The Digital Versatile Disc, which was developed by nine companies, including Sony, Panasonic and Toshiba, was launched in September 1985. In appearance, it resembles a Compact Disc and has a similar spiral track, although the track on a DVD is 71⁄2 miles long and is more closely spaced than that of a CD, giving it a much greater data capacity.
Discs containing pre-recorded video, audio or data are usually in 3.1 or 4.7 inch form, other sizes often being unsuitable for modern drives. The read-only (RO), write-once read-many (WORM) and read-write (RW) formats are as follows:-
|DVD-ROM||RO||4.7 GB *8.5 GB •9.4 GB •17 GB|
|DVD-R||WORM||3.95 GB 4.7 GB *8.5 GB •9.4 GB|
|DVD-RW||RW||4.4 GB 4.7 GB •9.4 GB|
|DVD-RAM||RW||2.6 GB •5.2 GB 4.7 GB •9.4 GB|
|DVD+R||WORM||4.7 GB *8.5 GB •9.4 GB|
|DVD+RW||RW||4.7 GB •9.4 GB|
|DVD+RAM||RW||4.7 GB •9.4 GB|
• Type 1 (double-sided): not supported by some drives, requires user to ‘flip’ disc
* Single-sided, dual-layer: not suitable for all disc writers, as used for DVD-Video
Type 2 (single-sided) discs in single-layer form are suitable for all drives
As you can see, the actual selection of a drive can be a complex business, particularly since the various competing DVD read-write formats aren’t compatible with all types of drive (see below).
A single-sided (Type 2) DVD provides 2.6 GB, 4.4 GB or 4.7 GB of storage space, whilst the double-sided (Type 1) variety, which is really a double-thickness disc in the form of a ‘sandwich’, has a capacity of between 5.2 and 9.4 GB. Unfortunately, the latter can’t be used in some kinds of drive and usually require the user to turn the disc over by hand.
A dual-layer disc provides increased capacity without you having to ‘flip’ the disc. A single-sided disc of this kind gives a capacity of 8.5 GB, increasing to 17 GB for the double-sided version.
Any kind of DVD drive should read all types of CD, as well as the DVDs shown in the following table:-
|DVD-RAM||√ •||√ •||√||×||√|
√ Drive accepts disc
× Drive does not accept disc
† Some older DVD drives and video players don’t accept DVD-R discs
• Accepts non-cartridge disc or 2.6 GB disc extracted from cartridge
? Some drive models accept disc
Note: Drive must be 12× speed or higher to read DVD-RAM discs
Most modern DVD-RW drives can write to at least CD-R, CD-RW and DVD-R, as well as to DVD-RW, although some older DVD-RAM drives can’t record onto CD-R or CD-RW discs. The majority of modern drives, including those fitted in Mac OS computers, can read virtually any kind of DVD, although, at the time of writing, Apple drives only record onto DVD-R discs. However, Mac OS X 10.3.3, or higher, provides support for DVD+R, so you can use a suitable external DVD+R drive.
This emerging format, not currently supported by many DVD drives, provides a capacity of 15 GB on each layer of an HD-DVD disc, so a single-sided dual layer disc accommodates 30 GB, with a possible double-sided disc holding up to 60 GB. This increased track density is achieved by using a blue laser, instead of the red 650 nanometre (nm) wavelength variety used in a standard DVD drive or the 780 nm version in a CD player. However, drives designed for HD-DVD discs should also work normally with older types of discs.
Developed by Sony, this non-standard format, not supported by many drives, is designed to compete with HD-DVD. It uses a blue-violet laser with a wavelength of 405 nm, which means that it can hold 25 GB on each layer of a BD disc, offering the future possibility of storing 100 GB on a double-sided dual-layer disc. It’s assumed that such drives will be able to play most types of DVD, but presumably not HD-DVD.
A DVD drive has a tray or slot for the disc, although your drive may not accept every type of DVD or some kinds of content.
All DVDs are played from the centre of the disc. As the drive’s laser head moves towards the centre of the disc, the disc’s rotational speed, also known as its angular velocity, increases automatically. This means that the speed of the disc in relation to the laser doesn’t change whilst the DVD plays. This constant linear velocity (CLV) recording system ensures that all data is transferred at a fixed rate, giving consistent results across the entire disc.
A typical drive plays DVD-ROMs at
12× the speed of the earliest CD drives. With an integral 512 KB buffer, this usually gives a data transfer rate of around 15.8 MB/s and an access time of 110 ms. Most DVD drives employ increased speeds for CDs, often as high as
40× speed for a CD-ROM or CD-R, or
24× when writing a CD-RW disc. However, they’re much slower when writing to a DVD, commonly running at
8× with DVD+R,
4× with DVD-R or DVD+RW and
2× with DVD-RW. At the time of writing, many DVD drives write to single-layer DVDs at
8× speed, but this falls to
2.4× with a dual-layer disc.
Most drives don’t accept every kind of disc, although Apple’s SuperDrive, used in many Macs, records onto both CD-RW and DVD-R discs. Unfortunately, early versions of this drive are only in ‘tray’ form, so can’t be fitted in some machines.
Although older DVD-RAM discs may appear in a caddy, also known as a cartridge, all modern drives only accept ‘bare’ discs. A Type 1 cartridge, with its double-sided disc, is sealed, making it impossible to use in a normal ‘slot’ or ‘tray’ drive. However, a Type 2 cartridge can be opened, allowing you to extract the single-sided disc and use it in a modern drive.
12×normally requires discs to be inserted in caddy before they can be played.
Most modern machines have built-in drives that accept DVDs. However, if you don’t have such a drive, you can install your own internal drive inside the computer. If this isn’t possible, you can connect an external drive via a spare USB or FireWire port. Unfortunately, the USB 1.0 interface, as found on older machines, is too slow for connecting a DVD drive.
As with Compact Disc, there are several types of DVD, as described below.
Often in the form of a read-only DVD, this kind of disc is used purely for recording sound. This technology, as promoted by Toshiba and other companies, is in competition with the Super Audio CD (SACD) format, although many DVD players can play both types of disc. 20-bit encoding is normally used, with surround sound provided by Dolby Digital 5.1 as standard, although some discs also convey Digital Theater Systems (DTS) surround sound (see below).
DVD-A offers much greater playing times than CD-A, giving up to 36 hours with Dolby Digital encoding. A recently advertised disc contains the equivalent of 16 CDs, providing 18 hours of continuous music.
This kind of disc can be in the form of DVD-ROM, DVD-R, DVD-RAM, DVD-RW or DVD+RW (see below). A 4.7 GB disc holds up to 133 minutes of video by means of MPEG-2 compression, which is decoded using computer software or special hardware, the latter sometimes in the form of a PCI card in older machines.
Dolby AC-3 coding provides Dolby Digital 5.1 surround sound for left, centre and right loudspeakers, plus two rear speakers and a sub-woofer, whilst some DVDs also include Digital Theater Systems (DTS) surround sound. DVD-Video discs can include multiple sound tracks in up to 8 languages, as well as subtitles in up to 32 languages.
The DVD drives fitted in Macintosh computers are often locked into a world region by means of the RPC Phase 2 (RPC-2) system. In an attempt to minimise piracy of DVDs, these drives limit the number of times you can play discs produced in other parts of the world, unlike an RPC-1 drive where such restrictions aren’t imposed. However, the makers of DVD are perfectly at liberty to create a region-free disc that can be played in any drive without requiring you to switch region.
As supplied, an RPC-2 drive lets you change region up to five times, after which you must apply a manufacturer’s reset. Up to five such resets can be applied, allowing 25 region changes in total, but after this you’re locked into the current region.
RPC-2 technology is in the drive’s firmware, which means it can’t be modified using normal software. However, RPC-1 firmware, which removes the RPC-2 restrictions, is available for almost all drives installed in Apple computers.
DVD has a much greater capacity than Compact Disc (CD), providing between 2.6 GB and 17 GB of storage (although most discs hold 4.7 GB) and an increased data transfer rate, typically 15 MB/s or higher.
DVD-R uses dye technology, very similar to that used in CD-R, which means that discs are cheap and readily available.
4×speed. Unfortunately, blank discs rated at
4×may not work in older
2×drives, although with Apple SuperDrive mechanisms this can be fixed by means of a firmware update.
This format, used in consumer products, is similar to CD-RW. As with DVD-R, the discs are cheap and easy to obtain.
Unlike other rewritable DVDs, you can move files directly on or off a DVD-RAM disc, without having to use a ‘burn’ process. However, this means that the discs are often incompatible with other types of DVD drive. DVD-RAM works in a similar way to a magneto-optical (MO) disk, but uses a lower power laser to change the phase of the media from transparent to crystalline. It has one particularly unusual feature: a wobbled groove, moulded into the disc during manufacture.
This group of formats are backed by Hewlett-Packard/Compaq, Philips, Mitsubishi, Sony and Pioneer, although the latter also support DVD-R and DVD-RW. As with DVD-RAM, a ‘burn’ process isn’t employed, which means that data can be transferred directly, as in a standard disk drive. DVD+RW normally offers 4.7 GB on each side of a disc and is backwards-compatible with other forms of CD and DVD. However, some DVD drives don’t accept DVD+ discs.
MacWorld magazine (UK), IDG Communications, 2003
©Ray White 2006.