Xv. Speech Communication A. Detection of Dsb Signals Occupying the Same Rf Spectrum

In a recent paper, 1 Bridges and Zalewski have discussed an approximate procedure for reducing the interference of one DSB signal upon another when the signals have overlapping spectra. They compared their scheme with various procedures for taking advantage of bandwidth SNR tradeoffs and concluded that it could be used for optimizing bandwidth utilization through the addition and later separation of many DSB signals. Contrary to the results and conclusions of Bridges and Zalewski, we shall show that since DSB is a linear modulation procedure that creates two shifted versions of the modulating signal spectrum, two DSB signals with overlapping spectrum may be perfectly separated by a finite, linear demodulation procedure. Because of the linearity of the modulation and demodulation process, there is no justification for comparison with nonlinear schemes nor for extending this procedure for more signals. The procedure could have important applications for protection of one DSB or AM signal against intentional or chance interference by another. The ability to separate two overlapping DSB signals is recognized for the singular case of quadrature modulation, when the two signals have the same carrier frequency but one is 900 out of phase with the other. In this case orthogonal detection is used to remove one signal and recover the other. This same technique may be used, even if the carriers are not 900 out of phase, as long as there is some phase difference.