A crossfade is a transitional effect between two movie clips, two still images, or two audio tracks. The crossfade gently transitions by overlaying the events at the transition point so that both are visible or audible at the same time before the first event entirely disappears into the second.
A crossfade differs from a simple fade transition in that the latter does not employ a point of overlay where both events are present at once. The first event fades out, then the second event fades in consecutively. With a crossfade the transition occurs concurrently with one event “crossing into” the next.
Crossfades can be used in video editing to join clips or a series of movie files, such as home movies taken with a digital camcorder. It is also a popular transition effect for producing slideshows of still pictures. In addition to being a pleasant effect, a crossfade is a poignant way to show the passage of time.
A crossfade transition can be short in duration or longer. Many factors play into the best length for any given instance depending on the effect the author wants to create, pacing of the clip, and content of the events. A visual crossfade that is too short will make the viewer feel like the clip is sped up, and a transition that is too slow will make the clip appear to drag.
Disc jockeys (DJs) also use crossfading to transition from one song to the next ‘without missing a beat.’ By using crossfades to mix music tracks the music literally never stops. The crossfade is the key to keeping a dance club on its feet.
A successful audio crossfade must match the beats per minute of both audio tracks before fading up the new track into the exiting track. If the beats aren’t aligned, the new track muddies the old one and dancers can’t keep pace because there is no clear beat. DJs commonly wear headphones on one ear and use controls to speed up or slow the incoming track before fading up the volume into the existing track. Both tracks play overlaid for a series of beats as the new track increases in volume and the old one decreases.
Audio crossfades are more challenging than visual crossfades. Most dance music hovers around 120 BPM for the very reason that it makes the DJs job easier, and record producers of dance music want club play. The challenge is tougher when two very different audio tracks must be overlaid, such as crossfading between genres of music, or for example, from 120 BPM to a ballad. In most cases a longer crossfade is used to give listeners (or dancers) a chance to wind down from the old track and transition into the new.