HD Monitors
Mar 1, 2009 12:00 PM, By Steve Mullen
Figure 1. Film mode deinterlacing of 24p
Click to enlarge
In last month's article, 24p judder, we explored solutions for the 24p judder issue that results from using low-cost cameras. This article will be a continuation of that discussion.
Deinterlacers used by flat-panel monitors must correctly process 720p60, 24p carried within 720p60, 1080i60 and 24p carried within 1080i60. In the first case, frames are simply passed though the deinterlacer and, if necessary, upscaled to 1920 x 1080.
Interlaced video must, of course, be deinterlaced. When not carrying 24p, deinterlacing is a relatively simple process. When carrying 24p, a more complex process is used that requires a deinterlacer to correctly detect the presence of 2:3 pulldown.
Deinterlacing 24p video
When 2:3 pulldown cadence is detected, a deinterlacer must switch from video mode to film mode. (See Figures 1 and 2.) In film mode, 540-line Fields 1 and 2 are combined to make one 1080-line progressive frame, which is displayed twice (green). Next, 540-line Fields 3 and 4 are combined to make another 1080-line progressive frame, which is displayed three times (blue). Four fields have now been converted to five progressive frames.
Figure 2. Video mode deinterlacing of 24p
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Next, Field 5 (red) is skipped, and Fields 6 through 10 (except Field 8) are converted to another progressive five frames. Through this process, eight fields (four film frames) are converted to 10 progressive frames.
Unfortunately, more than 80 percent of 125 monitors tested by Home Theater magazine failed a 2:3 pulldown detection test. The high rate of pulldown detection failure raises a red flag to those editing 24p material on flat-panel monitors.
When unable to detect 2:3 pulldown, a flat-panel monitor falls back to video mode deinterlacing. The result, as shown in Figure 2, is the display of three frames within a stream of 10 frames that contain pictures from two different film frames (gold). The outcome is 18 frames every second with degraded quality.
Figure 3. 60Hz and 120Hz LCD monitor operation
Click to enlarge
Many flat-panel monitors, however, offer a way to bypass the deinterlacer a 1080p24 input connector. This connection accepts 24 progressive frames each second. (See top row in Figure 3.) To use a 24p connection, your computer's graphics card, or hardware connected to your computer, must be able to output 24p via an analog or digital connection.
LCD monitors
No matter whether a 1080p24 input is used or pulldown detection is perfect, LCD technology has a liability perceived motion blur. The top row in Figure 3 shows four progressive frames. As shown in the second row, each presentation is visible for 1/60th of a second. Our eyes use the presentation of every new frame (top row) to determine the motion vector of a moving object. Following this vector, our eyes track the object.
An LCD monitor presents a continuous image until it is replaced by another picture. Each picture leaves a trail on the moving retinas we experience as monitor motion blur.
By using electronics and LCD panels that operate at 120Hz, motion blur can be reduced using several techniques. The third row of Figure 3 shows the 1/120th of a second presentation of a black frame every other presentation time. Each presentation erases the previous image from the retina, thereby decreasing motion blur. Because display brightness is significantly reduced, this technique is typically employed only by professional LCD monitors where low display brightness is not an issue
The fourth row in Figure 3 shows every other frame (green) generated by electronics that interpolate these frames from adjoining frames. This technique has two advantages. First, when there is motion, each interpreted picture is different from the preceding one; therefore, each picture replaces the image on the retina, thereby reducing motion blur. (Next-generation panels will operate at 240Hz.)
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