When Judy Collins sang "Who Knows Where the Time Goes?" in 1968, her bright voice brought out the song's pretty melody and sad lyrics. When Sandy Denny sang the same song, she brought out something more than prettiness or sadness: a haunting sense of wanting to stop time and helplessly realizing that one can't. Denny's voice wasn't as shiny on the surface as Collins', but her smoky alto contained swirling undercurrents Collins never hinted at.

Denny also wrote the song, the best known product of a troubled and undervalued but nonetheless rich career. Of all the female singer-songwriters in folk-rock's first generation, only Denny can be considered Joni Mitchell's peer. Best remembered for her crucial contributions to Fairport Convention, Denny also recorded a series of solo albums and led a number of short-lived bands before her accidental death at age 31 in 1978. Unfortunately, most of those records are out-of-print and hard to find.

Denny's widower, Trevor Lucas, and Fairport producer Joe Boyd have filled this vacuum with a lavish four-record box set that gathers Denny's last songs, rarest recordings, unheard studio tracks and unreleased live performances from 1967 through 1977. The set, "Who Knows Where the Time Goes?" (Hannibal HNDX 5301), makes a persuasive case that Denny was a major artist.

Denny was part of London's mid-'60s Bohemia, hanging out with John Renbourn and Paul Simon.

The scene then was abuzz over the example of Bob Dylan and the Byrds, who had adapted American folk roots to rock 'n' roll. For a couple of years, everyone was playing Dylan tunes and other American songs before they realized they could adapt British folk roots much the same way.

Denny was a folk club regular and worked with the Strawbs when Richard Thompson and Simon Nicol, a couple of Byrds fanatics, invited her to join their fledgling band, Fairport Convention, in 1968. When the band turned toward a more traditional, more British sound, Denny's vocals and writing had the sense of history and mysticism to make it work.

After three classic albums in a year and a half with Fairport, Denny left to form a new band with her Australian boyfriend, Trevor Lucas. Named after a Fairport song, Fotheringay was an extension of the Fairport sound, with a sharper focus on Denny. They recorded one excellent album and began a second before breaking up. Denny then sandwiched four solo albums around an oldies revival band called the Bunch and a brief reunion with Fairport.

The box set provides just a taste of her Fairport work, assuming quite sensibly that you should already own those three classic albums.

As a result, the new anthology doesn't really work as a "best of" collection; it omits such gems as "Come All Ye," "Fotheringay" and "I'll Keep It With Mine," one of the best Dylan covers ever recorded. Also missing is Denny's raging duet with Robert Plant on Led Zeppelin's "Battle of Evermore."

The set does a much better job of fleshing out the rest of her career. Denny's pre-Fairport folk career is represented by a couple of nicely understated BBC live recordings. The unrealized potential of Fotheringay is suggested by four songs from their only album and three songs from their unreleased second album, plus two live performances.

When Denny died, her popularity had declined dramatically, but her talent was intact. That is obvious from the three songs representing her last album, 1977's "Rendezvous," which is now almost impossible to find. The best of those songs, "One Way Donkey Ride," employs Steve Winwood's piano and Denny's pensive vocal to ponder both the poor and herself without pity.

Also included are six demos for songs she wrote between 1972 and '76 but never recorded. "The Music Weaver" is a fond tribute to Richard Thompson; "What Is True?" is a desperate grasping for an elusive love; "Full Moon" is one of the most grandly romantic songs she ever wrote.

Ultimately, all of Denny's best songs ask the question, where has the time gone? Denny was never interested in nostalgia but in the difficulty of establishing firm connections in a world that is always slipping away from us. Because she pursued this dilemma so honestly and so passionately, her music will outlast that of singers with better voices and songwriters with more clever lines.

Simon Nicol still holds Fairport Convention together 19 years after he cofounded the band. Nicol has always been overshadowed by larger talents such as Denny, Thompson and instrumental wiz Dave Swarbrick, but on Fairport's newest album, "Gladys' Leap" (Varrick, VR-023), Nicol steps out from those shadows to display his modest but valuable gifts. He has a deep, deadpan voice that hums with the droll and fateful irony of Fairport's best songs.

Three of the eight songs were written or cowritten by Ralph McTell, who as a singer is often too earnest for his own good. Nicol's understated delivery allows the lovely melodies and sly irony of McTell's songs to emerge on their own. The two songs about injustice in olden times and especially the gorgeous Irish folk tune "Bird From the Mountain" rank with the best numbers McTell has ever written.

The Albion Band's Cathy Lesurf, who sings Denny's songs at the annual Fairport reunion, lends a guest vocal on the jaunty "My Feet Are Set for Dancing." Fiddler Ric Sanders fills in for the absent Swarbrick, and bassist Dave Pegg and drummer Dave Mattacks provide the firm but unobtrusive folk-rock rhythm. All in all, this is one of the best Fairport albums since Denny and Thompson left the band.