LONG AND MERCIFULLY gone are the early days of what used to be called "educational television," when a flick of the UHF snob-knob would get you black-and-white flickering lectures on "Insects of the Amazon" and "Experimental Urdu." Today, public television has shown itself to be both professional and popular -- with creative, lively programming and often strong financial support from it's local viewers. But now the combination of inflation and the cable television revolution are posing tough questions for public broadcasting. If public television is going to stay alive and well, the industry will have to rethink not only its financial structure, but also the relationship of its programming to new technologies.

The immediate threat is the financial one: production costs are outstripping revenues at many individual public TV stations. Corporate contributions have either leveled off or dropped, and congressional appropriations are falling short of the inflation rate. Though Channel 26 here has managed relatively well with its modest annual budget, other key stations in the Public Broadcasting Service system have had to drop programs and staff to make ends meet.

Competition from cable television is taking its toll on public broadcasting too. Corporations as well as the established commercial networks can play to specialized groups quite profitably on cable as well as on video cassettes and discs. For example, a big firm that may have been underwriting a public television program might now prefer to team up with a commercial network for the exclusive cable rights to all symphony concerts in a city, or all basketball games. If so, wouldn't this accomplish what public broadcasting was established to do?

For viewers who can pay for it, yes; but a significant aspect of public television -- worth preserving -- is the one public access it offers to cultural and other special programming. To that end, public television can, and should, seize upon the new technologies to generate new sources of money. Some stations already are selling certain program series -- Julia Child's is one -- on discs and cassettes. Others are studying ways to lease out certain broadcast time for commercial entertainment services.

One of the most ambitious ideas comes from PBS President Lawrence Grossman, who has proposed the formation of a "Grand Alliance" between the country's most-respected cultural institutions and PBS that could take advantage of the cable, cassettes and discs. Joint investments could produce a steady supply of concerts, plays and special-audience documentaries that not only could appear on PBS and cable, but could be sold in cassettes. While it's not clear just how swiftly the cultural institutions may leap to join this venture, this is the sort of imaginative thinking on which any successful future for public broadcasting will depend.