Also called phase modulation, phase shifting is a type of modulation that uses phase changes to produce a signal pattern. Each phase change occurs as the carrier wave reaches the zero (center) point. If the wave is traveling up, a phase change sends the wave back down to its negative value, thereby creating two wave troughs in a row, instead of a trough and then a crest. If the wave is traveling down, a phase change sends the wave back up to its positive value, creating two wave crests in a row.