Transfer-Function Measurement Using Maximum-Length Sequences
A comprehensive analysis of transfer-function measurement methods based on maximum-length sequences is presented. Maximum-length sequence stimulated measurements employ efficient cross-correlation between input and output to recover the system impulse response. The results so obtained is shown to be identical to the system's direct response to a simple periodic square pulse but with noise and distortion immunity comparable to Time-Delay Spectrometry. By appropriate modification of the excitation signal, the system's DC component simultaneously recovered by the same cross-correlation operation. Another modification correctly measures systems containing unipolar transducers such as spark gaps and hammers. An algorithm is disclosed which computes a smoothed energy-time curve directly from the impulse response and a low-cost instrument performs these and other measurements on loudspeakers and acoustic spaces.
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