The DVD has become the de facto replacement for the VHS dub. DVD authoring has become an extension of most nonlinear editing software. This has made something that started out as a very arcane, expensive and specialized task into something that is easy enough for any video professional to master. When you produce a video DVD, you are actually building a creative product that must conform to the DVD-Video spec. This is one of the categories within the overall DVD format specification that includes other forms, such as DVD-ROM and DVD-Audio. Creating a disk with the full nuances of the DVD-Video spec has been addressed in such high-end applications as Sonic Solutions’ DVD Creator. Newer, more user-friendly programs like Adobe Encore DVD, Apple DVD Studio Pro 2, Sonic’s own Reel DVD and Sony’s DVD Architect now offer enough of these same DVD authoring features to satisfy the requirements of over 90% of all the types of DVDs typically produced.
The full DVD spec deals with complex elements like program managers, title sets and so on, but these newer authoring tools have adopted a more streamlined approach – dividing your assets into tracks (for video and audio elements) and menus. You are generally limited to 99 different assets – far less than the full DVD spec would actually allow, yet well within the requirements of most commercial and corporate DVDs. This is especially true considering the significant drop in price from the early days of DVD tools.
Most of the popular authoring applications offer the main features, like motion menus, subtitles and multiple languages; but, not all permit some of the more advanced features, including 24fps encoding, multiple camera angles and AC-3 Dolby surround audio tracks. Obviously, there is still room for the advanced products, but any of the sub-$1,000 software tools will do the trick for most producers.
Encoding
DVD program assets are made up of elementary stream files. Each video segment has a compressed MPEG2-format video file (.m2v) and, if there is audio with the program, at least one audio file (.wav). When multiple languages are used, there will be more than one audio file for each video element. For surround audio, there will be both a stereo track, as well as an AC-3 encoded audio file for the surround mix. In the past, these files had to be created prior to any authoring. Ground-breaking DVD tools, like Sonic’s DVD Creator, employed real-time encoding hardware (which is still used), but fast software encoding has now become an acceptable alternative. Many authoring tools let you import DVD-compliant files, which have been compressed using a separate application – or work with regular AVI or QuickTime files and apply built-in encoding at the last stage of the authoring process.
DVD videos can be either NTSC or PAL and the frame size is the same as DV: NTSC – 720 x 480 (non-square pixel aspect ratio). The compression of DVDs should typically fall into the range of 4.0 to 7.0 Mbps (megabits per second). Higher data rates are allowed, but you generally won’t see much improvement in the video and higher bit rates often cause problems in playback on some DVD players, especially those in older laptop computers. There are three encoding methods: constant bit rate, one-pass variable and two-pass variable.
Constant bit rate encoding is the fastest because the same amount of compression is applied to all of the video, regardless of complexity. Variable bit rate encoding applies less compression to more complex scenes (a fast camera pan) and more compression to less complex scenes (a static “talking head” shot). In two-pass encoding, the first pass is used to analyze the video and the second pass is the actual encoding pass. Therefore, two-pass variable bit rate encoding will take the longest amount of time. During the encoding set up, a single bit rate value is entered for constant bit rate encoding, but two values (average and maximum peak rates) are entered for variable.
The quality of one type of encoding versus another depends on the quality of the encoding engine used by the application, as well as the compression efficiency of the video itself. The former is obvious, but the latter means that film, 24P and other progressive-based media will compress more cleanly than standard interlaced video. This is due to the fact that interlaced video changes temporal image information every 60th of a second, while film and 24P’s visual information updates only 24 times a second. As a result, compression at the exact same bit rate will appear to have fewer artifacts when it is applied to film and 24P media, than when applied to interlaced media. Add to this the fact that film material has grain, which further hides some of these compression artifacts. The bottom line is that a major movie title can often look great with a much lower bit rate (more compressed) than your video-originated corporate training DVD – even with a higher bit rate. Most standard DVDs will look pretty good at a bit rate of around 5.5 Mbps, which will permit you to get about 60 to 90 minutes of material on a 4.7 GB general-purpose DVD-R.
Authoring
Creating the interactive design for a DVD is a lot like building a web site. Menus are created which are linked to video assets. Clicking a menu button causes the player to jump from one point on the DVD (menu) to another (video track). Menus can be still frames or moving video, but must conform to the same constraints as any other video. If they don’t start out as video, they are turned into video in the final DVD build. By comparison a web site, CD-ROM or even DVD-ROM might have HTML-based menus of one size and QuickTime or AVI video assets of a different size. This isn’t allowed in a DVD-Video, because the DVD must be playable on a set-top DVD player as video. Motion menus must be built with loop points, since the longer the menu runs, the more space it will consume on the disk. Thirty seconds is usually a standard duration for a loop. A slight pause or freeze occurs in the video when the disk is triggered to jump back to the start of the loop. This is a buffer as the DVD player’s head moves between two points on the disk’s surface.
Lay out your design on paper first. A flowchart is a great idea, because this will let you see how one or more menus connect to the videos in the most efficient manner. Although any type of flowcharting software program will work, it is often just as simple to do this on paper. On the other hand, a nicely printed visual flowchart goes a long way in explaining to a client how the viewer will navigate through a complex DVD. Links can be created as buttons or drop zones. A button is a graphic element added to a menu within the authoring tool, while a drop zone is a hyperlink area added to an imported video. You can create videos or animations to be used as menus and turn graphic or video portions into linkable “hot spots” by adding drop zones on top of the video.
Clicking on a button or drop zone activates a jump to another menu, a video track or a point within a video track (chapter point). Part of the authoring task is to define what actions occur when you click on the control keys of a standard DVD handheld remote. You must define what happens when the viewer clicks the Menu, Title or Return keys, as well as which direction the cursor travels when the arrow keys (up, down, left, right) are used. Although the degree of control over these parameters varies with different software applications, all the contenders must let you define the next options. You have to set the First Play – the file that plays when you first pop in the DVD. You also have to set the target destination for the end of each video file. This determines whether the video continues on to another video or jumps back to a menu – and if so, which one. Remember that if you have more than one menu, you will have to add buttons and navigation commands to go between the menus.
Finishing
Most authoring tools let you run a simulation of the DVD as you configure it. This lets you proof it to see if all the links work as intended. When the authoring is complete, the next step is to “build” the disk. This is an automatic process in which the application checks to see if your authoring has any errors and then “muxes” (multiplexes) the audio, video and menu files. The muxing stage is where the VOB (video object) files are created. You can see these files if you explore the folders of a non-encrypted DVD. If your DVD tool features built-in encoding, that step generally occurs during the building process.
When a project is built, it can be saved to your hard drive as a disk image or burned to a recordable DVD. Although there are a handful of different recordable DVD formats, the DVD-R general-purpose disks seem to be the most universal. These are typically rated at 4.7 GB, but actually hold about 4.3 GB of data. If you require more capacity, you will have to advance to a dual-layer or dual-sided DVD (9 GB and higher). These cannot be burned on a DVD recorder and not all authoring tools handle these formats. If they do, you can prepare a disk image to be saved to a DLT tape, which would be sent to a commercial DVD replication facility. When you burn a DVD-R on a recording drive, be aware that the burning speed is based on the rating of the media. 1X blanks will only burn at the 1X speed, 2X disks at 2X speed and so on. If you saved the build as a disk image, you will be able to use a disk burning utility like Roxio and burn multiple DVD copies. Although encoding and recording times vary, it is not out-of-line for a one-hour DVD to require as much as four hours from the time you start the build until the DVD has finished the recording step.
We’ve only scratched the surface; so to learn more, check out the white papers and technical documents that are available on the Pioneer, Sonic Solutions and Mitsui web sites. A great reference is DVD Production, A Practical Resource for DVD Publishers by Philip De Lancie and Mark Ely, available from Sonic Solutions. User guides and tutorials – especially those included with Adobe Encore DVD and Apple DVD Studio Pro 2 – are also quite helpful. Start small and move up from there. Soon you, too, can be a master of the DVD world.
© 2004 Oliver Peters
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