問70
頻出度★★☆
コンピュータの補助記憶装置である DVD 装置の説明として, 適切なものはどれか。
平成25年度 春期 ITパスポート試験 問70
テクノロジ系コンピュータ構成要素 › メモリ
This question's explanation and your answers so far.
解説
問 70
Incorrectア
Incorrect. A DVD device can read CD-ROMs. A DVD is an optical disc with the same 12 cm diameter as a CD, and the basic principle of shining laser light on the recording surface and reading the changes in the reflected light is also the same. The difference lies in the wavelength of the laser used for reading and writing: a DVD uses a red laser of about 650 nm, while a CD uses an infrared laser of about 780 nm. For that reason the optical pickup of a DVD drive is built so that it can switch between both wavelengths, and it can read not only DVDs but also CD-ROMs, CD-Rs, and music CDs (backward compatibility). Saying that it cannot read them because of the nature of the recording method is contrary to fact.
Incorrectイ
Incorrect. DVD devices have been miniaturized and can indeed be installed in notebook PCs. In fact, slim built-in drives with reduced thickness have been fitted into many notebook PCs, and external portable drives connected by USB are also widely used. The reason that more recent notebook PC models come without a built-in optical drive is not that miniaturization is technically difficult, but that the means of distribution and storage have shifted to network delivery, USB flash drives, and the cloud, so the need for an optical drive itself has fallen.
Incorrectウ
Incorrect. A DVD uses laser light for both reading and writing. To write, a strong laser beam is applied to change the recording layer (in DVD-R and similar media the dye layer is changed chemically, while in DVD-RW and DVD-RAM the recording layer undergoes a phase change between a crystalline and an amorphous state); to read, a weak laser beam is applied and differences in reflectivity are detected. The description of reading with laser light and writing with magnetism fits the magneto-optical disc (MO): an MO records by applying a magnetic field while heating the recording layer with laser light, and reads by using the change in the polarization of the laser light (the magneto-optical Kerr effect). This statement confuses DVD with MO.
- エ
正解 The correct answer is エ. Like a CD, a DVD is a kind of optical disc (optical medium) that is read and written with laser light, and it is used as secondary storage. A major feature of DVDs is that several types of media with different properties are standardized for different uses. DVD-ROM is read-only media on which the data is stamped at the factory before shipping, as with commercial software or movies, so the user can only read it. DVD-R and DVD+R are write-once (recordable) media that the user can write to just once, after which nothing more can be appended or rewritten. DVD-RW, DVD+RW, and DVD-RAM are rewritable media whose data can be erased and written again any number of times. Therefore the statement in エ, “Multiple types of media can be used, such as read-only ones and ones that can be written to repeatedly”, is a correct description of a DVD device. It is also worth remembering that the types of media a DVD drive can handle differ by model: there are drives that can only read (DVD-ROM drives) and drives that can also write (such as DVD Super Multi drives).
ここがポイント
- A DVD is an optical disc used as secondary storage, and both reading and writing are done with laser light. Its media can be classified into three kinds by writing characteristics: read-only DVD-ROM; write-once (recordable) DVD-R / DVD+R, which can be written to only once; and rewritable DVD-RW / DVD+RW / DVD-RAM, which can be rewritten repeatedly. CDs have the same three-way classification of CD-ROM / CD-R / CD-RW.
- A DVD drive uses a shorter wavelength than a CD (a red laser of about 650 nm), so it can record at a higher density than a CD (about 780 nm) and handles about 4.7 GB on a single-sided, single-layer disc. Because its optical pickup supports both wavelengths, it also has the backward compatibility to read lower-tier media such as CD-ROMs. Note as well that it is the magneto-optical disc (MO) that reads with laser light and writes with magnetism; its recording principle differs from that of a DVD, so do not confuse the two.
用語