HD Monitors
Mar 1, 2009 12:00 PM, By Steve Mullen
Second, several 120Hz LCD monitors are able to display a full 1080 lines of vertical resolution even when objects are in motion. (Most 120Hz displays offer about 580 lines of vertical resolution when there is motion, and 60Hz LCD monitors measure at only 330 lines.)
The interpolation technique, however, has two disadvantages. First, there is a possibility the interpolator will generate image artifacts. Second, the interpolator essentially temporally upsamples the video, thereby eliminating the desirable low temporal sampling judder in 24fps media. This loss shifts the viewing experience from film-via-video looking similar to film to film-via-video looking like video.
Figure 4. 120Hz 5:5 pulldown
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Figure 4 shows an alternate technique supported by some 120Hz LCD monitors. It uses what is called 5:5 pulldown. Each frame is repeated five times, generating a 120Hz refresh rate without 2:3 pulldown artifacts. While this technique eliminates visible 2:3 pulldown judder, it has a downside.
As the display refresh rate increases, the image sampling rate input by our eyes also increases. The higher the sampling rate, the smoother the motion appears. Even though 5:5 pulldown presents only 24 new pictures per second, the repeated four pictures push the overall sampling rate high enough to shift our experience from that of film to that of video.
Plasma monitors
Plasma monitors refresh at 60Hz, although one brand (Pioneer) offers the option of 72Hz when displaying 24p. When running at 72Hz, as shown in Figure 5, each frame is repeated three times — 3:3 pulldown. (Pioneer monitors can obtain 24p via a 1080p24 input or from a 1080i60 signal using its film-mode deinterlacer.)
Figure 5. 72Hz 3:3 pulldown
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Pixels that are illuminated follow a cycle of charge, activation and discharge (erase). During each pixel's brief activation period, its brightness is determined by pulse-code modulation. Motion blur is low because from the time of discharge through the display of all other rows, and including the pixel's next charge, the pixel is dark.
When operating at 72Hz, a plasma monitor replicates the viewing experience created by a film projector that employs triple-bladed shutter.
Eliminating 2:3 pulldown judder
Figure 6. 3:3 pulldown with alternating black frames
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It has long been a goal to view 24fps media on a video monitor without the contamination of 2:3 pulldown. Although 120Hz LCD displays that offer 5:5 pulldown are marketed as meeting this goal, their very high refresh rate detracts from the film experience.
A plasma display operating at 72Hz, or potentially an LCD monitor that can switch to a 144Hz refresh rate for 24p media, correctly replaces 2:3 pulldown judder with filmic motion judder. (See Figure 6.) Equally important, the inherent temporal sampling judder of 24fps media is preserved.
Figure 7: 60p progressive 2:3 pulldown
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Pulldown is also employed with 720p60 video. To move 24p video over a 60p connection, 36 extra frames are added each second. Figure 7 shows four progressive video frames in the top row. In the second row, frames are transferred ($) or repeated ((). Deinterlacing is not required as each 720-line frame can be sent directly to a flat-panel. The 2:3 cadence creates visible pulldown judder on 60Hz displays.
To eliminate pulldown judder, the monitor's deinterlacer can generate 3:3 (72Hz) or 5:5 (120Hz) pulldown. (In a similar fashion, 720p30 and 1080p30 require the use of 2:2 or 4:4 pulldown to drive a 60Hz or 120Hz display.)
Steve Mullen is owner of Digital Video Consulting, which provides consulting and conducts seminars on digital video technology.
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