3. Sound-object prototypes

A string of terminals and structural symbols generated by the Bol Processor is converted to MIDI messages in the following way: each terminal represents a sequence of MIDI codes or a " musical gesture" recorded from a MIDI instrument. We call these sequences sound-object prototypes.

Sound-objects are generally not performed at the same speed they have been defined. Their time-scale depends both on the tempo of the performance and on certain properties assigned to their prototypes.

3.1 Metrical properties of sound-objects

The position of an object in relation to time streaks is calculated on the basis of some properties introduced now. We use the names of properties as per previous publications. These appear more explicitly in the new BP2 interface.

Property PivBeg means that the streak position is the first MIDI message of the object prototype. PivEnd means the last message. PivCent means the centre. PivBegOn means the first NoteOn. PivEndOff the last NoteOff. PivCentOnOff means the centre of the time-span interval starting on the first NoteOn and ending on the last NoteOff. PivSpec is a general case in which the time offset between the first MIDI message and the streak is specified. These properties are explicitly shown in the dialog Fig.2.

Fig.2 shows that the pivot location may either be defined absolutely (in milliseconds) or as a percentage of the object's duration. In the latter case, it will be modified according to the current time base of the performance if the object has properties OkRescale, OkCompress or OkExpand.

OkRescale means that this object may be expanded or compressed according to the time base. FixScale means it can't. OkCompress means it can be compressed, OkExpand that it can be expanded. (See explicit representation on Fig.2) Indeed, if OkRescale is true, then all other properties except FixScale are made true. The scaling of an object is expressed by the dilation ratio a. If a> 1 then the sound-object is performed slower than its prototype. The dilation ratio may be confined to a range deemed acceptable for the sound-object (see Fig.2). Periodical sound-objects are handled in a different way, see §2.5.

The value of amay be sent to the MIDI device (before the sound-object is performed) in case the sound processor is able to make sense of it for modifying parameters in synthesis algorithms. (Most commercial synths don't.) The current value of ais encoded logarithmically in range 0.01 to 100, and the MIDI controller value is

so that a = 0.01 will yield 0 and a = 100 yields 128. If you want this value to be passed to the sound processor each time this particular sound-object is performed, then check "Send dilation ratio to controller".

"Relocate at will" ( Reloc) means that the pivot of the object may be located far from the time streak if necessary. "Never relocate" forces location on the time streak. Relocation may also be allowed within specific limits (see Fig.2).

3.2 Topological properties of sound-objects

OverBeg means that the beginning of the sound-object may overlap the end of an object preceding it in the same sequence. OverEnd means that its end may overlap another object following it in the same sequence. TruncBeg means that the beginning may be truncated if it overlaps an object in the same sequence. TruncEnd means that the end may be truncated if it overlaps an object in the same sequence. These properties (see Fig.3) may also be assigned within absolute or relative limits.

Continuity properties are shown Fig.4. ContBeg means that the beginning of this object should join (or overlap) the end of a preceding one in the same sequence. ContEnd means the end of this object should join (or overlap) the beginning of a following one. These properties may be compensated with a tolerance for a gap preceding or following the object.

MIDI properties (see Fig.4) instruct BP2 how to deal with channel, transposition, articulation, volume and panoramic changes as far as this object is concerned.

Given these sets of properties (stored in the "-mi." file) and a setting of the time base, BP2 will try to set all objects in time. For this it may need to relocate some of the objects that have Reloc or BrkTempo properties, truncate other objects, etc. If there is no solution, BP2 may release some of the constraints. (See operational mode §5.2)