|Microsoft DirectX 9.0|
A channel is a destination for messages that are specific to one or more parts. A part might contain notes for a single instrument, or it might contain one or more waveforms.
Under the MIDI 1.0 standard, there are 16 MIDI channels, meaning that no more than 16 instruments can be playing at one time. To make more channels available to applications, DirectMusic creates channel groups. Up to 65,536 channel groups can exist at one time, each containing 16 channels, for a total of over one million channels. A port can be assigned any number of channel groups, up to its capability to support them. MIDI hardware ports have only a single channel group.
System-exclusive messages address all 16 channels within a channel group, but not other channel groups.
Every part in a DirectMusic performance plays on a performance channel, sometimes called a PChannel. The performance channel represents a particular MIDI channel in a particular group on a particular port. When a band is selected by a performance, each instrument in that band is mapped to a performance channel, so the part on that channel will play on that instrument.
When audiopaths are being used, identical performance channels on different audiopaths are mapped to different output channels.
The number of notes that can be played simultaneously is limited by the number of voices available on the port. A voice is a set of resources dedicated to the synthesis of a single note or waveform being played on a channel. In the event that more notes are playing than there are available voices, one or more notes must be suppressed by the synthesizer. The choice is determined by the priority of the voice currently playing the note, which is based on the priority of the channel. By default, channels are ranked according to their index value, except that channel 10, the MIDI percussion channel, is ranked highest.
Applications and synthesizers can set their own channel priorities.