Immittance Spectral Pairs (ISP) form a new set of parameters proposed for presenting the LPC filter. ISP is close to LSP and owns similar `niceties', yet it is different. For a filter of order n it consists of a `gain' and n-1 `frequency' parameters instead of n `frequency' parameters for LSP. In regarding LPC as a pseudo model for the vocal tract ISP may represent the immittance at the glottis, without posing, like LSP, artificial boundary conditions. In quantization experiments, ISP has been found to compare favorably with LSP. A study of interframe differentiation coding for ISP and LSP is included and it demonstrates the respective performances of the two sets.