Possessing a mezzo-soprano vocal range, Madonna has always been self-conscious about her voice, especially in comparison to her vocal idols such as Ella Fitzgerald, Prince, and Chaka Khan. Mark Bego, author of Madonna: Blonde Ambition, called her "the perfect vocalist for lighter-than-air songs", despite not being a "heavyweight talent." According to MSNBC critic Tony Sclafani, "Madonna's vocals are the key to her rock roots. Pop vocalists usually sing songs "straight," but Madonna employs subtext, irony, aggression and all sorts of vocal idiosyncrasies in the ways John Lennon and Bob Dylan did." Madonna used a bright, girlish vocal timbre in her early albums which became passé in her later works. The change was deliberate since she was constantly reminded of how the critics had once labelled her as "Minnie Mouse on helium". During the filming of Evita, Madonna had to take vocal lessons, which increased her range further. Of this experience she commented, "I studied with a vocal coach for Evita and I realized there was a whole piece of my voice I wasn't using. Before, I just believed I had a really limited range and was going to make the most of it."