1. Trace equalization involves adjusting the gain of different channels so that their average amplitudes (probably their rms amplitudes) over some analysis window are equal. 2. Cross-equalization involves matching the frequency spectrum of different channels to each other or to a predetermined curve, possibly including adjustments because of phase differences. 3. Filtering to correct for frequency discrimination in recording or playback such as the linear-with-frequency response inherent in magnetic-tape pickup from direct analog recordings.