Overcranking HDCAM 24p
By Conrad Denke
Digital cameras enable directors of photography to take advantage of a range of effects not available with film cameras, creating effects such as the slow-motion shot of a pistol being fired. Image courtesy of Sony.
On my monitor, I see bullets flying, glass shattering and cars exploding. It’s an action sequence in the trademark style of director Robert Rodriguez. My company, Victory Studios in Los Angeles, has just posted a trailer for his forthcoming Miramax release, Once Upon a Time in Mexico. This is the third movie in the Mariachi series, and the trailer may well be the world’s first to incorporate slow motion by means of “overcranking” the HDCAM 24p system.
Overcranking, as film professionals know, is the technology behind cinema slow motion. Overcranking occurs when movie film is shot at a high frame rate and displayed at a lower frame rate. The difference in picture rates creates the slow motion effect. While it has been easy to vary the picture rate in analog film production, the technique hasn’t been duplicated in digital cinema until recently.
Introduced just one year ago, Sony’s HDCAM 24 fps progressive (24p) system has quickly become a favorite of producers, directors and cinematographers. The system combines a full 1080 scanning lines with the 24 fps capture rate of film to deliver many of the technical and psychophysical benefits of the “film look.” It also offers fast, cost-effective production in all the ways that digital, electronic media can outperform analog, chemical-based media.
I got into high definition four years ago because I believed it would finally bring electronic production to the level needed for theatrical releases. I have not been disappointed. HD has become like a religion, and I want to convert the world. It’s great to see the HDCAM 24p system selected for many television episodics, theatrical releases, commercials and music videos that would previously have been shot on 35mm film. And that’s all the more reason to pursue overcranking in HDCAM 24p.
Digital overcranking is made possible by the availability of a range of image capture rates on the Sony HDW-F900 HDCAM 24p camcorder. The available rates include 23.98, 24, 25, 29.97 and 30 fps progressive, and 50, 59.94 and 60 fields/s interlace. Overcranking involves some ingenuity in post production. For example, you can shoot at the rate of 60 fields/s interlaced (60i). Then in post production you can convert the footage from 60i to 60p. When you play back the result at 24p, you get motion rendered at 40 percent of the original speed.
Of course, 60i is just one of the eight image capture rates available with the Sony HDW-F900. Other rates, especially 50i and 30p, give the cinematographer additional tools and possibilities.
HDCAM 24p overcranking at 60i or 50i requires interlace-to-progressive (I/P) conversion, a process that’s well proven, if not necessarily transparent. I/P conversion is already used when NTSC and PAL productions are “blown up” to film. The conversion can be performed by many nonlinear editing systems, digital disk recorders and software packages, including products from Avid, Adobe and Discreet. In a pinch you can even trick Sony videotape recorders into conversion using Sony’s Dynamic Motion Control.
It was at NAB last April that Roland House, the Arlington, VA, post production firm, demonstrated overcranking HDCAM 24p. Here at Victory Studios we did further tests using our Sony HDCAM 24p equipment and various software programs. Chief engineer Scott Thomas supervised work in our Seattle facility, while Victory Studios’ senior HD editor Walt McGinn conducted tests in our Los Angeles location. After many trials, we hit upon a combination that minimized motion artifacts, maintaining superb image quality.
Putting theory into practice
When director Robert Rodriguez chose HDCAM 24p to shoot Once Upon a Time in Mexico and chose Victory Studios to post the trailer, the concept of 24p overcranking faced a practical test. Rodriguez burst upon the Hollywood scene with El Mariachi, a violent epic shot in 16mm for a reported $7000. A self-described “rebel without a crew,” Rodriguez aims to shoot movies quicker, cheaper and with less crew than typical Hollywood productions. He completes 50 or more setups a day in an industry where 20 to 25 setups are the norm. One favorite Rodriguez trick involves using a wheelchair as a camera dolly. Just as Rodriguez is impatient with conventional film shooting schedules, he gets frustrated with the conventional film post production process. This makes him an ideal candidate for shooting on HDCAM 24p.
Rodriguez’s movies are also technically demanding. In productions ranging from Desperado and From Dusk Till Dawn all the way to Spy Kids, Rodriguez has featured highly choreographed fight sequences. A Rodriguez movie is full of gunfire, explosions, fireballs, crashes and stunts with bodies jumping, flying and falling. In fact, the demanding action sequences for Once Upon a Time in Mexico would really give our approach to 24p overcranking a thorough workout. Because the HDCAM 24p overcranking process involves interlace-to-progressive (I/P) conversion, there are some restrictions and limitations. These were anticipated by B. Sean Fairburn, SOC, an HD engineer and camera operator on the Mexico shoot. “You need to be careful with motion,” said Fairburn. “The cinematographer has to be sensitive to the speed and axis of motion, speed and motion of the camera, and the shutter speed.”
Fairburn explained that slower motion is always easier to handle. Motion toward or away from the camera is easier than a vertical or horizontal axis of motion. Subjects that move with the camera, as in car-mounted shoots, are also easier to accommodate. And longer shutter speeds generate motion blur that, when desired, can also obscure I/P conversion artifacts. Fairburn noted that these factors can enable cinematographers to get away with more, but in any case, the results have to be tested.
As we posted the Once Upon a Time in Mexico trailer, we had the opportunity to examine many of the overcranking effects. While it is always possible to blow up an individual frame and search for artifacts, in actual viewing the results are seamless. We’ve screened 24p scenes intercut with the converted overcranked 60i scenes and you’d be hard-pressed to tell the difference.
In fact, we quickly put together 20 scenes for an un-color-corrected rough edit of the trailer and screened it for senior Miramax management. They thought it was ready to go. As the months go by, I predict that this reaction will be typical. Others will share in the enthusiasm as more producers utilize HDCAM 24p overcranking. Even though you won’t find this capability in Sony’s operating instructions, I’m here to tell you there’s no need to wait to take advantage of the potential of HD overcranking. It’s a technology whose time has come.
Conrad Denke is CEO and founder of American Production Services and Victory Studios.
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