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DVD Bits & Bytes

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Created on: Aug 28, 2009 4:34 PM by CreateSpaceResources - Last Modified:  Oct 26, 2010 5:57 AM by CreateSpaceBlogger

DVD Bits & Bytes

 

My MPEG encoder is driving me mad! The first time I tried to burn a disc with the video, I spent two hours encoding, and then my authoring program told me "Insufficient space on DVD." Determined to make my deadline, I stayed late at the office and fiddled with the encoding settings, and then waited. My second disc burned quickly, but the video looked horrible. I'm on my sixth try right now. Forget deadlines, I just want my video to fit on the disc and look good! Is that too much to ask?

 

Set your MPEG bit rates too high, and your video won't fit on the DVD. Set them too low, and it isn't pretty. So how do you figure out the magic number?

 

Many tools automatically calculate the bit rate for you--but their defaults may not always be ideal, especially if you're going to be delivering your project on DVD-R. Other tools require you to figure it out yourself. We've provided an extremely easy-to-use bit rate calculator at the end of this article. You don't actually have to read the rest of this article to use the calculator, but it's worth understanding how it works, and you're likely to come back to this article if you run into questions about the numbers.

 

DVD is a digital medium; everything is stored in bytes. A DVD can only store a certain number of bytes before it's full--so it's important to avoid trying to put too many bytes on the disc.

 

When a DVD is played, those bytes are turned back into audio and video (among other things). To some degree, the more bytes played back per second, the higher the quality. So it's important to avoid putting too few bytes on the disc as well.

 

How Big?

As you can see, it's a balancing act. Making sure that the numbers work out is called bit budgeting. And as with any budget, the first step is to understand how much you have to "spend."

 

These DVD-Rs have "4.7 GB" printed right on the side of them. But when I tried to burn one with that much data, it wouldn't fit. What's up?

 

A billion bytes are 1,000,000,000--sometimes written as 1gB (note the lowercase "g"). However, computers use powers of two to represent large numbers, and a Gigabyte or "GB" (note the uppercase G) is 230, or 1,073,741,824 bytes.

 

DVD-5 is commonly stated as having "4.7 GB" of capacity, but strictly speaking that is incorrect. The exact number is 4,699,979,766 bytes. While that is approximately 4.7 billion bytes, it is actually 4.377 Gigabytes. We'll do our math in billions rather than "Gigas" to avoid this confusion.

 

DVD-5 and standard recordable DVDs, such as those used by CreateSpace, hold the same 4.7 billion bytes. Other formats hold more, as shown below: 

  • DVD-9: 8.54 billion bytes
  • DVD-10: 4.7 billion bytes per side, or 9.4 billion bytes total
  • DVD-18: 8.54 billion bytes per side, or 17.08 billion bytes total

 

Playing it Safe

While it would seem desirable to use every last byte on the disc, you probably want to avoid that for several reasons:

 

  1. It's not realistic to budget that tightly: you'd need to know the exact size of every file, which would take painstaking calculation and may not be entirely certain. Allowing a 1% safety margin avoids that, and covers non-motion menus and navigation overhead for simple projects.
  2. The outer rim of the disc is the most unreliable area. In manufacturing, the majority of errors occur on the outer edge of the disc. As DVDs are written starting in the center, the more data on the disc, the closer you get to the outer edge. By allowing an additional 2% safety margin, you avoid putting data near the rim, and thus avoid production problems.
  3. Speaking from experience, things will go smoother if you leave some extra room in your budget. In theory, you know how much audio and video you've got. But what if that 30-minute tape turns out to actually have 31 minutes of video? Or a 15-second trailer needs to get tacked on at the last minute? Allowing an additional 3% safety margin minimizes the odds of having to go back and re-encode everything to make room for these sorts of last-minute surprises.

 

Adding these up, it's a good practice to leave about a 5% safety margin. So on a DVD-5 (or DVD-R) with 4.7 billion bytes, do your budgeting assuming you have 4.46 billion bytes.

 

Speed Limits

There are limits not only on how much data you should put on a disc, but also on how fast that data should be played back.

 

The DVD specification states that players must accommodate up to a maximum of 10.08 mbps (million bits of data per second). This means that your audio and video (and subtitles, if any) must add up to no more than that number. However, many DVD-equipped computers don't do a very good of handling discs which are running at that limit, so 8.0 mbps is a safer maximum.

 

In a similar vein, DVD-R media is subtly different from replicated media. By sticking to an even lower maximum bit rate of 6.5 mbps, you're giving the player the chance to re-read some sectors if needed without interrupting the video. This provides similar benefits for scratched or dirty discs, making them a bit more likely to continue playing correctly.

 

Adding It Up

Everything you put on the disc takes up bytes, whether it's audio, video, menus, slides, or other content. Unless you have a lot of motion menus though, 99% of your storage will be used by your audio and video tracks.

 

Audio: In DVD Audio we recommend Dolby Digital audio at 192 kbps (0.192 mbps) for stereo. That saves you more than a million bits per second over using 1.5 mbps PCM, allowing higher video quality and capacity on your disc.

 

Video: In general, you want to use the highest possible bit rate for your video given the capacity of your DVD and the total duration of video that will be stored (including motion menus). The video bit rate is the main thing you need to figure out for each project.

 

Everything Else: Feel free to ignore non-motion menus and navigation (remember, we have a 1% safety margin to cover that). Add up the number of bytes for anything else on the DVD. Make sure you're adding up the exact number of bytes, not the less accurate number of Megabytes or Gigabytes.

 

The Math

The total number of bits per second, times the number of seconds, is the number of bits you're storing. Given that, here is one way to calculate a video bit rate: (((disc capacity in bits x 92%) - (# bytes of "everything else" x 8)) / total video duration in seconds) - audio bit rate in mbps.

 

So for example, a DVD-5 with 110 total minutes of video, and 192 kbps Dolby Digital audio: = ((34,591,851,078 bits - 0) / 6600 seconds) - 0.192 bps audio = 5.05 mbps video

 

That's the average bit rate at which we want to encode the video. If it's higher than the safe maximums (6.5 or 8.0 mbps, from "Speed Limits" above) then reduce it to that maximum bit rate. You probably won't want to do these kinds of calculations by hand all the time, so we've created this handy Bit Rate Calculator. This calculator takes into account everything we've discussed so far, and works for simple or intermediate projects.

 

CBR or VBR

Once you know the bit rate that you're working at, there's another important decision for encoding the video: whether to use Constant Bit rate (CBR) or Variable Bit rate (VBR).

 

Remember that some things are easier to compress than others? If you were to chop up your movie into 1-second-long pieces, you'd find that it's true on that timescale as well. CBR takes a simple, consistent approach: each second gets the same number of bits: the "average bit rate" we calculated above. VBR is a more sophisticated approach, which attempts to save bits from the easy sections, in order to have more bits to encode the harder sections. 2-pass VBR gets even more sophisticated, looking through the entire file beforehand to more accurately assess which parts are hard or easy. In short, 2-pass VBR gives the best results in some cases, but encoding this way takes longer.

 

Even with VBR, you don't want to exceed the safe maximum bit rate, 6.5 mbps. In general, you will benefit from using VBR video if your maximum bit rate exceeds your average bit rate by about 1 mbps or more. Otherwise, CBR will work just as well.

 

Summary

  1. Remember that a "Gigabyte" isn't quite the same as a billion bytes.
  2. Only budget for 95% of the available capacity--the unused 5% is a safety cushion.
  3. Don't exceed 6.5 mbps for duplicated (DVD-R).
  4. Use the Bit Rate Calculator included in this article.

 

Looking for more helpful information and advice? Check out our Resources, Community, and Help sections.

 

Permalink: http://www.createspace.com/resources/dvd_bitrates

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