Ghosting arises from the multipath reception of television images, via geographical, artificial barriers, or both, including hilly terrain, mountains, high-rise buildings, airplanes, etc. It impairs the perceived image quality severely as it manifests itself in the form of resolution deterioration, hue change, and sync instability. The multipath distortion in over-the-air broadcasting usually gets more severe as television signals pass through a cascade of television repeaters without ghost canceling operation. This paper focuses on the development of a combined system of both IF demodulator and ghost canceler for use in television repeaters. The ghost canceler should be suitable for application in television repeaters that dictate high quality. So our main purpose is to build a professional ghost canceler which features good cancellation, robustness, reliability, and stability based on the least mean square algorithm and higher resolution analog to digital, digital to analog converters, and an IF demodulator which is designed to combat carrier phase variation and gain sagging during the vertical blanking period using a novel keyed automatic gain/phase control technique, both in the presence of multiple, severe ghosts. We have subjectively assessed the image quality before and after ghost cancellation using our prototype IF demodulator and ghost canceler chain. The improvement is found to be about less than one and two grades on the average on the CCIR five-grade impairment scale for the cases of weak and strong ghosts, respectively.
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