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The spectrum modification technique is a digital procedure for placing the spectrum of any band-limited signal-analog or digital within an arbitrarily chosen set of (nonoverlapping) frequency intervals, the sum of whose lengths is not less than the original bandwidth of the signal. Because it can serve the function of several digital techniques, such as mixing, interpolating, and decimating, and can he realized in a finite impulse response (FIR) filter form, it may be suitably implemented by a programmable FIR filter having a flexible timing scheme. The technique is more computationally efficient for many of its applications than the conventional algorithms it replaces. In the application for which it was developed, it replaced a mixer and the concomitant filtering required to shift a signal spectrum a fraction of its bandwidth and cut the computational rate an order of magnitude.