Broadcast Engineering - February 2009 - (Page 26)

Production cliPs Digital hanDbook Camera components According to the BBC white papers, the perception of temporal sampling judder, as well as motion (25p) judder or pulldown (24p) judder, is determined by the edges of moving objects. (See the “24p pulldown judder” sidebar.) 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. Edges have relatively low spatial resolution, compared to 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 2 on page 24.) 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 Figure 3. Typical camcorder gamma curve Figure 4. Example of horizontal spatial resolution ability to obtain a minimum depth of field (DOF). Film and digital cameras, with their large frame size, offer a very 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. 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 3.) 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 a 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. Both vertical and horizontal components are further filtered to match the recording format used. The red curve in Figure 4 illustrates the horizontal spatial resolution from a typical low-cost 1440 x 1080 camcorder. As shown by the red curve in Figure 4, signal strength is already very low by the midpoint of the recording bandwidth. Video cameras have a sharpness 26 broadcastengineeringworld.com | February 2009 http://www.broadcastengineeringworld.com

Table of Contents Feed for the Digital Edition of Broadcast Engineering - February 2009

Broadcast Engineering - February 2009 Contents Viewing the Slowdown in 3-D Examine Workflows Tapeless Technology Digital Audio 24p and 25p Judder Video Routing: A Look at What's Next Managing AFD The Right Connections Solid State Logic's AWS 900+ SE and More... Advertisers Index

Broadcast Engineering - February 2009

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.