The earlier version of Bol Processor, namely BP1, was built on a model of "pattern grammars" enabling both the production and fast parsing of sentences (Bel 1987a-b). The grammar format in BP2 is much less restrictive as far as production is concerned. A sound interface has been implemented (using the MIDI standard) which is not geared towards any particular synthesiser; it accepts musical input from any MIDI device.

Below is a block diagram showing the interaction of modules in BP2:

Three fields are used for storing grammars, items generated by the inference engine and sound-object prototypes. Items are represented as structures (strings and sets) of symbols. Each symbol is mapped to a single sound-object prototype or a note. The interpreter generates MIDI codes or Csound score lines given the symbolic structure of an item and properties of sound-object prototypes.

Interpretation is in two stages. Musical items (which may be polyphonic) are represented as strings of symbols in a syntactic form called a polymetric expression. First, a mapping is calculated between the sound-objects contained in a polymetric structure and a set of symbolic dates . Symbolic time, here, is an arbitrary ordered set that permits the ordering of sound-objects. The mapping of symbolic to physical time is called the interpretation of polymetric expressions. In this process, missing information regarding the ordering of sound-objects (along symbolic time) is inferred (Bel 1991,1992a). Then BP2 proceeds to the time setting of the sound-object structure represented as a complete polymetric expression. Start/clip dates of all sound-objects are calculated, yielding the dates of MIDI messages or Csound events. Sound-object properties are taken into account during time setting only.

The block diagram indicates that an external control can be exerted on the inference engine, grammars and the interpretation module. Specific MIDI messages may be used to change weights in grammars, the time base and nature of time (striated/smooth). These messages may also be used for synchronising events during their performance and even assigning computation time limits. Such features are currently used in improvisational rule-based composition.

Several BP2's may be linked together and to other software devices such as MIDI sequencers. Messages on the different MIDI channels and Apple Events may be used for communicating between machines or for controlling several sound processors. It must be kept in mind that a "sound-object" is not necessarily a sound-generating process. Depending on the implementation it may contain any kind of control/synchronisation messages as well.