A question about the 'negative filter coefficients'.

[ChanglongZheng]

Hi, firstly thank you for your excellent work. I had a question when I read the 'Spectral Analysis' part in your paper, as follow,

I wonder what effects will be caused when the negative filter coefficients exist, also in other papers I find they often avoid to generate the negative filter coefficients.

Looking forward to your insightful understanding, thanks! :)