Frame rate has been one of the major differences between film and television. While both systems wish to present enough information to their audiences to portray motion, they also are limited by practical restraints. Although film's 24 fps rate was chosen as a good compromise between cost and the end result, television also was heavily influenced by another parameter. The frequency of AC power — 50 Hz in Europe and 60 Hz in North America — was used to lock the video frames in order to avoid hum bars scrolling down viewers' pictures.
Although the need for such technical restraints on television have been engineered away, the legacy has lived on. Film and television have lived parallel lives until only recently. Wisely, both the ATSC and DVB digital television standards included 24 fps formats, but that was of little use until manufacturers made suitable equipment, and both program makers and filmmakers took an interest. Now the paths are converging, and a bridge between film and television is being formed under the 24p banner. Does this mean one of the media, film, is dead? A look at the technical issues and practical feedback provides the answer.
What is 24p?
24p is shorthand for 24 fps, progressive scan. It says nothing about picture size, but there is an assumption that it is high-definition — 1080 × 1920 pixels. However, the ATSC defines 1080, 720 and 480-line formats using 24p. DVB also defines these and adds 576 lines. So 24p can be standard definition as well as high definition.
In practice, the SD use of 24p is limited by both the lack of recorders and direct outlets. Although some disk-based systems such as Avid's Symphony and Film Cutter as well as Quantel's Editbox can operate at 24p, their practical use is mainly as offline for HD or film work. For this, inputs can be provided by down converted outputs from HD cameras.
By far the greatest interest in 24p is at HD. This received a boost in May 2000 when ITU-R BT.709-4 was approved, which included 1080 × 1920 images with 16:9 aspect ratio at 23.97, 24, 25, 29.97 and 30 fps progressive scan. These were designated as ‘preferred video formats for new productions and international exchange’ — or Common Image Formats. This encouraged the growing interest in 24p as a universal production format — even in countries where there is no HD broadcast. Hereafter, for simplicity, 24p implies the 1080 × 1920 image size.
The importance of 24 fps
Long ago, television learned the basics of how to transfer film material to the smaller screen. The initial plan involved pointing a camera at a projected film. Then came the telecine. Frame rates had to be converted, so for 25 fps television, film runs 4.2 percent faster, and 30 fps television uses 3:2 pulldown to map 24 frames onto 60 fields of television every second.
With these methods widely in use and well accepted, working from video at 24 fps seemed a relatively straightforward, although not a totally established, process. This familiarity certainly helped it to rapidly gain acceptance and support from both manufactures and production. Although allowed in ATSC and DVB, there are no TV stations broadcasting at 24 fps. This is partly historic as there is still not much equipment to handle the presentation needs of 24p. Also, the look of 24p is different to, say, 60I. Is the public ready for this?
24p is used as a production, mastering and distribution format but not for transmission — except in the case of E-cinema.
Progressive verses interlaced
In the not-so-early days of television, interlace fulfilled two vital roles: allowing a fast enough image refresh rate to portray fast movement and reducing perceived image flicker. Interlace effectively doubles the vertical refresh from 25 or 30 fps to 50 or 60 odd and even fields per second without increasing bandwidth or affecting the horizontal resolution. Even vertical resolution is not affected until there is movement. Then it is reduced by around 30 percent as successive fields show different positions for objects in the picture. Interlace also requires special attention in DVEs for areas such as freeze frame and zoom — requiring compromises and/or more processing. Another drawback is revealed by fine horizontal detail whose edges appear to dance vertically as they happen to appear in, say, the odd fields, but not the even (as odd and even are displaced by one line). This latter artifact makes interlace unsuitable for displaying the detail shown on computer displays.
Progressive scans have the advantage of always displaying their full vertical resolution; irrespective of movement, they do not dance on detail and do not require any processing for freezes. They always look very clean but require twice the bandwidth to display the same resolution for the same vertical refresh rate as interlace. This is one reason why progressive vertical refresh rates are usually half that of interlaced, resulting in less smooth portrayal of movement. Just as with movies, fast pans are to be avoided, and fast action appears to judder.
In addition to computer folk, film people generally prefer progressive scans because they are nearer to film where each frame is a distinct snapshot in time — not spread over two ‘half’ frames separated in time.
If 24p were displayed simply as frame 1, line 1 thru 1080, and then frame 2, line 1 thru 1080, the flicker would render the video unwatchable. So, just as cinemas double shutter, displaying each frame twice, 24p needs to be displayed at twice the vertical rate. Processing in the display system rather than transmitting each frame twice can achieve this goal. Such processing was not affordable until a decade or so ago, so 24p could not have been widely used by consumers until more recently.
Interlace and progressive both have their advantages and disadvantages. As television is moving toward multi-format operations, producers can pick the format that best suits their needs. Until physical restrictions such as bandwidth and storage can be removed or engineered to become non-issues, there will continue to be a need for the economies of interlace as well as the accuracy of progressive scans.
24PsF
Although the 24p picture format has many advantages it will need to share studio facilities, including recorders, switchers, DVEs, monitors and general infrastructure with other formats. To facilitate this, 24p Segmented Frame records each progressive frame as two segments — one of even lines (2, 4, 6, 8 etc.) and the other of odd lines (1, 3, 5, 7, etc.). This way the signal can be easily handled by HD studio equipment designed for interlace while preserving progressive images. Note that this is not interlaced as the image is still scanned progressively (1, 2, 3, 4, 5…) all in one continuous scan. There is not the 1/48-second time difference in capture there would be between fields of odd and even lines if interlace were used.
24PsF helps in many ways — right through to the monitor. As mentioned, 24p displayed as such is unacceptable. However, the 24PsF offers twice the vertical refresh rate and produces a very acceptable display without the need for any further processing. Most TV monitors are designed for this rather than pure progressive signals.
Digital cinematography
Film-style shooting is one application for 24p. Everyone has heard of George Lucas's use of Sony CineAlta 24p cameras for the later “Star Wars” movies. There now is a rapidly growing list of movies shot in this format. Although not offering exactly the same look as film, the results have been well accepted by the viewing public. Some can see the results via digital projectors — completing a scene-to-screen digital chain.
Discussion continues about the merits of using digits instead of film. Video is seen by some as lacking the flexibility of film. For example, the latitude of film negative is larger than video. Although cinemas can only show up to around seven stops of contrast — limited by the ambient lighting — the negative can hold as much as nine or even 12 stops, which allows a selection of the contrast range to be selected for printing. Thus, some degree of over or under exposure can be corrected in the lab. However, a big difference in the 24p shot is that the results can be seen live or instantly replayed from tape. This fast feedback has been exploited by digital cinematographers to readjust lighting so the recorded material is optimized on location — usually far more effective than lab corrections. This interactivity and assurance of a good take is much appreciated.
The inability to over crank video to produce slow-motion effects also is cited by film users who can use up to 100 fps or more, for some effects. As stated, many fixed frame rates now are available with HD as it heralds a new era of flexibility. Significantly, Panasonic offers variable image capture rates of 4-33, 36, 40 and 60 fps with a 720-line, progressive scan camera. However, there is, and will surely always be, areas where film is the only answer.
The cameras and lenses are not only designed to have a familiar feel for film people, but they include many ‘film-friendly’ adjustments and features. HD cameras in general benefit from higher resolution, 16:9 lenses, and for film, different ‘Cine’ gamma can stretch sensitivity into the high and low lights. So digital cinematography already has branched from mainstream HD.
Practical issues
HD is a growth industry. In Europe, there currently are more than 80 companies working in HD — despite a total lack of any HD transmission. This reflects a number of factors: the substitution of HD acquisition for film-for-television production, recognizing the need for HD picture quality for long-term archive shelf life, digital cinematography replacing film and international demand.
Home, a post house in London, already has gained much HD experience. David Klafkowski, director of operations, said the last 18 months were a big learning curve. A key challenge was making the equipment and the technical infrastructure all work together in the various formats. Here, a reference signal routing matrix has proved pivotal. Major equipment includes HDCAM and D5 HD VTRs as well as a Sierra Quickframe HD disks for uncompressed recording — all supporting a linear edit suite with a Snell & Wilcox switcher and Pandora Pixi HD digital color processor. This is used in longform edits, while nonlinear and effects-based work is carried out in an Avid DS|HD. Home also offers HD film transfers via its Philips Spirit HD Datacine.
In Bristol, Films at 59 was the UK's first online HD facility. It recognized the potential of HD for Natural History programming and so established HD post facilities as well as production support including HDCAM hire. Its HD post work has ranged from feature-length productions conformed in the linear suite, which includes a Snell & Wilcox switcher and Digital Vision color processor, to more effects-based productions and pop promos making use of its DS|HD. Up-conversions and several HD reworks, from film scan onwards, have produced HD versions of SD programs for international clients. On the shooting side, Gina Fucci, managing director, reports HD costs as now equal to super 16 and the camera far less cumbersome than 35mm — important for wildlife locations.
European film-for-television shoots have traditionally been at 25 fps, and this is now reflected in increasing use of 25p, 24p being used where the emphasis is on cinema release, via film or digits, or international distribution. As Klafkowski comments, “The ‘p’ is the important thing.” Certainly, 24p is recognized for its Universal Master qualities: avoiding the need for both ‘PAL’ and ‘NTSC’ film transfers and offering high-quality results for all world standards and an excellent master for all deliverables. Just one 24p scan, and subsequent down-conversions do the job. Fucci points out, “Multiple standards give people a real choice, and in the end, the business is about people.”
Is film dead?
At the quality-end of the market, there is SD video (Digital Betacam and DVCPRO 50), film and HD. Each has its own look and advantages, so HD now is a supplementary medium — adding to choice. Many shoot on super 16 or 35mm film and transfer immediately to 24p or 25p, and others may shoot directly with HD. It is a matter of look, distribution, budget and personal preferences. Already some producers are putting HD masters on the shelf to protect their long-term value, and the number of 24p scene-to-big screen movies is rapidly increasing. The digital production chain looks attractive compared with film's, but film distribution will continue until digital projector costs come closer to those of film.
So, clearly film is not yet dead. But digital production and distribution already have established advantages. Soon film will be squeezed to the extremities of the chain, to acquisition and exhibition. However, Klafkowski managed a sneak viewing of some of his 24p tapes on a very big screen and reports the results as “stunningly good.” With such technology already in place, film's only long-term use will be reduced to certain specialist areas of acquisition.
Bob Pank is a television industry journalist. He can be reached atbob@pank.demon.co.uk.
