Pop stars have been donating songs to charities for a long, long time, but we're getting a bumper crop of good-cause collections nowadays. The Band-Aid/Live-Aid/Farm-Aid/"We Are the World" phenomenon has taken root and is bearing fruit in album form.

Big names and new names alike contribute to these sets or perform at concerts from which the music is drawn. The challenge for the album producers is to avoid pulling together too mixed a bag or, vice versa, one too limited in scope and style. The challenge for listeners is to find a project that is at once entertaining and (considering why the albums are being created in the first place) not too depressing.VARIOUS ARTISTS; "Nobody's Child" (Warner Bros.).Maybe you've read or seen the many news stories about the thousands of severely neglected children - some with AIDS - in grossly understaffed Romanian orphanages. "Nobody's Child" aims to raise money to buy medical supplies and food and to improve facilities for those orphans through the Romanian Angel Appeal.

Olivia Harrison, Barbara Bach, Yoko Ono and Linda McCartney - wives and a widow of ex-Beatles - launched this effort. And, with a few notable exceptions, the album's contents definitely lean to pop's British old guard: George Harrison with the Traveling Wilburys and in a duet with Paul Simon; Eric Clapton; Elton John; the Bee Gees; Ringo Starr, etc. The only vocal by a woman comes from Edie Brickell; Guns N' Roses and Billy Idol provide two of the rare contemporary rock entries.

As entertainment, "Nobody's Child" has precipitous ups and downs. The opening title number by the Wilburys, while topical, is surprisingly the weakest song in the set, maybe the least engaging the Wilburys - Harrison, Bob Dylan, Jeff Lynne and Tom Petty - have recorded. Contributions from Clapton and Van Morrison must certainly be leftovers from their recording sessions. And four of the 15 songs in the 64-minute collection were recorded live, with two of the performances derived from NBC's "Saturday Night Live" and "Late Night With David Letterman."

But the gems on "Nobody's Child" are priceless.

Guns N' Roses' "Civil War" opens with a classic line from the Paul Newman film "Cool Hand Luke" - then explodes. A great rock song. Stevie Wonder's solo piano-and-vocal "Feeding Off the Love of the Land" shows he has lost none of his ability to go straight to the head and the heart. And although the sound quality is not outstanding, Paul Simon's duet with Harrison on "Homeward Bound" is a treasure.ROGER WATERS & VARIOUS ARTISTS; "The Wall: Live in Berlin" (Mercury-PolyGram); produced by Nick Griffiths and Roger Waters.

"The Wall" was devised to be performed live, to both generate and tap into the "warm smell of confusion, that space cadet glow," so to speak. This new two-disc set, recorded July 21 during a concert in the one-time no-man's land between East and West Berlin (the site of an even more fabled tumbling wall . . . ), successfully revives the grandiose glory of Pink Floyd's 1980 original.

The tale, about a head-case musician's psychological retreat to isolation and madness behind a "wall" built of hurts, fears and regrets, definitely ranks as one of the rock era's enduring creations. The first album zoomed to No. 1 and generated a surprise chart-topping single in "Another Brick in the Wall, Part II." Next came a strange experimental movie version from director Alan Parker. Now we have this live version, put together by ex-Pink Floyd leader-songwriter Roger Waters on behalf of the Memorial Fund for Disaster Relief.

Besides Waters, the cast reads like an international "Who's Who" of rock. Germany's Scorpions kick things off with a faithful version of "In the Flesh," and Cyndi Lauper, Thomas Dolby, Sinead O'Connor, Joni Mitchell (there's a name out of the past for you), Bryan Adams, Paul Carrack, Van Morrison and flutist James Galway all contribute. Ex-Band members Garth Hudson, Rick Danko and Levon Helm reunite to play prominent instrumental and vocal roles, and "The Trial" sequence features Dolby, Tim Curry ("Rocky Horror's" Frank N Furter), Marianne Faithfull and actor Albert Finney.

The rock-band-for-the-occasion includes guitarists Rick DiFonzo and Snowy White, who perform well . . . but still can't quite erase memories of David Gilmour's soul-penetrating style. In ample support we get, amazingly enough, the East Berlin Radio Orchestra and Choir and the Military Orchestra of the Soviet Army, orchestrated and conducted by arranger and film composer Michael Kamen.

The crowd's roar is nicely modulated throughout this seamless production, while the music, lyrics and Pink Floyd-ish sound effects remain clear in the foreground, giving listeners the participatory feel vital to good live recordings. All of the songs except "The Show Must Go On" survive, most with only mild revisions, if that. Intact are references to Waters' World War II childhood and the loss of his father in the war - and his recurrent anti-fascist warnings, which could have put a chill on things, considering the German setting, but didn't seem to.

In places, the casting choices seem questionable (Sinead O'Connor singing from the male point of view on "Mother"?), but all in all, epically, impeccably performed, "The Wall: Live in Berlin" works very well, thank you.VARIOUS ARTISTS; "Knebworth: The Album" (Polydor-PolyGram); produced by Chris Kimsey and Steve Smith.

Like Live-Aid in 1985, the concert in Knebworth, England, on June 30 offered a televised parade of pop and rock stars performing for a good cause - in this case, two of them, the Nordoff-Robbins Music Therapy Centre for mentally, physically and autistically handicapped children and the new Brit School for Performing Arts in Great Britain. But Live-Aid didn't generate an album, probably because of enormous legal complications. Knebworth has - one clocking in at 2 hours and 17 minutes on CD.

On the tube, if you happened to catch the show, the concert produced many a video memory: Jimmy Page joining ex-Led Zep mate Robert Plant on stage, Paul McCartney as the center of an extended audience-participation version of "Hey Jude" and, with night enveloping the stadium, Pink Floyd spinning its magic amidst sweeping lasers as rain began to fall.

"Knebworth: The Album" has many a highlight as well, although it isn't all great stuff beginning to end. For instance, few of us are likely to complain about getting four songs from Plant, but three from Status Quo, a group barely known outside of Britain? (Unless you remember the original "Pictures of Matchstick Men" from way back in 1968 . . . not performed here.) No one else gets more than two, unless you count Phil Collins' solo and Genesis stints together.

The concert's roster of stars is impressive, and the musicians don't restrict themselves to their biggest hits; instead of a recitation of the familiar, the event proves to be a good-natured rock variety show. Tears for Fears offer "Everybody Wants to Rule the World," and then "Badman's Song" from the "Seeds of Love" album. Plant performs "Hurting Kind" and "Tall Cool One," and also an acoustic "Liar's Dance" and, with Page, "Wearing and Tearing." Collins and Genesis present "Mama" then launch into a showy, spirited medley of '60s pop hits. Dire Straits' one and only number is the rock-bluesy "Think I Love You Too Much," not exactly an extract from the Top 10.

Elton John, McCartney, and David Gilmour and Pink Floyd, stadium concert veterans all, turn in fine live performances, as does Eric Clapton, whose "Sunshine of Your Love" would be one of the album's best . . . if not for an interminable drum solo by Steve Ferrone. Some things worked much better at the actual live event than on the recorded memento. So it goes.