To cancel the lower-frequency portions of the noise, noise-cancelling headphones use active noise control or ANC. A microphone captures the targeted ambient sounds, and a small amplifier generates sound waves that are exactly out of phase with the undesired sounds. When the sound pressure of the noise wave is high, the cancelling wave is low (and vice versa). The opposite sound waves collide and are eliminated or "cancelled". Most noise-cancelling headsets in the consumer market generate the noise-cancelling waveform in real time with analogue technology. In contrast, other active noise and vibration control products use soft real-time digital processing.