SECONDARY STORAGE
SECONDARY STORAGE
DEVICES
DEVICES
S
S
econdary storage
econdary storage
is a category of is a category of computer storagecomputer storage. It is used to store data . It is used to store data that is not inthat is not in active use. It is usually slower and has higher capacity thanactive use. It is usually slower and has higher capacity than primary storage primary storage, and is almost always, and is almost always non-volatile
non-volatile..
Storage devices in this category include: Storage devices in this category include:
•
• CDCD,, CD-R CD-R ,, CD-RWCD-RW •
• DVDDVD •
• Floppy disk Floppy disk •
• Hard disk Hard disk •
• Magnetic tapeMagnetic tape •
• Paper tapePaper tape •
• Punch cardPunch card •
• Flash memoryFlash memory
1. CD
1. CD
A
Acompact disccompact disc (or (or CDCD) is an) is an optical discoptical disc used to store digital data, originally developed for used to store digital data, originally developed for storing
storing digital audiodigital audio..
A standard compact disc, often known as an
A standard compact disc, often known as an audio CDaudio CD to differentiate it from later to differentiate it from later variants, storesvariants, stores audio data in a format compliant w
audio data in a format compliant with theith the red book red book standard. An audio CD consists of severalstandard. An audio CD consists of several tracks stored using 16-bit
tracks stored using 16-bit PCMPCMcoding at a sampling rate of coding at a sampling rate of 44.1 kHz. Most compact discs are44.1 kHz. Most compact discs are 120 mm in diameter, which can
120 mm in diameter, which can store up to 74 minutes of audio.store up to 74 minutes of audio. Compact disc technology was later adapted for use as a
Compact disc technology was later adapted for use as a data storage devicedata storage device, known as a CD-ROM, known as aCD-ROM..
Contents
Contents [[hidehide]] 1 History 1 History 2 Physical details 2 Physical details 3 Audio format 3 Audio format 4 Storage capacity 4 Storage capacity 5 Recordability 5 Recordability 6 Copy protection 6 Copy protection 7 Naming conventions 7 Naming conventions 8 See also 8 See also
An image of a
An image of a compact disccompact disc- Pencil included for scale- Pencil included for scale
History
History
The compact disc was developed in
The compact disc was developed in 19791979byby PhilipsPhilipsandand SonySony. Philips developed the general. Philips developed the general manufacturing
manufacturing process process, based on their earlier , based on their earlier LaserdiscLaserdisctechnology, while Sony contributed thetechnology, while Sony contributed the error-correction
error-correctionmethod.method.
Early compact disc prototypes produced by Philips were 115
Early compact disc prototypes produced by Philips were 115 mm in diameter, with a 14 mm in diameter, with a 14 bitbit resolution and a 60 minute capacity.
resolution and a 60 minute capacity. Sony insisted on a 16 bit resolution and Sony insisted on a 16 bit resolution and 74 minute capacity,74 minute capacity, which increased the size of the disc to 120 mm. The reason for the increase in capacity is rumored which increased the size of the disc to 120 mm. The reason for the increase in capacity is rumored to be to hold even
to be to hold even the slowest versions of the slowest versions of BeethovenBeethoven's's 9th Symphony9th Symphony.. Compact discs were first mass produced in
An image of a
An image of a compact disccompact disc- Pencil included for scale- Pencil included for scale
History
History
The compact disc was developed in
The compact disc was developed in 19791979byby PhilipsPhilipsandand SonySony. Philips developed the general. Philips developed the general manufacturing
manufacturing process process, based on their earlier , based on their earlier LaserdiscLaserdisctechnology, while Sony contributed thetechnology, while Sony contributed the error-correction
error-correctionmethod.method.
Early compact disc prototypes produced by Philips were 115
Early compact disc prototypes produced by Philips were 115 mm in diameter, with a 14 mm in diameter, with a 14 bitbit resolution and a 60 minute capacity.
resolution and a 60 minute capacity. Sony insisted on a 16 bit resolution and Sony insisted on a 16 bit resolution and 74 minute capacity,74 minute capacity, which increased the size of the disc to 120 mm. The reason for the increase in capacity is rumored which increased the size of the disc to 120 mm. The reason for the increase in capacity is rumored to be to hold even
to be to hold even the slowest versions of the slowest versions of BeethovenBeethoven's's 9th Symphony9th Symphony.. Compact discs were first mass produced in
Physical details
Physical details
Compact discs are made from a 1.2
Compact discs are made from a 1.2 mmmmthick disc of thick disc of polycarbonate polycarbonate plastic plastic coated with a muchcoated with a much thinner
thinner aluminiumaluminium(originally(originally goldgold, which is sometimes still used for its data longevity) layer , which is sometimes still used for its data longevity) layer which is protected by a film of
which is protected by a film of lacquer lacquer . The lacquer can be. The lacquer can be printed printed with awith a labellabel. Common printing. Common printing methods for compact discs are
methods for compact discs are silkscreeningsilkscreeningandand offset printingoffset printing. CDs are available in a range of . CDs are available in a range of sizes but by far the most common is 120
sizes but by far the most common is 120 mm inmm in diameter diameter , with a 74 minute audio capacity and a, with a 74 minute audio capacity and a 650 MB data (See
650 MB data (See storage capacitystorage capacity).). The information on a standard CD
The information on a standard CD is encoded as a spiral track of is encoded as a spiral track of pits pits moulded into the top of themoulded into the top of the polycarbonate layer (The areas between pits are kno
polycarbonate layer (The areas between pits are known aswn as landslands). Each pit is approximately). Each pit is approximately 125
125 nmnm deep by 500 nm wide, and varies from 850 nm to 3.5deep by 500 nm wide, and varies from 850 nm to 3.5 μmμm long. The spacing between thelong. The spacing between the tracks is 1.5 μm. To grasp the scale of the pits and land of a CD, if the disc is enlarged to the size tracks is 1.5 μm. To grasp the scale of the pits and land of a CD, if the disc is enlarged to the size of a stadium, a pit would be
of a stadium, a pit would be approximately the size of a grain of sandapproximately the size of a grain of sand. The spiral begins at the. The spiral begins at the center of the disc and proceeds
center of the disc and proceeds outwards to the edge, which allows outwards to the edge, which allows the different size formatsthe different size formats available.
available.
A CD is read by focusing a 780 nm
A CD is read by focusing a 780 nm wavelengthwavelengthsemiconductor laser semiconductor laser through the bottom of thethrough the bottom of the polycarbonate layer. The difference in height between
polycarbonate layer. The difference in height between pits and lands is one quarter of pits and lands is one quarter of thethe wavelength of the laser light, leading to a half-wavelength
wavelength of the laser light, leading to a half-wavelength phase phasedifference between the lightdifference between the light reflected from a pit and from its surrounding land.
reflected from a pit and from its surrounding land. The destructiveThe destructive interferenceinterferencethis causes reducesthis causes reduces the intensity of the reflected light compared to when
the intensity of the reflected light compared to when the laser is focused on just a land. the laser is focused on just a land. ByBy measuring this intensity with a
measuring this intensity with a photodiode photodiode,, one is able to read the data from the disc.one is able to read the data from the disc. The pits and lands themselves do not represent the zeroes and ones of
The pits and lands themselves do not represent the zeroes and ones of binary data binary data. Instead a change. Instead a change from pit to land or land to pit indicates a one, while no change indicates a zero. This in turn is
from pit to land or land to pit indicates a one, while no change indicates a zero. This in turn is decoded by reversing the
decoded by reversing the Eight-to-Fourteen ModulationEight-to-Fourteen Modulation used in mastering the disc, finallyused in mastering the disc, finally revealing the raw data stored on
revealing the raw data stored on the disc.the disc.
Audio format
Audio format
The
The data formatdata formatof the disc, known as theof the disc, known as the ''Red Book Red Book '' standard, was laid out by thestandard, was laid out by the DutchDutch
electronics
electronicscompany Philips, who own the rights to thecompany Philips, who own the rights to the licensinglicensingof the 'CDDA' logo that appearsof the 'CDDA' logo that appears on the disc. In broad terms the
on the disc. In broad terms the format is a two-channel stereo 16-bitformat is a two-channel stereo 16-bit PCMPCMencoding at a 44.1 kHzencoding at a 44.1 kHz sampling rate
sampling rate.. Reed-Solomon error correctionReed-Solomon error correction allows the CD to be scratched to a certain degreeallows the CD to be scratched to a certain degree and still be played back.
and still be played back.
The unusual sampling rate of 44.1
The unusual sampling rate of 44.1 kHz is inherited from a method of kHz is inherited from a method of converting digital audio intoconverting digital audio into a video signal for storage on video
a video signal for storage on video tape, which was the most affordable way to tape, which was the most affordable way to store it at the timestore it at the time the CD specification was being developed. This technology could store 3 samples in a single the CD specification was being developed. This technology could store 3 samples in a single horizontal line. A standard
horizontal line. A standard NTSC NTSC video signal has 245 usable video signal has 245 usable lines per field, and 60 fields alines per field, and 60 fields a second, which indeed works out at 44,100 samples/second. Similarly
second, which indeed works out at 44,100 samples/second. Similarly PALPALhas 294 lines and 50has 294 lines and 50 fields, which also gives 44,100 samples/second. This system could either
fields, which also gives 44,100 samples/second. This system could either store 14-bit samples withstore 14-bit samples with some error correction, or 16-bit samples with almost no error correction. There
some error correction, or 16-bit samples with almost no error correction. There was a debate over was a debate over whether to use 14- or 16-bit samples when
Hence, the decision to use the 16-bit, 44.1 kHz sampling rate. The Sony PCM-1630, an early CD Hence, the decision to use the 16-bit, 44.1 kHz sampling rate. The Sony PCM-1630, an early CD mastering machine, was just a modified
mastering machine, was just a modified U-MaticU-MaticVCR.VCR.
Storage capacity
Storage capacity
The
The compact disc specificationcompact disc specificationrecommends a linear velocity of 1.22 m/s and a track pitch of 1.59recommends a linear velocity of 1.22 m/s and a track pitch of 1.59 micrometres. This leads to a maximum audio program length
micrometres. This leads to a maximum audio program length of 74 minutes on a 1of 74 minutes on a 120 mm disc, or 20 mm disc, or around 650 MB of data
around 650 MB of data on a CD-ROM. However, in oon a CD-ROM. However, in order to allow for variations inrder to allow for variations in manufacturing, a disc with data appearing
manufacturing, a disc with data appearing slightly more densely is allowable. By deliberatelyslightly more densely is allowable. By deliberately making a disc with this density, we can
making a disc with this density, we can increase capacity and remain within or near increase capacity and remain within or near spec. Using aspec. Using a linear velocity of 1.1975 m/s and a track pitch of 1.497 micrometres leads to a new maximum linear velocity of 1.1975 m/s and a track pitch of 1.497 micrometres leads to a new maximum capacity of 79 minutes and 40
capacity of 79 minutes and 40 seconds, or 702 MB. Although seconds, or 702 MB. Although such discs allow for little variation insuch discs allow for little variation in manufacturing, they are generally reliable and only a small number of players are known to reject manufacturing, they are generally reliable and only a small number of players are known to reject them.
them.
Some blank discs (see
Some blank discs (see recordabilityrecordability) are available in 90 and even 99 minute configurations.) are available in 90 and even 99 minute configurations. Besides the increased density of their tracks, these run into two
Besides the increased density of their tracks, these run into two other technical problems. The firstother technical problems. The first is that the maximum capacity a disc can
is that the maximum capacity a disc can declare itself as having is, according to thedeclare itself as having is, according to the recordable CDrecordable CD
specification
specification,, just under 80 minutes. The second just under 80 minutes. The second is that timing markers on the disc with a is that timing markers on the disc with a valuevalue between 90 and 99
between 90 and 99 minutes are normally used to indicate to the minutes are normally used to indicate to the player it is reading the beginningplayer it is reading the beginning of the disc, not the end.
of the disc, not the end. These problems, as well as variable compatibility with CD recorders andThese problems, as well as variable compatibility with CD recorders and software, mean discs larger than 80 minutes are g
software, mean discs larger than 80 minutes are generally regarded as a niche product.enerally regarded as a niche product. Another technique to increase the c
Another technique to increase the capacity of a disc is store data in the apacity of a disc is store data in the lead out groove that islead out groove that is normally used to indicate the end of
normally used to indicate the end of a disk, and an exa disk, and an extra minute or two of recording is oftentra minute or two of recording is often
possible. However, these discs can cause problems in playback when the end of the disc is reached. possible. However, these discs can cause problems in playback when the end of the disc is reached.
Recordability
Recordability
Injection molding
Injection moldingis used to mass produce compact is used to mass produce compact discs. A 'stamper' is made from the originaldiscs. A 'stamper' is made from the original media (audio tape, data disc,
media (audio tape, data disc, etc.) by writing to a photosensitive dye with a laser. etc.) by writing to a photosensitive dye with a laser. This dye is thenThis dye is then etched, leaving the data
etched, leaving the data track. It is then plated to make track. It is then plated to make a positive version of the CD. Polycarbonatea positive version of the CD. Polycarbonate is liquified and injected into the mold cav
is liquified and injected into the mold cavity where the stamper transfers the pattern of pits andity where the stamper transfers the pattern of pits and lands to the polycarbonate disc. The
lands to the polycarbonate disc. The disc is then metallized with aluminum and lacquer disc is then metallized with aluminum and lacquer coated.coated. However, there are also
However, there are also CD-recordableCD-recordablediscs which can be recorded by adiscs which can be recorded by a laser laser beam using a CD-R beam using a CD-R writer (most often connected to a computer, though standalone units are also available) and can be writer (most often connected to a computer, though standalone units are also available) and can be played on most compact disc players. CD-R recordings are permanent and cannot be recorded played on most compact disc players. CD-R recordings are permanent and cannot be recorded
more than once, so the
more than once, so the process is also called "burning" a CD. process is also called "burning" a CD. (See also(See also CD burner CD burner andand overburning
overburning.).) CD-RW
CD-RWis a medium that allows multiple recordings on the is a medium that allows multiple recordings on the same disc over and over again. same disc over and over again. A CD-A CD-RW does not have as great
RW does not have as great a difference in the reflectivity of lands and bua difference in the reflectivity of lands and bumps as a pressed CD or amps as a pressed CD or a CD-R, so many CD audio players cannot
CD-R, so many CD audio players cannot read CD-RW discs, although the majority of standaloneread CD-RW discs, although the majority of standalone DVD
Recordable compact discs are injection molded
Recordable compact discs are injection molded with a "blank" data spiral. A phwith a "blank" data spiral. A photosensitive dye isotosensitive dye is then applied, and then the discs are metallized and lacquer coated. The write laser of the CD burner then applied, and then the discs are metallized and lacquer coated. The write laser of the CD burner changes the characteristics of the dye to allow the read laser of a standard CD player to see the data changes the characteristics of the dye to allow the read laser of a standard CD player to see the data as it would an injection molded
as it would an injection molded compact disc.compact disc.
Copy protection
Copy protection
The compact disc specification does not include any
The compact disc specification does not include any copy protectioncopy protectionmechanism and discs can bemechanism and discs can be easily duplicated or the contents "ripped" to a
easily duplicated or the contents "ripped" to a computer. Starting in earlycomputer. Starting in early 20022002, attempts were, attempts were made by record companies to market "
made by record companies to market "copy-protected" compact discs. These rely on deliberatecopy-protected" compact discs. These rely on deliberate errors being introduced into the data recorded
errors being introduced into the data recorded on the disc. The intent is that the on the disc. The intent is that the error-correction inerror-correction in a music player will enable music to be
a music player will enable music to be played as normal, while computer played as normal, while computer CD-ROMCD-ROMdrives will faildrives will fail with errors. This approach is the subject of an
with errors. This approach is the subject of an evolutionaryevolutionary arms racearms race or or cat-and-mouse gamecat-and-mouse game — — not all current drives fail, and copying software is being
not all current drives fail, and copying software is being adapted to cope with these adapted to cope with these damaged datadamaged data tracks. The recording industry then works on further approaches.
tracks. The recording industry then works on further approaches.
Philips have stated that such discs, which do not meet the Red Book specification, are not Philips have stated that such discs, which do not meet the Red Book specification, are not permitted to bear the
permitted to bear the trademarkedtrademarkedCompact Disc Digital AudioCompact Disc Digital Audio logo. It also seems likely thatlogo. It also seems likely that Philips' new models of CD recorders will be designed
Philips' new models of CD recorders will be designed to be able to record to be able to record from these 'protected'from these 'protected' discs. However, there has been great
discs. However, there has been great public outcry over copy-protected discs because many public outcry over copy-protected discs because many see itsee it as a threat to
as a threat to fair usefair use..
2. CD-R
2. CD-R
ACD-R (Compact Disc-R ecordable) is a thin (1.2 mm) disc made of polycarbonate with a 120 mm or 80 mm diameter that is mainly used to store music or data. However, unlike conventional CD media, a CD-R has a core of dye instead of metal.
A standard CD-R has a storage capacity of 74 minutes of audio or 650MiB of data (though MB is printed on CDs as the binary prefixes haven't caught on in the industry, MB will be used in this
article). Non-standard CD-Rs are available with capacities of 80 minutes/703MB, which they achieve by exceeding the tolerances specified in the Orange Book CD standards. Most CD-Rs on the market are of the latter capacity. There are also 90 minute/790MB and 99 minute/870MB discs, though they are rare.
The polycarbonate disc contains a spiral groove to guide the laser beam upon writing and reading information. The disc is coated on the side with the spiral groove with a very thin layer of a special dye and subequently with a thin, reflecting layer of silver , a silver alloy or gold. Finally, a
protective coating of a photo-polymerizable lacquer is applied on top of the metal reflector and cured with UV-irradiation.
A specially designed type of CD-ROM drive, called a CD-R drive, CD burner , or CD writer can be used to write CD-Rs. A laser is used to etch ("burn") small pits into the dye so that the disc can
later be read by the laser in a CD-ROM drive or CD player. The laser used to write CD-Rs is an infrared laser which emits laser radiation at a wavelength of 780 nm. The reflectivity in the pit area is different (lower) than for the unchanged dye area, because the refractive index of the dye is
lowered upon "burning" a pit. Upon reading back the stored information, the laser operates at a low enough power not to "burn" the dye and an optical pick-up records the changes in the intensity of the reflected laser radiation when scanning along the groove and over the pits. The change of the intensity of the reflected laser radiation is transformed into an electrical signal, from which the digital information is recovered ("decoded"). The decomposition of the dye in the pit area through the heat of the laser is irreversible (permanent). Therefore, once a section of a CD-R is written, it cannot be erased or rewritten, unlike a CD-RW. A CD-R can be recorded in multiple sessions.
Brief history
The CD-R was invented in 1988 by the Japanese company Taiyo Yuden. First CD-Rs were produced in 1994. Among the first manufacturers were the companies Taiyo Y uden, Kodak ,
Maxell, and TDK . Since then, the CD-R was further improved to allow writing speeds as fast as 54x (as of 2004) relative to the first 1x CD-Rs. The improvements were mainly due to optimisation of special dye compositions for CD-R, groove geometry, and the dye coating process. Low-speed burning at 1x is still used for special "audio CD-Rs", since CD-R audio recorders were
standardized to this recording speed.
There are three basic formulations of dye used in CD-Rs.
1. Cyanine dyes were the earliest ones developed, and their formulation is patented by Taiyo Yuden. Cyanine dyes are naturally green in color, and are chemically unstable. This makes cyanine discs unsuitable for archival use; they can fade and become unreadable in a few
years. Many manufacturers use proprietary chemical additives to make more stable cyanine discs.
2. Azo dye CD-Rs are blue in color, and their formulation is patented by Mitsubishi Chemicals. Unlike cyanine, azo dyes are chemically stable, and typically rated with a lifetime of
decades.
3. Phthalocyanine dye CD-Rs are usually silver or gold. The patents on pthalocyanine CD-Rs are held by Mitsui and Ciba Specialty Chemicals. These are also chemically stable, and often given a rated lifetime of hundreds of years.
Note that unfortunately, many manufacturers add additional coloring to disguise their cyanine CD-Rs, so you cannot determine the formulation of a disc based purely on its color. Similarly, a gold reflective layer does not guarantee use of phthalocyanine dye. Note also that rated CD-R lifetimes are estimates based on accelerated aging tests, and lifetime can vary considerably based on how you store the discs.
For optimum lifespan, CD-Rs should be stored vertically to prevent warping, inside archival plastic cases which use a ridged ring around the spindle which grips the disc. This ridge prevents
the surface of the disc from coming into contact with anything during storage. Discs should be stored in cool, dark conditions, with controlled humidity. Avoid using any kind of label on the CD surface, and avoid use of printed inserts using anything other than water-based inks.
Although the CD-R was initially developed in Japan, most of the production of CD-R had moved to Taiwan by 1998. Taiwanese manufacturers supplied more than 70% of the worldwide
production volume of 10.5 billion CD-Rs in 2003.
There was some incompatibility with CD-Rs and older CD-ROM drives. This was primarily due to the lower reflectivity of the CD-R disc. In general, CD drives marked as 8x or greater will read CD-R discs. Some DVD players will not read CD-Rs because of this change in reflectivity as well.
3. CD-RW
In computing and data storage,Compact Disc Rewritable, or CD-RW, is a rewritable version of CD-ROM. Whereas standard prerecorded compact discs have their information permanently
stamped into an aluminium reflecting layer, CD-RW discs have a phase-change recording layer and an additional aluminium reflecting layer. A laser beam can melt crystals in the recording layer into a non-crystalline amorphous phase, or anneal them slowly at a lower temperature back to the crystalline state. The different reflectance of the resulting areas make them appe ar like the 'pits' and 'lands' of a standard CD.
A CD-RW drive can write about 700MiB of data to CD-RW media an unlimited number of times. Most CD-RW drives can also write once to CD-R media. Except for the ability to completely erase a disc, CD-RWs act very much like CD-Rs and are subject to the same restrictions; i.e., they can be extended, but not selectively overwritten, and must be closed before they can be read in a
normal CD-ROM drive. A variation of UDF formatting allows CD-RWs to be randomly read and written, but limits the capacity to about 500MB.
Note that unlike CD-Rs, CD-RW discs are non-standard, in that they do not meet the Orange Book standards for CDs. Hence CD-RW media cannot be read by CD-ROM drives built prior to 1997 due to the reduced reflectivity (15% compared to 70%) of CD-RW media. CD-RW is also more expensive than CD-R, and so CD-R is sometimes considered a better technology for archival purposes. The write-once nature of CD-Rs also ensures that data cannot be accidentally modified
or tampered with, and encourages better archival practices. However, due to the crystalline layer of CD-RWs (as opposed to the organic material used in CD-Rs), disc manufactures claim longer durability and better data safety of CD-RWs.
4. DVD
DVDis an optical disc storage media format that is used for playback of movies with high video and sound quality and for storing data. DVDs are similar in appearance to compact discs.
Contents [hide] 1 History 2 Technical information 3 DVD-Video 3.1 Security 3.2 Region codes 4 DVD-Audio 5 DVD types
6 DVD players and recorders 7 Competitors and successors 8 See also
9 Bibliography 10 External links
History
During the early 1990s there were two high density optical storage standards in development; one was the Multimedia Compact Disc (MMCD), backed by Philips and Sony, and the other was the Super Disc (SD), supported by Toshiba, Time-Warner , Matsushita Electric, Hitachi, Mitsubishi Electric, Pioneer , Thomson and JVC. IBM led an effort to unite the various companies behind a single standard, anticipating a repeat of the costly format war between VHS and Betamax in the 1980s. The result was the DVD format, announced in September of 1995. The official DVD
specification was released in Version 1.0 in September , 1996. It is maintained by the DVD Forum, formerly the DVD Consortium, consisting of the ten founding companies and over 220 additional members. The first DVD players and discs were available in November of 1996 in Japan and in March of 1997 in the United States.
By the northern spring of 1999, the price of a DVD player had dropped below the $300 mark. At that point Wal-Mart began to offer DVD players for sale in its stores. When Wal-Mart began
selling DVDs in stores, DVDs only represented a small part of their video inventory; VHS tapes of movies made up the remainder. As of 2004, the situation is now reversed. Most retail stores mainly offer DVDs for sale, and VHS copies of movies make up a minority of the sales. The price of a DVD player has dropped to below the level of a typical VCR ; a low-end player can be purchased for as little as $40 in a number of retail stores.
In 2000, Sony released its PlayStation 2 console in Japan. In addition to playing video games developed for the system, it was also able to play DVD movies. In Japan, this proved to be a huge selling point due to the fact that the PS2 was much cheaper than many of the DVD players
available there. As a result, many electronic stores that normally didn't carry video game consoles carried PS2s. Following on with this tradition Sony have decided to implement one of DVD's possible successors, Blu Ray, into their next PlayStation console currently known as the
PlayStation 3
"DVD" was originally an initialism for "digital video disc"; some members of the DVD Forum believe that it should stand for "digital versatile disc", to indicate its potential for non-video
applications. Toshiba, which maintains the official DVD Forum site, adheres to the interpretation of "digital versatile disc." The DVD Forum never reached a consensus on the matter, however, and so today the official name of the format is simply "DVD"; the letters do not "officially" stand for anything.[1] (http://www.dvddemystified.com/dvdfaq.html )
Technical information
A DVD can contain:
• DVD-Video (containing movies (video and sound)) • DVD-Audio (containing high-definition sound) • DVD-Data (containing data)
The disc medium can be:
• DVD-ROM (read only, manufactured by a press) • DVD+R/RW (R=Recordable once, RW = ReWritable) • DVD-R/RW (R=Recordable once, RW = ReWritable)
• DVD-RAM (random access rewritable; after-write checking of data integrity is always
The disc may have one or two sides, and one or two layers of data per side; the number of sides and layers determines the disc capacity. As of 2004, the double sided formats have almost disappeared from the marketplace.
• DVD-5: single sided, single layer, 4.7 gigabytes (GB), or 4.38 gibibytes (GiB) • DVD-9: single sided, double layer, 8.5 GB (7.92 GiB)
• DVD-10: double sided, single layer on both sides, 9.4 GB (8.75 GiB)
• DVD-14: double sided, double layer on one side, single layer on other, 13.2 GB (12.3 GiB) • DVD-18: double sided, double layer on both sides, 17.1 GB (15.9 GiB)
The capacity of a DVD-ROM can be visually determined by noting the number of data sides, and looking at the data side(s) of the disc. Double-layered sides are usually gold-colored, while single-layered sides are silver-colored, like a CD.
Each medium can contain any of the above content and can be any layer type (double layer DVD-R is announced for 2004, while double layer DVD+DVD-R discs are already on the market, though scarce and expensive).
The DVD Forum has created the official DVD-R(W) standards. But as the licensing cost for this technology is very high, another group was founded: the DVD+RW Alliance who created the DVD+R(W) standard with lower licensing costs. At first, DVD+R(W) media were typically more expensive than DVD-R(W) media, but the prices have become very comparable.
The "+" (plus) and "-" (dash) are two similar technical standards that are partially compatible. As of 2004, both formats are equally popular, with about half of the industry supporting "+", and the other half "-". It is open to debate whether either format will push the other out of the market, or whether they will co-exist indefinitely. All DVD readers are supposed to read both formats (though real-world compatibility lies around 90% for both formats), and most current DVD writers can write both formats.
Unlike compact discs, where sound (CDDA, Red Book ) is stored in a fundamentally different fashion than data (Yellow book et al.), a properly authored DVD will always contain data in the UDF filesystem.
The data transfer rate of a DVD drive is given in multiples of 1350 kB/s, which means that a drive with 16x speed designation allows a data transfer rate of 16 x 1350 = 21600 kB/s (21.09 MB/s). As CD drive speeds are given in mulitples of 150 kB/s, one DVD "speed" equals nine CD "speeds", i.e. 8x DVD drive should have data transfer rate similar to a 72x CD drive. In physical rotation terms (spins per second), one DVD "speed" equals three CD "speeds", so the amount of data that are read during one rotation is three times larger for DVD than for CD and 8x DVD drive has the same rotational speed as 24x CD drive.
Note that both CD and DVD disks and drives usually have constant rotational speed while reading and data density on the track is also constant; as linear (meters per second) track speed grows at outer parts of the disk proportionally to the radius, the maximum data rate specified for the
drive/disk is achieved only at the end of the disk's track (disks are written from inside). Average speed of the drive therefore equals to only about 50-70% of the maximum nominated speed.
DVD-Video
DVD-Video discs require a DVD-drive with a MPEG-2 decoder (eg. a DVD-player or a DVD computer drive with a software DVD player). Commercial DVD movies are encoded using a combination of MPEG-2 compressed video and audio of varying formats (often multi-channel formats as described below). Typical data rates for DVD movies range from 3-10 Mbit/s, and the bitrate is usually adaptive. A high number of audio tracks and/or lots of extra material on the disk
will usually result in a lower bitrate (and image quality) for the main feature.
The audio data on a DVD movie can be of the format PCM, DTS, MPEG audio, or Dolby Digital (AC-3). In countries using the NTSC standard any movie should contain a sound track in (at least) either PCM or Dolby AC-3 formats, and any NTSC player must support these two; all the others are optional. This ensures any standard compatible disc can be played on any standard compatible player. The vast majority of commercial NTSC releases today employ AC-3 audio.
Initially, in countries using the PAL standard (e.g. most of Europe) the sound of DVD was supposed to be standardized on PCM and MPEG-2 audio, but apparently against the wishes of Philips, under public pressure on December 5, 1997, the DVD Forum accepted the addition of Dolby AC-3 to the optional formats on discs and mandatory formats in players. The vast majority of commercial PAL releases employ AC-3 audio by now.
DVDs can contain more than one channel of audio to go together with the video content. In many cases, sound tracks in more than one language track are present (for example the film's original language as well as a dubbed track in the language of the country where the disc is being sold). With several channels of audio from the DVD, the cabling needed to carry the signal to an amplifier or TV can occasionally be somewhat frustrating. Most systems include an optional digital connector for this task, which is then paired with a similar input on the amplifier. The selected audio signal is sent over the connection, typically over RCA jacks or TOSLINK , in its original format to be decoded by the audio equipment. When playing compact discs, the signal is sent in S/PDIF format instead.
Video is another issue which continues to present problems. Current players typically output analog video only, both composite video on an RCA jack, as well as S-Video in the standard
connector. However neither of these connectors were intended to be used for progressive video, so yet another set of connectors has started to appear in the form of component video, which keeps the three components of the video, one luminance signal and two color difference signal, as stored on the DVD itself, on fully separate wires (whereas s-video uses two wires, uniting and degrading the two color signals, and composite only one, uniting and degrading all three signals).
Additionally, the connectors are further confused by using a number of different physical connectors on different player models, RCA or BNC, as well as using VGA cables in a non-standard way (VGA is normally analog RGB, not component). Even worse, there are often two sets of component outputs, one carrying interlaced video, and the other progressive. In Europe and
other PAL areas, SCART connectors are typically used, which carry both composite and analog RGB intelaced video signals, as well as analog 2-channel sound on a single multiwire cable, and which offer a reasonable compromise between video quality -- which is superior to S-Video though inferior to progressive component video -- and cost.
DVD Video may also include one or more subtitle tracks in various languages, including those made especially for the hearing impaired. They are stored as images with transparent background which are overlayed over the video during playback. Subtitles are restricted to four colors
(including transparency) and thus tend to look cruder than permanent subtitles on film.
DVD Video may contain Chapters for easy navigation (and continuation of a partially watched film). If space permits, it is also possible to include several versions (called "angles") of certain scenes, though today this feature is mostly used -- if at all -- no t to show different angles of the action, but as part of internationalization to e.g. show different language versions of images containing written text, if subtitles won't do.
A major selling point of DVD Video is that its storage capacity allows for a wide variety of extra features in addition to the feature film itself. This can include audio commentary that is timed to the film sequence, documentary features, unused footage, trivia text commentary, simple games and film shorts.
Security
Most DVD-Video titles use Content Scrambling System (CSS) encryption, which is intended to discourage people from making perfect digital copies to another medium or from bypassing the region control mechanism (see below). Discs can also specify that the player use Macrovision, an analog anti-copying mechanism that prevents the consumer from copying the video onto a VCR tape by using a deliberately-defective signal which may also cau se problems for some projection TV's as well as older television models. This alone would not prevent the duplication of DVDs in their entirety without decrypting the data, given suitable equipment, a lthough "consumer-grade" DVD writers deny this ability by refusing to duplicate the tracks on the disc which contain the decryption keys.
The CSS system has caused problems for the inclusion of DVD players in strictly open source operating systems, since open source player implementations can not officially obtain access to the decryption keys or license the patents involved in the CSS system. Proprietary software players may also be difficult to find on some platforms. However at least one successful effort has been made to write a decoder by reverse engineering, resulting in DeCSS. This has led to long-running legal battles and the arrest of some of those involved in creating or distributing the DeCSS code, through the use of the U.S. Digital Millennium Copyright Act, on the grounds that such software could also be used to facilitate unauthorized copying of the data on the discs.
Region codes
DVD movies can contain a region code, denoting which area of the world it is targeted at, which is completely independent of encryption. The commercial DVD-video player specification dictates
that players must only play discs that contain their region code. This allows the film studios to set different retail prices in different markets and extract the maximum possible price from consumers. With region coding, studios can dictate release schedules and prices around the world. However, many DVD players allow playback of any disc, or can be modified to do so. Region coding pertains to regional lockout, which originated from the video game industry.
Region
code Area
0 Playable in all regions
1 United States, Canada, and U.S. territories
2 Western Europe, Greenland, South Africa, Lesotho, Swaziland, Japan, Egypt, and the Middle East
3 Southeast Asia, South Korea, Hong Kong, Macau, Indonesia, Philippines, Taiwan 4 Australia, New Zealand, Oceania, Mexico, Central America, South America
5 Russia, other Former Soviet Union countries, Eastern Europe, the Indian subcontinent, Mongolia, North Korea, the rest of Africa
6 People's Republic of China 7 Reserved for future use
8 International venues such as aircraft, cruise ships, etc.
See a world map showing region codes (http://www.robertsdvd.com/world.gif)
European Region 2 DVDs may be sub-coded D1 through D4. "D1" identifies a UK-only release. "D2" and "D3" identify European DVDs that are not sold in the UK and the Republic of Ireland. "D4" identifies DVDs that are distributed throughout Europe.
Region 0 designates no actual region, but it is used as shorthand for a disc meant to be playable on all players. On such a disc, the actual region coding is R1/2/3/4/5/6. In the early days, region 0 players were created that would allow any region disc to be played in them, but studios responded by adjusting regioned discs to refuse to play if the player was determined 0 (since no player should
anyway). This system is known as Regional Coding Enhancement or just RCE.
Many view region code enforcement as a violation of WTO free trade agreements; however, no legal rulings have yet been made in this area. However, many manufacturers of DVD players now freely supply information on how to disable the region code checking, and on some recent models, it appears to be disabled by default.
The Flaming Lips' CD and DVD release of Yoshimi Battles the Pink Robots, an early album that employed the DVD-audio format
DVD-Audio is a new format to deliver high-fidelity audio content on a DVD. It offers many channels (from mono to 5.1 surround sound) at various sampling frequencies and sample sizes. Audio on a disc can be 16, 20, or 24 bit and can be at sampling rates of 44.1, 48, 88.2, 96, 176.4, or 192 kHz (the highest sampling rates of 176.4 and 192 kHz are limited to stereo only). In addition, different sampling sizes and frequencies can be used on a single disc. Audio is stored on the disc in LPCM format or is losslessly compressed with Meridian Lossless Packing. The DVD-Audio
player may downmix surround sound to stereo if the listener does not have surround sound. DVD-Audio may also feature menus, still images, slideshows, and video. Also, DVD-DVD-Audio discs
usually contain Dolby Digital or DTS versions of the audio (with lossy compression, usually
downsampled to lower sampling sizes and frequencies) in the DVD-Video section. This is done to ensure compatibility with DVD-Video players.
The introduction of the DVD-Audio format angered many early-adopters of the DVD format. While the DVD-Audio discs do have higher fidelity, there is debate as to whether or not the difference is distinguishable to typical human ears. DVD-Audio currently forms a niche market, probably due to requiring new and rather expensive equipment. DVD-Audio is currently (as of
2003) in a format war with SACD. Most market observers believe the winner of the war will eventually supplant the Compact Disc due to its superior playback capabilities, unless a new and superior format takes over from either.
DVD types
A DVD-RAM is easily recognized due to the numerous rectangles on its surface.
• DVD-ROM discs are pressed similarly to CDs. The reflective surface is silver or gold
double-sided single-layered and double-double-sided double-layered. As of 2004, new double double-sided discs have become quite rare.
• DVD recorders started to become available in Japan during 1999, and in the rest of the
world soon after, with a familiar battle for format dominance beginning. As with the
adoptance of USB, Apple computer was one of the early adopters of the technology. DVD recorders require a special unit to write and can use 1 or 2 disc sides (the disc capacity is measured in GB/side):
o DVD-R discs can record up to4.7 GBin a similar fashion to a CD-R disc. It is supported by the DVD Forum. Once recorded and finalized it can be played by most DVD-ROM players.
o DVD-RW discs can record up to4.7 GBin a similar fashion to a CD-RW drive. Supported by the DVD Forum.
o DVD-RAM (current specification is version 2.1) require a special unit to play 4.7
or 9.4GB recorded discs (DVD-RAM disc are typically housed in a cartridge). 2.6GB discs can be removed from their caddy and used in DVD-ROM drives. Top capacity is9.4GB (4.7GB/side). Supported by the DVD Forum.
o DVD+R discs can record up to4.7 GBsingle-layered, single-sided DVD+R disc.
This is currently up to 16x speed. Like DVD-R you can record only once. Supported by the DVD+RW Alliance.
o DVD+RW discs can record up to4.7 GBwith up to 4x speed. Since it is rewritable
it can be overwritten several times. It does not need special "pre-pits" or finalization to be played in a DVD-Player. Supported by the DVD+RW Alliance.
o DVD+R DL is a derivate of DVD+R that uses dual layer recordable discs to store
up to 8.5 GBof data. Supported by the DVD+RW Alliance.
All above formats are also available as 8 cm (3 inch) sized DVD mini discs (not mini-DVD, which describes DVD data on a CD) with a disc capacity of 1.5 GB.
DVD players and recorders
Modern recorders often support additional formats, including DVD+/-R/RW, CD-R/RW, MP3, SVCD, JPEG, PNG, SVG, KAR and MPEG4 (DivX/XviD). Some also include USB ports or flash memory readers. Many are priced at under $/ € 100.
Competitors and successors
There are two successors to DVD being developed by two different consortiums: The Blu-ray Disc and HD-DVD.
On November 18, 2003, the Chinese news agency Xinhua reports the final standard of the Chinese government-sponsored Enhanced Versatile Disc (EVD) and several patents around it.
On November 19, 2003 the DVD Forum decided with eight to six votes that HD-DVD is the HDTV successor of the DVD.
5. FLOPPY DISK
Afloppy disk is a data storage device that comprises a circular piece of thin, flexible (hence "floppy") magnetic storage medium encased in a square or rectangular plastic wallet. Floppy disks are read and written by a floppy disk drive or FDD, not to be confused with "fixed disk drive", which is an old IBM term for a hard disk drive.
Contents [hide] 1 Background
2 History
2.1 Origins, the 8-inch disk 2.2 The 5¼-inch minifloppy
2.3 The 3½-inch Micro Floppy Diskette 2.4 The 3-inch Compact Floppy Disk 3 Structure
4 Compatibility
5 More on floppy disk formats 5.1 Using the disk space efficiently 5.2 The Commodore 128
5.3 The Commodore Amiga 5.4 The Acorn Archimedes 5.5 12-inch floppy disks 5.6 4-inch floppies
5.7 Auto-loaders
5.8 Floppy mass storage 5.9 2-inch floppy disks
5.10 Ultimate capacity, speed 6 Usability
7 The floppy as a metaphor 8 Floppy disk/drive trivia 9 See also
10 References 11 External links
Background
Floppy disks, also known as floppies or diskettes (a name chosen in order to be similar to the word "cassette"), were ubiquitous in the 1980s and 1990s, being used on home and personal
computer ("PC") platforms such as the Apple II, Macintosh, Commodore 64, Amiga, and IBM PC to distribute software, transfer data between computers, and create small backups. Before the popularization of the hard drive for PCs, floppy disks were often used to store a computer's
operating system (OS), application software, and other data. Many home computers had their primary OS kernels stored permanently in on-board ROM chips, but stored the disk operating
system on a floppy, whether it be a proprietary system, CP/M, or, later, DOS.
Disk type Year Capacity
8-inch 1971 80 kB 8-inch 1973 256 kB 8-inch 1974 800 kB 8-inch dual-sided 1975 1MB 5¼-inch 1976 110 kB 5¼-inch DD 1978 360 kB 5¼-inch QD 1984 1.2 MB 3-inch 1984? 320 kB 3½-inch 1984 720 kB 3½-inch HD
Historical sequence of floppy-disk formats, ending with the last format (3½-inch HD) to be ubiquitously adopted.
By the early 1990s, the increasing size of software meant that many programs were distributed on sets of floppies. Toward the end of the 1990s, software distribution gradually switched to CD-ROM, and higher-density backup formats were introduced (e.g., the Iomega Zip disk ). With the arrival of mass Internet access, cheap Ethernet, and USB "keydrives", the floppy was no longer necessary for data transfer either, and the floppy disk was essentially superseded. Mass backu ps were now made to high capacity tape drives such as DAT or streamers, or written to CDs or
DVDs. One unsuccessful (in the marketplace) attempt in the late 1990s to continue the floppy was the SuperDisk (LS120) with a capacity of 120 MB while the drive was backward compatible with standard 3½-inch floppies.
Nonetheless, manufacturers were reluctant to remove the floppy drive from their PCs, for
backward compatibility, and because many companies' IT departments appreciated a built-in file transfer mechanism that always worked and required no device driver to operate properly. Apple Computer was the first mass-market computer manufacturer to drop the floppy drive from a
computer model altogether with the release of their iMac model in 1998, and Dell made the floppy drive optional in some models starting in 2003. To date, though, these moves have still not marked the end of the floppy disk as a mainstream means of data storage and exchange.
External USB-based floppy disk drives are available for computers without floppy drives, and they work on any machine that supports USB.
Floppy disks are almost universally referred to in imperial measurements, even in countries where metric is the standard. [Note: Throughout this article, the "K" is u sed to indicate the "binary kilo" (1,024).]
History
Origins, the 8-inch disk
An 8-inch floppy disk looks exactly like a big 5¼-inch disk (shown), with a partly exposed
magnetic medium spun about a central hub for reading. The flexible plastic cover contains a cloth inner liner to brush dust from the medium.
In 1967 IBM gave their San Jose, California storage development center a new task: develop a simple and inexpensive system for loading microcode into their System/370 mainframes. The 370s were the first IBM machines to use semiconductor memory, and whenever the power was turned off the microcode had to be reloaded ('magnetic core' memory, used in the 370s' predecessors, the
System/360 line, did not lose its contents when powered down). Normally this task would be left to various tape drives which almost all 370 systems included, but tapes were large and slow. IBM wanted something faster and more purpose-built that could also be used to send out updates to customers for $5.
David Noble, working under the direction of Alan Shugart, tried a number of existing solutions to see if he could develop a new-style tape for the purpose, but eventually gave up and started over. The result was a read-only, 8-inch (20 cm) floppy they called the "memory disk", holding
80 kilobytes (KB). The original versions were simply the disk itself, but dirt became a serious problem and they enclosed it in a plastic envelope lined with fabric that would pick up the dirt. The
new device became a standard part of the 370 in 1971.
A Japanese inventor, Yoshiro Nakamatsu (aka Dr. NakaMats), claims he independently came up with the floppy disk principle back in 1950, and so a sales license had to be acquired by IBM when they started manufacturing their floppy disk systems twenty years later.
In 1973 IBM released a new version of the floppy, this time on the 3740 Data Entry System. The new system used a different recording format that stored up to 256 KB on the same disks, and was read-write. These drives became common, and soon were being used to move smaller amounts of data around, almost completely replacing magnetic tapes.
When the first microcomputers were being developed in the 1970s, the 8-inch floppy found a place on them as one of the few "high speed" 'mass storage' devices that were even remotely affordable to the target market (individuals and small businesses). The first microcomputer operating system, CP/M, originally shipped on 8-inch disks. However the drives were still expensive, typically costing more than the computer they were attached to in early days, so most machines of the era used cassette tape instead.
This began to change with the acceptance of the first standard for the floppy disk, Ecma International-59, authored by Jim O'Reilly of Burroughs, Helmuth Hack of BASF and others. O'Reilly set a record for maneuvering this document through ECMA's approval process, with the standard sub-committee being formed in one meeting of ECMA and approval of a draft standard in the next meeting three months later. This standard later formed the basis for the ANSI standard, too. Standardization brought together a variety of competitors to make media to a single
interchangeable standard, and allowed rapid quality and cost improvement.
By this time Alan Shugart had left IBM, moved to Memorex for a brief time, and then again in 1973 to found Shugart Associates. They started working on improvements to the existing 8-inch format, eventually creating a new 800 KB system. However profits were hard to find, and in 1974 he was forced out of his own company.
Burroughs Corporation was meanwhile developing a high-performance dual-sided 8-inch drive at their Glenrothes, Scotland, factory. With a capacity of 1 MB, this unit exceeded IBM's drive
capacity by 4 times, and was able to provide enough space to run all the software and store data on the new Burrough's B80 data entry system, which incidentally had the first VLSI disk controller in the industry. The dual-sided 1MB floppy entered production in 1975, but was plagued by an
industry problem, poor media quality. There were few tools available to test media for 'bit-shift' on the inner tracks, which made for high error rates, and the result was a substantial investment by Burroughs in a media tester design that they then gave to media makers as a quality control tool, leading to a vast improvement in yields.
The 5¼-inch minifloppy
In 1975, Burroughs' plant in Glenrothes developed a prototype 5.25-inch drive, stimulated both by the need to overcome the larger 8-inch floppy's asymmetric expansion properties with changing humidity, and, to reflect the knowledge that IBM's audio recording products division was
demonstrating a dictation machine using 5.25" disks. In one of the industry's historic gaffs,
Burroughs corporate management decided it would be "too inexpensive" to make enough money, and shelved the program.
In 1976 one of Shugart [Assoc.]'s employees, Jim Adkisson, was approached by An Wang of Wang Laboratories, who felt that the 8-inch format was simply too large for the desktop word processing machines he was developing at the time. After meeting in a bar in Boston, Adkisson
asked Wang what size he thought the disks should be, and Wang pointed to a napkin and said "about that size". Adkisson took the napkin back to California, found it to be 5¼ inches (13 cm) wide, and developed a new drive of this size storing 110 KB.
The 5¼-inch drive was considerably less expensive than 8-inch drives from IBM, and soon started appearing on CP/M machines. At one point Shugart Assoc. was producing 4,000 drives a day. By 1978 there were more than 10 manufacturers producing 5¼-inch floppy drives, and the format quickly displaced the 8-inch from most applications. These early drives read only one side of the disk, leading to the popular budget approach of cutting a second write-enable slot and index hole into the carrier envelope and flipping it over (thus, the "flippy disk ") to use the other side for additional storage.
Tandon introduced a double-sided drive in 1978, doubling the capacity, and a new "double density" format increased it again, to 360 KB.
For most of the 1970s and 1980s the floppy drive was the primary storage device for
microcomputers. Since these micros had no hard drive, the OS was usually from one floppy disk, which was then removed and replaced by another one containing the application. Some machines using two disk drives (or one dual drive) allowed the user to leave the OS disk in place and simply change the application disks as needed. In the early 1980s, 96 track-per-inch drives appeared, increasing the capacity from 360 to 720 KB. These did not see widespread use, as they were not supported by IBM in its PCs. In 1984, along with the IBM PC/AT, the quad density disk appeared, which used 96 tracks per inch combined with a higher density magnetic media to provide
1.2 megabytes (MB) of storage. Since the usual (very expensive) hard disk held 10–20 megabytes at the time, this was considered quite spacious.
By the end of the 1980s, the inch disks had been superseded by the 3½-inch disks. Though 5¼-inch drives were still available, as were disks, they faded in popularity as the 1990s began. On
most new computers the 5¼-inch drives were optional equipment. By the mid-1990s the drives had virtually disappeared as the 3½-inch disk became the pre-eminent floppy disk.
The 3½-inch Micro Floppy Diskette
The non-ferromagnetic metal sliding door protects the 3½-inch floppy disk's recording medium. Throughout the early 1980s the limitations of the 5¼-inch format we re starting to become clear as machines grew in power. A number of solutions were developed, with drives at 2-inch, 2½-inch, 3-inch and 3½-3-inch (50, 60, 75 and 90 mm) all being offered by various companies. They all shared a number of advantages over the older format, including a small form factor and a rigid case with a slideable write protect catch. Amstrad incorporated a 3-inch 180 KB single-sided disk drive into their CPC and PCW lines, and this format and the drive mechanism was later "inherited" by the ZX Spectrum +3 computer after Amstrad bought Sinclair Research. Later models of the PCW featured double-sided, quad density drives while all 3-inch media were double-sided in nature with single-sided drive owners able to flip the disk over to use the other side. Media in this format remained expensive and it never caught on with only three manufacturers producing media -Amstrad, Tatung and Maxell.
Things changed dramatically in 1984 when Apple Computer selected the Sony 90.0 × 94.0 mm format for their Macintosh computers, thereby forcing it to become the standard format in the United States. (This is yet another example of a "silent" change from metric to imperial units; this product was advertised and became popularly known as the 3½-inch disk, emphasizing the fact
that it was smaller than the existing 5¼-inch.) The first computer to use this format was the HP-150 of 1983. By 1989 the 3½-inch was outselling the 5¼-inch.
The 3½-inch disks had, by way of their rigid case's slide-in-place metal cover, the significant
advantage of being much better protected against unintended physical contact with the disk surface when the disk was handled outside the disk drive. When the disk was inserted, a part inside the drive moved the metal cover aside, giving the drive's read/write heads the necessary access to the magnetic recording surfaces. (Adding the slide mechanism resulted in a slight departure from the previous square outline. The rectangular shape had the additional merit that it made it impossible
to insert the disk sideways by mistake, as had indeed been possible with earlier formats.) Like the 5¼-inch, the 3½-inch disk underwent an evolution of its own. They were originally offered in a 360 KB single-sided and 720 KB double-sided double-density format (the same as then-current 5¼-inch disks). A newer "high-density" format, displayed as "HD" on the disks
themselves and storing 1.4 MB of data, was introduced in the mid-80s. IBM used it on their PS/2 series introduced in 1987. Apple started using "HD" in 1988, on the Macintosh IIx. Another advance in the oxide coatings allowed for a new "extended-density" ("ED") format at 2.88 MB introduced on the second generation NeXT Computers in 1991, but by the time it was available it was already too small to be a useful advance over 1.4 MB, and never became widely used. The 3½-inch drives sold more than a decade later still used the same format that was standardized in 1989, in ISO 9529-1,2.
Not long after the 2.88 MB format was declared DOA by the market, it became obvious that users had a requirement to move around ever increasing amounts of data. A number of products
surfaced, but only a few maintained any level of backward compatibility with 3½-inch disks. Insite Peripherals' "Floptical" was the first off the blocks, offering 20, 40 and ultimately 80 MB devices that would still read and write 1.4 MB disks. However, the drives did not connect to a normal floppy disk controller, meaning that many older PCs were unable to boot up from a disk in a Floptical drive. This again adversely affected adoption rates.
Announced in 1995, the "Super Disk" drive, often seen with the brand names Matsushita (Panasonic) and Imation, had an initial capacity of 120 MB. It was subsequently upgraded to 240 MB. Not only could the drive read and write 1.4 MB disks, but the last versions of the drives could write 32 MB onto a normal 1.4 MB disk (see note below). Unfortunately, popular opinion held the Super Disk disks to be quite unreliable, though no more so than the Zip drives and
SyQuest Technology offerings of the same period. This again, true or otherwise, crippled adoption. Thus 3½-inch disks are still widely available. As of 2004 3½-inch drives are still common
equipment on most new PCs. On others, they are either optional equipment, or can be purchased as after-market equipment. However, with the advent of other portable storage options, such as Zip disks, USB storage devices, and (re)writable CD's the 3½-inch disk is becoming increasingly
obsolete. Some manufacturers have stopped offering 3½-inch drives on new computers as standard equipment. The Apple Macintosh, which popularized the format in 1984, began to move away from it in 1998 with the iMac model. Possibly prematurely, since the basic model iMac of the time only had a CD-ROM drive giving users no easy access to removable media. This made USB-connected floppy drives a popular accessory for the early iMacs.
The formatted capacity of 3½-inch high-density floppies was originally 1440 kibibytes (KiB), or 1,474,560 bytes. This is equivalent to 1.41 MiB (1.47 MB decimal). However, their capacity is usually reported as 1.44 MB by diskette manufacturers.
In some places, especially South Africa, 3½-inch floppy disks have commonly been called stiffies or stiffy disks, because of their "stiff" (rigid) cases, which are contrasted with the flexible "floppy" cases of 5¼-inch floppies.
The 3-inch Compact Floppy Disk
A now unused semi-proprietary format, the 3-inch Compact Floppy was a format used mainly on the Amstrad CPC, PCW and ZX Spectrum computers while these machines were still supported, as well as on a number of exotic and obscure CP/M systems such as the Einstein or Osborne
computers and occasionally on MSX systems in some regions. The disk format itself was not more capient than the more popular (and cheap) 5¼" floppies, but was more reliable thanks to its hard casing.
Their main problems were their high prices, due to their quite elaborate and complex case mechanisms and low nominal capacities, as well as their being bound to using specifically designed drives, which were very hard to repair or replace.
Eventually, the format died out along with the computer systems that used it.
Structure
A user inserts the floppy disk, medium opening first, into a 5 ¼-inch floppy disk drive (pictured, an internal model) and moves the lever down (by twisting on this model) to close the drive and
engage the motor and heads with the disk.
The 5¼-inch disk had a large circular hole in the center for the spindle of the drive and a small oval aperture in both sides of the plastic to allow the heads of the drive to read and write the data. The magnetic medium could be spun by rotating it from the middle hole. A small notch on the right hand side of the disk would identify whether the disk was read-only or writable, detected by a mechanical switch or photo transistor above it. Another LED/phototransistor pair located near the center of the disk could detect a small hole once per rotation, called the index hole, in the magnetic disk. It was used to detect the start of each track , and whether or not the disk rotated at the correct speed; some operating systems, such as Apple DOS, did not use index sync, and often the drives designed for such systems lacked the index hole sensor. Disks of this type were said to be soft sector disks. Very early 8-inch and 5¼-inch disks also had physical holes for each sector, and were
termed hard sector disks. Inside the disk were two layers of fabric designed to reduce friction between the media and the outer casing, with the media sandwiched in the middle. The outer casing was usually a one-part sheet, folded double with flaps glued or spot-melted together. A catch was lowered into position in front of the drive to prevent the disk from emerging, as well as to raise or lower the spindle.
The 3½-inch disk is made of two pieces of rigid plastic, with the fabric-medium-fabric sandwich in the middle. The front has only a label and a small aperture for reading and writing data, protected by a spring-loaded metal cover, which is pushed back on entry into the drive.
The 3½-inch floppy disk drive automatically engages when the user inserts a disk, and disengages and ejects with the press of a button, or by motor on the Apple Macintosh.
The reverse has a similar covered aperture, as well as a hole to allow the spindle to connect into a metal plate glued to the media. Two holes, bottom left and right, indicate the write-protect status and high-density disk correspondingly, a hole meaning protected or high density, and a covered gap meaning write-enabled or low density. (Incidentally, the write-protect and high-density holes on a 3½-inch disk are spaced exactly as far apart as the holes in punched A4 paper (8 cm),
allowing write-protected floppies to be clipped into European ring binders.) A notch top right ensures that the disk is inserted correctly, and an arrow top left indicates the direction of insertion. The drive usually has a button that, when pressed, will spring the disk out at varying degrees of force. Some would barely make it out of the disk drive; others would shoot out at a fairly high speed. In a majority of drives, the ejection force is provided by the spring that holds the cover shut, and therefore the ejection speed is dependent on this spring. In PC-type machines, a floppy disk can be inserted or ejected manually at any time (evoking an error message or even lost data in some cases), as the drive is not continuously monitored for status and so programs can make
assumptions that don't match actual status (ie, disk 123 is still in the drive and has not been altered by any other agency). With Apple Macintosh computers, disk drives are continuously monitored by the OS; a disk inserted is automatically searched for con tent and one is ejected only when the
software agrees the disk should be ejected. This kind of disk drive (starting with the slim "Twiggy" drives of the late Apple "Lisa") does not have an eject button, but uses a motorized mechanism to eject disks; this action is triggered by the OS software (e.g. the user dragged the "drive" icon to the
"trash can" icon). Should this not work (as in the case of a power failure or drive malfunction), one can insert a straight-bent paper clip into a small hole at the drive's front, thereby forcing the disk to eject (similar to that found on CD/DVD drives).
The 3-inch disk bears a lot of similarity to the 3½-inch type, with some unique and somehow curious features. One example is the rectangular-shaped plastic casing, almost taller than a 3½-inch disk, but narrower, and more than twice as thick, almost the size of a standard compact audio cassette. This made the disk look more like a greatly oversized present day memory card or a standard PCMCIA notebook expansion card, rather than a floppy disk. Despite the size, the actual 3-inch magnetic-coated disk occupied less than 50 per cent of the space inside the casing, the rest being used by the complex protection and sealing mechanisms implemented on the disks. Such
mechanisms were largely responsible for the thickness, length and high costs of the 3-inch disks. On the Amstrad machines the disks were typically flipped over to use both sides, as opposed to being truly double-sided. Double-sided mechanisms were available, but rare.
Compatibility
The three physical sizes of floppy disks are incompatible, and disks can only be loaded on the correct size of drive. There were some drives available with both 3½-inch and 5¼-inch slots that were popular in the transition period between the sizes.
However there are many more subtle incompatibilities within each form factor. Co nsider, for example the following Apple/IBM 'schism': Apple Macintosh computers can read, write and format IBM PC-format 3½-inch diskettes, provided suitable software is installed. However, many IBM-compatible computers use floppy disk drives that are unable to read (or write) Apple-format disks. For details on this, see the section "More on floppy disk formats".
Within the world of IBM-compatible computers, the three densities of 3½-inch floppy disks are partly compatible. Higher density drives are built to read, write and even format lower density
media without problems, provided the correct media is used for the density selected. However, if by whatever means a diskette is formatted at the wrong density, the result is a substantial risk of
data loss due to magnetic mismatch between oxide and the drive head's writing attempts.
The situation was even more complex with 5¼-inch diskettes. The head gap of a 1.2 MB drive is shorter than that of a 360 KB drive, but will format, read and write 360 KB diskettes with apparent success. A blank 360 KB disk formatted and written on a 1.2 MB drive can be taken to a 360 KB drive without problems, similarly a disk formatted on a 360 KB drive can be used on a 1.2 MB drive. But a disk written on a 360 KB drive and updated on a 1.2 MB drive becomes permanently unreadable on any 360 KB drive, owing to the incompatibility of the track widths. There are several other 'bad' scenarios.
Prior to the problems with head and track size, there was a period when just trying to figure out which side of a "single sided" diskette was the right side was a problem. Both Radio Shack and Apple used 360 KB single sided 5¼-inch disks, and both sold disks labeled "single sided" were certified for use on only one side, even though they in fact were coated in magnetic material on both sides. The irony was that the disks would work on both Radio Shack and Apple machines, yet
