Abstract: A method and device for very quickly and accurately determining the fundamental frequency of an input analog electrical signal. The method first uses sparse range autocorrelation to determine the note which is closest to the fundamental frequency. It then uses fine range autocorrelation and interpolation to calculate more precisely the exact pitch. Smoothing is employed for both the sparse range determination and the subsequent fine range determination to reject spurious signals. Because the sparse autocorrelation produces good results with merely one or two full cycles of the fundamental frequency, the initial sparse determination can be made in less than ten milliseconds and this is updated with a fine determination less than two milliseconds later.

Abstract: A circuit for multiplexing three or more audio band electrical signals into two audio band electrical signals which can be provided to a stereo input and a circuit and computer method for demultiplexing the signals. In particular, six signals from the six strings of a guitar are multiplexed for stereo audio analog input to a sound card in a personal computer where the signals are digitized and then, using software running on the computer, demultiplexed into six signals from which the frequencies above 4.5 kHz have been lost.