4.2 What are the features and speeds of DVD-ROM drives?
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This article is from the DVD Formats FAQ, by jtfrog@usa.net (Jim Taylor) with numerous contributions by others.
4.2 What are the features and speeds of DVD-ROM drives?
Unlike CD-ROM drives, which took years to move up to 2x, 3x, and faster
spin rates, faster DVD-ROM drives began appearing in the first year. Most
1x DVD-ROM drives have a seek time of 90-200 ms and access time of 100-250
ms. 1x DVD-ROM drives provide a data transfer rate of 1.321 MB/s
(11.08*10^6/8/2^20) with burst transfer rates of up to 12 MB/s or higher.
The data transfer rate from a DVD-ROM disc at 1x speed is roughly
equivalent to a 9x CD-ROM drive (1x CD-ROM data transfer rate is 150 KB/s,
or 0.146 MB/s). DVD spin rate is about 3 times faster than CD (that is, 1x
DVD ~ 3x CD), but almost all DVD-ROM drives increase motor speed when
reading CD-ROMs, achieving 12x or faster performance. 2x DVD-ROM drives are
available (providing a transfer rate of 22.2 Mbps or 2.6 MB/s from DVDs,
equivalent to an 18x CD-ROM rate). Most 2x DVD-ROM drives read CD-ROMs at
20x (max) speeds and higher. 4x, 4.8x, 5x, 6x, 8x, and 10x drives are also
available, although they usually don't achieve a sustained transfer rate at
their full rating. A 5x drive can theoretically transfer data at 55.4 Mbps
or 6.4 MB/s, equivalent to a 45x CD-ROM data rate. Most 4x and faster
DVD-ROM drives read CD-ROMs at 32x (max).
The bigger the cache (memory buffer) in a DVD-ROM drive, the faster it can
supply data to the computer. This is useful primarily for data, not video.
It may reduce or eliminate the pause during layer changes, but has no
effect on video quality.
In order to maintain constant linear density, typical CD-ROM and DVD-ROM
drives spin the disc more slowly when reading near the outside where there
is more physical surface in each track. (This is CLV, constant linear
velocity.) Some faster drives keep the rotational speed constant and use a
buffer to deal with the differences in data readout speed. (This is CAV,
constant angular velocity.) In CAV drives, the data is read fastest at the
outside of the disc, which is why specifications often list "max speed."
Note: When playing movies, a fast DVD-ROM drive gains you nothing more than
possibly smoother scanning and faster searching. Speeds above 1x do not
improve video quality from DVD-Video discs. Higher speeds only make a
difference when reading computer data, such as when playing a multimedia
game or when using a database.
Connectivity is similar to that of CD-ROM drives: EIDE (ATAPI), SCSI-2,
etc. All DVD-ROM drives have audio connections for playing audio CDs. No
DVD-ROM drives have been announced with DVD audio or video outputs (which
would require internal audio/video decoding hardware). In order to hook a
DVD-ROM PC to a television and a stereo receiver, the decoder card or the
video card must have a TV video output and an audio output. Some cards have
SP/DIF outputs to connect to digital audio receivers. If there's no video
output, a TV scan converter can be connected to the VGA output.
Almost all DVD-Video and DVD-ROM discs use the UDF Bridge format, which is
a combination of the DVD MicroUDF and ISO 9660 file systems. The OSTA UDF
file system will eventually replace the ISO 9660 system originally designed
for CD-ROMs, but the bridge format provides backwards compatibility until
more operating systems support UDF.
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