TRANSITIONING FACILITIES IN THE DTV AGE
May 1, 2007 12:00 PM, BY JOHN LUFF
As we approach the end of analog broadcasting, a scant 22 months away, it is useful to look at what it takes to build a workflow and facility that seamlessly integrates HD into an SD world. Broadcasters are increasingly building news operations around the inevitability of HD local production, with SD slowly becoming the legacy format.
HD represents a totally new medium in addition to the obvious technical differences. It requires careful attention to sound and picture quality, both of which were not as urgent in the past.
Changes in the studio
HD cameras provide less depth of field given identical focal length, shooting distance and f-stop. This can greatly affect planning a new set and the production process. Items out of focus on an SD set would still be visible on an HD set. In addition to the increase in resolution, this effect puts significant pressure on keeping sets in pristine condition and managing the contrast in the set design to achieve the intended look.
Lighting also contributes significantly to managing the contrast and apparent detail. HD studio cameras have about the same sensitivity as SD cameras, but to achieve depth of field similar to SD shots, it may be necessary to lower the lighting levels.
Even when HD cameras are used to create downconverted images for SD, the sharpness of the SD pictures increases. Don't assume that because the output is eventually SD, the issues, including depth of field differences, no longer exist.
Remember that sharpness and resolution are not the same. Apparent picture sharpness is related to the square of the area under the MTF curve, while resolution is a limiting value for detail that can be seen. (See Figure 1.)
Figure 1. Sharpness is related to the square of the area under the MTF curve.
Courtesy Canon USA.
Talent will have to learn new makeup techniques that can stand up to higher resolution. Some traditional makeup approaches used for SD look almost theatrical when shot with HD cameras. Lighting can be important in softening what might be perceived as excess resolution that makes facial features, shall we say, more distinct. This is an important issue to programmers and production professionals, as well as on-air talent, and it cannot be minimized.
Aspect ratio
In addition to the factors related to the physics of optics and HDTV scanning, there are other critical issues. The most obvious issue is the wide aspect ratio of HD images. Studio sets may need to mimic the aspect ratio to look natural. Dual SD and HD outputs will be around for some time into the future, so whether at the transmitter or at the set-top converter, it is important to protect the image for later formatting to SD. This technique is often called center safe.
The need to cut content from regular programming has caused most broadcasters to opt for a center cut from the HD image for delivering the SD copy, keeping the top and bottom of the frame aligned, and cutting off one-eighth of the frame from each side. (See Figure 2 below and Figure 3 on the next page.) This results in an over-sampled NTSC picture, with excellent picture quality and a reasonable compromise between the best resolution possible and the most appropriate rendition of the scene.
Tapeless workflow
Other parts of the transition are not quite as settled as aspect ratio and staging issues. News operations are in the throws of conversion to so-called tapeless workflows. Unfortunately, just as momentum is building and stations are making that transition more smoothly, an immature HD news production infrastructure creates new barriers that deter progress.
HD news requires about three times the storage bandwidth of SD. HDV recorders eat up about 25Mb/s, and Panasonic DVCPRO-HD, generally on P2 cards, requires four times the bandwidth. Sony's XDCAM-HD records at 18Mb/s to 35Mb/s on Blu-ray disks.
This is happening at a confusing time in the industry. Hard disk storage is now as cheap as videotape. According to Screen Digest, disk storage of HD content is more than 20 percent cheaper than popular tape formats. By 2008, it is expected to be less than 30 percent of the cost of storage on tape. This offsets the cost of switching to nonlinear workflow and will reverse the adverse economics that HD news faces this year.
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