REGGAE USED to be defined simply as a beat, in much the same way as were calypso, samba and cumbia. But as the music and its performers continue to grow, reggae is reflecting as wide a variety of interests and influences as any contemporary pop genre.

Take, for example, UB 40. They are the most popular reggae band in their native Britain, in part thanks to their sure-handed skank, but mostly because they know how to augment that traditional reggae afterbeat with rhythmic touches borrowed from funk and disco.

"Nkomo a Go Go," the brashly propulsive instrumental from their new album, "Geffrey Morgan," is a case in point. Although the melodic focus is a ska-styled horn line, the groove is supplemented by dub bass, throbbing Afro-pop percussion and the latest in drum machine technology.

It's a heady blend, but UB 40's pop instincts invariably pull the music's diverse energies together. Whether using sweetly harmonied vocals for contrast against the ominous pulse of "Riddle Me," or coaxing a buoyant melody out of a toast like "You're Not an Army," they understand the rules of both pop and reggae, and never undercut one for the sake of the other. Nor do they skimp on content; the best of the songs here tackle tough issues with an impressive combination of clarity and common sense.

Social commentary is nothing new in reggae, of course, but an increasing number of reggae artists are learning to voice their opinions in terms that carry weight outside the realm of Jah Rastafari. Pablo Moses' "Tension," for example, attacks such problems as international conflict and crime in the streets without once dismissing all such evils as the inescapable tolls of Babylon. That's not to say that Moses doesn't believe in the righteousness of Rastafari -- "Watch Out" makes that much plain -- but neither does he apply it as a cure-all.

Better still, Moses has no trouble parlaying his serious subjects into lighthearted numbers, as in "Bomb the Nation," which promises to "bomb them with reggae nitro." Given the explosive power of his backing band, that's no small threat.

Still, the most persistent growth in reggae has been in the area of lovers' rock, the music's sweetest, most soulful side, and that's where Don Carlos is currently making his mark. Because his voice has more of an edge than most crooners', he's capable of a far wider range of expression than most of his competition. As a result, the songs on "Just a Passing Glance," his latest album, carry far more weight than their melodies alone might have, from the charismatic title track to the romantic lament "I Just Can't Stop." UB 40 -- "Geffrey Morgan" (A&M SP 5033); PABLO MOSES -- "Tension" (Alligator AL 8311); UB 40 and Moses appear Saturday at 8 at the Warner. DON CARLOS -- "Just a Passing Glance" (RAS 3008); appearing Friday at Kilimanjaro's Heritage Hall with the Itals and Roots Radics.