Short Time DFTとHilbert変換器への応用に関する研究

Abstract

Reducing the spectrum occupancy over radio channels is as well known as important to prevent from exhausting radio spectrum resources. The Hilbert transformer used in SSB or RZ SSB provides with indespensable function to eliminating one side spectum from modulated signals to efficiently reduce the spectum occupancy without any distortions (1). Therefore, many investigations are keenly studied on realizing the Hilbert transformers (2). Unfortunately, most of them have been concerned with approximation in the way of adjusting phase shifting of all pass filters to their arguments being π/2 radian with certain amount of phase shifting error. Where these approximated Hilbert transformers are adopted to such SSB radio transmission systems, the approximation error degrades speech quality and expands frequency bandwidth over radio channels. An exact realization of Hilbert transformers is succesfully discussed with employing instantaneous spectrum based on the "short time Discrete Fourier Transform (ab. in ST-DFT)" and is examined to be free from error of phase shifting via computer simulations (3,4). High speed processing for the ST-DFT Hilbert transformers is also introduced to save great deal of computing power with multi-rate signal processing.