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Bitrates in DVD Studio ProDetermining the proper rate for your tracks and reencoding assets
For most people, a maximum bitrate seems relevant only when considering whether the content will be able to fit on a disc. That, of course, is easy enough to figure out. But regardless of what fits on a disc, there are still limitations imposed on you by DVD players in general that aren't capable of displaying content beyond a certain bitrate. And the problem isn't just with the video streams in your tracks. It also involves audio streams, multiple angles and even subtitles.
We'll take a look at some of the issues involved and cover what you should know before you encode to save yourself time and frustration. And, at the end of this article, I'll show you how to reencode assets that have been processed through DVD Studio Pro's background encoding without having to go in and replace all your assets manually in the event that you get a "Bitrate Too High" error during the build process.
When you plan for encoding, you need to be conscious of several factors. The first, of course, is whether your content will fit onto a single disc at any given bitrate. Apple has a formula for determining a maximum bitrate for this concern, which is 560/minutes=max bitrate. So, if your video is 100 minutes, the maximum bitrate would be 560/100=5.6 Mbps. But that's just the video portion, and it doesn't take into account anything else you put on the disc, such as audio, which will cost you another 1.5 Mbps for 16-bit stereo audio (less, of course, for encoded Dolby Digital or DTS audio, depending on your encoding settings). That brings you down to 4.1 Mbps for the video.
So work it out. A hundred minutes is 6,000 seconds. Multiply that by 5.6 (the combined bitrate for audio and video), and you get 33,600. One byte equals 8 bits, so that's 4,200 MB, which, divided by 1,024, comes out to about 4.102 GB. That sounds like a wide margin for a 4.7 GB disc, but it's not. A 4.7 GB disc really only holds 4.377 GB. That's not a lot of wiggle room if you have subtitles, DVD-ROM content or multiple, long, video-intensive menus.
But at least you're pretty much safe. And there's always DVD Studio Pro's disc capacity meter to help keep you within the realm of feasibility.
And, of course, there are plenty of utilities out there to help you calculate your bitrate. One of the free ones for Mac OS X is Railhead Design's BitrateCalc, which you can download for free by clicking here.http://www.railheaddesign.com/pages/software/bitratecalc/bitratecalc.html
BitrateCalc lets you calculate your target video bitrate
taking audio and other overhead into consideration.
Where another type of bitrate problem can arise is when you're using video of a shorter duration.
Bitrate maximum and player compatibility
Using the formula above, if you have 30 minutes of video, your disc has enough capacity to handle video theoretically encoded at 18.66 Mbps, which, of course, can't be done at all. The maximum bitrate for DVD video is 9.8 Mbps average, and none of Apple's encoders even bother to go that high.
Why? Because 9.8 Mbps is going to cause problems. Even the 9.0 Mbps maximum in Compressor, QuickTime Pro and DVD Studio Pro's built-in encoder can cause problems. DVD players can't handle more than about 10 Mbps for the combined bitrate of all streams in a single track--including audio and subtitles, and even less for tracks containing multiple angles.
What's more, some players can't even handle that. And some can't even handle moderately high sustained bitrates without dropping frames. So you have to be conservative when figuring out your the total bitrate you're going to shoot for, or you may wind up with problems when playing a disc on some players. Apple puts this conservative estimate at a maximum 9.2 Mbps for video, audio and subtitles combined.
Assuming you go for the maximum (9.2 Mbps total), that means the following:
Video (one stream): 7.7 Mbps
Audio (one stream): 1.5 Mbps
Total: 9.2 Mbps
Which is great. But this assumes that you have only one audio stream in your track and one angle. (Incidentally, subtitles in general take up a negligible amount of bandwidth, something along the lines of 0.01 Mbps per stream. But they can add up with multiple language streams, and they can get larger when using graphics in the subtitle stream.)
Bitrates and multiple audio streams
But what happens if you have two audio streams in your track? Surely that can't count against you, since you're only playing one stream at a time anyway. Ah, but it does indeed count against you. So, if you have two 16-bit stereo audio streams, this means:
Video (one stream): 6.2 Mbps
Audio (two streams): 3.0 Mbps
Total: 9.2 Mbps
Is this bad? No. You can get great image quality out of a 6.2 Mbps video stream, assuming you're using a solid two-pass variable bitrate encoder, which can exceed this maximum limitation when it's important, while keeping the average bitrate at or below the maximum. Exceeding the maximum is perfectly acceptable with a VBR encode, as long as you keep a moderate average. But don't go nuts. I've had players simply freeze up on me when the bitrate got too high.
One way to overcome these problems, incidentally, is to use A.Pack to convert your audio from AIFF to Dolby Digital, which will cut the audio bitrate down to something reasonable, like 448 Kbps (or lower), while providing an outstanding audio experience, with or without surround channels enabled.
And converting an AIFF audio file to an AC-3 format couldn't be simpler. Simply launch A.Pack (in your Applications folder); choose the audio coding mode (say 2/0, which means left and right channels, no surround); select your data rate; and then just drag your file onto the little rectangles in the interface. When you do so, A.Pack will ask you which channels to use for each speaker. Select the proper one (left or right), and then do the same with the next channel.
When you're done, hit the Encode button, and you have yourself some fantastic, low-bandwidth audio.
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