Digital Versatile Disc (DVD)

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 7½ 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:-

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).

Capacity

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.

• A recordable 4.7 GB single-sided disc is sometimes known as a General Type 2 Disc.

Compatibility

Any kind of DVD drive should read all types of CD, as well as the DVDs shown in the following table:-

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.

• Many modern computers have a DVD/CD-RW drive, sometimes known as a Combo drive, that reads DVDs and can write to CD-RW discs, although a DVD-R/CD-RW drive (also called a SuperDrive in Apple computers) is preferable. A drive that records to both DVD-R and DVD+R discs is known as a DVD±R drive.
• A DVD-Video recorder contains a DVD-RAM, DVD-RW or DVD+RW drive, again posing compatibility problems, although standard DVD-R and DVD-RW discs aren’t a problem.
• Only a modern DVD±RW drive can write to 8.5 GB dual-layer discs. Older types, including those fitted in currently-manufactured Mac OS machines, can only record onto single-layer discs.

High Density DVD (HD-DVD)

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.

Blue-ray Disc (BD)

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.

DVD Drive Hardware

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.

• The slot-loading mechanism in many modern machines only usually accepts standard 3.1 inch or 4.7 inch discs. Other ‘novelty’ sizes of disc may fail to work or may damage the drive.
• Excessive noise sometimes occurs if the disc’s screen-printed label results in an uneven weight distribution, although this rarely causes any harm to the drive or the disc.

Speed

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.

Matching Discs to Drives

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.

• Before buying a DVD drive, always check what kinds of discs it accepts.
• Modern machines employ special software to play DVD-Video discs, usually with the assistance of a suitable video card in the computer. In some instances, an older machine can also be made to work by installing a DVD card.

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.

• A drive whose speed is less than 12× normally requires discs to be inserted in caddy before they can be played.

Drive Connections

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.

• External DVD-R drives aren’t recognised by the iDVD or Apple DVD Player applications. In addition, such drives, when connected to a machine that lacks an internal DVD drive, can’t be used to play commercial DVDs.

DVD Types

As with Compact Disc, there are several types of DVD, as described below.

DVD-Audio (DVD-A)

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.

• Dolby’s MLP Lossless system can be used to compact PCM data without any loss of data, thereby doubling the available space on a DVD-A disc.

DVD-Video (DVD-V)

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.

• Some DVD players contain a Dolby Digital 5.1 decoder, with audio outputs for surround sound loudspeakers, whilst most models have S/PDIF socket for connecting an external home cinema decoder-amplifier. The DTS Digital Out variation of this connector can also be used with an external DTS-compatible decoder.
• When playing DVD-Video discs on some computers the picture gets out of step with the sound, an effect known as loss of lip-sync, which is usually caused by failings in the decoding software. In the Classic Mac OS this can be minimised by updating the system or by turning off background processes such as AppleTalk or virtual memory (VM).
• A proposed form of self-destruct DVD-Video disc, known as an ez-D, can be used instead of renting a VCR tape. Such discs contain a basic movie without ‘extra content’ and have a red coating that goes black 48 days after leaving the protective packaging, thereby preventing further use. A clever idea, although not very good for the environment.
• DVD-Video discs can also be used to present a slide show on a standard DVD-Video player. Apple’s iDVD application can be used to prepare such a disc: if you select Always Add Original Slideshow Photos to DVD under the Slideshow section of Preferences it puts the original image files onto the DVD in a folder called DVD-ROM Contents.

DVD-Read Only Memory (DVD-ROM)

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.

• A standard DVD-ROM drive reads most types of CD or DVD. However, some models can’t accept DVD+RW discs, or, in some instances DVD-RAM discs (see below). It’s been said that some DVD-ROM drives can’t play CD-R discs, since the latter require a longer wavelength laser, although this doesn’t seem to be the case with modern drives.
• DVD uses the universal disk format (UDF), as supported by Mac OS 8.1 or higher.

DVD-Recordable (DVD-R)

DVD-R uses dye technology, very similar to that used in CD-R, which means that discs are cheap and readily available.

• A dual-layer disc can’t be used to for recording in an older drive.
• Most DVD-R drives play standard DVD-ROM discs, although there can be problems with double-sided discs.
• Many DVD-R drives record at 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.

DVD-ReWritable (DVD-RW)

This format, used in consumer products, is similar to CD-RW. As with DVD-R, the discs are cheap and easy to obtain.

• Some types of DVD-ROM drive may refuse to read this kind of disc.
• The rate for recording DVD-RW discs is sometimes limited to 2× speed.

DVD-Random Access Memory (DVD-RAM)

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.

• The number or read/write cycles to a DVD-RAM disc is limited by the long-term effect of a laser heating up the disc, although it can still be used up to 100,000 times.
• Most DVD-RAM drives can also record DVD-R and CD-R discs.
• When used in a DVD-Video recorder, a DVD-RAM disc can store up to 12 hours of video.

DVD Plus (DVD+R/RW/RAM)

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.

• The term DVD±RW is sometimes used to refer to drives that work with both DVD-RW and DVD+RW discs.
• DVD+ discs aren’t supported by Mac OS 9.x or by older versions of Mac OS X, although these discs can be used as a backup medium in Mac OS X 10.3. Fortunately, most current computer drives can’t ‘burn’ DVD+ discs anyway.

Reference

MacWorld magazine (UK), IDG Communications, 2003

©Ray White 2006.