A heterodyne filter for the speech frequency range is described. It can be used as a band pass filter of continuously variable low and high frequency cut‐offs adaptable for any desired transmission band in the frequency range 40–4000 c.p.s. and with more than 60‐db attenuation and with cut‐offs of a maximal slope of 1 db per c.p.s.
Band width and low frequency cut off are varied by two separate controls. Band elimination filtering can also be obtained with continuously variable mid‐frequency but with band width restricted to either 300 or 800 c.p.s.
The variable filtering is performed by single sideband transmission with successive filtering in two modulation units and demodulation in a third unit. Distortion products from impurities in transmission and modulation are kept at a −60‐db level and random noise at a −85‐db level, relative to a single test tone.
The filter was designed for articulation tests and general speech research purposes and also as a wave analyzer of continuously variable band width, 45–4000 c.p.s. in the frequency range 40–2000 c.p.s. The maximum permissible speed in analysis of sustained sounds, governed by the inverse square of band width law, is discussed with regard to the quality of the analyzing band pass filter.