Sony Corp. says it will introduce its first HD Radio products in July, joining the growing group of companies seeking to make the next-generation digital radio technology a standard feature in audio products over the next several years. HD Radio is new form of digital radio broadcasting that allows radio stations to deliver extra music content on up to four side channels that piggyback on the frequency it already uses. So far, more than 1,200 radio stations in the United States have adopted the technology, according to the HD Digital Radio Alliance, a group of broadcasters promoting the new format. A listener, however, needs a special radio receiver to hear HD Radio signals, which also boasts improved sound quality. About 50 models are available already, often sold as part of a car stereo or home audio component, according to the alliance.