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.
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.
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.
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.)
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 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 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.)
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
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.
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.