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Core MIDI Services and MIDI Server Services

Core Audio uses Core MIDI Services for MIDI support. These services consist of the functions, data types, and constants defined in the following header files in CoreMIDI.framework:

• MIDIServices.h

• MIDISetup.h

• MIDIThruConnection.h

• MIDIDriver.h

Core MIDI Services defines an interface that applications and audio units can use to communicate with MIDI devices. It uses a number of abstractions that allow an application to interact with a MIDI network.

A MIDI endpoint (defined by an opaque type MIDIEndpointRef) represents a source or destination for a standard 16-channel MIDI data stream, and it is the primary conduit for interacting with Core Audio services. For example, you can associate endpoints with tracks used by the Music Player API, allowing you to record or play back MIDI data. A MIDI endpoint is a logical representation of a standard MIDI cable connection. MIDI endpoints do not necessarily have to correspond to a physical device, however; an application can set itself up as a virtual source or destination to send or receive MIDI data.

MIDI drivers often combine multiple endpoints into logical groups, called MIDI entities (MIDIEntityRef). For example, it would be reasonable to group a MIDI-in endpoint and a MIDI-out endpoint as a MIDI entity, which can then be easily referenced for bidirectional communication with a device or application.

Each physical MIDI device (not a single MIDI connection) is represented by a Core MIDI device object (MIDIDeviceRef). Each device object may contain one or more MIDI entities.

Core MIDI communicates with the MIDI Server, which does the actual job of passing MIDI data between applications and devices. The MIDI Server runs in its own process, independent of any application. Figure 2-4 shows the relationship between Core MIDI and MIDI Server.

In addition to providing an application-agnostic base for MIDI communications, MIDI Server also handles any MIDI thru connections, which allows device-to device chaining without involving the host application.

If you are a MIDI device manufacturer, you may need to supply a CFPlugin plug-in for the MIDI Server packaged in a CFBundle to interact with the kernel-level I/O Kit drivers. Figure 2-5 shows how Core MIDI and Core MIDI Server interact with the underlying hardware.

The drivers for each MIDI device generally exist outside the kernel, running in the MIDI Server process. These drivers interact with the default I/O Kit drivers for the underlying protocols (such as USB and FireWire). The MIDI drivers are responsible for presenting the raw device data to Core MIDI in a usable format. Core MIDI then passes the MIDI information to your application through the designated MIDI endpoints, which are the abstract representations of the MIDI ports on the external devices.

MIDI devices on PCI cards, however, cannot be controlled entirely through a user-space driver. For PCI cards, you must create a kernel extension to provide a custom user client. This client must either control the PCI device itself (providing a simple message queue for the user-space driver) or map the address range of the PCI device into the address of the MIDI server when requested to do so by the user-space driver. Doing so allows the user-space driver to control the PCI device directly.

For an example of implementing a user-space MIDI driver, see MIDI/SampleUSBDriver in the Core Audio SDK.

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