Holiday pops arrived at Heinz Hall on Thursday in a collaborative concert by the River City Brass Band and the Bach Choir of Pittsburgh. Both groups specialize in playing different venues, but performing in Heinz Hall is relatively new for each and is a significant indication of the two groups' growth as musical entities.

RCBB music director Denis Colwell and Bach Choir artistic director and conductor Thomas Douglas shared the conducting duties -- in one instance, passing the baton in the middle of an audience sing-along. In doing so, they displayed an uncanny but utterly coincidental similarity of stick technique: relaxed and graceful, yet always communicative and expressive.

On Thursday, the downsized contingent of singers was insufficiently miked. Diction and sometimes the entire choral sound were lost when the band played anything above pianissimo. The choir sounded best in a cappella pieces, giving particular warmth and color to the Welsh lullaby "Suo Gan."

The RCBB played with its customary exactitude and artistry. Their best moment was a vigorous performance of the robust polonaise from Rimsky-Korsakov's opera "Christmas Eve."

Bass trombonist Ron Spang played brilliantly in an arrangement of "Frosty the Snowman," laying out one dazzling riff after another. Tenor Marc Shulman sang a precise rendition of Handel's "Every Valley" despite Douglas' hurried tempo.

The program turned serious with Glenn Rudolph's "The Dream Isaiah Saw," commissioned by the Bach Choir after the Sept. 11 attacks. Rudolph is a local composer whose work should be heard more often. Based on a Thomas Troeger poem, "The Dream Isaiah Saw" begins with lyric intensity, with each verse building to a gripping, cathartic climax.

Sing-alongs have become a major part of any RCBB concert. For this holiday series, the audience got a special treat: The concert ended with them on their feet, singing an enthusiastic rendition of the Hallelujah Chorus from Handel's "Messiah."

The concert repeats at 3 p.m. tomorrow in Baldwin High School.