SOME 40 years ago Rudolf Serkin made a beautiful recording of Beethoven's Fifth Piano Concerto, the so-called "Emperor," with Bruno Walter and the New York Philharmonic. In the early years of microgroove he made a similarly beautiful one with Eugene Ormandy and the Philadelphia Orchestra, and still later, yet another, in stereo, with the Philharmonic again, under Leonard Bernstein. Last January, just two months shy of his 78th birthday, Serkin recorded his fourth "Emperor," this time digitally, with Seiji Ozawa and the Boston Symphony Orchestra, and it is surely his best yet, musically as well as sonically (Telarc 10065).

The new "Emperor" seems to have an aura about it, as if Serkin had been able to go back over the last four decades and combine all the outstanding strengths of his various performances of the work. The solo playing is vigorous and assured, tempered by an unselfconscious dignity that is the strongest justification for the sobriquet applied to this Concerto. His collaboration with Ozawa and the Bostonians (who, incidentally, recorded a splendid "Emperor" with Christopher Eschenbach a few years ago) is a fully integrated partnership, with the sort of give-and-take that does more than merely sustain momentum.

With this release Telarc adds the Boston Symphony to the impressive roster of first-rate American orchestras it has been recording in standard repertory works. There is no blockbuster emphasis here, but simply the most effective use of the Soundstream digital process for a recording that preserves the most lifelike sound in the most natural balance. There are more than a few fine recordings of the Concerto that cost less than the $18 asked for this one, but I can't imagine anyone who makes the investment feeling he didn't get his money's worth.

The newest recording of Mahler's First Symphony, surprisingly, is not a digital one and, perhaps more surprisingly still, is a performance by a French orchestra. Lorin Maazel conducts the Orchestre National de France, of which he is music director, on CBS M 35886. It is a straightforward, well-paced reading, appropriate to the youthful vigor of the work and recalling the way no less a Mahlerian than Bruno Walter used to take it in that respect.

The next recording of this work to reach us, by the way, will be digital, and will again be under an American conductor; Telarc has in preparation a Mahler First played by the Saint Louis Symphony Orchestra under Leonard Slatkin, which ought to be turning up in a month or two.

In the meantime, an utterly different sort of symphony is represented on a disc in Philip's "Living Baroque" series: four little symphonies composed by Frederick the Great, played by the Pro Arte Orchestra of Munich under Kurt Redel (9502.057). These three-movement symphonies might easily be taken for works by the King's tutor, Carl Heinrich Graun, by those who claim any familiarity with that composer's music, or taken for Carl Phillipp Emanuel Bach by the rest of us.

No. 3 in D major, the only one for the four symphonies calling for flutes, oboes and horns as well as strings (and apparently the only one that has been recorded before), opens with what sounds like an echo of Emanuel Bach's Hamburg symphonies -- but we'd better make that "pre-echo." Frederick composed his symphonies in or about 1732; Emanuel was only 18 then, didn't reach Berlin until 1738, and didn't enter the King's service until 1740. So we might consider that C. P. E. Bach was really influenced, as a symphonist, by his royal employer.

In any event, these little symphonies are handsome and eminently worth listening to.