frequency response error, phase error, phase delay error and group delay error. However, in the most common applications of the VFDs, the time domain instantaneous delayed samples are estimated. In this thesis, for such applications, the relations between the above frequency domain errors (in particular the frequency response error, phase error and group delay error), and the time domain errors are investigated. The design criteria of the VFD filters for this particular application are identified. Besides the investigation on the design criteria, a new VFD filter design approach is proposed. In this approach, the full band input signal is split into several subbands by a newly proposed filter bank. By shifting each subband a proper phase, the VFD is realized by combining necessary subbands. The proposal VFD technique can be incorporated with the variable bandedge characteristic with little extra complexity. Hence, the filters with simultaneously variable bandedges and fractional delays (VBFDs) are obtained. In the design of the VBFD filters, the split subbands can be either kept or discarded to form the variable bandedges. However, this requires the bandwidth of the individual band of the filter bank to be narrower than the transition bandwidth of the VBFD filters. By introducing a shaping filter to the last retained band to form the transition band of the VBFD filters, the bandwidth of the individual band of the filter bank may be relaxed to about twice of the transition bandwidth of the VBFD filters. Compared with the existing VBFD filter design techniques, the proposed approaches significantly reduce the computational complexity of the VBFD filters.