HNR modification fails for low F0 values

[ljuvela]

If input F0 to Synthesis contains low but non-zero values (around 10Hz or so), the program will crash and give the following error:

gel: interp.c:84: ERROR: x values must be monotonically increasing
HNR modification ...Default GSL error handler invoked.

This is caused by multiples of F0 in harmonics analysis falling into a single FFT bin (hence x values not monotonically increasing).

User fix: manually clip your F0 values to be above 40Hz.

Developer fix: clip F0 values in Synthesis and add a warning message.