Magical 5-inch Discs

This post was published more than a few years ago (on 1999-09-11) and may contain inaccurate technical information, outmoded thoughts, or cringe takes. Proceed at your own risk.

For the past month-and-a-half, I have been sharing my joy with a new techno-toy to anyone who will listen. Unfortunately, the technology is still new enough that is hasn't become commonplace, and I find myself explaining the basics to those I hope to understand my giddiness. The technology? DVD.

It's understandable that DVD isn't well understood. The companies that created it are still bickering over new standards such as DVD-Audio and competing recordable formats such as DVD-RAM and DVD-RW. But, I'm getting ahead of myself. Let's start at the beginning…

To the man on the street, the technology is pretty transparent. The DVD player accepts a disc the same size as a CD, which can contain a full-length feature film on only one side of a disc. Most DVDs also offer a menu from which you can select to play the film or access special features such as production artwork, behind-the-scenes featurettes, an alternate audio track with the director's commentary or a second language, subtitles, or any number of other features. Some DVDs even offer a multiple angle feature— in the new Ghostbusters DVD release, you can see a scene before and after the special effects were added by hitting the angle button; or as, of course, pioneered by the porn industry, you can view various camera angles on the same… (ahem) action. DVDs can also be encoded to take best advantage of the newest widescreen TVs.

Now, how does all this fit on one side of a 5" disc when LaserDiscs had trouble fitting a long feature on two sides of a 12" disc? Well, to start (and this may come as a surprise to most folks)— LaserDiscs are analogue (BACK THIS UP). Sure, they use a laser to read the signal on the disc, but the tracks are encoded similar to how audio is encoded on an LP. The width and variations in the track feed the information to the player.

On the other hand, DVDs are completely digital, just like audio CDs. In fact, the formats are very similar to start— DVDs use the same little pits in the surface that CDs do to designate the ones and zeroes of the digital stream (almost all digital data is encoded with only 1s and 0s— this is called binary data). The DVD format takes it an evolutionary step further, using a smaller laser to read the data, so the pits can be smaller, allowing more to be compressed on one disc. Beyond that, DVDs can have two sides, and on each side, two layers (the top layer is semi-transparent, allowing the laser to focus on the top or bottom layer as needed). These advances add up to a staggering amount of data that can be held on a DVD. Whereas a CD-ROM can only hold 650 MB of data, a full-blown dual-sided, dual-layered DVD can hold 17 GigaBytes (GB), or the equivalent of 26 CD-ROMs.

DVDs are encoded with a video compression scheme (or “codec”) called MPEG–2. This sort of compression is “lossy”, meaning that not all or the visual information in the original picture is kept. This codec analyses each frame of video to determine what the human eye would see in that picture. Believe it or not, your eye doesn't take everything in a picture in at first glance— the main areas of concentration are those of contrast or motion. This codec then applies more compression (losing some information) to the parts of the image your eye won't notice, while at the same time using lighter compression on the parts that your eye will be drawn to.

A similar scheme is applied to the audio on DVDs, using a codec called AC–3, or as the public better knows it, Dolby Digital. The good folks at Dolby Labs perfected this particular “perceptual coding” scheme, which keeps only the audio that stands out to your ear. Very similar to the immensely popular MP3 audio format popularised by the Web, it uses an incredibly complex theory of how the harmonics of some frequencies mask other frequencies— I subject I am not qualified to delve into. If you're interested in more information on how they work this magic, take a look at Dolby's web site (BACK THIS UP) where they have plenty of information on the matter.

Some purists state that they can see a degradation of image and sound on DVDs as compared to LaserDisc. While it is true that if you look at your TV from a couple inches away, you may notice the compression, or may see a small pixel or two "glitch" every once in a great while (and only on discs that were encoded improperly), on the whole, the DVD gives you a visibly better picture than VHS and, in a lot of cases, LaserDisc.

I realise that I may have lost a lot of you back there— I'm not the greatest teacher when it comes to technology. Suffice to say that DVDs give you a crisp image and great sound on a disc about the size of a CD, and everyone should get one!