed to be inserted by changing the image on the matrix rather than by turning out the backlight. This would work on fast TN matrixes, but would not on MVA and PVA matrixes with their too slow transitions into black and back again. Such transitions take mere milliseconds on today’s TN matrixes, but take as long as 10 milliseconds on even the best monitors on *VA matrixes. Thus, the insertion of a black frame would take longer on them than the period of repetition of the main image frames, and BFI technology wouldn’t work. Moreover, it’s not even the frame repetition period (16.7 milliseconds at 60Hz, which is the standard refresh rate of LCD monitors) but our eyes that limit the maximum duration of the black frame. If the black frames last too long, the monitor will be flickering just as a CRT monitor with a scan rate of 60Hz does. I don’t think anyone would like it.

By the way, it’s not quite correct to say that BFI doubles the frame rate as some reviewers claim. The matrix’s own frequency grows up according to the number of added black frames, but the image frame rate still remains the same. And from the graphics card’s point of view, nothing changes at all.

So when BenQ introduced its FP241WZ monitor on a 24” PVA matrix, it turned to have a different technology that served the same purpose: the black frame is inserted not with the matrix, but with the backlight lamps. The lamps just go out for a while at the necessary moment.

This implementation of BFI is not affected by the matrix’s response time, so it can be successfully employed in TN as well as any other matrixes. Talking about the FP241WZ, it has 16 horizontal independently-controlled backlight lamps behind the matrix. As opposed to CRT monitors with their bright band of scanning running through the screen as you’ve seen above in the photographs with a low exposure value, BFI uses a dark band. At each given moment, 15 out of the 16 lamps are alight, and one is turned off. Thus, a narrow dark band runs across the screen of the FP241WZ during each frame:

Why did they choose this implementation rather than lighting up only one lamp (this would emulate a CRT monitor exactly) or turning off and lighting up all the lamps at once? Modern LCD monitors work at a refresh rate of 60Hz, so an attempt to accurately copy a CRT would produce a badly flickering picture. The narrow dark band is synchronized with the monitor’s refresh rate (the matrix above each lamp shows the previous frame the moment before the lamp is turned off, but when this lamp is lit up again, the matrix already shows the new frame) and this partially makes up for the above-described persistence-of-vision effect, but without a noticeable flicker.

This modulation of the backlight lamps worsens the monitor’s max brightness somewhat, but that’s not a problem as modern LCD monitors have a good reserve of brightness (it may be as high as 400 candelas per sq. m in some models).