Frequency-Band Multiplication or Division and Time-Expansion or Compression by Means of a String Filter
Friedrich Vilbig · The Journal of the Acoustical Society of America · 1952
A frequency band multiplication or division has been performed by string filters. An exact relationship between the input and output frequencies has been shown. Two different methods have been used. The first multiplies frequencies by rectification with the aid of a push-pull electrostatic pick-up. Alternating plus-minus polarizing techniques are employed to suppress the undesirable sum frequencies. The resolution of such a doubling system, which will not pass the sum frequencies, is approximately 1.5 times the band width of the single filters. However, the band width is limited by the transit time of speech. The second method multiplies and divides by exciting the harmonics or subharmonics of the single strings. This is done by employing light, stiff “fingers” to disturb the free oscillations of the damped strings. The excitation and the resulting frequency band division and multiplication are investigated. It is shown that in order to get time compression or expansion, it is only necessary to record the multiplied or divided frequency bands and play them back with the corresponding diminished or increased speeds. The results given have a general application to similar type filters such as reed, crystal or magnetostrictive.