Instantaneous Compandors on Narrow Band Speech Channels

W HEN a signal such as speech is instantaneously compressed in amplitude, harmonics and cross modulation products are generated which extend the frequency spectrum of the original signal by many octaves. It is proposed to demonstrate that the additional products thus generated are necessary for the distortionless recovery of the original signal. Then the conditions will be examined under which this broadband signal can be transmitted without distortion through a bandwidth no wider than that occupied by the spectrum of the uncompressed speech. Finally, some of the practical aspects of using intantaneous compandors on narrow band speech channels will be considered, with emphasis on the nature of the transmission requirements placed on the medium. The advantages to be obtained from the use of instantaneous compandors have already been presented by Mallinckrodt. BANDWIDTH VS DISTORTION If a single frequency tone is compressed by a 2 to 1 compressor, 2 and then the fundamental alone is expanded, it can be shown that the resultant 3rd harmonic distortion is only 13 db below the fundamental. Expansion of both the fundamental and the 3rd harmonic output of the compressor will reduce this 3rd harmonic distortion to 29 db below the fundamental. Expansion of the fundamental plus the 3rd and 5th harmonics will reduce the 3rd harmonic distortion in the recovered signal to 45 db below the funda1 C. O. Mallinckrodt "Instantaneous Compandors," B.S.T.J., Vol. X X X , No. 3, July 1951. 2 In a 2 to 1 compressor, the output amplitude is the square root of the input amplitude.