Optical disc recording technologies
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Optical disc authoring requires a number of different technologies working in tandem, from the media to the firmware to the control electronics of the drive. This article discusses some of the more important technologies.
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[edit] Types of recordable optical disc
There are numerous formats of recordable optical disc on the market, all of which are based on using a laser to change the reflectivity of the recording medium in order to duplicate the effects of the pits and lands created when a commercial optical disc is pressed. Emerging technologies such as holographic data storage and 3D optical data storage aim to use entirely different data storage methods, but these products are in development and are not yet widely available.
The earliest form is magneto-optical, which uses a magnetic field in combination with a laser to write to the medium. Though not widely used in consumer equipment, the original NeXT cube used MO media as its standard storage device, and consumer MO technology is available in the form of Sony's MiniDisc. This form of medium is rewriteable.
The most common form of recordable optical media is write-once organic dye technology, popularized in the form of the CD-R and still used for higher-capacity media such as DVD-R. This uses the laser alone to scorch a transparent organic dye (usually cyanine, phthalocyanine, or azo compound-based) to create "pits" (i.e. dark spots) over a reflective spiral groove. Most such media are designated with an R (recordable) suffix. Such discs are often quite colorful, generally coming in shades of blue or pale yellow or green.
Rewritable, non-magnetic optical media are possible using phase change alloys, which are converted between crystalline and amorphous states (with different reflectivity) using the heat from the drive laser. Such media must be played in specially tuned drives, since the phase-change material has less of a contrast in reflectivity than dye-based media; while most modern drives support such media, many older CD drives cannot recognize the narrower threshold and cannot read such discs. Phase-change discs are designated with RW (ReWriteable) or RE (Recordable-Erasable). Phase-change discs often appear dark grey.
[edit] Recording modes
Optical discs can be recorded in Disc At Once, Track At Once, Session at Once (i.e. multiple burning sessions for one disc), or packet writing modes. Each mode serves different purposes:
- Disc At Once: writes the entire disc in one pass; preferred for duplication masters
- Track At Once: writes individual tracks with a gap between tracks; used for audio CDs
- Session At Once: writes and finalizes multiple sessions on one CD; usually not supported for CD Audio, and not universally supported by authoring software
- Packet writing: writes data to the medium on demand (see below)
[edit] Connection technologies
Unlike early CD-ROM drives, optical disc recorder drives have generally used industry standard connection protocols. Early computer-based CD recorders were generally connected by way of SCSI; however, as SCSI was abandoned by its most significant users (particularly Apple Computer), it became an expensive option for most computer users. As a result, the market switched over to Parallel ATA connections for most internal drives; external drives generally use PATA drive mechanisms connected to a bridge inside the case that connects to a high-speed serial bus such as FireWire or Hi-Speed USB 2.0. A few very high-speed drives, mostly DVD recorders, use Serial ATA.
Standalone recorders use standard A/V connections, including RCA connectors, TOSlink, and S/PDIF for audio and RF, composite video, component video, S-Video, SCART, and FireWire for video. High-bandwidth digital connections such as HDMI are not yet commonplace.
[edit] Overburning
Overburning is the process of recording data past the normal size limit.
Many disc manufacturers extend a recordable disc to leave a small margin of extra groove at the outer edge. This lead-out was originally intended to provide tolerance for the read head of an audio CD player should it overseek, by providing a padding of up to 90 seconds of silent digital audio.
Recording onto the lead-out is possible with some combinations of CD recorder and authoring software. The actual amount of data that a disc will hold depends ultimately on the recordable media and varies somewhat between brands of disc, with lead-out accounting for up to 10% of the total disc capacity.
Almost all CD-ROM drives are capable of reading from the lead-out. For this reason, software publishers have on occasion shipped their software on similarly oversized compact discs to reduce packaging costs. Oversized discs have also been used as a form of copy protection because it is more difficult to record copies of them.
The overburn options in CD recording software must be used when burning to the non standard 90 and 99 minute blanks that are now available as the data structures in the ATIP do not allow such sizes to be specified.
[edit] Buffer underrun protection
A buffer underrun occurs during recording if the recorder runs out of data in the recording buffer. Once the laser is on, it cannot stop and resume flawlessly; thus the pause necessitated by the underrun can cause the data on the disc to become invalid. Since the buffer is generally being filled from a relatively slow source, such as a hard disk or another CD/DVD, a heavy CPU or memory load from other concurrent tasks can easily exhaust the capacity of a small buffer. Therefore, buffer underrun protection was implemented by various individual CD/DVD writer vendors. With this technique, the laser is indeed able to stop writing for any amount of time and resume when the buffer is full again. The gap between successive writes is extremely small. [1]
Since the techniques for protecting against buffer underrun are proprietary and vendor-specific, technical details vary. However, most vendors use one or the other of two basic approaches. Some CD burners have the ability to resume burning after a buffer underrun has taken place. But even without such a capability included in the hardware, nearly all CD burners now on the market have the ability to record at slower speeds. Many CD-burning software tools can prevent buffer underrun by monitoring the state of the buffer, and slow down the recorder as needed to avoid running out of data in the buffer. Even when the hardware has the capability to resume writing after an underrun event happens, it is safest to regard this as a fallback in case prevention fails.
Fortunately for users today, most CD-recording hardware and software now on the market is much better than it once was at preventing buffer underrun problems. Also hard drive and CPU speeds have increased by a greater ratio over the last decade than has the speed of optical drives, so most systems today are better at keeping up with the optical drives anyway. In the early 1990s recording a CD was always a somewhat risky endeavor -- one disconnected from any networks, disabled any background programs, etc., before starting to record a CD, and was very careful not to do anything else while the recording was in progress. Even with all precautions, making a useless "coaster" was annoyingly common. Today, recording a CD is much more reliable.
Another way to protect against the problem, when using rewritable media (CD-RW, DVD-RW, DVD-RAM), is to use the UDF file system, which organizes data in smaller "packets", referenced by a single, updated address table, which can therefore be written in shorter bursts.
[edit] Packet writing
Packet writing is a technology that allows optical discs to be used in a similar manner to a floppy disk. Packet writing can be used both with once-writeable media and rewriteable media. Several competing and incompatible packet writing disk formats have been developed, including DirectCD and InCD. Proposed standards include Universal Disk Format and its proposed extension Mount Rainier.
[edit] Specific proprietary technologies
[edit] BURN-Proof
BURN-Proof (Buffer Underrun-Proof) is a proprietary technology for buffer underrun protection developed by Sanyo.[1]
[edit] SafeBurn
SafeBurn is a proprietary technology for buffer underrun protection developed by Yamaha Corporation.
[edit] Power Burn
Power Burn is a proprietary technology for buffer underrun protection, developed by Sony. Features:
- Buffer underrun protection: When a buffer underrun occurs, the drive suspends writing. The drive memorizes the end writing point and timing, and immediately resumes writing from that exact point when sufficient data is filled in the buffer memory.
- Protection from write errors caused by shock and vibration: PowerBurn's Shock Proof technology pauses writing when the device is moved, and resumes after the drive becomes stable. This allows it to work in a mobile environment.
- Optimization of write conditions: The drive detects characteristics of each individual medium and optimizes all key writing conditions such as writing speed, laser power and write strategy.
[edit] JustSpeed
JustSpeed is a technology that automatically adapts the write speed to the medium inserted. It is supported by Nero Burning ROM version 6.
[edit] Longevity
Store-bought recordable/writable optical media use lasers in optical drives to change dyes in/on the optical media whereas factory-manufactured optical media use more permanent molds/casts. Thus, data storage is merely temporary with store-bought optical media as opposed to factory-manufactured optical media. One must be wary of storing family photos and videos (or any other miscellaneous important data) solely on optical media as it may not last as long as one would expect or anticipate. Good alternatives would be to additionally backup one's media using other media technologies and/or investing in non-volatile memory technologies.
- See also: Compact_disc#Recordable_CD
- See also: CD-R#Expected_lifespan
[edit] References
[edit] External links
- Understanding CD-R & CD-RW by Hugh Bennett
- Understanding Recordable & Rewritable DVD by Hugh Bennett
