You don't have to look very hard to find admirable attributes in Diane Delin's violin work. On an instrument notorious for its problematic intonation, she maintains a strong, pure tone, even when improvising. In other words, even when making an instantaneous decision about which note will come next, she will still play it in tune. She swings lightly but surely at quick tempos and, more impressive, on ballads. She varies her phrasing and, when she lands herself in some melodic rut (an occupational hazard of improvisation), she scurries out of it almost as quickly. She has borrowed not only from the first and most expectable jazz violinists but also the more recent ones, such as Jean-Luc Ponty and John Blake, fitting their innovations into her own style without mimicking them. Delin has one serious problem, though: she doesn't get to play nearly as much pure jazz as she'd like--not even on her self-produced album of last year, Another Morning, on which she spent some of her time in fusion land in hopes of gaining some airplay. Delin's residency in Chicago--and the low profile she unwillingly keeps--raises an intriguing question: Just how many top-flight jazz violinists can a city this size support? The deserving Johnny Frigo, who sounds better than ever at 79 (though he's briefly out of commission after getting hit by a van while crossing the street), gets most of the attention; meanwhile, his heir apparent performs just a couple of real jazz gigs a year--which makes her appearance this week all the more a must. Friday and Saturday, 8:30 PM, Metropole Lounge, Fairmont Hotel, Illinois Center, 200 N. Columbus; 565-7444.