The DJ mixer crossfader was originally developed as a control for implementing smooth fades from one program source to another, but where did the idea come from? Evolution of the DJ Crossfader article tells the history and development of crossfader in DJ mixers.
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Tomi Engdahl says:
A Series of New Apple Inventions Reveal a DJ-Styled Crossfading Feature
http://www.patentlyapple.com/patently-apple/2012/02/a-series-of-new-apple-inventions-reveal-a-dj-styled-crossfading-feature-advanced-camera-sensors-possible-3d-motion-tv-remo.html
Portable electronic devices are increasingly capable of performing a range of audio operations in addition to simply playing back streams of audio. One such audio operation, crossfading between songs, may take place as one audio stream ends and another begins for a seamless transition between the two audio streams. Typically, an electronic device may crossfade between two audio streams by mixing the two streams over a span of time (e.g., 1-10 seconds), during which the volume level of the first audio stream is slowly decreased while the volume level of the second audio stream is slowly increased.
Some electronic devices may perform a beat-matched, DJ-style crossfade by detecting and matching beats in the audio streams.
Apple’s invention relates to methods and devices for efficient beat-matched, DJ-style crossfading between audio streams. For example, such a method may involve determining beat locations of a first audio stream and a second audio stream and crossfading the first audio stream and the second audio stream such that the beat locations of the first audio stream are substantially aligned with the beat locations of the second audio stream. The beat locations of the first audio stream or the second audio stream may be determined based at least in part on an analysis of frequency data unpacked from one or more compressed audio files.