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The bandwidth compressor-expander developed by Gabor more than 25 years ago is reexamined. It is shown that the compressor or expander can be simulated by comb filtering followed by frequency translation of the tooth contents. With a proper choice of window function, the comb teeth do not overlap and no ambiguity is introduced by the coding process. When applied to television messages, with the line harmonics chosen as preferred frequencies, compression or expansion is best described as providing a line frequency standards conversion. The combined compression-expansion system gives a reduced transmission bandwidth at the expense of vertical resolution and provides a flexible technique for the two-dimensional smoothing of television pictures in which simultaneous control of vertical and horizontal resolution is possible. A sampled signal form of the compressor-expander can be realized in digital form by using shift registers as delay lines. The digital version permits simple control of the compression ratio and preferred frequencies.
Date of Publication: Mar 1976