The problem of synchronous fractional sample rate conversion (FSRC) of a digital signal by L/M, where L and M are coprime integers [6,10], is revisited. Based on a novel approach two different efficient causal block implementations of FSRC are concurrently derived and compared with each other. While the computational load of both structures, being performed in an LTI MIMO subsystem at the subnyquist rate F-o/L = F-i/M, is identical, their group delay is always different. By column and row shifts of the matrix representation of the MIMO subsystem it is possible to transform each structure into any arbitrary implementation with changed group delay. Moreover it is shown that, by structural manipulation of the signal flow graph of the MIMO subsystem, both implementations ultimately require the same amount of computation and storage in spite of different group delay. Finally, by using Nyquist(L)filters, the maximum number of input samples to FSRC: is retained at its output. (C) 2001 Elsevier Science B.V. All rights reserved.