TOKYO — Sony Corp. and a group of five other companies announced Thursday the specifications for a rewritable optical disc, which is designed to offer greater compatibility and memory than a rewritable digital videodisc (DVD) format backed by the international consortium that developed DVD technology.Sony, along with Hewlett-Packard Co., Philips Electronics N.V., Mitsubishi Chemical Corp., Ricoh Co. Ltd. and Yamaha Corp. jointly announced specifications for what they call the “phase-change rewritable” 120mm optical disc. The new disc format will be for products known as DVD+RW drives. The disc has an initial storage capacity of 3.0 gigabytes per side, which is greater than the 2.6 GB per side for the DVD random access memory (DVD-RAM) format reached by the 10-member DVD Forum. In a press release, the companies said their disc allows DVD read only memory (ROM) drives to read DVD+RW discs “at virtually no additional cost.” “The six companies are widely recognized as industry leaders in CD-recordable and CD-rewritable technology and represent a combined share of more than 75% of the drive market,” the release said. The disc was developed by HP, Sony and Philips. DVDs are being billed as replacements for videocassettes, CDs and CD-ROMS because they offer much greater storage capacity. DVDs also promise to be a major consumer item for electronics companies. Japanese industry estimates say that by 2000 the market for DVDs should be about $25.6 billion, with rewritable DVDs accounting for 20% to 30% of the total. An international standards body called the European Computer Manufacturers Assn. has received specifications on the new disc proposed by the six companies and the DVD-RAM format reached by the DVD-Forum. The two formats are not compatible. Many industry observers liken the format war to the battle over Beta and VHS formats for videocassettes.