The string quartets of Be'la Barto'k can make or break a chamber ensemble. Elements of Bach counterpoint, Beethoven thematic development and Debussy sonority fragment or coalesce around folklike melodies. Getting the notes right is an achievement in itself; having four voices play with equanimity amid Barto'k's violent shifts is a feat, one which the Eder String Quartet from Budapest accomplished handily Saturday evening at the Library of Congress.

It didn't hurt that this group shares the same ethnic heritage as the composer. Or that it has recorded all of Barto'k's quartets. In its reading of his Quartet No. 1, Op. 7, it approached sixth-sense status charting a course through the music. Yet when a single instrument performed an expressive melody, as happened with the violin in the opening movement, there was no break in the action.

Collectively, they waited until the final movement to really dig in their heels with fiery bowing, a tactful measure used earlier in Jana'cek's Quartet No. 2 ("Intimate Letters"). There exists a real temptation for emoting in this work, a four-movement billet-doux that captures Jana'cek's intense feelings for a woman 38 years his junior. Heartstrings practically replace the traditional gut variety. The Eder String Quartet never completely lost its head; restraint prevailed at first, even in the initial giddy excitement of love in bloom. They reserved their exclamation points for the finale, in which violinists Pa'l Eder and Erika To'th fanned the romantic flames.

Webern's "Langsamer Satz" (Slow Movement) began the program somewhat inauspiciously. Written under the watchful eye of Schoenberg, Webern's student piece has a bit of Brahms and a bit of his teacher's "Verkla rte Nacht" about it. The quartet's rendition was suavely executed.