15 May 2002: changed value of dipwidth in period_11.m (1.9 --> 1.85) to 
avoid error condition.

20 June 2002: plot square root of aperiodicity ratio, rather than ratio 
itself

1 June 2003: new version of YIN.  The implementation is improved (faster, 
more robust, bugs corrected).  YIN now processes files a chunk at a 
time, so it can handle larger files.

28 July 2003: bug fixes.

1 Sept 2003: put sf routines in private/