CARL NIELSEN's name, as well as his music, was virtually unknown here as recently as 30 years ago, and when he was first tentatively "discovered" there was a good deal of doubt as to whether his work was really exportable. It was not until just before the centenary of his birth (1965) that his music was taken up on a meaningful scale by important musicians outside Denmark. By now the Danish master's symphonies and concertos are performed all over the world and his Wind Quintet has established itself as a repertory staple; all these titles are well represented on records, and there are two "integral" recordings of the six symphonies. Nielsen's important contributions in the realms of opera, choral music and th string quartet, however, remain little known or totally unknown outside of Denmark, and it is only quite recently that the operatic and choral gaps in his discography have been filled.

They have ben filled, though, with genuine distinction. Two years ago the English record company Unicorn, in cooperation with the Danish State Radio, brought out the first recording of the first of Nielsen's two operas, Saul and David, a superb production with Boris Christoff as King Saul and the late Jascha Horenstein conducting. Now, from the same source, there is a similarly splendid recording of Maskarade, with an all-Danish cast, conducted by John Frandsen (UN3-75006, three discs).

In contrast to the serious Biblical opera of 1902, Maskarade, introduced four years later, is a light-hearted comedy of manners, based on a play by Lidvig Holberg, the so-called "Moliere of the North." The setting is Copenhagen on a spring day in 1723, the time of the carnivallike masquerade in which class distinctions were suspended so that members of all social strata could mingle. A young man (Leander) rebels at marrying the young woman his father has chosen for him (Leonora), falls in love with an unknown beauty he meets at the masquerade, and eventually discovers that his masked beloved and his father's choice are one and the same. The subplots involving the older generation and the servants add both warmth and spice to what is essentially a celebration of the Danish character in the same goodhearted sense in which The Bartered Bride celebrates that of the Czechs.

The overture, heard in Washington on two occasions in the last year or so, and the dance sections which have turned up on records now and then give a fair idea of the flavor of the work, but the opera in its entirety is a glorious revelation, without a dull page. It seems to combine the most endearing and exhilarating qualities of such seemingly disparate works as Hansel und Gretel, Die Fledermaus and The Marriage of Figaro as well as the Smetana Bride.

Frandsen, whose fine performance of Nielsen's Sinfonia espansiva circulated here briefly on the Epic label in the late '50s, clearly loves Maskarade, and all his associates among the solo singers and members of the Danish State Radio Symphony Orchestra and Chorus just as clearly share his affection for it. The wit, vivacity and joy Nielsen and his librettist Vilhelm Andersen put into the work are realized to a degree that would gladden the heart of any composer, and cannot fail to gladden ours.

Ib Hansen, Gurli Plesner, Tonny Landy, Edith Brodersen, Gert Bastian and Mogans Schmidt Johansen head the cast. Landy's is the only name likely to be at all familiar to American listeners (he is in Antal Dorati's Philips recording of Hayden's La fedelta premiata ), but all participants seem born for their respective roles - and, indeed, have sung them in staged productions. The recording itself, taped last June following a concert performance on the composer's birthday, sounds first rate as mastered adn pressed in this country, and the set comes with a booklet containing helpful essays and photographs as well as a bilingual libretto. You don't have to be Danish, or even an opera-lover, to love Maskarade.

Nielsen's two early choral master works, the Hymaus amoris, Op. 12, and Sovnen ("Sleep"), Op. 18, have been recorded by the venerable Danish conductor Mogens Woldike for EMI, with the same orchestra and chorus heard in Maskarade and with Landy and two of the other singers featured in the opera included among the six soloists in the Hymnus amoris . Angel so far has said nothing about domestic release of this disc, but it may be found in local shops in a fine English pressing (Odeon import ASD-3358, encoded for "SQ" quadraphonic playback).

The "Hymn to Love," with a text devised largely by Nielsen himself and translated into Latin at his request, calls for a children's chorus in addition to the soloists, adult chorus and large orchestra, and is a work in praise of love as felt at four stages of human life. Sleep is a somewhat less amibitious but also quite touching setting of poem by Johannes Jorgensen, for adult chorus and orchestra only. Both works may be grand pantheistic edific that was to rise in Nielsen's symphonies. More to the point, both - the Hymnus in particular - are powerfully and poignantly effective in their own right and deserve the widest exposure. They could not be in better hands.

Deutsche Grammophon has released a record on which a Danish string quartet named for the composer plays the first and last of Nielsen's four string quartets - Op. 13 in G minor and Op. 44 in F (2530,920; also on cassette, 3300.920). They are stunning performances, splendidly recorded, and the other two quartets are sure to follow.