Compatibility issues

The DVD format has been dogged by compatibility problems from the very beginning. Some of these have now been addressed but others, in particular those concerning the rewritable and video variants, persist and look as though they might escalate to become the same scale of issue as the VHS vs Beta format war was for several years in the VCR industry.

Incompatibility with some CD-R and CD-RW discs was an early problem. The dies used in certain of these discs will not reflect the light from DVD-ROM drives properly, rendering them unreadable. For CD-RW media, this problem was easily solved by the MultiRead standard and fitting DVD-ROM drives with dual-wavelength laser assemblies. However, getting DVD-ROM drives to read all CD-R media reliably presented a much bigger problem. The DVD laser has great difficulty reading the CD-R dye because the change in reflectivity of the data at 650nm is quite low, where at 780nm it's nearly the same as CD-ROM media. Also the modulation at 650nm is very low. Designing electronics to address this type of change in reflectivity is extremely hard and can be expensive. By contrast, with CD-RW the signal at 780nm or 650nm is about one quarter that of CD-ROM. This difference can be addressed simply by increasing the gain by about 4x. This is why CD-RW was originally proposed by many companies as the best bridge for written media to DVD from CD technology.

DVD-R Video discs can be played on a DVD-Video player, as well as a computer that is equipped with a DVD-ROM drive, a DVD-compliant MPEG decoder card (or decoder software) and application software that emulates a video player's control functions. A recorded DVD-ROM disc can be read by a computer equipped with a DVD-ROM drive, as well as a computer equipped for DVD video playback as described above. DVD Video components are not necessary, however, if DVD Video material is not accessed or is not present on a disc.

By the autumn of 1998, DVD-ROM drives were still incapable of reading rewritable DVD discs. This incompatibility was finally fixed in so-called 'third generation' drives which began to appear around mid-1999. These included LSI modifications to allow them to read the different physical data layout of DVD-RAM or to respond to the additional headers in the DVD+RW data stream.

Speed was another issue for early DVD-ROM drives. By mid-1997 the best CD-ROM drives were using full CAV to produce higher transfer rates and lower vibration. However, early DVD-ROM drives remained strictly CLV. This was not a problem for DVD discs as their high density allows slower rotational speeds. However, because CLV was also used for reading CD-ROM discs the speed at which a CLV-only DVD-ROM drive could read these was effectively capped at eight-speed.

These issues resulted in a rather slow roll-out of DVD-ROM drives during 1997, there being a six-month gap between the first and second drives to come to market. However, by early 1998 second-generation drives were on the market that were capable of reading CD-R and CD-RW discs and with DVD performance rated at double-speed and CD-ROM performance equivalent to that of a 20-speed CD-ROM drive.

With the early problems solved, the initial trickle of both discs and drives was expected to become a flood since the manufacture of DVD discs is relatively straightforward and titles from games and other image-intensive applications are expected to appear with increasing regularity. However, in 1998 progress was again hampered by the appearance of the rival DIVX format. Fortunately this disappeared from the scene in mid-1999, fuelling hopes that a general switch-over to DVD-based software would occur towards the end of that year, as DVD-ROM drives reached entry-level pricing and began to outsell CD-ROM drives.

The following table summarizes the read/write compatibility of the various formats. Some of the compatibility questions with regard to DVD+RW will remain uncertain until product actually reaches the marketplace. A 'Yes' means that it is usual for the relevant drive unit type to handle the associated disc format, it does not mean that all such units do. A 'No' means that the relevant drive unit type either doesn't or rarely handles the associated disc format:

As of mid-2001, DVD-RAM drives were still able to write to their own media only. This contrasts unfavorably with DVD-RW drives, which are capable of writing to CD-R, CD-RW, DVD-R(G) media as well as to their own DVD-RW discs, and with DVD+RW drives, which are expected to have similar capability.