Optical Storage Media: CDs and DVDs
Optical Storage Media: CDs and DVDs |
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Barton F. Branstetter IV, MD has reported no financial interest, arrangement or affiliation with a commercial organization that may have a direct or indirect influence in the subject matter of this presentation.
Objectives
Contents
There are many ways to move information from one computer to another. Storage media have evolved from the humble origins of floppy disks to modern compact discs (CDs) and Digital Versatile Discs (DVDs). When they were first introduced, CDs and DVDs could only be read on personal computers; expensive equipment was required to create the discs. Now, however, the technology for writing ("burning") discs is standard on any new computer. Modern software has made the process of creating and managing CDs and DVDs simple for anyone with basic computer skills.
This article describes the relevant terminology for creating and managing CDs and DVDs. Step-by-step instructions for creating discs are outlined, and alternative storage media are discussed.
The computer industry is rife with jargon that can create confusion, even for experienced users. The first step in understanding CD/DVD technology is to review the slang that has arisen around the technology. (For simplicity, I use the term "CD" instead of "CD or DVD" in the definitions.)
AIFF: Audio Interchange File Format is an audio file format used predominantly by the Apple Macintosh operating system. When these files are saved on a PC, they use the extension ".aif". An AIFF file contains raw audio data, channel information (monophonic or stereophonic), bit depth, and sample rate.
Aspect Ratio: The ratio of width to height of a video image. Common aspect ratios include "wide screen" (aka "letterbox", the standard aspect ratio for movies) and "full screen", in which an entire monitor or TV screen is filled with a width:height ratio of 4:3
Buffer Underrun: An error caused when the CD writer cannot receive data from the computer fast enough. Burners need to write data at a constant rate, and they cannot slow down to wait for more input. This problem is encountered with fast CD writers using slow connections. You can overcome this problem by slowing down the CD writer.
Burning: Writing data to a CD. The term derives from the physical process that occurs when a laser heats an element of the disc and changes it from transparent to opaque. CD writers are called "burners".
Burning Software: Software used to organize the data that is to be written on the CD, and to subsequently control the burning process.
Burning Speed: The speed at which a CD writer burns data to a blank CD, relative to the reference speed of 1.3 m/sec. This speed is a constant linear velocity (CLV), unlike the constant angular velocity used by vinyl records. Thus, the CD or DVD spins slower when the reading laser is on the outside of the disc, and faster when the laser is on the inside of the disc. Note that DVDs have different reference CLV: 4.0 m/sec. Disc drive speeds are listed as a multiple of the reference speed (e.g., a 4x drive reads at 4 times the reference speed). Burners are listed with their write speed followed by their read speed, e.g. 24X / 48X . If the burner can also erase and re-write, the rewrite speed goes in between, e.g. 24X / 8X / 48X .
Burn Proof Technology: This is a proprietary name from Sanyo, an acronym coined from the phrase "Buffer Under RuN-Proof". Other manufacturers have similar technology with different names. Burn-proofing may consist of hardware and/or software solutions. Hardware devices can anticipate a buffer under-run, for example when the buffer dwindles to 10 percent full. The hardware will then put the recorder into suspend mode, wait for more data to fill the buffer, and then prepare a restart by seeking the last recorded portion of the disc. Software methods divert free system resources to monitoring the CD burning. While not as robust as hardware for under-run protection, software solutions are usually sufficient.
CD: Compact Disc. The most basic optical storage medium available to consumers, a CD stores 650-800 Megabytes of data. (A 3.5" floppy disc stores 1.44 Mb.) The more formal term is CD-ROM (Compact Disc Read Only Memory)
CD-R: Recordable CD. Once data has been burned onto a CD-R, it cannot be erased. More data may be added, however. See also: "Pressed CD", "CD-RW", and "CD-ROM". In reference to a drive, CD-R means a computer component capable of burning recordable CDs.
CD-ROM: Compact Disc Read Only Memory is the more technically accurate term for a pressed CD, but it is usually applied only to data CDs, not audio CDs.
CD-RW: Rewritable CD. These CDs can be erased and recorded repeatedly. In reference to a drive, it means a computer component capable of burning rewritable CDs, erasing them, and then burning them again. CD-RW drives can read both CD-R and CD-RW discs, but the opposite is not true - older CD-R drives cannot read CD-RW discs.
Closed disc: (aka Finalized disc) A disc that will no longer accept data, even if there is more space on the disc. Closing the disc is necessary for some applications, such as audio discs.
Coaster: The term used by burning enthusiasts to describe a failed CD (totally dead). Coasters are useless, except to keep condensation off the coffee table.
Codec: See Decode.
Decode/Encode: Transform data from one format to another. For example, an MP3 file needs to be decoded before it can be written to an audio CD intended for a car stereo. A CODEC (COmpression/DECompression) is the software that performs this task.
Disc at Once (DAO): A process in which a CD is created in toto without suspending the writing laser. At the conclusion of a DAO, the disk is considered "closed". See also "Packet Writing", "Track at Once".
DVD: When it was first introduced in 1996, DVD stood for Digital Video Disc, but as technology advanced, the more general term Digital Versatile Disc took over. DVDs have the same physical dimensions as CDs, but much greater storage capacity. Double-sided DVDs with two layers of data on each side can store 17 Gigabytes of data.
DVD-Audio: Launched in mid-2000, this audio-only storage format is similar to CD-Audio, but offers higher sound quality. 16, 20 and 24-bit samples rates of 44.1 to 192KHz are available, compared to 16 bits and 44.1KHz for CDs. DVD-Audio can also contain music videos and graphics.
DVD-Video: Popular MPEG2 video format that is designed to be used to store movies on a DVD and can be played on most DVD players.
Encode: see "Decode"
External/Internal Device: Most burners fit inside your computer in a front drive bay (i.e., internal device). However, if your internal bays are filled, or you wish an easier setup, you can purchase an external device which sits outside the computer and is connected with a cable such as USB or FireWire.
FireWire: There are two popular connection cables between computers and their external devices: USB and FireWire. Connection speeds are as follows:
USB: 12 Megabits per second (Mbps)
FireWire: 400 Mbps
USB2: 480 Mbps
FireWire2: 800 Mbps
The first three are in common usage, with FireWire2 approved as a standard in 2002. Most PC devices use USB, whereas Macintosh devices frequently use both. Both USB and FireWire are plug-and-play technologies, meaning that you don't need to open the computer and install an internal card to use these devices.
Finalize: See "Closed Disc"
Internal Device: See "External/Internal Device"
ISO: The International Organization for Standardization (ISO) is a non-governmental global organization established in 1947 that works to develop standards across goods and services.
ISO 9660: A widely used data interchange format adopted in 1987. CDs created in this format can be read by UNIX, Macintosh and Windows computers. ISO 9660 is inadequate for the higher capacity of recordable and erasable DVDs.
Mixed Mode CD: A CD containing both data and audio tracks. Examples of mixed mode CDs include games. Track one usually contains the data and the remainder of the tracks contain normal CD Audio.
MPEG: The Moving Picture Experts Group (MPEG) is a working group under the International Organization for Standardization (ISO) that sets standards for encoding audio and video in digital format.
MPEG2: MPEG2 is a second set of flexible compression standards created by the MPEG group that takes advantage of the fact that over 95% of digital video is redundant. MPEG2 handles this by using higher bit rates (i.e., higher quality) for more complex pictures and lower bit rates for simple pictures.
MP3: Moving Picture Experts Group Layer 3, MP3 is a compression format for digital music. MP3 can attain compression ratios of 12:1, allowing almost CD quality stereo music to take up only about 1MB per minute of sound. Unfortunately, MP3 is only recognized by computers, so it cannot be used on audio CDs intended for stereo systems.
Open Disc: see "Closed Disc"
Overburning: Writing excessive data to a CD, taking it beyond its stated capacity. This allows you to write a few extra minutes of audio to most high quality CDs, but it is totally media dependent.
Packet Writing: A method of writing data on a CD in small increments, as opposed to writing large blocks of information as required by the Disk-At-Once method. Packet writing allows small packets (or chunks) of data to be written one packet at a time, lessening the chance of buffer underrun. Packet writing is good only for data tracks, as audio disks require burning with Disk-at-Once or Track-at-Once. See also "Disc at Once", "Track at Once".
Pressed CD: The traditional method of creating CDs, requiring expensive factory equipment. Pits and bumps are pressed into the incipient CD from a glass mold. CD-R and CD-RW, in contrast, use lasers to change the opacity of chemicals embedded above the reflective surface.
Read Speed: This is the maximum speed at which a CD writer or standard CD-ROM drive can READ data. It is expressed as a multiple of the speed of a standard audio CD (e.g. 24X).
Rip: To take audio or video from a CD or DVD and store it in another format on the hard drive.
SCSI: Small Computer System Interface is a set of evolving standard electronic interfaces that allows personal computers to communicate with peripheral hardware such as disk drives, tape drives, CD-ROM drives, printers, and scanners. SCSI has largely been replaced by USB and FireWire.
Session: Whenever you write a batch of data to a CD it is called a "session". If you leave the disc "open", you may choose to write more data later. This is then a "multi-session" disc. The number of sessions is limited only by the physical capacity of the disc. Be warned, however, that each session requires a table of contents (TOC) which can use up to 30 MB of disc space.
TOC: Table of Contents, a listing of files written to a disc at the end of a burn session that tells the operating system what's on the disc and where it is physically located. It is from the TOC that file and directory listings are generated. This is the CD equivalent of a hard-drive File Allocation Table (FAT).
Tracks: Data is written onto CDs in a single long spiral, just like a vinyl LP record. As on the LP, there are breaks in the data that divide the CD into "tracks". Unlike LPs, CD tracks start in the center of the disc and extend out.
Track at Once (TAO): Track at once records a CD track by track (each track in a single pass), turning off the writing laser only between tracks. See also "Disc at Once", "Packet Writing".
USB: see FireWire
Video Object File (VOB): The file format used for DVD-Video. It is recognized by any DVD player.
Wave: A Wave file is an audio file format created by Microsoft that has become a standard PC audio file format for everything from system and game sounds to CD-quality audio. A Wave file is identified by the file name extension ".wav". Used primarily by PCs, this format has been accepted as a viable interchange medium for other computer platforms, such as Macintosh, allowing content developers to freely move audio files between platforms. Wave is an uncompressed raw audio data format that also stores information about the file's number of tracks (mono or stereo), sample rate, and bit depth. Wave files are a type of CDA, that is, they can be recognized by car and home stereos.
MEDIA -- FILE FORMATS:
There are three basic categories of file format: data, audio, and video. Data files include the familiar word processor documents, PowerPoint files, spreadsheets, images, etc. In contrast to the many types of data file formats, there are only a few commonly-used audio and video formats. Audio file formats include AIFF (for the Macintosh), MP3, and WAV. Stereos and other non-computer audio devices require an uncompressed format such as WAV or AIFF, which we will give the more generic name CD Audio (.CDA).
This means that if you save your audio files to a CD in a format such as MP3, you won't be able to play them in your stereo. Most CD burning software will try to prevent this error by forcing you to decide whether you are making an "audio CD" or a "data CD" at the beginning of the burning process. If you decide on an "audio CD", only the CDA format will be used. (Some software allows "mixed CDs", which combine CDA and data files. This is an advanced technique beyond the scope of this syllabus.)
Video file formats include MPEG, AVI (Video for Windows), MOV (QuickTime Video format for the Macintosh), WMV (Windows Media Video), and DVD Video. This last format is the standard for non-computer DVD video players.
MEDIA - CD vs DVD vs NON-OPTICAL MEDIA
CDs can store 650 Megabytes (MB) of data on a 74 minute CD, or 700 MB on an 80 min CD. This is 450 times the capacity of a 3.5" floppy disc, and over 6 times the capacity of a standard Zip disc. DVDs store up to 4.7 Gigabytes (GB) of data. Furthermore, DVDs can be made with multiple layers of reflective material, doubling the capacity to 8.5 GB. Even furthermore, DVDs can be made double-sided, increasing their capacity to a staggering 17 GB. Some older computer users don't have this much data on their entire hard drive. Unfortunately, CDs cannot burn in multiple layers, and cannot be double-sided, so they are limited to 650 MB.
The balance is not entirely in favor of DVDs, however. DVDs are slower to burn, and the DVD readers are not yet ubiquitous. In contrast, you would have a hard time finding a computer without a CD drive. The DVDs themselves cost much more, particularly DVD-RWs. Video DVDs are encoded with a "region", which is intended to prevent videos released exclusively in one part of the world from being sold in other countries before the official release dates. The disc and the drive must be set to the same region for the disc to play. DVD drives are permitted a limited number of region switches (in case you move to a different country). It is best not to switch regions unless making a permanent change. Region encoding does not apply to data discs.
Although this article is focused on optical media, let us not forget that there are many other ways to move data around. Traditional floppy drives and Zip drives ("magnetic media") are much faster than CD burners. If you are repeatedly moving data back and forth between computers, the write speed of magnetic media may be a substantial advantage.
Another option is memory sticks ("solid-state media"). These items are smaller than a cigarette lighter and can be placed on a keychain. They store several hundred MB of data on a memory chip (no moving parts). Memory sticks usually plug into a USB port, and their data transfer rate is high. The only downside is that the USB ports on some computers have narrow openings that won't accommodate the memory stick.
Remote server access is the simplest way to move data. You don't even have to carry any media with you! This is accomplished by posting the files to a server that can be accessed at the destination. Emailing files to yourself, and then downloading them as attachments, also works if you have remote access to your email. Email is frequently size-limited, however, so this may not work with larger files.
MEDIA -- HOW A BURNER WORKS:
Optical media (CDs and DVDs) consist of a layer of reflective material surrounded by layers of translucent plastic. A low-power laser beam (the "read laser") is directed at the reflective layer, and if it bounces back, the computer considers that a binary "1". If, however, the laser beam is absorbed or deflected, that represents a "0".
The read laser can be disrupted by bumps in the reflective layer (in the case of a pressed disc), or by darkening part of a translucent layer in the disc (in the case of CD-R or CD-RW). This special translucent layer is called the phase-change layer. Darkening ("burning") foci on the phase-change layer is the responsibility of a high-power laser (the "write laser").
On a CD-R disc, this process is irreversible - the darkened foci will remain burned forever. On CD-RW discs, the burning layer is a polymorphous crystal that can be converted into liquid phase and returned to a translucent state. This is accomplished with a medium-power laser (the "erase" laser") that holds the polymorphous crystal at its crystallization temperature.
The name of a CD drive indicates the types of laser it has: CD-ROM drives have only a read laser; CD-R drives have read and write lasers; CD-RW drives have read, write, and erase lasers.
One compromise of CDRW disks is that they reflect less light than CD-ROMs or CDRs. Consequently, CDRW discs can only be read on CD players that are capable of reading the laser specification required by CDRW discs. An older CD-ROM player that came installed in your computer might not read your colleague's CDRW disc. Audio CD players almost certainly won't read CDRW discs.
Discs come in different colors because different companies hold unique patents on the specific chemical dyes used in their media, and the different colors result from these unique dye formulations. Some companies make their own discs, while others simply license their formula to other companies who actually make the discs. Although there is no technical advantage to one color over another, you may find that some brands work better with your audio CD players.
HARDWARE -- DRIVES
There are three types of CD drives: CD-ROM, CD-R, and CD-RW. CD-ROM drives will only read discs, not burn them. CD-R drives will read any disc and write to CD-R discs, which is a permanent medium (no erasing). CD-R drives will not write to CD-RW discs. CD-RW drives will accept any of the discs. CD-R drives are becoming hard to find as the price of CD-RW drives drops.
The speed at which a burner can record data is printed on the package as "nX", where n is the burning speed relative to the play speed of an audio CD. An "8X" CD-R is one that burns data at 8 times the reference speed. Actually, a CD-R will have two numbers, e.g., 12X48X, where the first number is the write speed and the second number is the read speed. CD-RW drives have three numbers, e.g., 10X4X32X. This is usually write speed, rewrite speed, and then read speed.
The nomenclature for DVD burners is similar, but there is an additional complexity: two competing formats are available, DVD-RW and DVD+RW. Get a drive that supports both formats! DVD-ROM drives can read CDs, and most DVD burners can burn CDs.
Combination drives include the ability to burn CDs (usually CD-RW) and read DVDs. They are only marginally more expensive than CD-RW drives, so they are usually worth the extra cost.
HARDWARE -- CONNECTIONS AND CABLES:
CD and DVD writers can be connected to the computer in one of two ways. With an "internal drive", the burner is installed inside the computer itself, in a built-in drive bay, and gets power from the computer's power source. Internal drives are connected to the computer's motherboard with internal cables. The second type of drive is an "external drive", in which the burner sits physically outside of the computer case and has its own power source. External drives are more expensive.
With external drives, a special cable is needed to send data to the motherboard. These different types of connections include SCSI, USB, USB 2.0, FireWire (current, or 400), and FireWire 2 (next, or 800). The choice of cable depends on your computer model, its age, and what type of motherboard and installed hardware you have. Although any of the connections will work, some have distinct advantages over others.
You can view the CDR writer/burner as a car, where the connection cable is the road. If you have a Ferrari, but you're driving it on a gravel road, the advantages of the Ferrari are lost. Driving a Model T on the German Autobahn has a similar disadvantage. Thus, it makes economic sense to try and match the connection cable to the external device.
The different types of connection cables have different transmission speeds. SCSI (small computer systems interface, pronounced "scuzzy") is the most common computer interface to be used on parallel cables. Although their maximum transmission speed is somewhat faster than standard USB (universal serial bus), SCSI cables are becoming less popular because of the portability and easy setup of the more advanced cables.
USB: 12 Megabits per second (Mbps)
SCSI: 20 Mbps
FireWire: 400 Mbps
USB2: 480 Mbps
FireWire2: 800 Mbps
So, which is the best for performance vs. cost? As with most technology, you get what you pay for - faster drives cost more, and the newest, fastest drives are overpriced. It is still prudent to match the speed of your burner to the speed of your connection. USB works for write speeds of 4-12X, SCSI works for 10-12X, and Firewire and USB2 work up to 48X.
There is one other caveat when buying cables. The cable must match the plug on the computer. A FireWire cable will not work in a USB plug, and a USB2 cable plugged into a standard USB plug will only work at the slower speed. All the parts - the computer, the cable, and the drive - must match up. Note that although SCSI, USB and FireWire are available on both PCs and Macs, Macintosh products tend to favor FireWire and PC components tend to favor USB. Older versions of Windows (95, NT) may not support USB and FireWire.
A minor consideration is the cost of the media itself. CDRW discs are much more expensive than CDR discs. Keep in mind, the more discs one buys, the cheaper they become, and discs sold without jewel cases (the translucent rectangular cases) are much cheaper than those with jewel cases. Considering the huge amount of data that can be stored on CDs, even CDRWs are cheap, costing less than $1.00 each in bulk. Although disc manufacturers will claim otherwise, the brand of disc doesn't have much impact on the reliability of the media.
COMPATIBILITY:
"How come it won't read my CD?"
The biggest compatibility issue for CDs derives from the lower reflectivity of CD-RW discs. These discs cannot be played in audio systems and older CD-R drives. Newer CD-R drives can likely read any disc, but if you want to be sure that your recipient can read your disc, use a CD-R disc instead of a CD-RW disc. Note that CD-RW drives can write to either CD-R or CD-RW discs, but CD-R drives require CD-R discs.
Another compatibility issue arises from "multi-session" CD-R drives. A multi-session disc is one which was burned but not finalized, then burned again later. Older CD-R drives do not support multi-session, and may not read the later data sessions.
The third compatibility issue is "mini-CDs". Instead of the conventional 12-cm diameter, these discs are 4-8 cm in diameter. The smaller versions are also called "business card CDs"; they are designed to fit into your wallet. Unfortunately, tray-loader CD drives will not reliably play these smaller discs.Most drives are now tray-loading.
In summary, to ensure the best chance of having your data show up at your destination, use a standard-size CD-R disc with only one burning session.
The situation is somewhat more complex for DVDs. Three different data formats have been developed: DVD-RW, DVD+RW, and DVD-RAM. For a few years, it was unclear which format would become standard, and it looked to be a competition similar to VHS vs. BetaMax. Fortunately, newer drives will read all formats, which seems to have solved the problem. When buying a DVD drive, be sure to get a drive that supports at least DVD- and DVD+ formats. DVD-RAM is becoming less popular.
You should be aware of two other DVD formats: DVD-Video, which is used by the movie industry; and DVD-Audio, which is a higher-quality alternative to CD-Audio. Modern DVD drives will recognize these formats.
When buying DVD discs, make sure that the type of disc (DVD+RW or DVD-RW) matches the type of drive you have. Even if your drive can read several formats, it may not write in all of them, so you have to match your media to your drive. The newest drives can read and write in multiple formats.
DVD+RW discs do not suffer from the low reflectivity of CD-RW discs, and the design of the DVD+RW format focused on compatibility. Thus, DVD+RW discs can be interpreted by most consumer electronics. This is the main advantage of DVD+ technology over DVD- technology. The advantage of DVD- technology is greater adherence to international standards.
SOFTWARE:
In the past several years, there has been a proliferation of burning software, both commercial and freeware. User interfaces and error checking have advanced to the point that even computer novices can create CDs and DVDs. If your new computer comes with a burner, it probably comes with the burning software already installed. If you buy a drive as an upgrade, the software will often come with it.
The most popular software for both the PC and Macintosh platforms is made by Roxio. On the PC, the software is called "Easy CD & DVD Creator"; on the Macintosh it is called "Toast". These products are relatively inexpensive, and they are much more robust than their freeware/shareware counterparts.
A Walk-through of CD Burning:
Burning CD-ROMS can be easy, and provides for a very cheap way of storing very large amounts of data. Any file format can be written to a CD, and audio discs can also be created from computer files for use in car or home stereos. With some attention to detail, you can rather effortlessly burn CDs for a handful of cents. To repeat, I would recommend purchasing a CD writer matched to the fastest transfer mechanism already present in your computer. But keep in mind, you still may not be able to burn CDs at the fastest speed setting your new burner is capable of. In the beginning, always utilize the "simulation" capability to calibrate your system. I also recommend spending the money on COTS, as it truly makes the process simpler and can substantially decrease the chances of encountering a buffer under-run.
http://entertainment.howstuffworks.com/cd-burner.htm
http://www.pctechguide.com/09cdr-rw.htm
http://www.pctechguide.com/10dvd3.htm
http://www.disctronics.co.uk/technology/dvdintro/dvd_intro.htm
I would like to acknowledge Dr. Gregory Katzman, who wrote the previous version of this article. The organization (and much of the content) is attributable to him.
Copyright © 2004 American Society of Neuroradiology, www.asnr.org