  Many experiments, and in particular gravitational wave detectors, produce
continuous streams of data whose frequency representations contain discrete,
relatively narrowband coherent features at high amplitude. We discuss the
application of digital Fourier transforms (DFTs) to characterization of these
features, hereafter frequently referred to as lines. Application of DFTs to
continuously produced time domain data are achieved through an algorithm
hereafter referred to as EFC for efficient time-domain determination of the
Fourier coefficients of a data set. We first define EFC and discuss parameters
relating to the algorithm that determine its properties and action on the data.
In gravitational wave interferometers, these lines are commonly due to
parasitic sources of coherent background interference coupling into the
instrument. Using GEO 600 data, we next demonstrate that time domain
subtraction of lines can proceed without detrimental effects either on features
at frequencies separated from that of the subtracted line, or on features at
the frequency of the line but having different stationarity properties.
