CD-ROMSECTION 2.3.7

A CD-ROM vade mecum

“Vade mecum” is from the Latin, meaning “go with me.” So hold on loosely, but don’t let go*, as we venture into the weird and wild world of CD-ROMs…

* Fun fact: Most people believe that .38 Special the gun was named after .38 Special the band, but it’s actually the other way around! I know, right?

Background

CD-ROM stands for Compact Dis … actually if you don’t know this one by now, you’re in the wrong line of work.

Recorded music was an early killer app for CD-ROMs. Unfortunately, for most of you the introduction of CD technology left you crestfallen, as you were expected

to buy the entire Huey Lewis and the News catalog on CD despite the fact that you already owned not only every official Huey release, but also numerous rare bootlegs on vinyl, cassette, and 8-track tape.

Nevertheless, CD sales were a veritable cash cow for the music industry from their inception in the 1980s through their heyday in the 90s. Sales were bolstered by the calculated move to release a CD which only had one good song on it, in blatant violation of the "all killer, no filler" precept that defined 70's classic rock albums.

This was eventually remedied by Napster, iTunes, yada yada.

Of course, a CD is simply a medium for storing data – musical or otherwise – so let’s get down to brass tacks and see what makes these puppies tick.

Freaking CD-ROMs, how do they work?

To create the disc, a laser burns small holes (0.8 millionths of a meter in diameter) on the surface of the disc, creating pits where the laser burned a hole and lands where the disc is unburned. The burning of the pits on one side causes bumps to form on the other side. There is only one track on a CD-ROM, since the laser burns in a continuous spiral, the

same way that a vinyl record has a single groove.

This groove would be ~3.5 miles long if stretched into a straight line.

To play back the disc … A light that’s ¼ the wavelength of the height of a pit shines on the bottom of the disc

(where the pits are bumps sticking out).

(cont. next slide)

Freaking CD-ROMs, how do they work?(continued..)

When transitioning from pit to land (or conversely), light that is reflected from a bump will be shifted ½ a wavelength ahead of light reflecting off the land, causing destructive interference in the light which will cause it to look dark.

Anytime such destructive interference (i.e., darkness) is observed, the detector treats that as a 1. If no interference is observed during a clock cycle, it is interpreted as a 0. (When not in transition between pits/lands the light looks the same, so it’s not the pits/lands themselves that are used to encode information, but the transition between them.) See the diagram on the right:

101: An Introduction to Binary Numbering.

Each 8-bit byte is encoded as a 14-bit symbol, with the symbol to data byte correspondence handled by a hardware LUT (look up table).

A sequence of 42 consecutive symbols is grouped into a 42*14 = 588 byte frame (with just 192 bits representing data and the other 396 bits for error correction/control, so only ~1/3 of the information on the CD represents raw data).

The Yellow Book standard groups 98 frames into a sector, which is the largest grouping of data besides the CD itself. (Other standards like Green Book and the High Sierra with Rock Ridge* extensions refined this by adding more advanced multimedia capability and a cross-platform file system, respectively.)

Error-correcting occurs at all three levels: symbol, frame, and sector. The hierarchy percolates upwards: First single-bit errors are corrected at the symbol level, then bursts that cross

symbol boundaries are corrected at the frame level, and finally, any residual errors are corrected at the sector level.

* The name “Rock Ridge” was chosen as an homage to the Mel Brooks film “Blazing Saddles.” What was your favorite scene? Mine was the “I Get No Kick from Champagne” musical number. Of course, the scene where they were sitting around the campfire farting was pretty good. Then again, when that guy punched out the horse, that was really funny. And in “Space balls,” when those guys were “combing the desert,” that was great. “We ain’t found ----!!!” But I digress: as far as I know there is no optical data standard with a name inspired by “Space balls.”

CD-R / CD-RWSECTION 2.3.8/2.3.9

CD- Recordable

By the mid 1990s, CD recorders became a common device.

Individuals or startup companies have a method to produce their own small-run CD-ROMs.

CD- Recordable

Feature: write once, read multiple times.

Structure: Printed label, Reflective layer, Dye layer, Polycarbonate Layer.

The color of CD-Rs

Dye Color + Reflective Layer => Final Color

Cyanine + Gold => Green

Cyanine+ Silver => Green/blue

Phthalocyanine + Gold => Gold

CD- Recordable Write

CD-R laser is turned up to high power (8–16 mW).

When laser hits a spot of dye layer, it will create dark spot which is called pit.

The intact dye layer is called land.

CD- Recordable Read

CD-R laser is turned up to low power (0.5 mW).

The change between pit and land will generate data.

CD- Rewritable

Feature : Write multiple times

Structure(difference between CR-RWs and CR-Rs)

Can CD-RWs replace CD-Rs?

CD-RW Blank is more expensive than CD-Rs.

Most of us have experience at deleting files on hard disk by accidently. This will not happen to CD-Rs.

DVD / Blu-RaySECTION 2.3.10/2.3.11

DVD(Digital Versatile Disk/Digital Video Disk)

Digital optical disc storage format developed in 1995.

Invented and developed by Philips, Sony, Toshiba and Panasonic.

DVD’s are 120mm injection-molded polycarbonate discs .

Contains pits and lands that are illuminated by laser diode and read by a photo detector.

CDs vs DVDs

DVDs have

Smaller pits(0.4 microns versus 0.8 microns for CDs).

A tighter spiral(0.74 microns between tracks versus 1.6 microns for CDs).

A red laser(at 0.65 microns versus 0.78 microns for CDs).

Formats of DVDs available

The versatile needs of Philips, Sony, Toshiba and Time Warner lead to four different combinations of DVDs:

Single-sided , single-layer(4.7 GB).

Single-sided, dual-layer(8.5 GB).

Double-sided, single-layer(9.4 GB).

Double-sided, dual-layer(17 GB).

Dual layering technology

Dual layered recording allows the DVD discs to store significantly more data than the single-layered discs.

A dual-layer disc differs from its usual DVD counterpart by employing a second physical layer within the disc.

The drive with dual-layered capability accesses the second layer by shining the laser through the first semitransparent layer.

The lower layer needs slightly larger pits and lands to be read reliably, so its capacity is slightly smaller than the upper layer’s.

Double-sided discs

Double-sided disks are made by taking two 0.6-mm single-sided disks and gluing them together back to back.

To make the thicknesses of all versions the same, a single-sided disk consists of a 0.6-mm disk bonded to a blank substrate.

Advantage of DVDs over CDs

Greater capacity Faster data transfer Flexibility Better video

Blu-Ray

The successor of DVD is Blu-Ray. Blu-Ray gets its name from the blue laser it uses instead of red one used by

the DVDs. The blue laser has a shorter wavelength than the red one, which allows it to

focus more accurately and thus support smaller pits and lands. Single-sided Blu-Ray disks hold about 25 GB of data, double-sided can hold

about 50 GB. The data rate is about 4.5 MB/sec.

Blu-Ray(continued)

The reasons for slower transition to Blu-Ray from DVDs:

Blu-Ray is expensive than DVDs. A high-definition television and appropriate connection cables are also

required to take advantage of Blu-Ray disc. Confusion over firmware updates.

Advantage of Blu-Ray over DVDs

Greater storage capacity. Better quality. Blu-Ray can hold 3D movies.

Credits slide

Christopher Igoe        CD-ROM (section 2.3.7)

Min Zhang                  CD-R / CD-RW (section 2.3.8/2.3.9)

Gouthami Nandikur   DVD / Blu-Ray (section 2.3.10/2.3.11)