24p judder
Feb 1, 2009 12:00 PM, By Steve Mullen
Camera components
According to the BBC white papers, the perception of temporal sampling judder, as well as motion (30p) judder or pulldown (24p) judder, is determined by the edges of moving objects. Hard edges create distinct moving objects. These increase our perception of judder. Therefore, any aspect of a camera's optical or electronic components that increases edge sharpness inherently increases the perception of all types of judder.
Figure 3. Modulation transfer function (MTF)
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Edges have relatively low spatial resolution compared with fine detail. The perception of judder is increased by an unfavorable balance between a band of midspatial frequencies and an upper band of high spatial frequencies carrying fine detail.
A modulation transfer function (MTF) describes the relation between image contrast and spatial resolution. (See Figure 3.) An MTF curve's shoulder starts high and rolls off to a long foot. The higher the frequency the roll-off begins, the more fine detail passes through a lens.
Expensive cinema lenses have an extended MTF that transmits images with loads of fine detail. The lens on a less expensive camera has a lower frequency roll-off that significantly attenuates fine detail.
A camera's sensor size determines its ability to obtain a minimum depth of field (DOF). Film and digital cinema cameras, with their large frame size, offer a shallow DOF. Next come video cameras with 2/3in chips. At the bottom of the heap are cameras with 1/3in or 1/4in chips, which are unable to suppress background judder because they have an inherently deep DOF.
Figure 4. Typical camcorder gamma curve
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The perception of judder depends on image contrast, which is a function of a camera's gamma. Moderate-cost video cameras allow the selection of several gamma curves. (See Figure 4.) Panasonic has equipped its DVCPRO HD and P2 camcorders with a sophisticated Tele Gamma mode for use where the content will be viewed on televisions. (These camcorders also feature a different Cine Gamma mode for use where the content will be transferred to 35mm film.)
It seems obvious that an HD camera's sensor(s) should have resolution equal or greater than the recording resolution. At low frame rates, however, cameras that use horizontal and vertical green shift to quadruple the number of pixels may yield less judder because their softer video attenuates edge sharpness. (This softness may not be desirable at high field or frame rates.)
All video cameras incorporate a low-pass anti-aliasing filter to prevent aliasing when a sensor's signal is digitized. The more sophisticated the filter, the steeper the filter's slope. Conversely, an inexpensive filter rolls off more slowly. The former allows a high cutoff frequency and thus less lost detail. The latter forces the turnover point to be further below the Nyquist frequency, thereby causing a significant loss of fine detail.
Figure 5. Example of horizontal spatial resolution
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Both vertical and horizontal components are further filtered to match the recording format used. The red curve in Figure 5 illustrates the horizontal spatial resolution from a typical low-cost 1440 × 1080 camcorder.
As shown by the red curve in Figure 5, signal strength is already very low by the midpoint of the recording bandwidth. Video cameras have a sharpness (detail enhancement) control that adjusts the amount of boost applied to the signal. In Figure 5, the green and blue curves represent, respectively, normal (midpoint) and maximum sharpness. The boost expands the area under the curve — thereby increasing overall image sharpness — and lifts the higher frequencies, thereby preventing loss of fine detail.
Unfortunately, even at a normal setting, the horizontal frequency response curve has a moderately large peak within the frequency range that creates judder. (See the gray zone in Figure 5.)
More sophisticated camcorders have separate controls for detail enhancement and aperture correction. While the detail control alters edge sharpness, the aperture control alters the amount of fine detail. These controls enable a camera operator to balance edge detail and fine detail to minimize judder.
Figure 6. Anti-judder correction applied (blue)
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Figure 6 illustrates the judder band (orange) plus three representative response curves: film (purple), a digital cinema camera (black) and the response of this camera with negative detail enhancement (blue). Negative detail correction, as offered by Sony HDCAM and CineAlta camcorders, reduces the perception of judder.
Until low-cost HD camcorders incorporate the ability to dial-in negative detail and yet not reduce the amount of fine detail, a camera operator can try to eliminate excessive judder by setting sharpness midway between minimum and normal. Figure 5 shows this curve by a series of purple dots. (Setting sharpness at the minimum, as is often done in an effort to create a film look, simply strips video of fine detail, as shown by the red curves in Figures 5 and 6.)
Another judder reducing solution is to include the use of appropriate optical filters and/or a slightly slower shutter speed that increases motion blur. Likewise, a camera operator can control camera motion while the director controls the movement of objects within the frame.
Steve Mullen is owner of Digital Video Consulting, which provides consulting services and publishes a series of books on digital video technology.
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