Spectral properties and band-limiting effects of time-compressed TV signals in a time-compression multiplexing system

Time-compression multiplexing (TCM) has recently been proposed for application in multiple TV transmissions through satellites. It is advantageous over frequency-division multiplexing because of its relative immunity to nonlinear transponder effects. Here we study two important and fundamental aspects of TCM — the spectral properties and band-limiting effects on the time-compressed signal. We derive the output spectrum of a time-compressed signal and show that if the original input signal is assumed to be: (i) band-limited to B Hz, (ii) segmented into T-second intervals before time compression by a factor of α(α ≥ 1), and (iii) 1/T ≪ B, then essentially all the spectral power in the output time-compressed signal is contained in the bandwidth |f| ≥ αB Hz. This result is applicable to the TV case. Numerical examples on various types of spectra are also presented. Using the TV example, we further demonstrate that the ripples created by low-pass filtering the time-compressed signal up to αB Hz are small, and interburst interference due to these ripples can be kept negligible with a small guard time (about 2 percent of the burst duration) between different signal bursts. We also provide a brief discussion on some interesting spectral properties of time-compressed signals in spectrum-expansion applications.