Linear-phase filtering for playback of recorded folded-spectrum video

摘要

An apparatus and a method for reproducing television images from recordings of folded-spectrum video signals uses a zero-crossing detector to convert an FM signal encoding folded-spectrum luminance signal to a PPM signal having the folded-spectrum luminance signal in its baseband, accompanied by the harmonics of the FM carrier and their respective sidebands. The PPM signal is digitized by an analog-to-digital converter, after passage through an anti-aliasing filter that is used to suppress only higher frequencies that would otherwise be sampled at less than Nyquist rate during digitization. Except for this anti-aliasing filtering, the suppression of the harmonics of the FM carrier and their respective sidebands is deferred until after digitization. Linear-phase digital filtering is then used to suppress the harmonics of the FM carrier and their respective sidebands, and the folded-spectrum luminance signal supplied in response to this digital filtering is unfolded to restore it to its original full bandwidth. This procedure permits the group delay characteristic below the folding-carrier frequency to be kept constant, thereby avoiding phase error as would interfere with the unfolding of the folded signal into the original full bandwidth.