Planning the Digital Smorgasbord
First article in weekly series The Media Giants, examining business strategies of major media conglomerates, focuses on AOL Time Warner; chief executive Gerald M Levin outlines plans to deliver media smorgasbord to every home in nation, using subscription-based, on-demand approach, interview; company stands alone among media rivals because it controls major Internet service and major cable television operation; chairman Stephen M Case cites plan to knit together PC, TV, phone and stereo, interview; co-chief operating officer Richard D Parsons says goal is to create new industries; graphs of company's, stock and financial data; tables comparing company's roles in various media industries; photo (L)