a modulator , and demodulator , apparatus and method for use in a multiple sub - channel communication system is taught . a commutator is employed for fractionally sampling , or distributing , signals from , or to , a multiple channel polyphase filter . the filter is coupled with a discrete fourier transform , or its inverse , such that the relationship between the base - band sampling rate of a plurality of sub - channel signals , the frequency spacing of the sub - channel signals , and the sampling rate of a composite signal can be related by any rational number , thereby freeing designers to optimize system design respecting channel spacing , bandwidth , and signaling rates . the advantages of the present invention are realized by adjusting the interpolation and decimation rates of the filter , and by adjusting the resolution and decimation rates of the transform .