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Compression and Expansion

April 5th Fact-of-the-Day

Only the peaks of an RF envelope can be heard when the peak-to-peak amplitude of an SSB signal is only slightly stronger than the noise level, because portions of the envelope that are weaker than the noise are almost totally masked from human detection by the noise. Loosing the information contained in the weaker portions of a transmitted signal can significantly reduce intelligibility. Transmitting a stronger signal is an obvious solution, but that isn't always possible. Another solution is to modify the linearity of the transmitted envelope such that more of the information to be sent is compressed into stronger regions near the peaks and then to correct the resulting linearity distortion on the receiving end with a complimentary expander. That method is most effective with white noise and is of little or no value with impulse noise such a lightning crashes, because good performance depends on a relatively constant noise floor. ©2005 Martek International All rights reserved.


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