High-fidelity low-pass finite-impulse-response filters

The paper describes a method of designing a low-pass finite-impulse response (moving-average) filter having a flat passband and an arbitrarily contoured stopband. The method effectively separates the design of the passband and the stopband, with independent control over the properties of each. The filter causes a controllably small amount of distortion when applied to a smooth input and is especially useful in connection with data rate conversion in those cases when the object is to process with precision a low-frequency signal. The method permits direct placement of stopband zeros, convenient for locating 'don't care' bands in first stage data-rate- reduction filters, or suppressing large discrete sources of interference. The theory depends on the expansion of the filter weighting sequence (impulse response) as a power series of central-difference operators. (Author)