[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. A 1x DVD-ROM drive provides 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 physical spin rate is about 3 times faster than CD (that is, 1x DVD spin ~ 3x CD spin), but most DVD-ROM drives increase motor speed when reading CD-ROMs, achieving 12x or faster performance. A drive listed as "16x/40x" reads a DVD at 16 times normal, or a CD at 40 times normal. DVD-ROM drives are available in 1x, 2x, 4x, 4.8x, 5x, 6x, 8x, 10x, and 16x speeds, although they usually don't achieve sustained transfer at their full rating. The "max" in DVD and CD speed ratings means that the listed speed only applies when reading data at the outer edge of the disc, which moves faster. The average data rate is lower than the max rate. Most 1x DVD-ROM drives have a seek time of 85-200 ms and access time of 90-250 ms. Newer drives have seek times as low as 45 ms.

* "Disc write time" is the approximate theoretical time it takes to write a DVD-5, which doesn't include software overhead, time to write leadout, etc.

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.

Rewritable DVD drives (see 4.3) write at about half their advertised speed when the data verification feature is turned on, which reads each block of data after it is written. Verification is usually on by default in DVD-RAM drives. Turning it off will speed up writing. Whether this endangers your data is a subject of debate. Verification is off in DVD-RW and DVD+RW drives.

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 called 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 called 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 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 of DVD drives is similar to that of CD drives: EIDE (ATAPI), SCSI-2, etc. All DVD drives have audio connections for playing audio CDs. No DVD drives have been announced with their own DVD audio or video outputs (which would require internal audio/video decoding hardware).

Almost all DVD-Video and DVD-ROM discs use the UDF bridge format, which is a combination of the DVD MicroUDF (subset of UDF 1.02) 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.

DVD-ROM drives and DVD recordable drives have an RCA connector or a 4-pin flat (Molex) connector to send analog audio to the audio card in the PC. This is just like the connecter on a CD drive, and in fact it's only for playing audio CDs. The audio from DVDs comes through the computer, not out of the drive. Playing audio from a CD used to require the analog audio output, but most PCs can now play digital audio directly from the CD so the analog connector is not needed.