Interactive Music Sequencer Design

Making a Computer Music Language

Event Blocks
Music can be described as events or commands occurring over time. We can build our music command language with "event blocks" composed of three elements: time, event type, and event details. The time is stored relative to previous events and is called a delta-time, the event type is identified with a number, and the event details are zero or more parameters that are defined by the event type.

Figure 1 - Event Block

Delta-Time

Event Type

Event Parameters

With this basic design we can describe any groupings of events and time intervals. Building our computer music language is now a task of choosing how to store the delta-time, what event types, and what details are needed for each event type.

MIDI (Musical Instrument Digital Interface) Music
The MIDI specification has been around for a long time. Most composition and sequencing software provides compatibility with MIDI input and output ports, and these products also provide options to save and load MIDI files.

Since the MIDI specification defines a delta-time storage format and a useful set of event types and parameters, we should start with this model as a reference. This will make it easier to convert the MIDI language to our custom music language.

Keep in mind that our goal is to create music directly on a machine with audio capabilities, so we want to take what is useful from the MIDI design, but not restrict ourselves to its features. We are also likely to get music from composers in the MIDI file format, so we will want to convert the MIDI file music language to our own custom language.

MIDI Events
The MIDI specification defines several events called the channel voice messages, and there are also meta-events defined in the MIDI file format 1.0 specification. Some of the meta-events simply contain text string data. I summarize the MIDI events and meta-events we are interested in Table 1, Table 2, and Table 3.

Table 1 - MIDI Channel Voice Message

Event Type	Event Parameters
Note Off	Note Number
	Release Velocity
Note On	Note Number
	Attack Velocity
Pitch Wheel	Pitch Bend LSB
	Pitch Bend MSB
Control Change	Controller ID
	Controller Value
Program Change	Program Number
Poly Key Pressure	Note Number
	Pressure Value
Channel Pressure	Pressure Value

Table 2 - Basic MIDI Event Types

Meta Event Type	Meta Event Parameters
End of Track
Set Tempo	Tempo Number
Time Signature	Numerator
	Denominator
	MIDI Clocks per Metronome Click
	32nd Notes in a MIDI Quarter Note
Key Signature	Sharps/Flats Indicator
	Major or Minor Indicator

Table 3 - Text MIDI Event Types

Meta Event Type

Text Event

Sequence/Track Name

Instrument Name

Lyric

Marker

Cue Point

Of the MIDI channel voice messages, the control change event is unique in that it specifies an additional event identifier, the controller ID. Examples of common controller ID numbers are 7, which refers to volume and 10, which refers to pan position. Many controller ID numbers are not commonly used and we can have musicians insert control change messages with controller IDs to mark the music in special ways.

The basic meta-events listed give us ways to get time organization details and tempo changes. The text meta-events give us ways to get custom data embedded in strings. Composers can put special track settings in the track name strings or special kinds of playback commands in lyric strings. These methods will save us from having to write custom tools for composers when deadlines are looming.

MIDI Channel and Tracks
The "channel" in MIDI channel voice messages refers to the fact that these messages are sent with a channel number from 1 to 16 (0 to 15). The channel number is used by MIDI devices to route commands. We are not routing commands to MIDI gear, so we won't talk about channels anymore. Type 1 MIDI files have any number of tracks where each track contains a series of event blocks. It is the track concept that is useful for our data organization.

Sequences, Tracks, Events, Instruments, and Voices
This is where we leave the world of MIDI and talk about our custom music sequencer language. A quick summary of the terminology used in the rest of this paper is in order: A Sequence is a collection of Tracks that run simultaneously. Each Track is a sequence of Events that control the current Instrument. Certain types of Track Events will turn on and off Voices of the current Instrument. Some types of Track Events will modify all active Track Voices. You can think of an Instrument as the default settings given to a Voice that is turned on with a NoteOn Event. I will present a list of class definitions that display this structure in the implementation section.

Event Type Possibilities
Table 4 lists many event type ideas so you can get a taste of the possibilities. There are a lot of details that could be discussed ast to how these event types might be implemented, but I will just mention the options here and move on to the other parts of this discussion.

Table 4 - Event Type Ideas

Event Type	Notes
Basic	These are like traditional MIDI events.
Note Off	Note Number
	Release Velocity Optional
Note On	Note Number
	Attack Velocity
SetTrackVolume	Value
SetTrackPitchBend	Value
SetTrackPan	Value
SetTrackEffect	Any Effect Type
	Value
SetInstrument	Program Change
Track End	Loop to Start if the Sequence is Flagged
Test Values	We can have an array of test values at the sequence level or track level (the value could have a name and have the string mapped to a lookup number)
Set Test Value	Sequence or Track Test Value
	Test Value ID
	Value
Inc Test Value	Sequence or Track Test Value
Dec Test Value	Sequence or Track Test Value
DecZ Test Value	Do Not Decrement Below Zero
Advanced
SetSequenceVolume	Value
SetSequenceEffect	Any Effect Type
	Value
SetListenerWorldPosition	Sets 3D Pan and Volume
SetTrackWorldPosition	Sets 3D Pan and Volume
SetInstrument If	Sequence or Track Test Value
Position Marker	Marker Name (String Mapped to Number)
Jump to Position Marker	Marker Name (String Mapped to Number)
Jump to Position Marker If	Sequence or Track Test Value
	Marker Name (String Mapped to Number)
State Change	Fades, Ducking, Mute Groups
Change Sequence State	Set immediate state or set a target state and interpolation time
Change Sequence State If	Sequence or Track Test Value
Change Track State	Set immediate state or set a target state and interpolation time
Change Track State If	Sequence or Track Test Value
Change Voice State	Set immediate state or set a target state and interpolation time
Change Voice State If	Sequence or Track Test Value
Arrangement
Jump to Track	Jump to a New Track Data
Jump to Track If	Sequence or Track Test Value
Gosub to Track	Jump to New Track Data, Returns When Done
Gosub to Track If	Sequence or Track Test Value
Callback
Callback	Calls Game Code, Could Change Test Values
Callback If	Sequence or Track Test Value