Explain that an optical disc (光ディスク) is a medium read and written with a laser beam, and tell it apart from semiconductor memory, which stores data in electric circuits.
Rank the three generations of optical discs (CD, DVD, Blu-ray Disc) by capacity at the same 12cm diameter.
Use the ending of a disc's name (ROM / R / RAM) to tell whether it is read-only, writable once, or rewritable many times.
A CD (Compact Disc) is a 12cm optical disc (光ディスク) whose data is read by shining a laser beam onto the recording surface of the spinning disc and detecting changes in the reflected light.
Spin the disc
Shine a laser on the surface
Reflection strength changes with the surface
Turn light changes into 0s and 1s
Those changes are converted into an electric signal and read out as a string of 0s and 1s.
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On a read-only CD, tiny dents called pits run in a spiral across the recording surface, and the difference in reflection where a pit is or is not present encodes the data. It holds about 650 to 700MB, less than the later DVD and Blu-ray Disc. It first spread as a music format and was later adopted for storing computer data.
AnalogyLike a fingertip running over Braille, it uses light to trace the fine bumps on the disc surface.
How to rememberThe name comes from Compact Disc: it was far smaller (compact) than the vinyl records that came before it.
Exam points
Mix-upCD names a family of disc formats, and within it are types with different writing behaviour, such as read-only CD-ROM and write-once CD-R.
A CD-ROM is a CD whose contents are recorded at manufacture; the user can only read it and can neither write to it nor erase it.
The contents are stamped from a master disc into pits on the recording surface, so identical discs can be mass-produced cheaply. That made it a long-standing choice for handing out unchanged content to many people, such as software and dictionary data. It is also non-volatile, keeping its data without power.
AnalogyLike a printed, bound book: you can read it, but you cannot add your own writing to its pages.
How to rememberROM stands for Read Only Memory, so the name says outright that it is memory you can only read.
Exam points
Mix-upA CD-ROM has its contents fixed at the manufacturing stage, while with a CD-R the user writes onto a blank disc; the two differ in who records the data and when.
A CD-R is a write-once CD on which the user can write data only once into a dye-based recording layer (記録層).
Blank disc with a dye layer
Apply a strong laser
Heat alters the dye, leaving a mark
Read the marks with a weak laser
To read, a weak beam picks up the difference in reflection between altered marks and the rest.
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When the drive fires a stronger laser beam, the heat alters the dye and changes how it reflects light, and that change is the recording. The altered spots cannot be restored, so anything written can be neither erased nor overwritten. As long as unwritten space remains, more data can be added there later.
AnalogyLike a farewell card signed in permanent marker: you can add messages in the blank space, but what is written stays.
How to rememberThe R stands for Recordable, marking it as a CD that users can record on themselves.
Exam points
ExamWriting to a CD-R works by using the heat of the drive's laser beam (レーザー光) to alter the dye in the recording layer (記録層).
A DVD is an optical disc that keeps the same 12cm diameter as a CD but raises recording density so that one side with one layer holds about 4.7GB.
It uses a red laser with a shorter wavelength than a CD's, making the recorded marks smaller and packing them closer, which gives it about seven times the capacity of a CD. A version with two recording layers holds about 8.5GB on one side. It became a common format for films and other video, and for storing large amounts of data.
AnalogyLike writing smaller and tighter with a fine pen so the same notebook holds more.
How to rememberDVD stands for Digital Versatile Disc, and true to "versatile" it carries both video and data.
Exam points
ExamA DVD drive is not tied to one kind of disc; it can handle several disc types with different writing behaviour.
ExamA DVD is a non-volatile (不揮発性) storage medium whose data stays when the power is off.
A DVD-ROM is a DVD whose contents are recorded at manufacture, and the user can only read it.
The contents are stamped onto the surface from a master disc, which suits making and distributing large runs of the same disc. It has been used for packaged films and for releasing large software. Following the DVD standard, it holds about 4.7GB on one side with one layer.
How to rememberThe name splits into "DVD + ROM", and the ROM at the end means Read Only.
Exam points
ExamAs the ROM in its name (Read Only Memory) indicates, a DVD-ROM is a read-only (読出し専用) DVD.
A DVD-R is a write-once DVD on which the user can write data only once.
Like a CD-R, it records by using laser heat to alter a dye recording layer, so written areas cannot be erased. It holds about 4.7GB on one side with one layer and is used for saving video or keeping data that should stay unchanged.
How to rememberIt carries the same "-R" as CD-R, a sibling that inherits the same write-once nature.
A DVD-RAM is a DVD whose recorded data can be rewritten any number of times, so files can be added, deleted and overwritten again and again.
Name ending
Can the user write?
Examples in this unit
ROM
Not possible (read-only)
CD-ROM, DVD-ROM
R
Once only (write-once)
CD-R, DVD-R
RAM / RW
Any number of times (rewritable)
DVD-RAM
Its recording layer uses a material that switches between a crystalline and a non-crystalline state depending on how it cools after laser heating, and the difference in reflection between the states encodes the data (phase-change recording). Because the state can be switched over and over, the same spot can be rewritten repeatedly. DVD-RW is also rewritable, but DVD-RAM is built to make small-unit reads and writes easier, much like a hard disk.
AnalogyLike a whiteboard, where you can write and wipe the same surface over and over.
How to rememberRAM stands for Random Access Memory, a name that means any location can be read or written. It shares the name with the RAM of main memory, but it works differently.
Exam points
ExamA DVD-RAM is an optical disc that records by changing the state of its recording layer (記録層) with a laser, and its storage method differs from semiconductor memory (半導体メモリ), which holds data in electric circuits.
Mix-upA DVD-RAM lets the same spot be rewritten again and again, whereas a DVD-R cannot change anything already written.
A Blu-ray Disc (ブルーレイディスク, BD) is an optical disc that uses a blue-violet laser to raise recording density, holding about 25GB per layer on a 12cm disc.
Optical disc
Laser used
Typical capacity (one layer)
CD
Near-infrared light (longest wavelength)
About 700MB
DVD
Red light
About 4.7GB
Blu-ray Disc
Blue-violet light (shortest wavelength)
About 25GB
The shorter a laser's wavelength, the finer it can be focused, so switching from the DVD's red to a shorter blue-violet wavelength lets the recorded marks shrink and sit closer together. With two layers it reaches about 50GB, used for high-definition video and large data. BD also comes in types such as read-only BD-ROM, write-once BD-R and rewritable BD-RE.
How to rememberBlu-ray comes from "blue ray", naming the disc after the colour of the laser it reads and writes with.
Exam points
ExamAmong 12cm optical discs, the Blu-ray Disc (ブルーレイディスク) has the largest capacity, followed by DVD, with CD the smallest.