Created attachment 307971 [details] [review]
crossfade: New example for crossfading audio tracks using audiomixer

/*
 * crossfade: An example application to play a list of audio files with
 * cross-fade between each file.
 *
 * This example application uses the audiomixer element combined with
 * GstController and pad probes to play a list of audio files in such a way that
 * the audio is cross-faded while switching between files.
 *
 * Each file is played by a decoder bin inside each CrossfadeItem. This bin has
 * a queue right before its srcpad which has a "max-size-time" of
 * 2 * CROSSFADE_TIME_MS. When an EOS is detected on the sinkpad of this queue,
 * the next CrossfadeItem is attached to audiomixer, the correct pad offset is
 * set, and fadein/fadeout is scheduled using GstControlSources.
 *
 * A crossfade can also be scheduled immediately by switching to the next item
 * "early" using crossfade_app_activate_next_item_early().
 *
 * The list of CrossfadeItems can be edited and added to dynamically; except the
 * currently-playing item and the next one (which has been prepared already).
 *
 * Bugs:
 * - If the size of the file being played is smaller than 3*CROSSFADE_TIME_MS
 *   and the file was played with a fadein crossfade (i.e., it was not the first
 *   file to be played), the queue before audiomixer which is used to detect a
 *   file going EOS will contain buffers of a total time less than
 *   CROSSFADE_TIME_MS. The crossfade will hence happen in whatever buffer-time
 *   is available.
 * - If CROSSFADE_TIME_MS is set to a value less than the duration of a single
 *   buffer, no crossfade will happen, but the switch will be gapless.
 */

This is a decent example of how to use audiomixer for gapless/crossfade playback in an audio player. Although the example expects files, this method of doing crossfade/gapless should also work with arbitrary URIs.