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Music Player API

The Music Player API allows you to arrange and play a collection of music tracks. It consists of the functions, data types, and constants defined in the header file MusicPlayer.h in AudioToolbox.framework.

A particular stream of MIDI or event data is a track (represented by the MusicTrack type). Tracks contain a series of time-based events, which can be MIDI data, Core Audio event data, or your own custom event messages. A collection of tracks is a sequence (type MusicSequence). A sequence always contains an additional tempo track, which synchronizes the playback of all tracks in the sequence. Your application can add, delete, or edit tracks in a sequence dynamically. Each sequence must be assigned to a corresponding music player object (type MusicPlayer), which acts as the overall controller for all the tracks in the sequence.

A track is analogous to sheet music for an instrument, indicating which notes to play and for how long. A sequence is similar to a musical score, which contains notes for multiple instruments. Instrument units or external MIDI devices represent the musical instruments, while the music player is similar to the conductor who keeps all the musicians coordinated.

Track data played by a music player can be sent to an audio processing graph, an external MIDI device, or a combination of the two. The audio processing graph receives the track data through one or more instrument units, which convert the event (or MIDI) data into actual audio signals. The music player automatically communicates with the graph's output audio unit or Core MIDI to ensure that the audio output is properly synchronized.

Track data does not have to represent musical information. For example, special Core Audio events can represent changes in audio unit parameter values. A track assigned to a panner audio unit might send parameter events to alter the position of a sound source in the soundstage over time. Tracks can also contain proprietary user events that trigger an application-defined callback.

For more information about using the Music Player API to play MIDI data, see “Handling MIDI Data.”

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