"I found a dream that I could speak to," Joni Mitchell sang Tuesday at the Rosemont Theatre. Has she ever. Backed by a 70-piece orchestra, the former folk-rock queen marked her emergence as a sophisticated singer of standards with as strong and stylish a show as one could have hoped for--one that in conceptually tracing the arc of a love affair tapped even deeper personal feeling than the confessional material that made her famous.

For an artist who has long identified herself with jazz, performing her new album song by song may have seemed a bit lacking in the spontaneity and flexibility that define the genre. But from the start, Mitchell made it clear that she was wagging the material, not the other way around.

It's been a long time since a singer--pop, jazz, whatever--approached classics such as "Stormy Weather" and "You're My Thrill" from such a position of strength. Even in expressing vulnerability, she refused to indulge in the usual game of soliciting sympathy or giving in to need.

Making brilliant use of the special qualities of her age-thickened voice--the sexy throb and ruffled phrasing, the quiver and smoky sigh--she brought a lyricist's insight and sense of incident to the words. That was true whether she was indulging in the pluckiness of "I Wish I Were in Love Again" or following in the disillusioned path of Billie Holiday on "You've Changed."

Most of the songs were ballads. When she put on a brighter face and swung a big band arrangement, as she did on "Sometimes I'm Happy," she lit up the room. One or two more swingers might have been just the thing to keep some of those fans expecting the old Joni from departing the new one early.

The orchestrations by Vince Mendoza (who conducted) were not done any favors by the harsh sound mix, which inflicted its worst damage on the high end of the music. The sense of intimacy between singer and band was seriously compromised as a result. But on the darker arrangements, which recalled Nelson Riddle's arrangements for Frank Sinatra's breakthrough themed albums of the mid-'50s, the strings had a gravity to which Mitchell responded.

The performance, which climaxed with odds and ends including her "Be Cool," included reworked versions of two of her classics: "Both Sides Now" and "A Case of You." They showed off her interpretive skills even more than the standards.

On the former song, she created a sense of naturalness by pushing the words into an offhand conversational flow. On the latter tune, more so than on the album, she passed the difficult test of transforming a collegiate, hippie-era favorite into a powerful adult confession.

Now that she has climbed the orchestral mountain, it would be interesting to see Mitchell, whose supporting cast of jazz players included noted young trumpeter Wallace Roney, take the next logical step and perform with a quartet or quintet. Having proved herself to be in her element in a larger setting, she deserves to treat herself to a freer and more demanding one.