I am not exactly sure what nonnegative zero-phase response means. If a filter is zero-phase (i.e. symmetric and non-causal), then what does nonnegative imply? And what are the conditions to satisfy it?
Context: This is an error I get in Matlab when trying to design min-phase filters: Error using firminphase>checknonnegative (line 79) The filter has to have a nonnegative zero-phase response.
Error in firminphase (line 38) checknonnegative(b);
Error in firgr>genfilter (line 323) hRet = firminphase(hRet, s.zero, 'angles');