Bit budgeting allows the developer to plan for the video and audio content, and the client to brace for the cost of the project.
DVD is a high-density optical storage technology for video, audio and data. The application of DVD-Video, as developed by a consortium of manufacturers now known as the DVD Forum (www.dvdforum.org), allows the storage and playback of feature length films and long-form video content on set-top boxes and computer DVD drives. The interactive component of the standard also extends its functionality well beyond standard linear tape formats such as VHS. Adoption of the standard has grown steadily in applications such as entertainment, training and corporate communications.
The DVD-Video specification
With influences from Hollywood and broadcasters, targets for the DVD-Video specification included high-resolution, full-motion video and CD-quality audio. They adopted standard compression technologies such as MPEG for better compatibility across platforms. Video formats can be in NTSC or PAL and maintain a 4:3 or 16:9 aspect ratio.
DVD-Video is most often found in the MPEG-2 format, which offers high quality through an efficient coding process. For broadcasters this translates to easy repurposing of digital television content as the DTV standard is rolled out. Typical average data rates for video are 4Mb/s, but can go as high as 10Mb/s. The process of determining the target bit rate and achieving a high-quality encoded video is covered later in this article.
Standard digital audio formats also have been included in the DVD-Video specification. They include Dolby Digital Sound (DDS), MPEG and pulse code modulation (PCM). The format developed by Dolby Laboratories can be produced as stereo or as surround sound for a true “theater experience.”
DVD media options
Manufacturers helped to define the media options for the DVD specification but, again, the driving force was the quality and amount of video that the disc could hold. Most DVDs today use the DVD-5 format, which offers a capacity of 4.7GB and has become relatively inexpensive to produce and duplicate for widespread distribution of feature length movies.
A capacity of 8.5GB is achieved on a single-sided disc using the DVD-9 format. This is manufactured as a dual layer, so the laser that reads the disc can reach both layers without turning it over. The density of a DVD-9 disc is slightly less than twice that of DVD-5 because the pits and bumps that are placed on the disc as data on the bottom layer need to be slightly more spread out in order for the laser to read through to the top layer.
Higher-capacity discs, like the DVD-10 with 9.4GB, are double-sided, meaning the disc needs to be turned over in the player in order to access information on both sides. Programming interactivity between the sides is difficult, so DVD-10 is most often used for recording the same content in two different formats, one on each side — for instance, a widescreen and a standard 4:3 version of a program.
Manufacturing of DVD-18 discs is still somewhat imprecise. The drive to improve this process may yet come from the high-definition television market, where four times the capacity will be required just to get a feature-length HD film onto one disc. DVD-18 would provide 17GB.
Developing a DVD
Advanced planning can lead to a more successful DVD project. Prior to encoding and authoring, it's best to determine the scope of the project. This includes how much video and audio will need to fit on the disc, how much interactivity needs to be built in, and what the overall look and feel of the title will be. A storyboard is useful for determining the project's complexity and hence the length of development time it may take.
With a plan in place, the assets of the project can be assembled and a bit budget can be determined for the disk. Bit budgeting allows the developer to plan for the video and audio content, and the client to brace for the cost of the project. Generally, a five to 10 percent overhead is given to menus, textual content and authoring requirements. If the video content is two hours or less, including extraneous clips such as movie outtakes or actor interviews, then the designer is pretty well assured of fitting the material on a DVD-5. Anything beyond this will be easier to lay out on a DVD-9. If cost is an issue, an experienced compressionist can usually achieve high quality at lower average bit rates in order to keep the project on a DVD-5.
Real-time encoding of the MPEG video is generally undertaken for medium- to large-scale operations. Often this is a two- or three-pass operation whereby the entire video is analyzed, perhaps preprocessed and finally encoded at a variable bit rate, or a constant bit rate if the video content is an hour or less.
For smaller-scale projects, MPEG encoding can be done in software. Motion estimation operations can be speeded up, at the expense of accuracy, but this can lead to a lower-quality encode. It's best to get a good demonstration and understanding of a software encoding application in order to determine its effectiveness.
The quality of the source and the amount of preprocessing also can affect the quality of the encoded MPEG. Most studios will use a Digital Betacam source and apply sophisticated noise reduction to the video prior to encoding. This step can take more time, but also can lead to significantly higher-quality results.
There are many breeds of DVD authoring applications that allow the producer to assemble the assets and create the interactive links for the project. Assets include menu screens, video and audio content, images, and textual information. Again it's important to plan this stage in order to avoid complicated menus or navigational dead ends.
The authoring process entails creating all the links between menus and the content. A front page of the title contains a menu of items for the user to select. The Chapter sub-menu consists of links programmed into a long video. The links are usually visual representations that may take the form of framegrabs from the video itself, graphics and text, or even short motion video clips that can be looped. Other sub-menus might allow the user to select language tracks for the audio or subtitles, actor biographies or interviews, and other extraneous content.
The menus are often created in Photoshop, or After Effects if they include motion graphics. In most authoring programs they are considered layered images, with a background, a subpicture overlay and a button description layer. The subpicture overlay consists of the images or motion video that is used to portray the buttons.
It's important to remember to include a common interface for linking back to the main menu, or even some of the sub-menus. There are standard definitions for many of these operations in the DVD specification.
A trained compressionist/authoring expert can turn around a relatively simple title in a week or two. A DVD-R can then be burned for testing. Generally, overall production will take four to six weeks, resulting in a glass master and a check-disc package for final approval.
The future of DVD
Broadcast-quality video and CD-quality audio are the most attractive features of the DVD specification. Interactivity extends the standard far beyond the consumer VHS format as well. Most believe DVD will co-exist with other interactive video environments like the Internet because it caters to high quality and ease of use for entertainment purposes.
Barb Roeder is president of BarbWired LLC. She can be reached through her website,www.barb-wired.net.