Tech Talk - Smart Computing
Q What is an optical drive?
A This type of removable media drive reads and often writes data on plastic CDs, DVDs, and/or BDs (Blu-ray Discs) loaded through a slot or, more typically, a retractable tray. Most computers have one, excepting the majority of netbooks and some very thin notebooks.
An optical drive “reads” a spinning disc by shining light—a laser, in fact—on its data side. Invisible to the naked eye is a spiral track, like the groove of a vinyl record, which contains a pattern of either indentations pressed into an aluminum layer (in prerecorded discs) or dark spots “burned” into a layer of dye in front of reflective aluminum (in recordable and rewriteable discs). The read laser reflects differently from the pits (indentations or dark spots) and lands (unaltered parts of the pressed layer or recording dye), making a pattern that indirectly translates into the ones and zeroes of binary digital data.
CD/DVD/BD drives fit in bays and enclosures for 5.25-inch wide devices. Laptops and some portable external drives use much thinner “slim” models. Older drives used the IDE (Integrated Drive Electronics) interface as a data link to the PC. Today’s models use SATA (Serial Advanced Technology Attachment).
Q What are DVD+R, CD-R, BD-RE, etc.?
A These are disc formats to which you can write data files of any type, and/or music or video intended for playback in consumer CD, DVD, and BD players. You can use burning software to put content on blank CD/DVD/BDs, or a recent OS (operating system) such as Windows Vista/7 to write to CD and DVD media.
Optical drives that can record data to blank discs are called burners. When “burning” a disc, a write laser changes the state of the recording dye at points where the data stream calls for a pit. These dye changes are permanent in a recordable (R) disc but reversible in a rewriteable (RW or RE) disc.
Recordable discs are write-once media, which in this case means you can continue to add data to a disc until it’s full. More specifically, you can record more than one session of data to a CD-R, DVD-R, DVD+R (which is practically the same as DVD-R for most purposes), or BD-R if there’s room and if your burning software didn’t finalize the disc after writing the last session. (If you do add data to a CD-R, DVD±R, or BD-R, it’s important to enable the multisession burning option in the software to avoid the possibility of making the earlier data on the disc inaccessible. If this happens, however, some file recovery applications can retrieve the data from the “lost” session or sessions.)
Some drives can use software that securely erases recordable discs by burning their entire dye layers with the write laser. Rewriteable formats include CDRW, DVD-RW, DVD+RW, and BD-RE. You can erase and rewrite these, as their classification implies. Many drives also support the niche rewriteable format DVD-RAM.
Prerecorded software discs, such as program installation CDs, have the suffix ROM (read-only memory). Vendors also describe read-only drives as CD-ROMs, DVD-ROMs, and BD-ROMs.
Regarding backward compatibility, virtually all DVD drives can read CDs, and practically all BD drives can read both DVDs and CDs. Backward compatibility for data writing is more mixed. For instance, there are combo drives that can read all three formats (CD/DVD/BD) but only write the lesser two (DVD and CD).
Q How much data can a CD, DVD, or BD hold?
A CD-R/RWs store up to 700MB with some exceptions. DVD±R/RWs can hold 4.7GB on one side, or 8.5GB if they’re DL (dual- or double-layer) discs. Don’t confuse double-layer discs with double-sided ones, which you can flip over to record twice as much data, such as with a 9.4GB DVD-RAM.
A BD-R/RE drive uses a blueviolet laser to detect and make smaller pits than a CD/DVD drive’s longer-wavelength red laser can. Hence BD can handle 25GB or 50GB (DL) per disc. Because a BD’s pits are so fine, each disc requires a special protective coating on its data side to repel fingerprints and minimize scratches, which can misdirect the read/write laser.
You can also think of disc capacity in terms of video or music duration. A store-bought audio CD might contain about 80 minutes of music, while a typical movie DVD or BD may have about two to three hours of film and features. Note that the same discs can hold more content encoded at a lower quality level. It follows that a BD can carry a much higher-quality version of a movie than a DVD although the video length may be the same.
Finally, there are 1.4GB miniDVDs and other media made for portable devices such as DVD camcorders. Mini discs are also sold in doublesided, 2.8GB form.
Q What do specs such as 48X and 22X mean?
A These are speed ratings based on the read or write speeds of the very first CD, DVD, and BD drives. For instance, the initial maximum read speed of CD-ROM drives was 150KBps (kilobytes per second). Therefore, an early 1X unit could read a CD-ROM at up to 0.15MBps (megabytes per second), while a modern drive with 48X CD read speed can read at up to 7.2MBps/
For DVD, 1X equals 1.35MBps; a 24X DVD burner, therefore, can write compatible media at up to 32.4MBps. A 12X BD burner, meanwhile, may write BD-R at 12X (54MBps) and BD-RE at 2X (9MBps), where 1X BD is 4.5MBps. Manufacturers list their drives’ write, read, and rewrite speeds separately for each supported disc format.
Note that a drive can only read CDs, DVDs, and BDs near top speed when it accesses the data that was written last to a full disc. In the same vein, a burner can only write data near its top speed when the disc is almost full. The reason for both is that optical drives record data to discs starting near the center hub and working outward. When the disc is spinning at a given speed, the outer parts of the data track will send more bytes past the read/write lens in one revolution than will the inner parts of the track.
Q Are recordable and rewriteable discs playable in a standalone player?
A It depends on the player’s capabilities, and also the format you chose for writing the content to the disc. If you created a disc in CD-Audio or DVD-video format, for instance, most standalone players will play it.
However, some disc formats are more broadly supported than others. For example, many car stereos can play CD-Rs, but not CDRWs. Similarly, a standalone DVD recorder may support DVD-RW but not DVD+RW. If you simply burned files to a disc, or you opted to make a jukebox CD with raw MP3 songs on it, you’ll only be able to play it in devices with support for that type of file. For example, recent car stereos with MP3 support can play about eight to 11 hours of music from one CD-R.
BY MARTY SEMS
Smart Computing
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