'Soldier Girls' is funny, informative; The most engaging current movie turns out to be a documentary; Coppola and high tech

True to his belief that the future of mass communication rests on high technology, Francis Coppola recently participated in a New York demonstration of high-definition television - or HDTV - presented by CBS and the Japanese Broadcasting Corporation. Coppola contributed a short film from his Zoetrope studio, shot with the latest and most finely tuned equipment Hollywood, New York , and Tokyo have to offer. Flashing across the TV screen via 1,125 ''scanning lines'' (conventional TV uses a mere 525) and registering with crystal clarity that's sharper than anything you can get on a standard video setup, the Coppola presentation looked just dandy.

But the flimsy story - a silly little fable about a despondent man and a mermaid - unwittingly raised the big question that media moguls often avoid: Is anyone planning to improve the content of the shows and movies we'll be watching when the latest technological toys have invaded our living rooms? With all the talk from Coppola and others about the revolutionary ''electronic cinema,'' it's high time ideas rather than semiconductors became the most integral part of the mass-communications process.