YOU HAVE HEARD Diana Halprin playing the violin, whether you know it or not. If you saw "Carrie" or "Dressed to Kill," you heard her in the sound track. If you have listened to commercials for TWA or Pan-Am or 7-Up, Pepsi Cola, Michelob, Datsun or Ford, her violin was part of the experience.

Diana Halprin made her debut with the Philadelphia Orchestra at age 7 and was hailed by a critic as "a musical miracle." At 18, on the brink of a virtuoso career, she rejected it to get married. At 20, divorced and unemployed, she thought she was "a has-been." Today, at 35, she is a very active free-lance studio musician in New York -- a profession whose top members earn well over $100,000 per year.

Halprin prefers not to give precise figures on her income, which tends to fluctuate. She says that it's a bit less than the magic "six figures," but comfortable enough to let her ease back into the classical career she dropped half a lifetime ago. Friday night, she will be playing the Tchaikovsky Violin Concerto in Caracas, Saturday night a Bartok sonata at the Library of Congress.

The comeback began in December 1979 with a recital in New York, featuring works of such composers as Schubert, Bach and Faure. The New York critics were impressed. "Miss Halprin's playing was first-class from start to finish . . ." one reported. "Intonation, articulation, phrasing -- Miss Halprin had them all thoroughly in hand . . . [her] return to the concert stage is an occasion for rejoicing."

Here is a review she got a few years earlier: "Diana can pump out enough rock-and-roll, blues, and honest-to-God classical music of an evening to render even the most jaded of music freaks limp and groaning in fulfillment." That evening she happened to be playing in a nightclub with a pop group called J. S. Blue.

"To me," says Halprin, "there is no contradiction between having that review in Screw magazine and playing Bartok. People have a way or boxing themselves into little corners, but the world is varied and everything in the world is a continuum. Certainly music is."

Besides blues and rock, commercial jingles, sound tracks and classical sonatas and concertos, Halprin's continuum has included long stretches in the orchestras of both of New York's major opera companies, the City Opera and the Metropolitan, pit-band work for musical comedies and backup work with such singers as Barry Manilow, Joni Mitchell, James Taylor, Roberta Flack, Carly Simon and Harry Belafonte. That may sound like a lot of experience for a 35-year-old, but she has been performing for a long time.

Diana Halprin was not quite born with a violin in her hands, but almost. "My father was a fiddle player," she recalls. "He studied with Leopold Auer, and he started me at 3. They gave me a toy piano when I was 2, discovered that I had perfect pitch, and were very happy, but it took another year for them to find a fiddle that was small enough for me to play. By the time I was 4, I had given more than 50 public performances, at amateur hours and social occasions -- things like the Brahms Hungarian Dances and 'Hatikvah' and 'Gypsy Airs.' When I was 5, they took me to New York to play for Ivan Galamian, and he put me into the Curtis Institute for two years. I auditioned for the Curtis at 5 with the Mendelssohn concerto, with Efrem Zimbalist playing the orchestral part on piano."

After two years at Curtis and her debut with the Philadelphia Orchestra, Galamian took her to the Juilliard prep school in New York. During the next 10 years, he taught her practically every concerto in the classical repertoire. She is able to play nearly 25 concertos with a few days for preparation, but at 18, on the brink of a solo virtuoso career, she married a fellow musician, turned down scholarship offers from Juilliard and Sarah Lawrence and moved with her husband to Kansas City, where she became a teacher at the University of Missouri.

Two years later, divorced and back in New York, she faced the challenge of earning a living. "If you're 20 years old and not active in a virtuoso career -- forget it," she says. "I didn't know the right people or how to go about it, and I didn't have the money to launch a career. I couldn't find a patron. There aren't any patrons anymore for individual artists. If you give to an organization, it's tax-deductible, but not if you help an individual."

So she auditioned for the City Opera, was accepted, worked there for four years and finally quit during a tempestuous rehearsal. "It was a paycheck, but it was a trap," she says. "Then a few years later, the Met trapped me for two years. I worked in the pits of Broadway shows and it was the pits, but it was one way into doing jingles -- radio and television commercials -- which is very pleasant and very rewarding financially."

Her introduction to commercial free-lancing came through a fellow-violinist, Harry Cykman, who became her mentor and now lives with her and lets her use his 18th-century violin for concerts while she shops for one of her own. Cykman, winner of the prestigious Naumberg award, concertmaster at Radio City Music Hall for six years, and a protege of Fritz Kreisler, had been free-lancing in commercials while still doing occasional classical performances and he brought Halprin into that tight little musical world.

Tight is the right word for commercials, which come in two lengths: 28 seconds or 58 seconds, depending on how much air time the advertiser wants to buy. Free-lance musicians are usually hired for one hour, with options for overtime in 20-minute increments. The standard scale is $56 for a one-hour segment, and that allows the advertiser to use the music for 13 weeks in one commercial. Additional payments are sent to the musicians if the same taping is used in another commercial or for a second 13-week period -- and that's when the money starts to look good. "My big blockbuster was a TWA commercial," Halprin says. "I must have made $8,000 to $10,000 on it; maybe more. I didn't count it. After you've been in this business awhile, every week you get from the union lots and lots of little checks, and they add up."

Most of the studios she works in are located on the west side of Manhattan, between 43rd and 57th streets, conveniently close together. "At first," says Halprin, "maybe you do one jingle every two weeks, then maybe 1 1/2 every two weeks. Now, three or four is a really lousy week for me -- a very scary week. The best week I ever had was 16 or 17 jingles, but you can't do that very often -- for one reason, because frequently there are time conflicts. If you're doing two or three dates in a day, it's handy just to run across the street or around the corner to the next studio. There is one studio across town, Howard Schwartz's near Grand Central Station, and for us a trip like that is a schlep, like going to Washington. It's a good studio, though; they serve us fresh pastries and real cream for our coffee, and we love them dearly. You really get to the point where you judge a studio by the quality of its coffee."

Some of the world's most expert musicians work in the New York studios: such classical recording artists as violinist Emanual Vardi and violinist Aaron Rosand. Some have given up jobs that most musicians would yearn for, like Peter Gordon, who formerly played horn with the Boston Symphony. And some are able to combine free-lance work with a regular position, like Fred Slotkin, first cellist of the New York City Ballet.

Sometimes the music is easy -- "a lot of footballs," says Halprin (using the musician's slang term for whole notes), "but other times you walk in and look at the score and it's all over the fiddle. Those arrangers know every trick in the book, and if it's needed, they will use it." The music is always sight-read. There is seldom more than a few days' notice of a recording date and fairly often the call comes on the same day as the session.

Some classical musicians look down on this kind of work and refuse to accept it even when they are having financial problems. "I know one violinist with a very prominent, rising young string quartet in New York," Halprin says. "He and his wife put up a sign in their apartment building, 'Starving musicians will paint your apartment,' but he refuses to do jingles." '

She cannot accept that attitude, not only because she finds the work enjoyable and the technical standards very high, but also because it is a way to survive and she is a survivor.

"Making a living and playing the violin are two very different things," she says. "It's expensive to make a career. Without the financial comfort the jingles give me, I couldn't do it. And I really want to survive, one way or another. I don't intend to cry into a pillow and go into a corner and die."