The Project—Transcoding
Overview
Welcome to the chapter about what many consider to be the Black Magic of DVD authoring: transcoding. We’re going to take a short tour of this seemingly mystical art and show you how to penetrate the fog of mystery quickly and easily using the tools that are available right within Adobe Encore DVD 2.0.
Transcoding is the process that Encore uses to turn non-DVD-compliant assets into assets that match the worldwide DVD standard. What that really means is that all the video, audio and graphics files that you want to become part of your DVD must be transformed into assets that can fit on a blank DVD and then be displayed on a TV by any DVD player in the world. Think about transcoding this way:
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Transcoding shrinks huge video and audio files so that they can fit on a blank DVD.
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Transcoding ensures that the shrunken files can be played back by any DVD player.
The mysterious part of transcoding is how to shrink the video and audio so that when the finished disc is played back, the quality of the picture and sound is good enough to make the audience go “Oooh” and “Aaah” instead of “Ewww” and “Gross”. The simplest solution to this mystery is to let Encore keep the Transcode Settings for each asset at Automatic. This is the default setting and it is applied to each asset upon import.
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Asset transcode settings set to Automatic
With the Automatic setting, Encore continually recalculates the total disc space required for the project, adjusting the Transcode Settings as content is added to the program in order to maintain the highest video and audio quality while making sure that everything will fit on the disc. These calculations occur behind-the-scenes and require no user input.
But before you can rely on Encore’s automatic transcoding, you need to tell Encore what kind of blank DVD you will be using. There are two types of blank media that are widely available today: single layer and dual layer. Here are the capacities of each:
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A single-layer disc, or DVD-5, can hold 4.7 gigabytes (GB) of data.
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A dual-layer disc, or DVD-9, can hold 8.54 GB of data.
Select the Disc panel and choose the disc capacity from the drop-down box next to the disc graphic. If your finished project will be put on a DVD-5 disc, select 4.7 GB. If your project will be put on a DVD-9 disc, choose 8.54 GB. The default is 4.7 GB. We will put our finished project on a 4.7 GB DVD-5 disc.
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Make sure to tell encore the correct size of your blank media
Now look at the whole top section of the Disc panel: Encore gives both a graphical and a numerical indication of used and free space. In the disc graphic, the space required for assets that need to be transcoded is shown in red, and the space required for assets that have already been transcoded is shown in green. The space required for untranscoded assets is only an estimate; Encore won’t know the exact size of a transcoded asset until transcoding is complete. The estimate is a good one, however, and you can count on its accuracy. Encore updates the space required whenever menus, timelines or slideshows are added to or deleted from the project.
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The Disc panel showing a graphical and numerical representation of the disc space used for the project so far
Transcode Settings
As good as it is, automatic transcoding is not a panacea. So when would you want to abandon Encore’s automatic transcoding logic?
If you are planning to put your project on a dual-layer DVD-9 disc, then the answer is probably, “Never”. The space available on a DVD-9 translates into a 2-hour or longer main program, plus plenty of supplemental material, all transcoded at the highest possible quality. For DVD-9, therefore, transcoding is essentially a set-it-and-forget-it operation. And since Encore chooses the Automatic setting by default when an asset is imported, you can even skip the set-it part! However, many authors began with, and are still heavily invested in DVD-5. The most common situation requiring a solution other than automatic transcoding would be a very long program that is to be authored on a DVD-5 disc.
The 4.7 GB storage limit for a DVD-5 can present a problem for DVD authors if the total time for all the program material on the disc exceeds approximately one and one-half hours. For total program times less than that, a normal DVD-5 disc can hold all of the program material transcoded with outstanding video and audio quality. Discs produced at this high-quality setting will be virtually indistinguishable from the original source video and audio.
Once the total program time exceeds one and one-half hours on a DVD-5 disc though, compromises must be made. These compromises will affect the video assets far more significantly than the audio assets. Encore’s default audio transcoding scheme is Dolby Digital, which produces superb audio and very small file sizes. There are minimal space-saving benefits to adjusting the audio transcode settings, so we will focus on video transcoding.
The villain that steals your available disc space is an asset’s bit rate. Bit rate is usually measured in megabits per second, or Mbps, and is a standard measure of video quality. Better quality is obtained by using higher bit rates when assets are transcoded. But higher bit rates also use up more disc space than lower bit rates do. With relatively short programs, running out of space on the disc is not an issue and Encore’s automatic transcoding logic selects bit rates that are as high as possible to ensure quality that is as high as possible. But as program length gets longer and longer, automatic transcoding forces the bit rate lower and lower so that all the program material can still fit on the disc. At some point, the bit rate will get so low that the video quality is no longer acceptable.
| Note |
Encore’s automatic transcoding is smart about low bit rates, too. Encore will not automatically transcode any video asset at a bit rate below a certain threshold, even if the disc capacity is exceeded. Instead, Encore will refuse to build the project or burn the disc. But don’t fret about Encore’s obstinate attitude; that threshold level is so low that no one would want to watch the resulting video anyway. |
So what can you do to ensure acceptable video and audio quality on a DVD-5 when your program is long? The answer has three parts:
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Vary how each asset is transcoded.
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Determine the number of passes, or how many times Encore gets to examine the asset frame-by-frame.
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Choose which assets will be transcoded at higher bit rates and which ones will be transcoded at lower bit rates.
In Encore, you have two choices for how an asset will be transcoded: Constant Bit Rate (CBR) or Variable Bit Rate (VBR). CBR transcoding will transcode the entire asset at the same bit rate. VBR transcoding will transcode the asset at an average bit rate. VBR transcoding looks at the amount of detail and picture information in each frame of the asset as it goes along, varying the bit rate as necessary between set maximum and minimum values to maintain the best picture quality and the selected average bit rate. This analysis makes VBR slower than CBR. The advantage of using VBR is that, at lower bit rates, the video quality of the transcoded file will usually be noticeably better than the same file transcoded using CBR. At higher bit rates, there isn’t much difference in the video quality between assets transcoded with VBR and those transcoded with CBR.
Encore offers one-pass or two-pass transcoding. One-pass or two-pass defines whether you give the transcoder one or two chances to examine the asset frame-by-frame. If you choose two-pass, the transcoder will use the information from the first pass to refine its bit rate decisions during the second pass. Although it takes twice as long, choosing two-pass can further increase the video quality of the transcoded asset.
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Reserve two-pass transcoding for VBR. Two-pass doesn’t make sense for CBR transcoding. |
For parts one and two of the answer above, the choices are relatively straightforward: if you want to maximize quality, and at the same time minimize file size by using lower bit rates, choose two-pass VBR transcoding. But what about part three? How do we choose which assets should get transcoded at higher bit rates and which ones get transcoded at lower bit rates?
The answer is in the assets themselves. Video assets that have lots of motion across the frame, or lots of fine detail like forest scenes or scenes with water surfaces, are very challenging for any transcoder and require higher bit rates than other video assets. Keeping this in mind, you can choose to lower the bit rate for video assets that don’t have a lot of motion, or are less detailed. This allows you to increase the bit rate for more challenging video assets, thereby preserving the overall quality of the whole program.
So now we need to reach into Encore’s toolbox for transcoding presets that are easy to use and will produce good results. Here’s how to get to them:
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In the Project panel, right-click an asset that needs transcoding.
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Select Transcode Settings.
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Select the preset that you want to use.
The Project transcoding presets
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Apply these presets to your troublesome video assets and you will achieve maximum video quality in the minimum amount of space.
Transcoding Assets Within Encore
Once Encore knows what Transcode Settings to use for each asset, the actual transcoding can take place. All non-DVD-compliant assets have to be transcoded before the project can be built or burned to disc. Transcoding can take a lot of time, especially if you use two-pass VBR transcoding for large projects. It can be frustrating to tell Encore to begin building or burning the project, only to have to wait for any non-compliant assets to be transcoded. Adobe Encore DVD 2.0 makes background transcoding available to ease the pain of this time-consuming task. With background transcoding, you can tell Encore to transcode assets early on in the project while you continue to work on other tasks. To take advantage of background transcoding:
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Select an asset in the Project panel.
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Right-click on the asset and choose Transcode Now.
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Encore automatically opens the Progress panel and begins transcoding the asset.
There is no “free lunch”, however; background transcoding takes up memory and CPU cycles, which will slow down your system somewhat. Having a computer that exceeds the recommended system specifications for Encore will minimize any impact to your system from background transcoding.
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The Progress panel after selecting Transcode Now
Transcoding an asset will create a new file on your hard drive. When transcoding is complete, Encore points to the new, transcoded asset in the Project panel. The original file is left unchanged. If you ever decide that you need to transcode the original file in a different way, Encore allows you to revert to the original file. To Revert to Original:
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Right-click on the asset in the Project panel.
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The Revert to Original command
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Transcoding Assets Before Import
Another way you can save time on transcoding in Encore is to ensure that any assets that are imported into Encore are already DVDcompliant. For example, instead of exporting your movie from a nonlinear editor, or NLE, as an .avi file or .mov file (both of which will need to be transcoded by Encore), you might consider taking advantage of your NLE’s own transcoding capabilities to export a DVD-compliant video. When that asset is brought into Encore, it will not need to be transcoded.
There are two main drawbacks to transcoding outside of Encore:
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You have to make all the transcoding decisions yourself. You can’t take advantage of Encore’s automatic transcoding or built-in presets.
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Chapter point placement inside of Encore is less precise with material that has been transcoded outside of Encore. We will talk about adding chapter points in “The Project: Timeline Creation”.
There is another, more subtle, issue that can affect you if you transcode your assets external to Encore. Blank DVD disc capacities are listed in Gigabytes, or billions of bytes, and are abbreviated as GB. The capacity of a DVD-5 disc is 4.7 GB, or 4,700,000,000 bytes. If we write that number using powers of ten, it becomes 4.7 109 bytes. However, not all gigabytes are the same. Computer operating systems like Microsoft Windows calculate file sizes based on powers of two, not powers of ten like we just did. For example, you and I would think of a kilobyte as 1000 bytes, or 103 bytes. A computer thinks of a kilobyte as 210 bytes, or 1024 bytes. This difference between human thinking and computer thinking increases as file sizes get bigger and bigger. A file on your computer’s hard disk that would occupy the same space as a blank DVD-5 would be listed in Windows Explorer as 4.37 GB. As silly as it sounds, the end result is that you can’t fit 4.7GB of transcoded assets on an empty 4.7 GB DVD disc! For dual-layer discs, you can only fit 7.95 GB of transcoded assets on an 8.54 GB blank DVD-9.
The good news is that Adobe Encore DVD 2.0 reports file sizes in millions or billions of normal human bytes, not computer bytes, so that you don’t have to worry about this discrepancy. Once you import your DVD-compliant assets into Encore and create project elements with them, you can use the Project, Disc and Timeline panels as tools to judge exactly how much space they will take up on the finished DVD.
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Encore shows the bit rate and transcoded file size for an asset transcoded external to Encore
Filed under: DVD Authering
