Bandwidth compression eliminating frequency transposition and overcoming phase ambiguity

Abstract

Abstract 1,188,014. Bandwidth compression. WESTERN ELECTRIC CO. Inc. 11 Oct., 1967 [1 Nov., 1966], No. 46392/67. Heading H4R. In a bandwidth compression system the speech signal is split into a number of contiguous frequency bands to provide a subsignal S(t) which is treated as the real part of an analytic signal, # (t) = s(t) + j###S(t) where #S (t) is the Hilbert transform of s (t), and the signal which is transmitted is the real part of the square root of the analytic signal which is shown to be where a (t) is the instantaneous amplitude of the analytic signal # (t), and # (t) = a (t) ej# (t). The signal transmitted is limited in frequency band to half the input frequency band and is a signal from which the original frequency band may be synthesized. As shown in Fig. 1 the input speech band is split into contiguous frequency bands by filters 100, having a bandwidth such that no filter contains more than one formant. The resulting channel signals are applied to Hilbert transform networks 102, comprising appropriately adjusted transversal equalizers, and to a delay 101 of value equal to the delay imposed on the signal in network 102 to produce appropriately synchronous value s (t) and #S (t) from which, by squaring, adding, and square rooting, in 103, 104, 105 and 106 the instantaneous amplitude of the analytic function is derived. The resulting signal, a (t), is added to the original signal, s (t), in 107, multiplied by ¢ in 108, and the square root taken in order to obtain the real part of the square root of the analytic signal. To resolve the ambiguity introduced by the square rooting a switch 111 is arranged to invert the output of 110 every time signal s (t) goes through zero when the signal s (t) is positive using the circuit described with respect to Fig. 3 (not shown). It is shown that the original signal s (t) may be obtained from the transmitted real part of the square root of the analytic function simply by producing the imaginary part of the square root in Hilbert transform network 114, delaying the real part in 113 to synchronize it with the imaginary part, squaring both parts in 115 and 116 and taking the difference in 117. The resulting signals from the similar channels 1 to N are added in 10 and reproduced at 11. In a modification Fig. 4 (not shown) the amplitude of the real part of the analytic signal is restored to its original amplitude, a (t), prior to transmission by multiplying the output of the square root network 110 by the output of a similar square root network connected to the output of square root network 206. At the synthesizer the output of the subtractor 117, which is now a (t) s (t), is divided by a signal, a (t), derived by adding the outputs of networks 115 and 116 and taking the square root. The arrangement is said to produce instantaneous frequency division.

Inventors

• 👤 James L Flanagan
• 👤 Manfred R Schroeder

Bibliographic Details