Just over 20 years ago, Linda Ronstadt - the Seventies pin-up girl of US soft-centred rock - took a career diversion that provoked a lot of controversy, sold a lot of albums and began the process of repositioning her as a singer with more to offer than re-treads of "Heart like a Wheel". (After all, that wasn't even her song.)

Just over 20 years ago, Linda Ronstadt - the Seventies pin-up girl of US soft-centred rock - took a career diversion that provoked a lot of controversy, sold a lot of albums and began the process of repositioning her as a singer with more to offer than re-treads of "Heart like a Wheel". (After all, that wasn't even her song.)

Ronstadt sees it today as a life-saving decision. "I was trapped by rock," she states emphatically. "I hated arenas. That's not the way to present music. I just couldn't do it any more. I wanted to try theatres, where you can have an intimate rapport with your audience." Ronstadt, in contrast to many rock stars, cared about communicating, rather than watching her bank balance bulge. She also cared about the circumstances that being in rock music imposed on her. "There were also the issues of my voice, and what I was doing to it. You're expected to make a lot of noise. And there's the whole problem of what you do as a rock singer on stage. Because there simply isn't enough for singers to do. You know - hanging around on stage while the guitar-players get through their solos. It got to me."

But it made her famous, I suggest. "Yes. And it was very lucrative. But I wasn't very happy. The whole thing became... well, it was inappropriate, in the end."

Making a string of albums between 1982 and 1986 with Nelson Riddle, reaching back to standards made famous by Frank Sinatra, Peggy Lee, Rosemary Clooney and many others of the pre-rock era, gave Ronstadt the opportunity to do something she not only deemed appropriate, but loved. At the time it seemed an uncomfortable choice. There were rumours then that she was talked into it, for a start. But she regards it as something that arose naturally from her own life.

"I started my professional career singing folk music, then moved on to rock and country, but when I was a little girl, my mother and father had all these singles and albums sung by the greats, and that's what I heard, over and over. It's what I grew up with. Ella & Louis, that album they made together, Peggy Lee, Frank Sinatra - I loved Only the Lonely. I got Nelson from there. Same with Mexican music. I'm from a Mexican family and I always had that music around me, so it was all there when I made the Mexican album in 1987. My family were very musical: they all played an instrument, or sang, grandparents and all. My grandma was an opera fan; she was big on Maria Callas. My brother was a boy soprano."

She pauses for a moment, remembering those days. "In those days, US radio was amazing - you could get late-night anything: R&B, gospel, country music. I grew up in Arizona; we were by the border, so you could get Mexican stations, and even the Canadian ones. The radio stations had very powerful transmitters - you could pick them up."

But the 1980s collaborations with Riddle, although very popular (the three albums have sold more than six million copies between them in the US alone), were not received well. Ronstadt was judged to have been out of her depth: no tone, bad enunciation, little understanding of non-rock phrasing. She now wonders how "ready" she was then. But careers usually work that way: everything is a rush, an idea is acted upon quickly, completed, and then it's on to the next project. Ronstadt herself knew the circumstances weren't exactly ideal.

Comparing her new album, Hummin' to Myself, with What's New, the first of the three Riddle albums, she can see the pitfalls. "When I first started, in the Eighties, I really didn't know how to do it. With the song 'What's New', for example, I never sang it in my key! We had no time to check beforehand. Nelson did the arrangement, we went in and recorded - we did it three times, in fact - and kept the first take. That was the first time I'd ever sung it professionally! Live with a studio orchestra."

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I wasn't the sort of situation most artists would be game to try. "Well, some of the other tracks we tried to patch, but working like that with ballads in that genre is virtually impossible. There are so many rubato sections, there's no way you could use a click track, which is second nature to rock singers. So with that stuff it really was by the seat of my pants."

I wonder whether there were technical vocal hurdles to surmount as well. "Oh, yes. Like with the new one, 'Hummin' to Myself'. It's a totally different vocal technique: you're singing on the breath. I used to either work on falsetto or chest-belt voice. This type of singing mixes both. In fact, when you master it, it's a more natural technique, probably the most natural. On this album we recorded three tracks completely live - 'Miss Otis Regrets', 'Cry Me a River' and 'I'll Be Seeing You'. The others we worked on a little. You have to, otherwise you run out of voice, doing long sessions like that."

Ronstadt's return to the classic American ballad repertoire, this time with a small group ("When Nelson died, there was no one else I could collaborate with, so I stopped") of simpatico jazz musicians such as Christian McBride, David "Fathead" Newman and Lewis Nash, causes me to wonder what criteria she used when choosing the new record's repertoire. "Well, it was a combination of looking at what was left over from the Nelson Riddle projects - you know, things that just didn't translate well to the orchestra, that Nelson wasn't comfortable with scoring - that I still felt close to, and things that I'd often catch myself singing and think, 'I must record that one day.'"

This time, however, the singer was considerably more prepared for the challenges handed out by both singing this type of song and by doing it in that most exposed of situations, fronting a small acoustic jazz ensemble. For a start, she'd not only survived, but triumphed in, a Broadway production of The Pirates of Penzance, noting that "after eight shows a week, I wanted to do something else. My boyfriend had a huge jazz collection, so I took my time listening, making my selections." She is worried that this may make it sound like she was stuck for material. Not so. "I never try anything I didn't hear in my own living room when I was 10 or 11. Or on the radio. That's the stuff that feels entirely natural when I come to it again." So tunes such as "Miss Otis Regrets", "Day Dream" and "Never Will I Marry" get a relaxed and natural going-over, and although there are still some ugly moments where the sound of the voice doesn't quite match the lyricist's intentions, this set is of a different calibre altogether from that of the 1980s Riddle sessions.

Ronstadt may never have quite solved the swingin' Riddle, but she's certainly happy enough hummin' to herself loudly enough for us to tune in to.

'Hummin' to Myself' is out now on Universal