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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.
One popular audio cross fade technique that you see in the movies is when the next scene starts while there are a few seconds of audio, such as dialogue, carrying over from the current scene. You may also see the reverse, where the audio from the next scene bleeds into the current scene before the visuals have changed. Both of these techniques are used quite a lot to accomplish the same effect, to create a seamless audio transition from one scene to the next.
Some high end video recording software also lets you accomplish the same result, by putting the audio on multiple tracks, or letting you “drag” the audio portion of the previous clip to the next clip. It’s really cool when you get to use it in your home movies. It adds a touch of professionalism to your work.
The first camcorder I got was an analog camcorder and it had a single button that created a cross fade effect. I thought it was a novelty at first but when I used it I noticed it created a pleasing effect for my home movies, introducing a new setting and mood.
Nowadays of course all of that cross fade stuff is done in post production, along with hundreds of other kinds of transitions that stretch the limits of creativity. But I still prefer the simple cross fade for its silent and poignant simplicity.