  Many signal processing applications require digital filters with variable
frequency characteristics, especially the filters with variable bandwidth. Due
to their linear phase and inherent stability, variable bandwidth finite impulse
response (FIR) filters are the popular choice in majority of the applications.
Once a variable cutoff frequency (VCF) FIR lowpass filter is designed, variable
bandwidth bandpass / highpass / bandstop filters and reconfigurable filter
banks can be realized from the same. In this paper, we present a comprehensive
review of the existing variable cutoff frequency FIR filter design techniques,
including the developments in the recent two decades. We provide the basic
concepts, design and architectural details for each of these techniques and the
major developments / incremental works thereof. Qualitative as well as
quantitative comparisons are provided to assist the reader in choosing the most
suitable VCF filter design technique for a particular application.
