Barbara in 1961: Her songs stroke the brow and disturb the unconscious 

It was the incongruity that first struck my adolescent ear. There was a woman on the radio singing in French about a string of German names — Herman, Peter, Helga et Hans — her lovers, perhaps. With bog-standard O-Level French, I could hear from her slick delivery that she was struggling with some inexpressible moral dilemma.

There was no word of explanation from the DJ and no song of Barbara's other than "Göttingen" was ever played again on the BBC, at least not when I was listening. In France I heard her 1970s hit "L'Aigle Noir" ("The Black Eagle"), and thought little of it. They teach it now in primary schools.

In 2003, I heard the German Chancellor Gerhard Schröder quoting the lyrics of "Göttingen" at a ceremony in Versailles to mark the 40th anniversary of Franco-German amity. "I was a doctoral student in Göttingen when she came to sing," he recalled. "It went to our hearts, the start of a wonderful friendship between our countries." 

Politicians rarely credit others with their achievements, least of all a black eagle who hints at intimacies that cannot be openly expressed. As Schröder dispensed his bilateral bromides, I knew there was more to Barbara than met the ear. Eventually, I persuaded Radio 3 to let me make a documentary about her. Say what you like about the BBC, it is still prepared to go boldly into the collective unknown.