Best Bits: What's a '2-Hour' DVD Disc?

Does a blank 4.7GB DVD hold two hours of video? No. Yes. It depends.
A week doesn't go by without an e-mail showing up in my inbox announcing new low prices for Taka Yuden Modi (or some such brand name) 16x, 2-hour blank DVD discs. Unable to resist the marketing pressure, and afraid that the cost of DVDs will rise if oil goes any higher, I immediately send off an order for a hundred of them.

All right, you got me. Most of that is fiction. I do order blanks in 100-quantity, but it's rarely in response to an ad such as the one I just described. It seems to me that if a vendor can't get the capacity of a DVD disc correct, abandon hope all ye who order from here! What's that? You don't see any errors? It's that 2-hour thing. Hourly specifications may be fine for drug-store photo-processing, but it doesn't cut it in the world of blank DVDs.

Numbers Never Lie
A DVD is an optical disc, and like any other storage media, it has a set capacity. The two popular ones, at the moment, are single layer and dual layer, capable of holding 4.7GB and 8.5GB, respectively. (Let's not get into 9.4GB double sided, single layer and 17GB double sided, double layer just yet. You'll probably never see them and they'd just be a red herring anyway.) Logic should tell you that if a single layer, single sided, blank DVD disc holds two hours of video, then two hours of video should be 4.7GB in size.

How wrong could you be? First, let's knock out the size misinformation.

As with hard drives and other mechanical storage devices, DVD disc makers take pride in not understanding how things are sized. They think that 1GB is 1,000,000,000 bytes. After all, it has the requisite number of places and zeros. We in computing, however, know that a kilobyte is 1,024 bytes and that makes a gigabyte closer to 1,024,000,000 bytes.

Why do the manufacturers insist that their way is correct? Why put lipstick on a pig? They can advertise a 4.7GB (etc.) disc that will actually hold only about 4.38GB of computer-based content. (it's also easier for them to count in a decimal system than a binary number system as computers do.) And, getting more for your money always makes you feel better too, doesn't it?

That brings us to the crux of the matter: is two hours of DVD 4.7GB or 4.38GB? The true answer is neither "or both- with a dash of, "it depends," thrown in to spice things up.

Measuring By The Clock
Video is only measured by time when it hits the movie houses or your television screens, where it's treated as media. It's data when attempting to get it onto an optical disc. The size of data isn't measured in seconds or any other unit of time. It's measured in bytes. If you have a storage medium, like an optical disc, with a capacity of 4.38GB, that's how much video you can store on it. Is that two hours? That's where the "depends" enters the picture.

Let's say you record an hour of TV video. That video has a bit rate for both the visual and audio components. Let's say it's a typical NTSC broadcast and you're recording it in 720x480 resolution, in 4:3 format, with 24-bits of color, at the standard 29.97 frames per second, 48KHz, 16-bit stereo. You've set the recorder to use a variable video bit rate that maxes out at 8K bits per second for the video and a fixed 384Kbps for the audio recording rate.

When you're done, and assuming you're using the standard MPEG-2 10:1 compression ratio for the recording, your hour of video will be about 3.5GB in size. (That will vary some, depending on how much "fudge" time you leave before the scheduled start and after the scheduled end of the show.) Wait a minute! 3.5GB? That's only 1 hour on a 4.35GB DVD disc with some left over. So it's a one-hour DVD disc?

See how crazy it gets when you try to apply the wrong measuring tools? What if I told you that you could easily pack four one-hour TV shows on a single-layer, single-sided 4.7GB/4.38GB "two-hour" optical disc without breaking a sweat? What if I do that next time?

Bill O'Brien can be blamed for more than 2,000 articles on computers and technology topics. With his writing partner, Alice Hill, Bill co-authored "The Hard Edge," the longest-running (1992 to 2004) technology column penned by a techno duo. For more, go to

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