DVD authoring tips
RON BURDETT AND GARRET MAKI
08/30/98
Broadcast Engineering
Copyright 1998 by PRIMEDIA Intertec, a PRIMEDIA Company. All rights reserved.
Authoring a DVD differs significantly from preparing a title for VHS video release. With tape, the project typically requires making a telecine master of the film, preparing the trailers and other material, and then dubbing these elements into the studio's preferred format. In authoring a DVD, the role is more like that of a producer; in addition to preparing the master, we assisted the studio in determining how material would be organized on the disc, what features it would have, and the look and structure of its menu system.
A DVD project also differs from a conventional video project in that the capacity of a DVD is measured in bits rather than minutes. The amount of information that can be stored on a disc is less dependent on the running time of the film, than on the quality and the compression rate used to encode the film. As a result, a DVD needs to be carefully budgeted, and there is no analogy in video production for this.
Hardware and source material
DVD production requires a variety of hardware and software tools that are not typically used in video production. However, as in video production, the quality of the end product is related directly to the quality of the tools used. With Fargo, we compared several encoding systems, before choosing Nuko Powered by C-Cube. After carefully evaluating different authoring systems, we chose the Daiken system.
Another requirement of DVD production is enormous hard-drive capacity. In fact, the hard-drive capacity of the production system needs to be three times as great as the capacity of the disc being mastered. This is because it needs to be able to simultaneously hold the elemental files, the multiplexed files and the final files, each set equal to the size of the DVD disk. Needless to say, the system also requires a fast network.
Although virtually any type of source material can be used to produce a disc, the quality of the end product depends on the quality of the source material. The type of video source used also has a bearing on the compression ratio that will ultimately be used to encode it. Because DVD is a component medium, component video formats such as D-1 or digital Betacam produce the best results.
Menus and monitors
In laying out a menu system, small details can make a big difference in consumer satisfaction. For example, we had to decide whether to lay out the film's 17 chapter stops on a single menu page or on a series of pages. Ultimately, Polygram chose the multipage approach, as this allowed us to include relatively large key frames and titles for each chapter. It also meant that the choices on each page would appear in a single column, rather than in a grid, making navigation easier.
We also learned that it was important to consider how the menu screens would appear on a typical consumer's home television. A menu that looks great on a $20,000 monitor in a darkened edit bay may look significantly worse to a consumer viewing it at home. To ensure that this did not happen, we purchased a 20-inch consumer TV set of average quality and hooked it up to our PhotoShop workstation. It became our benchmark and allowed us to be sure the choices we made worked as well on the home television as they did on the professional monitor. For example, we found that we could only go down to a certain point size for things such as the actors' biographies before the words became illegible. It also helped in selecting colors for menus and highlights, because certain colors do not display as well on a television as they do on a computer monitor.
Bit budget
Armed with the DVD's assets, it was time to prepare a bit budget. A bit budget was used to allocate space on the disc for each of the various elements. Fargo was to be encoded onto a double-sided, single-layered DVD disc, which gave us 4.7GB of disc space per side. Thus, creating the budget was a straightforward math problem of dividing up this space. (It is important to note that the "GB" used to measure space on a DVD means "billions of bytes" and is not equivalent to "gigabytes," the binary unit used in computer applications and signifying 230 or ~1.07 billion bytes.)
In creating the bit budget, we began with the fixed items, the largest of which were the audio tracks. As DVD audio tracks are encoded at a fixed rate, determining the space we required was merely a matter of multiplying the bit rate by the total length of the audio tracks. For Fargo, the audio tracks were done in two-channel Dolby Prologic surround sound and AC-3 encoded at 192kb/s. The other fixed items included the subtitles, menus, closed captions and navigation data.
Encoding process
Once the fixed items were accounted for, the remaining space was available for the video. By dividing the remaining space by the total time length of the video (which again included the pan-and-scan and letterbox versions of the film and the trailers), we arrived at the maximum average bit rate that could be used in compressing the video. In this case, the value was 4.9Mb/s. But, rather than use all the available space, we chose to encode the video at an average rate of 4.8Mb/s. Doing so left a little extra space to correct any problems that might be found after the video was encoded. While an average rate of 4.8Mb/s was more than adequate for Fargo, keep in mind that there is no one bit rate that is ideal for all films; much depends on the source material.
The actual encoding of the video was done in a three-step process. The first pass produced a log of the 3:2 pull down. This essentially reversed the telecine process, eliminating the duplicate fields that were produced when the film was transferred to video. The second pass was used to evaluate the film from a compression standpoint. The encoder evaluated each scene for its relative complexity and determined whether it required compression at a rate greater or lower than the average. It then produced a file of the bit-rate graph. The actual encoding of the video was then done according to this graph in the third pass.
A compressionist monitored this process and looked for any visible compression artifacts. Scenes containing such artifacts were then encoded again at a higher bit rate, using the extra disc space we had set aside for this purpose.
Quality-control checking a DVD presents an interesting challenge. There are significant time-to-market challenges requiring fast turnaround and, unlike the software industry, we didn't have the luxury of sending beta copies to several thousand home users. Even though strict QC procedures can be built into each project, it is impossible to test a disc on every available player to uncover possible hardware conflicts.
It doesn't require special technical mastery to manage any of these issues. What is needed is a strong sense of design and the ability to view the final product from the consumer's perspective. Authoring a DVD involves hundreds of small choices and giving careful thought to each is essential to producing a great DVD disc.
Ron Burdett is president of Sunset Post, and Garret Maki is senior vice president of the firm's DVD division in Glendale, CA.
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