# MIDI data and the Playable interface

### MIDI data handling in TSE3

The TSE3 library has a single header file providing definitions for the elements of the standard MIDI specification. This is the file `Midi.h` (see the reference at the bottom of this page).

It defines the following elements:

The `Clock` data value type.
  Describes a time value in a song, as a number of ticks. These ticks are in units of a pulse. There is a fixed number of pulses per quarter note (PPQN) defined in `Clock::PPQN`. (A quarter note is another name for a crotchet.)
The `Event` template data value type.
  Provides a standard way to generate object which associate a data value (for example, a tempo change) with a time in `Clock`s.
The `MidiCommands` and other similar definitions.
  Definitions of all the MIDI status bytes (e.g. `MidiCommand_NoteOn`, system commands, controller values etc. These are: `MidiCommands`, `MidiSystemCommands`, and `MidiControlChanges`.
The `MidiCommand` data value class.
  This is the basic value type used to describe a single MIDI command, including it's statub byte, channel and port pair (which describe where the MIDI command should be sent), and any data bytes.
The `MidiEvent` data value class.
  This is built upon the `MidiCommand` class, but associates it with an event time in `Clock`s.
  Note that here we do not use the `Event` template class as a definition. This is because the `MidiEvent` contains a second `MidiCommand`, used if the first is a `MidiCommand_NoteOn`. If so, the second command holds the related `MidiCommand_NoteOff` event. This can be used by the TSE3 library to ensure that all MIDI note off events are sent for any scheduled note on event.``
The `TSE3MetaMidiCommand` definitions.
  These are a number of extension status bytes only understood by the TSE3 library which are used internally.

Used together, these definitions describe standard MIDI data, scheduled to some clock source. The TSE3 song components generate musical data in this format.

The kdoc documentation describes these classes. You can find further information about them there.

### The playable interface

Each component that comprises a Song can produce some sort of stream of MIDI data that needs to be scheduled to a timed output. To simplify this process they implement the `Playable` interface. This makes the Song structure use a form of the composite design pattern (GoF book).

The `Playable`PlayableIterator` that can iterate over the MIDI data in the output stream. This is a form of the iterator design pattern.`

Each different kind of `Playable` object provides it's own implementation of the `PlayableIterator` interface that knows how to generate the MIDI events.

The user can ignore the individual song components `Playable` interface, and meerly use the `Song`'s `PlayableIterator` that will in turn use the `Playable` iterators of all sub components to create a MIDI data stream for the whole song.

    +------------\     +------------+      creates +--------------------+
    | Interface  |_\   |  Playable  |--------------|  PlayableIterator  |
    |              |   +------------+              +--------------------+
    +------------- +         ^                               ^
                             |                               |
    +------------\           |                               |
    |   Example  |_\   +------------+              +--------------------+
    | Implemenation |  |    Song    |              |    SongIterator    |
    +---------------+  +------------+              +--------------------+

The data generated by a `PlayableIterator` object is in the form described above, as defined by the `Midi.h` header file.

### Streaming system exclusive MIDI data

System exclusive MIDI data is a particular nuisance. All sequencer systems have this problem. If you don't care about, or know about system exclusive MIDI data (or sysex data) then you can skip this section.

Any other form of MIDI data, for example `MidiCommand_NoteOn` events and the like, can be interspersed in any order. They are sent in whole atomic units (the `MidiEvent` or `MidiCommand` classes). They are easy to handle and stream around the system in `PlaybaleIterator` objects.

However, sysex events break this simple atomic data structure. They can be of arbitrary size, with a single start of sysex system byte at the start, and an end of sysex status byte at the end.

So how do we stream these around using the `PlayableIterator` class? Carefully, is the answer.

Sysex data has been designed to fit into the `Playable` architecture rather than be handled as a special case. However, there are certain restrictions involved in their use.

The start of a sysex block is naturally defined by a `MidiCommand` with `MidiComand_System` status byte and reason code `MidiSystem_SysExStart`. The `data1` byte contains the first data byte. `data2` is not used. If the event is held in a `MidiEvent` (rather than a single `MidiCommand` - this will be true if it is streamed from a `PlayableIterator`) then the second (note off) field is not used to hold extra values.

The next `MidiCommand` may need to be another sysex data byte. In this case the same status information is put in the `MidiComand` - although the playing `MidiScheduler` object knows not to send this again. `data1` contains this next data byte. More sysex data bytes may follow.

The stream carries on in this manner until the end of the sysex data block, when a `MidiComand` containing the `MidiSystem_SysExEnd` status information is put in the stream.

As stated above, sysex data cannot be rearranged. Nor can other MIDI events occur in the middle of them (you cannot shove a note on in the middle of a block of sysex). For this reason any `PlayableIterator` must take care to give each sysex `MidiCommand` exactly the same clock time to ensure no other events can get into the middle of the sysex stream.

The standard TSE3 `PlayableIterator` objects are designed in such a way that if all events have the same event time, events from other sources will not interrupt the stream.

### See also

- `Midi.h` for descriptions of the TSE3 representation of MIDI data.
- `Playable.h` for the definition of the `Playable` class.