DVD-RAM is the official format for rewritable DVDs. Unfortunately, there
are competing formats called things like "DVD-RW", "DVD+RW", and "+RW" which some
manufacturers have promoted. This has lead to a lot of confusion and
mis-information about DVD-RAM; this article attempts to outline the basic
facts.

Physical Properties

DVD-RAM drives generally fit in the same slot as a DVD-ROM or CD-ROM drive.
They are also available as external units, connected via USB, Firewire or
SCSI.

DVD-RAM discs are currently available in three formats -- single-sided,
holding 2.6GB; double-sided holding 2.6GB per side for a total of 5.2GB; or double-sided holding 4.7GB per side for a total of 9.4GB. The
discs are supplied in cartridges, which protect them from dust and
clumsiness.

Currently, all DVD-RAM discs must be inside their cartridges to rewrite
them. However, you can remove a single-sided DVD-RAM disc from its cartridge
and then read it in a DVD-ROM drive. Once you remove it, however, the DVD-RAM
drive will be more careful when using it, leading to slower rewrite speeds.

Because the DVD-RAM drive needs to accept discs in cartridges as well as discs
without cartridges, it doesn't use a normal tray like a DVD-ROM drive; instead,
it has two protruding 'fingers' which you slide the disc (or cartridge)
between. The fingers grip the disc by its edges, and then draw it into the
drive mechanism.

Compatibility

DVD-RAM drives will read CD, CD-R, CD-RW, DVD-ROM, audio CD, DVD Video, CDi
and PhotoCD discs. However, they won't read CDs which have unusual shapes --
such as rectangular "business card" CDs. This is because the discs can't be
reliably gripped by the 'fingers' which load and unload the drive.

5.2GB and 9.4GB DVD-RAM discs are only readable in DVD-RAM drives. 2.6GB DVD-RAM
discs can also be read in recent DVD-ROM drives (including any drive labelled
"Multi-Read" or "DVD Multi").

DVD-RAM drives cannot burn CD-R or CD-RW discs. The technology used to
write the data is fundamentally different. Some manufacturers are
experimenting with drives that use two separate lasers to allow them to write
both kinds of disc.

Today, you cannot copy DVD movies onto a DVD-RAM disc and play them back on
a home DVD player or computer. It may become technically possible when
"DVD Multi" drives start being used in home DVD players and computers.
However, it may still not be possible because of CSS and other protection
mechanisms used by the movie industry to control what equipment you can use to
watch the movies you buy.

You can, of course, copy your own digital movies in DV format to DVD-RAM,
and play them back straight from the DVD on your computer. (Yes, the format is
fast enough to do that.)

A computer with a DVD-RAM drive can boot from DVD-RAM, DVD-ROM or CD, if the
operating system allows it. Microsoft is working on native DVD-RAM support for
Windows; it's already supported on the Mac.

Using DVD-RAM

DVD-RAM works like a floppy drive. A really big floppy drive. First, you
insert the disc and format it. Formatting takes only a few
seconds, because DVD-RAM has a certain amount of formatting engraved into the
physical surface of the disc. (You can see this if you gently slide the window
of the cartridge open.)

Once the disc is formatted, you can drag and drop files onto it, just like a
floppy. Write and rewrite speeds vary by drive and operating system; as
of Q1 2000, on an Apple PowerMac G4 you get about 1MB/sec. This is slow, but
comparable to CD-RW or Zip. Read speeds on the same machine are the same as
for a 20x CD-ROM or 2x DVD-ROM.

It's important to note that there's no "burn" process, and you can erase
just a few files and then re-use that space. You can also leave the files
copying and do something else on the computer at the same time -- there's no
danger of "buffer underrun" turning the disc into a coaster. This makes
DVD-RAM far more convenient than CD-RW or the forthcoming DVD-RW.

DVD-RAM discs can be rewritten over 100,000 times. They are expected to
last for decades -- estimates from manufacturers vary between 20 and 100
years, depending on what kind of environment you store them in. Discs are likely to last longer if they are not erased and
rewritten, and if they're left in the cartridge.

Because DVD-RAM is an open standard, the discs tend to be cheap when
measured in price-per-megabyte.

Competing technologies

The DVD-RAM standard is finalized and approved by an international open
standards body, The DVD Forum.

The standard for eraseable writeable DVD, DVD-RW, has recently been
finalized. Drives and discs will likely become affordable and readily
available in 2001, and will have a capacity of 4.7GB. Like CD-RW, DVD-RW will
require packet writing or disc burning software. Like CD players which fail to
read CD-RW discs because of the lower reflectivity used, many DVD drives and
DVD players will be unable to read DVD-RW discs. DVD-RW drives still won't
write CD-R or CD-RW discs. It's also worth noting that DVD-RW will have half the record speed of DVD-RAM, and the discs will only be rewritable 1,000 times.

No other standard for fully rewritable DVD is finalized as of Q1 2001.
There is, however, a well-established standard for write-once DVD, called
DVD-R. Drives and mastering software for DVD-R currently cost many hundreds of US
dollars.

Sony, Philips and some other manufacturers promoted a format they
initially called "DVD+RW", subsequently renamed "+RW" for legal reasons. +RW
has a capacity of 3GB per side, with no plans for increasing capacity. No
currently shipping CD or DVD drive of any kind can read +RW discs. In addition,
+RW requires that you "burn" discs or use packet-writing software, like CD-RW.
Sony have recently abandoned plans to make +RW drives, but HP has sold a few.
+RW drives don't write CD-R or CD-RW either.

In summary: DVD-RAM is the only open standard, multi-gigabyte capacity DVD
format that's as convenient to use as a floppy disk, and compatible with the
DVD-ROM drives available since 1999. It's great for computer data storage, but
not for making discs for your DVD player.