You are puttering along back in 1992 with your 486SX/25, your 120 meg hard drive almost full to the brim, and you decide to plunk down $300 or more for a CD-ROM drive. You finally manage to get it installed, and pop in The 7th Guest, which tended to be packaged with the drives at that time. Holy CRAP! 650 megs on one shiny little disc!

CD-ROMs were actually special back then. Durable and massive in capacity. Whoever thought back then that today we'd be getting CD-ROMs as junk mail. Ahhh, nostalgia...
I once had a 1x CDROM. I used it to clean mailing lists by comparing them to the Post Office's CD of every address in the US. It's the sort of process I'd start at night, and it would finish in the morning.

I loaned the drive to my brother once I got a different job, and he broke it. I miss it now - I think it might be a collector's item! I wonder how much I could get for something like that on one of the auction sites?

Anyway, I felt like such an alpha geek, having one of the first CDROMS. Those were the days!
Like most computer backing store, a CD-ROM is just a huge load of 0s and 1s. The data is read with a laser in a CD-ROM drive. Here is a diagram.


\ \
 \ \   <- laser
  \ \



///\/////\\\\\////\\\\\/////\\\////\\\////\\\///
------------------------------------------------ CD-ROM

The laser iterates over the surface of the CD-ROM, shining on those dashes. They represent little reflective bits on the surface of the CD. The CD-ROM drive has sensors to catch this reflected light. If a certain sensor is activated, it's a 1. Otherwise, it's a . Nice and simple - the dashes are just reflecting the laser in the appropriate direction.

The reason CD-ROMs are read-only is because the process of "burning" these reflective surfaces could only take place once on a CD. Now, there are a hundred or so layers of the reflective material on CD-RWs, so one layer can be burnt totally away and a series of data burnt into the next layer. Cool.

CD-ROM stands for "Compact Disc Read Only Memory" and is a method for storing data on the audio-intended compact discs brought to us by Sony and Philips. There are a number of different types of CD-ROM, including stamped aluminum/plastic CD-ROMs, CD-Recordables or CDRs, CD-Rewritables or CDRW/CD-RWs, amd extended-play (from 70 up to about 90 minutes long) and three inch CDs of all of the above types. A drive which reads CD-ROM discs is also called simply a "CD-ROM".


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Pits and Lands on a Compact Disc, Greatly Magnified

CD-ROM technology also spawned the DVD, or Digital Versatile Disc, which is gradually superseding it, as well as some proprietary CD formats like the 1GB GD-ROM used by Sega's Dreamcast console game system.

PHYSICAL CHARACTERISTICS

There are 21-minute (80mm/3 inch), 63-minute, 74-minute, and 80-minute CD-Rs (120mm/4.72 inch)which translate into approximate data storage capacities of 184MB, 553MB, 650MB, and 700MB respectively.

Ordinary CD-ROMs are made of an aluminum sheet which is stamped (like a record) and then covered with plastic for durability. In theory, if a CD becomes scratched, the scratch can be buffed out of the plastic.

Recordable discs are made up of at least two plastic layers, a reflective metal layer, and a layer of organic dye, as well as two dielectric layers. Discs without a top plastic layer are highly susceptible to losing parts of the metal layer due to scratching or moisture. The organic layer comes in one of four compositions according to the CD-Recordable FAQ2:

  1. cyanine dye, which is cyan blue in color (hence the name);
  2. phthalocyanine and "advanced" phthalocyanine dye, which have a faint aqua tinge;
  3. metalized azo, which is dark blue;
  4. formazan dye, which is light green.

There are actually at least three other compositions I know of; Silver, Gold, and Red. I do not know what their dyes consist of.

Regardless, a CD-Rewritable is written by burning the organic layer with a laser in order to change some of it from transparency to opacity to emulate the pits and lands (think mesas) of an ordinary stamped CD.

Which color (and hence, which dye) is used in your CD-R makes a significant difference in its behavior because certain CDs are opaque to different frequencies of light, and different CD and CD-ROM players have laser tubes or LEDs which put out different frequencies.

CD-RWs have a pre-laid spiral track which the laser follows during recording. The spiral track makes 22,188 revolutions around the CD, with roughly 600 track revolutions per millimeter as you move outward. If you "unwound" the spiral, it would be about 3.5 miles long. 2

STANDARDS FOR CD-ROM CONTENT

There are a number of formats used with CD-ROMs. Most of them are not competing, but rather provide different feature sets for different needs. Like many standards, they are organized into color-identified books; the "red book", the "yellow book", and so on.

Red Book

This is the format for CDDA, or Compact Disc - Digital Audio. It is the standard format for Audio CDs. It is often combined with other formats on the same disc.

In one second of Red Book format audio there are 75 frames (or sectors) and each holds 2352 bytes of equivalent computer data. Red Book CDs have only one session which can contain no more than 99 digital audio tracks.

Red Book audio is 44.1kHz, 16 bit, and stereo.

Yellow Book

The Yellow book describes data CDs. It is further separated into Mode 1, the original data format, and Mode 2 which can be further divided into 2 forms: Form 1 and Form 2.

Green Book

This is the format for CD-i (Interactive CD), a multimedia computer system (essentially a console game system) from Philips.

Orange Book

The Orange Book describes the physical format for Recordable CDs. Part I describes CD-MO (Magneto-Optical) discs. Part II describes CD-WO (Write-Once), AKA CDR or CD-Recordable. This also includes the "hybrid" specification for Kodak's for Photo CD. Part III details specifications for CD-RW (ReWritable) discs.

White Book

The white book describes the Video CD format, a fixed-bandwidth MPEG1 video disc utilizing MPEG 1 Layer 2 audio (mp2).

Blue Book

The Blue book contains the format for CD Extra, a format which specifies two sessions; A Red Book audio session with no more than 99 audio tracks, followed by a Yellow Book data session. This format is popular for video games and music CDs which have additional material such as lyrics or a music video added to them.

CD-ROM/XA

XA stands for "extended architecture" and is a halfway point between the Yellow Book and the Green Book. Mode 1 CD/XA uses standard Yellow Book sectors. Mode 2 can be form 1 (2048 bytes of data, with error correction, for data) or form 2 (2324 bytes of data, no ECC, for audio/video). Form 2 is used to create SVCDs for example. Its lack of error correction is due to the fact that many video formats (including MPEG) mandate their own error correction, and using Mode 2 Form 2 will allow you to save the space used for ECC.

USES

CD-ROMs are used for nearly everything these days. Compact disc is the format of choice for music. CD-ROMs are the format of choice for software distribution. There are two video formats, Video CD/VCD and Super Video CD/SVCD. Many console video game systems use them; Sega Genesis, 3DO's 3DO, and Philips' CD-i were some of the first. Three inch CDs are cut down until they occupy no more space than a business card and used for utility and demonstration software.

The production of compact discs of all types has become so inexpensive that CDs with various types of content are handed out free in the checkout line of your local supermarket, usually try-and-buy packages for nationwide ISPs. CDs are also slowly replacing printed documentation, especially in technology. Digital documentation has a number of advantages over print media including searchability and a lower shipping weight.

Finally, outdated and damaged CDs frequently find new life in mosaics and mobiles, and other kitsch-art projects such as analog clocks.


References:

  1. Acer Peripherals, CD-ROM FAQ. Acer Peripherals, 2002. (http://www.benq.com.au/products/cdr/service/faq.htm)
  2. Andy McFadden's CD-Recordable FAQ. (http://www.cdrfaq.org/faq.html)

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