Watch out or the Tuning Police will get you.

As I understand it, at various periods there's been an interplay between music of the peasants and music of the courts. (I'm talking Old World here). So you had peasant musicians doing a version of what the court was doing, and then the court musicians would pick that up and copy it back. Mostly round dancing. So as far back as you like to look there have been people engaged in collecting music and bringing it back to use in a different setting. And alongside that yiou had the antiquarian collecting of ballads, up to and including Child.

But M.Ted is after something different - where and when does the tradition begin of collecting and preserving and reviving songs described as "folksongs"? And the answer to that has to be, for England and America, the end of the nineteenth century and the beginning of the twentieth, and most especially with Cecil Sharpe. And you can even arguably hone it down to a single occasion,the meeting of Cecil Sharpe with Morris Dancer William Kimber at Headington Quarry near Oxford on Boxing Day 1899 (I think it was.)