Audio: crossfade (0:06)
crossfade plays two rhythms and crossfades between them. The crossfade figure shows the waveforms of both rhythms and illustrates the effect that crossfading has on amplitude.
Crossfading is an envelope technique for transitioning between two sounds. It creates the auditory illusion by fading out one sound at the same time as fading in another.
In crossfade both rhythms are contained within their own ASR envelope. The actual crossfade occurs during the period when the two sounds overlap. This is when the release segment of the first rhythm overlaps with the attack segment of the second rhythm. The first rhythm reduces in amplitude whilst the second rhythm increases.
The shape of a fade is either a line or a curve. A line can only ever have one shape, it must be straight. A curve can take many shapes
Here are two of the most common shapes for a crossfade:
- Linear crossfade: the amplitude of each fade increases or decreases in a straight line. This approach is called the constant gain or equal gain method. It has the advantage of simplicity. The downside is that there is a loss of power in the middle of the fade, when the amplitude has reduced to one-half, but the power, which is proportional to the square of amplitude, has reduced to one-quarter. This means there is a distinct hole in the middle of the sound. This method is often used to crossfade between two tracks that have different sounds because it emphasises the fact that they are different.
- Logarithmic crossfade: the amplitude of each fade increases or decreases in a curve. The shape of the curve is such that power is evenly distributed between the two tracks throughout the whole of the fade. This approach is called the equal power or constant power method. It produces a much smoother sound than the linear method. This method is often used to crossfade between two tracks that are alike because it emphasises the fact that they are similar.
A linear crossfade is used in crossfade to emphasise the fact that the two rhythms are different.
Crossfading has practical uses. It helps a disc jockey (DJ) to transition between two recordings. It is used when sampling a loop to ease the transition from the end of the loop to the beginning by removing artefacts, clicks and pops, caused by a rapid transition between two significantly different amplitude levels. Writing a mashup, a mixture of two or more songs, is one of the creative uses of crossfading.