Different DVD Formats Explained
DVD-ROM
Like DVD discs, there is little to distinguish
a DVD-ROM drive from an ordinary CD-ROM drive as the only giveaway is
the DVD logo on the front. Even inside the drive there are more similarities
than differences: the interface is ATAPI or SCSI for the more upmarket
drives, and the transport is much like any other CD-ROM drive. CD-ROM
data is recorded near the top surface of a disc. DVD's data layer
is right in the middle so that the disc can be double-sided. Therefore,
the laser assembly of a DVD-ROM drive needs to be more complex than its
CD-ROM counterpart, to enable it to read from both CD and DVD media. An
early solution to entailed having a pair of lenses on a swivel: one to
focus the beam onto the DVD data layers and the other for reading ordinary
CDs. Subsequently, more sophisticated designs have emerged that eliminate
the need for lens switching. For example, Sony's "dual discrete optical
pickup" design has separate lasers optimised for CD (780nm wavelength)
and DVD (650 nm). Many Panasonic drives employ an even more elegant solution
which avoids the need to switch either lenses or laser beams by use of
a holographic optical element capable of focusing a laser beam at two
discrete points.
DVD-ROM drives spin the disk a lot slower than their CD-ROM
counterparts. However, since the data is packed much closer together on
DVD discs, the throughput is substantially better than a CD-ROM drive
at equivalent spin speed. While a 1x CD-ROM drive has a maximum data rate
of only 150 KBps, a 1x DVD-ROM drive can transfer data at 1,250 KBps,
which is just over the speed of an 8x CD-ROM drive.
DVD-ROM drives became generally available in early 1997
and these early 1x devices were also capable of reading CD-ROM discs at
12x speed - sufficient for full-screen video playback. As with CD-ROM,
higher speed drives appeared as the technology matured. By the beginning
of 1998, multispeed DVD-ROM drives had already reached the market, capable
of reading DVD media at double-speed, producing a sustained transfer rate
of 2,700 KBps, and of spinning CDs at 24-speed and by the end of that
year DVD read performance had been increased to 5-speed. A year later
performance had improved to six-speed (8,100 KBps) reading of DVD media
and 32-speed reading of CD-ROMs.
There is no standard terminology to describe the various
"generations" of DVD-ROM drive. However, second generation (or
DVD II) is usually used to refer to 2x drives also capable of reading
CD-R/CD-RW media and third generation (or DVD III) usually means 5x (or
sometimes 4.8x or 6x) drives, some of which are capable of reading DVD-RAM
media.
DVD-Video
DVD-Video titles are usually encoded from digital studio master tapes
to MPEG-2 format. MPEG-2 offers greater overall compression than MPEG-1
and yields a much sharper, cleaner picture. MPEG-2-encoded video commonly
uses 480 horizontal lines per frame (720 x 480 pixels), versus 425 lines
for laserdisc and 250 to 270 lines for VHS video.
A single-sided (DVD-5) DVD-Video disc was designed to
hold a typical feature-length movie - which averages 133 minutes. MPEG-2
encoding uses lossy compression that removes redundant information (such
as areas of the picture that don't change) and information that's not
readily perceptible by the human eye. The resulting video, especially
when it is complex or changing quickly, may sometimes contain visual flaws,
depending on the processing quality and amount of compression. With MPEG-2
compression a full-motion image needs a minimum video data rate of 3,500
Kbit/s. Digital surround-sound - centre, left, right left-rear and right-rear
directional channels, plus a non-directional subwoofer - requires a further
384 Kbit/s. Add additional storage for dialogue tracks in different languages
and subtitles and the required capacity increases to 4,692Kbits - or 586.5KB
- for every second of a 133-minute movie (a minimum of 4 Mbit/s being
required for high quality results). The sums work out to a total required
storage capacity of 4.68GB. Higher data rates can result in higher quality,
with almost no perceptible difference from the master at rates above 6
Mbit/s. As MPEG compression technology improves, better quality is being
achieved at lower rates.
The net result is that a movie played from DVD-Video should
look a good bit better than one played from consumer videotape and generally
better than laserdisc, assuming the picture has been encoded with at least
a reasonable degree of skill. Furthermore, DVD-Video titles typically
support multiple aspect ratios, allowing the viewer to choose from
at least a couple, such as 16:9 letterbox, wide-screen formats and a more
conventional 4:3 ratio. Furthermore, DVD-Video titles also typically let
you choose from up to eight different languages and from 32 different
sets of sub-titles.
For a dual layer disc (DVD-9) capacity
increases to 240 minutes. A double-sided disc (DVD-10) will hold
slightly more at 266 minutes, but the disc needs to be turned over to
play the other side. Many DVD movies have taken advantage of double-sided
discs by putting a version formatted for a normal TV or monitor with a
4:3 aspect ratio on one side and a widescreen version formatted for 16:9
aspect ratio on the other.
There are two ways of writing the DVD
data layers: parallel track path (PTP) and opposite track path (OTP).
In PTP discs both layers read from the inside of the disc to the outside,
whereas in an OTP disc the outer layer reads from the inside to out, and
then back in for the inner layer. This allows the drive to read both layers
almost continuously, with only a short break to refocus the pickup lens.
This is especially useful for DVD movies, where long play time without
interruption is needed.
In 1998 the spectre of another VHS vs
Beta-type confrontation in the DVD arena was raised when Digital Video
Express (DVE) - a partnership a partnership between one of the largest
US electronics retailers, Circuit City, and a prominent Los Angeles entertainment
law firm - announced an alternative movie format to DVD-Video. Known as
DIVX, the rival format took a pay-per-view approach to viewing movies
and quickly garnered the support of leading studios Disney, Paramount,
Universal and MGM.
DIVX
DIVX is essentially a limited-use, pay-per-view DVD technology.
Marketed as a more affordable DVD format - with the financial backing
of the Circuit City Stores retail chain - it allows a user to purchase
a DIVX disc at a minimal cost and view its contents for an unlimited number
of times within a 48-hour period. Once the 48 hours are up, the user is
charged for each additional use - DIVX machines having a built-in modem
which it uses to automatically call the central billing server to report
player usage roughly twice a month. Users have the option to purchase
the right to unlimited viewing - for a sum equivalent to the cost of a
DVD-Video disc. Given that a DIVX player is basically a DVD-Video player
with additional features to enable a pay-per-view mode of operation, it
not surprising that it's capable of playing standard DVD-Video discs.
Obviously a standard DVD player will not allow viewing of a DIVX disc.
In addition to the built-in modem the
typical DIVX player also contains decrypting circuitry, in order to read
the DIVX discs which are encoded with a state-of-the-art algorithm [Triple-DES].
Also the player is able to read a unique serial number off the disc which
is recorded on an area of the DIVX disc known as the Burst Cutting Area
(BCA), located in the inner most region of the disc. Annex K of Part 1
of the DVD Specification defines the BCA for a DVD. Essentially, this
can be used to record up to 188 bytes of data after the disc has been
manufactured. DIVX uses this number to keep track of the viewing period.
Despite some consumers balking at the
idea of having two different standards for digital discs, and others objecting
to the idea that they would have to keep paying for something they had
already purchased, DIVX appeared to be gaining acceptance amongst consumers
- with sales of the enhanced players reportedly matching those of standard
DVD units - when its backers pulled the plug on the format in mid-1999,
blaming inadequate support from studios and other retailers for the format's
demise. Its fate was effectively sealed once companies - including US
retail chain Blockbuster - announced plans to rent DVDs to consumers instead
of DIVX discs.
It might not have lasted long, but its
likely that DIVX played a useful role in creating a viable rental market
- essential for DVD-Video to become as popular as VHS. Furthermore, one
of its features - the use of the BCA - offers some interesting possibilities
future distribution of software on DVD-ROM discs. For example, it could
mean an end to the rigmarole of consumers having to manually enter a long
string of characters, representing a product's serial number, during software
installation. A unique vendor ID, product ID, and serial number can be
stored as BCA data, and automatically read back during the software installation
process. Storing a product's serial number as BCA data could also confer
important ant-pirating benefits - making it almost impossible to install
a software product without possessing an authentic copy of the disc.
DVD-Audio
It's interesting to note that the first optical storage medium made available
to the public was the now-familiar audio CD. Since then, the fields of
digital audio and digital data have been intertwined in a symbiotic relationship,
with one industry making use of the other's technology to their mutual
benefit. It took several years for the computer industry to realise that
the CD was the perfect medium for storing and distributing large amounts
of digital data, and it was not until well into the 1990s that CD-ROMs
became standard pieces of PC equipment.
With the latest PC industry-driven developments
in optical storage, the record industry is now looking to borrow from
the technology to find another way to make the public buy their album
collections all over again. The quest for higher fidelity CDs has produced
a number of standards which are battling with DVD-Audio to become the
next accepted standard. Amongst these are SACD (Super Audio CD) and DAD
(digital Audio Disc). When one of these finally wins out it could produce
discs with 24-bit resolution at a 96kHz sampling rate, as opposed to the
current 16-bit/44.1kHz format. The SACD format is backward compatible
with existing players - a fact which may help it in the battle for consumer
acceptance.
When DVD was released in 1996 there it
did not include a DVD-Audio format. Following efforts by the DVD Forum
to collaborate with key players from the music industry a draft standard
was released in early 1998. The DVD-Audio 1.0 specification (minus
copy protection) was subsequently released in spring of 1999.
DVD-Audio includes the option of PCM (pulse code modulation)
digital audio with sampling sizes and rates higher than audio CD. Alternatively,
audio for most movies is stored as discrete, multi-channel surround sound
using Dolby Digital or Digital Theatre Systems Digital Surround (DTS)
audio compression similar to the digital surround sound formats used in
theatres. DTS is an audio encoding format similar to Dolby Digital, requiring
a decoder, either in the player or in an external receiver. It accommodates
channels for a subwoofer plus five speakers - front left, front centre,
front right, rear left, and rear right - and some argue that because of
its lower compression level DTS sounds better than Dolby Digital. As with
video, audio quality depends on how well the processing and encoding was
done. In spite of compression, Dolby Digital and DTS can be close to or
better than CD quality.
DVD-Audio discs will be capable of carrying
video, like DVD-Video titles, as well as high quality audio files and
include limited interactivity. Capacity of a dual layer DVD-Audio will
be up to at least 2 hours for full surround sound audio and 4 hours for
stereo audio. Single layer capacity will be around half these times.
The first DVD-Audio products are expected
to appear in mid-2000. The delay is in part caused by the slow process
of selecting copy protection features (encryption and watermarking), complications
having arisen as a result of the Secure Digital Music Initiative (SDMI).
In late 1999 a decision was made as to which copy protection technology
would be used by SDMI, but a later version of the DVD-Audio specification
incorporating this is not expected until mid-2000.
Whilst DVD-Audio discs can be designed to work in DVD-Video
players, it's also possible to make a DVD-Audio disc that won't play at
all in a DVD-Video player. This is because the DVD-Audio specification
includes new formats and features that DVD-Video can't handle. The sensible
way forward is 'universal players' that can play both DVD-Video and DVD-Audio
discs - but these aren't expected for some time.
DVD-R Technology
DVD-R is a write-once format, meaning that data can be written to a disc
and stored without fear of accidental erasure. The fundamental technology
employed is similar to that used by CD-R, except that data is written at
a higher rate and density. DVD-R, like CD-R, uses a constant linear velocity
rotation technique to maximize the storage density on the disc surface.
This results in a variable number of revolutions per minute (RPM) as disc
writing/reading progresses from one end to the other. Recording begins at
the inner radius and ends at the outer. At "1X" speeds, rotation of the
disc varies from 1,623 to 632 RPM on 3.95 Gbyte media and 1,475 to 575 RPM
on 4.7 Gbyte media, depending on the record/playback head's position over
the surface. On 3.95 Gbyte media, the track pitch, or the distance from
the center of one part of the spiral information "track" to an adjacent
part of the track, is 0.8 microns, one-half that of CD-R. 4.7 Gbyte media
uses an even smaller track pitch of 0.74 microns. Read more about DVD-R Format.
DVD+RW Standard
DVD+RW has been standardized by 'DVD+RW Alliance' that
consists of eight leading companies including those recently promoted
CD-R/RW: Sony (Japan), Dell Computer (USA), Thomson Multimedia (France),
Hewlett-Packard (USA), Philips (Netherlands), Mitsubishi Chemical (Japan),
Yamaha (Japan), and Ricoh (Japan). With extensive experience in popularizing
CD-ROM/-R/RW, the Alliance has taken a serious consideration for user
comfort in operation, including compatibility with existing drives/players
and supporting Random Access suitable for data storage on PC. Providing
a High storage capacity is also an important consideration of the Alliance. Read more about DVD+RW here.
