Optical discs are shiny discs whose surfaces are not as smooth as they look. They have little microscopic pits in the surface, or some other irregularity, but it's on purpose (at least at first): the readers for these discs use lasers to translate the pits into data. To help guide the disc drive along, the disc features an equally microscopic spiral groove running from the center of the disc to the rim, with the pits and lands contained within this groove. If you've seen a toothed music box, a player piano, or a Fisher-Price toy record player with toy records, these work on the same principle, only on a much smaller scale.
The first commercially available format of optical disc was the LaserDisc, which encoded analog data. Unfortunately, they were too far ahead of their time, and sold poorly in a world that was still just getting used to VCRs. The technology, which was introduced by MCA DiscoVision in 1978, eventually ended up at Pioneer (where it was kept on life support well into the DVD era).
The first optical discs to actually catch on were Compact Discs (CDs for short), which encoded digital data instead of analog; this would set a precedent for all later formats of optical disc. They were introduced in 1982, but were slow to catch on due to player prices being generally expensive at first and only gradually decreasing between 1985 and 1989, with 1987 being the first year where the format outsold releases on the earlier vinyl record format. Writable versions showed up around 1990, but were far too expensive for anyone outside the CD mastering world to use until the first inexpensive PC writers came out around 1998.
Other important optical disc formats, in approximate order of introduction:
- DVDs (introduced in 1996 in Japan and 1997 internationally, accepted instantly)
- Universal Media Discs, aka UMDs, the disc format of the PlayStation Portable
- HD-DVDs, invented approx. 2006, now used mostly for computers, the apparent loser of the hi-def movie disc format war aganst Blu-ray
- Blu-ray, also invented approx. 2006, the apparent winner of the hi-def movie disc format war against HD-DVD (they became common around the end of 2009); official PlayStation 3 disc format.
The MiniDisc, invented around 1989 and brought to market in 1992, is an edge case - it looks optical, or would if you broke it out of its casing, and it's read with a laser, but it's written with a magnet, so it's both this and a Magnetic Disk. There were also bigger, non-proprietary "magneto-optical" discs made for PCs and Macs, but they never caught on all that well (at least not in the U.S.) and were doomed when the zip drive and cheap CD-Rs came about in the late 1990s, though they were popular with musicians and journalists as an alternative to DAT.
Optical discs are a cheap, effective, and reliable way to get big files from point A to point B. When the CD finally became popular for the first time, it was believed that they were Nigh-Invulnerable. This was never quite true, but CDs themselves and their more advanced kin don't wear out from use like vinyl records, magnetic tapes, or film stock (their players do, but not the discs themselves); on the computer side of things, a CD or DVD was (and still is) far less vulnerable to dust, debris, and magnetic fields than a magnetic disk of the same capacity.note These days, it costs almost nothing to print a CD or DVD (even the writable varieties); most of the cost of pre-recorded CDs and DVDs comes from the intellectual property. The dollar DVDs in dollar stores have public domain TV shows and films, and very little else.
Discs that can only be written on once but can be written on at home, called CD-Rs, use bursts of laser light, stronger than the normal laser, to burn dyes into light and dark spots. (If you don't keep your RW+ drive dusted, you will smell it happening.) This process also allows you to decorate the side you're not writing data on. RW+ discs, the ones that can be written on over and over, use the writing laser to modify a metal layer inside the disc, changing its "phase" from crystalline (which reflects light) to amorphous (which absorbs it), affecting the shininess enough to create "pits."
The most recent generation of discs - HD-DVDs and Blu-rays - can store tens of gigabytes on a single disc easily and reliably.
As of 2010, optical discs are the only economical way to sell high-quality digital music, video, and software in a large scale on a physical medium. Flash Memory cards can hold MP3s, photos, and small programs, but not CD-quality music, full-length movies, or video games that are trying to look like full-length movies. Well, they could, technically, do so, but given that the cost of a Flash card comparable to a Blu-ray disc in capacity is about ten times higher, this is hardly a sane thing to do, though with the recent drop in prices certain Flash Memory manufacturers like San Disk are testing the waters with digital music sales, which require less space and thus cheaper cards.
Vinyl records have made a comeback, but they're comparatively expensive to manufacture, the few remaining pressing plants are overburdened and the high demand combined with inherent imprecision of vinyl manufacturing means a high defect rate for LPs compared to optical media. If you thought that dips in vinyl quality control was just a symptom of the "Vinyl Revival", you'd be surprised to learn that this was an issue in the 1970s, at least for American pressings, according to a 1978 article
in Rolling Stone (European and Japanese pressings, then and now, are highly coveted by collectors due to their higher quality control). Vinyl records are also just as vulnerable to dust, debris and scratches as optical discs are, lacking any error correction. Most of the time, this will manifest as clicks, pops or other surface noise even as the disc remains playable, but more severe cases can result in skips or even "locked grooves", where the record repeats on a passage as the familiar "Broken Record" sound. In extreme cases, the stylus can be damaged. This is one reason many serious vinyl fans obsessively clean their records before playing. Records are also susceptible to warping, especially if stacked horizontally or exposed to heat.
Optical discs are not without their faults. Their players rely on moving parts. The discs can skip if bumped, and they can be scratched; CD players deal with this with a memory buffer. If shaken too much, the mechanism in an optical disc player can fail; this makes CD players and DVD players in your car costly.
Fingerprints, debris, oils, or minor scratches on an optical disc can cripple it, especially if the label side of a CD is scratched — the part of the CD that reflects the laser is directly under the label (DVDs and HD-DVDs have the data layer in the middle of the disc, making them less vulnerable to scratches; BDs store their data immediately beneath the surface of the shiny side, but cover it with an extremely hard anti-scratch coating). There's also the problem of disc rot, in which the reflective layer slowly oxidizes away with time, rendering the disc unreadable. Usually, this is the result of the reflective layer becoming exposed due to either a manufacturing error, a particularly bad scratch, or poor storage conditions (namely keeping them in a hot and humid place for too long— CD releases in the 80's even contained a disclaimer asking buyers to store them in a cool, dry location). Pressed CDs made before 1991 are reportedly more vulnerable than ones pressed in 1991 or later, though even a disc from 1982 will last indefinitely if stored and cared for properly. Most of the problems that left pressed discs susceptible to disc rot were ironed out by the DVD's introduction in 1996, though particularly extreme carelessness can still be costly. All CD-Rs so far can also suffer a similar kind of catastrophic failure if the dyes used to make them go bad or eat through the coating, with earlier CD-Rs being far more vulnerable. Rewritable discs can also be erased by the temperatures inside a hot car; other optical discs, left a bit longer inside such a car, just warp too much to fit comfortably into the disc player.
There is one other thing to keep in mind when we discuss optical media, and that is how the player handles the velocity of the disc. There are two methods:
Constant angular velocity, or CAV, means that the player always spins the disc at the same speed, just like a vinyl record player would. (Paul McCartney had his Run Devil Run CD labeled at 15,000 rpm.) On the plus side, it's easier to design a player that works that way, and the design is more reliable. On the minus side, you get a LOT of wasted space on the outer edges of the disc, as the head will travel faster over that section than the inner tracks. (Of course, less data on the edges means less data wrecked if you slip up while holding the disc by the edges....)
Constant linear velocity, or CLV, means that the player, instead of revolutions per minute, reads X pits per minute. Because the disk always travels the same linear rate under the laser, the disc can store more data in the outer edges. But it requires a spindle that can be spun at variable speeds - continuously variable speeds. Players designed for CLV discs can play CAV/rpm discs (and so every reader that a CD can fit in can play a CD); players designed for CAV/rpm discs usually don't play CLV discs because of the reliability factor - and fewer players often means less popularity.
CDs, DVDs and BluRay are all CLV formats on-disc, though most modern drives cheat a bit and use "zoned CAV" to make recording faster (optical drive controller hardware these days is smart enough to know how to adjust the bit rate to match, and just hike it a bit in the outer "zones" of the disc, where the linear velocity is higher). LaserDiscs could be found in either CAV or CLV forms; CLV discs couldn't do things like pause or search unless you had a more expensive player with "still store", whereas on CAV discs, the head could just hover in place like on a hard drive. Speaking of which, all hard drives are CAV, for the reasons stated above, and have been using the zoning trick for decades to stuff more data in (modern drives can have dozens of zones).
Floppy disks for the PC were CAV, but some other systems used CLV, sometimes for the same disks. For this reason, floppy disks had a slightly higher capacity when formatted for use with an Apple Macintosh or a Commodore 1541 drive. The cost of this was a markedly higher rate of drive failure. Of course, some clever programmers brought the zoning trick to the floppy world and wrote the drivers for the PC that used it, allowing to pack a whooping 800K on the standard DS/DD 5.25" PC floppy (that normally held just 360K), and up to 2.88M on the normally 1.44M 3.5" DS/HD one. The latter even got the official Sony/IBM blessing and was actually the final official amount that could be fit onto the 3.5" floppy.
