When the leading man sweeps the heroine into his arms, that may not be a dozen weeping violins you hear.
It may be a synthesizer - an electronic patchwork of keys, wires, knobs, buttons, and switches, bearing a name like Polymoog or Synclavier II.
It can imitate any instrument in the orchestra. It can sound like Artoo-Detoo conversing with E.T. It can make noises nobody has heard before.
And it's making a big dent in the movie-score monopoly once held by strings, woodwinds, and other traditional mainstays.
Since 1978, two synthesized scores have won Academy Awards - Vangelis's music for ''Chariots of Fire'' and Giorgio Morodor's for ''Midnight Express.'' Other recent synthesizer movies include the James Caan thriller ''Thief'' and two Paul Schrader dramas, ''Cat People'' and ''American Gigolo.'' Films with rock-music scores, such as ''Saturday Night Fever'' and ''Can't Stop the Music,'' also lean toward electronically oriented sound tracks.
Broadly speaking, synthesized sounds fit best in movies with outlandish atmospheres, such as melodramas and fantasies from ''Quadrophenia'' to ''The Exorcist.'' In science fiction, electronic music has seemed at home since the theremin spiced space operas of the 1950s, and Alfred Hitchcock opened new doors by using electronic sound effects in his bizarre fable ''The Birds.''
Today, some established composers avoid the synthesizer, even when setting an other worldly mood. According to film-music expert Terry Atkinson in American Film magazine, this is why such sci-fi outings as ''E.T.'' (scored by John Williams) and ''The Thing'' (scored by Ennio Morricone) feature normal orchestras instead. Yet, says Mr. Atkinson, the trend hasn't faded. ''Blade Runner'' sports music by Vangelis, and James Horner included synthesizers in his 88-piece orchestra for ''Star Trek: The Wrath of Khan.''
Meanwhile, the ''synth sound'' continues to spread toward new territory, showing up where it might be least expected. Perhaps the boldest experiment in ''Chariots of Fire'' is its integration of artificially generated music with a period story that would normally feature traditional instruments. It worked, leading many critics to praise the score for joining with the editing in climaxes that might otherwise have been far less effective. And it worked in record stores, where the ''Chariots'' score became a best selling album.
The synthesizer may have its greatest impact, however, as a behind-the-scenes tool for composers. Francis Ford Coppola, director of such dramas as ''The Godfather'' and ''Apocalypse Now,'' maintains that filmmakers will soon have electronic ''imaging'' devices at their command, enabling them to experiment with scenes and performances - and blueprint entire movies to the smallest detail - before shooting a single foot of film. Musicians already have such gadgetry in hand, as Atkinson points out, allowing symphony-length scores to be planned and ''auditioned'' in advance of actual recording. The finished product can then be played by a normal orchestra, if desired, or generated by the synthesizer itself.
If such films as ''Chariots of Fire'' and ''Midnight Express'' are any indication, synth music has moved far beyond the novelty status it had in the days of ''A Clockwork Orange,'' featuring electronic Beethoven arranged by Wendy Carlos, and such extravaganzas as ''Tommy'' and ''Lisztomania,'' both made by Ken Russell. Still, some listeners are concerned about overuse of electronics. And some composers agree, especially when cheap ''pop'' effects are the end result. Even synthesizer wizard Morodor is quoted by American Film as detesting ''that oing-boing that a lot of rock bands use.''
Similarly, filmmaker George Miller (director of ''The Road Warrior'') inveighs against rock scores in the current issue of Film Comment, suggesting that movies are ''visual rock 'n' roll, in a way'' but that actual rock sound tracks have not succeeded ''in a single case.'' Accordingly, the ''Road Warrior'' music has more of an ''establishment'' ring than any other element in the picture. Meanwhile, such a forward-looking composer as Steve Reich, whose music was heard in the PBS series ''Cosmos,'' rejects electronic musicmaking devices entirely, preferring the warmth and immediacy of a player in direct contact with an instrument.
Given such lingering conservatism, it seems unlikely that electronic sounds will take over Hollywood. But given two Oscars for synth scores in the past four years, there's sure to be plenty more movie music featuring the kinds of instruments listed in the credits for the ''Cat People'' sound track: Minimoog, Jupiter 8, drum machine, wind noises, Prophet 5, blaster beam. . . .