Data Representation

The data representation classes, illustrated in Figure 1-1, are used to encapsulate and organize music data.

MKNotes, MKParts, and MKScores form the core of music representation. Of paramount importance is the MKNote class: A MKNote object represents a musical note as a list of attributes, such as frequency, amplitude, and duration. Music applications use MKNote objects as a common currency: They're the basic package of musical information upon which the other objects act. MKPart and MKScore objects, as their names suggest, provide a means for organizing MKNote objects. The other data classes, MKEnvelope, MKWaveTable (and its progeny), and MKTuningSystem, are designed to help define MKNote object attributes:

• MKEnvelopes represent time-varying functions that can be used to continuously control the values of a MKNote's attributes (such as its amplitude and frequency).

• A MKWaveTable contains timbral information that's used during music synthesis on the DSP.

• A MKTuningSystem is a mapping of pitch names to specific frequencies, allowing an easy representation of alternate tunings.

The MusicKit defines an ASCII file format called scorefile that represents the music data objects as editable text in files on a disk. A few C-like programming constructs, such as variables and arithmetic operators, can be used in a scorefile to help create and fine-tune music data. You can also store music data as a Standard MIDI File.

Synthesis

Synthesizing music is potentially the most technically involved of the three MusicKit areas. At the easiest level, you can use and manipulate the software instruments, called MKSynthPatches, that are provided by the MusicKit. A MKSynthPatch subclass corresponds, roughly, to a voice preset on a MIDI synthesizer. However, the MusicKit MKSynthPatches are generally less confined than most MIDI presets: An enormously wide variety of sounds can be produced by the MKSynthPatches supplied by the MusicKit simply by varying the attributes of the MKNotes that they receive.

At a lower lever, you can design your own MKSynthPatch subclasses by interconnecting DSP synthesis modules that the MusicKit provides as objects called MKUnitGenerators. Finally, at the lowest level, you can design MKUnitGenerators yourself by writing MC56000 assembly language macros and using the dspwrap tool to turn the macros into subclasses of MKUnitGenerator. This last level falls below the boundary of the MusicKit and is described in Programming the DSP. The principal MusicKit synthesis classes are shown in Figure 1-2.

The MKSynthInstrument class isn't strictly part of the synthesis machinery. However, it provides an easy way to allocate and control MKSynthPatch objects.

An additional class, not shown in the illustration above, is MKOrchestra. An MKOrchestra represents an entire DSP; the standard configuration includes a single DSP, thus most applications will create but a single MKOrchestra object. It's through an MKOrchestra that all synthesis resources, such as MKUnitGenerators and MKSynthPatches, are allocated.

Performance

During a MusicKit performance, MKNote objects are acquired, scheduled, and rendered (or realized). These functions are embodied by objects of the MKPerformer, MKConductor, and MKInstrument classes:

• MKPerformer objects acquire MKNotes.

• Through messages scheduled with a MKConductor object, a MKPerformer forwards each MKNote it acquires to one or more MKInstruments. The MKConductor thus controls the tempo of the performance.

• An MKInstrument receives MKNotes that are sent to it by a MKPerformer and realizes them in some manner, typically by synthesizing them on the DSP or by sending them to an external MIDI instrument. Other types of realization include writing MKNotes to a scorefile or adding them to a MKPart.

MKPerformer and MKInstrument are abstract classes; each subclass specifies a particular means of MKNote acquisition or realization. The MusicKit provides a number of MKPerformer and MKInstrument subclasses.

Figure 1-3 shows the primary classes that are used to design a MusicKit performance.

In addition to the MKPerformer, MKConductor, and MKInstrument classes described above, five other classes are included in Figure 1-3: MKNoteSender, MKNoteReceiver, MKNoteFilter, MKSynthInstrument, and MKMidi.

• MKNoteSender and MKNoteReceiver objects are part of the implementation of MKPerformer and MKInstrument: They're the ports through which MKNotes are sent by MKPerformers and received by MKInstruments.

• A MKNoteFilter is a MKPerformer/MKInstrument hybrid; while it inherits from MKInstrument, it also implements MKPerformer protocol. Thus, it can receive MKNotes like an MKInstrument and then send them on to other MKInstruments, like a MKPerformer. MKNoteFilters are interposed between MKPerformers and MKInstruments and act as MKNote-processing modules.

• MKSynthInstrument is a subclass of MKInstrument that causes MKNotes to be realized on the DSP.

• A MKMidi object represents an external MIDI synthesizer that's attached to a computer through one of the serial ports. It can receive as well as send MIDI signals from and to the synthesizer it represents. While it inherits neither from MKPerformer nor MKInstrument, it implements their protocols and contains MKNoteSenders and MKNoteReceivers.

A number of other MKPerformer and MKInstrument subclasses are provided by the MusicKit. During a MusicKit performance, performance objects can be dynamically connected and reconnected. This allows you to mix and match MKNote sources with any means of realization. For example, the MIDI signals sent from an external MIDI synthesizer are automatically converted to MKNote objects by a MKMidi object. The MKNotes can then be sent to a MKSynthInstrument for realization on the DSP, or written to a scorefile by a MKScorefileWriter MKInstrument.