KEITH D. MARTIN
See also: To Do list
A screen shot from the Tcl/Tk interface that I built to let me examine the blackboard objects in detail and to help me understand what was really going on in the blackboard internals.
It would be pointless to attempt to build a system that performs transcription in general, but with some restrictions of the domain, transcription can be both possible and useful. Monophonic (one voice) transcription is not trivial, but has been solved as research problem to a large degree. In the 70s, Moorer built a system that was capable of transcribing pieces in two-voice polyphony, with some restrictions on the musical content of the signal. Some advances have been made since then, but multiple-voice transcription has largely been a pipe dream.
Currently, I am building a "blackboard" system which will be capable of transcribing piano performances of four-voice polyphony written in the style of 18th century counterpoint. This restriction of domain is useful for several reasons:
Overview of Implementation
The basic structure of a blackboard system is simple and very
intuitive. The system maintains a workspace where hypotheses are
formed and modified. Typically (and in the case of the current
system), the blackboard structure is hierarchical, with several
separate hypothesis levels in some abstraction hierarchy. In the
current system, the lowest level of the hierarchy contains "harmonic
track" hypotheses, which represent stable "sinusoids" found in the
acoustic signal. The next two levels, in order of increasing
abstraction, contain "harmonic partial" hypotheses and "note"
hypotheses. "Notes" are made up of sets of "harmonic partials", which
are found in the raw signal as "harmonic tracks".
Figure 1: One example of a
hypothesis abstraction hierarchy considered for this project
Outside of the blackboard workspace are a number of "knowledge sources" (KSs) which continually scan the workspace looking for opportunities to apply their knowledge. The name "blackboard system" comes from the metaphor of a collection of scientists standing at the blackboard collaborating to solve a problem.
In the system described here, KSs are fairly fine-grained, and can be described as rules, in the sense of a forward-chaining rule-based system. Each KS has a "precondition" (like the "if"-part of an "if-then" rule). When the precondition is satisfied, the KS becomes activated and "wants" to perform its "action" (the corresponding "then"-part). To keep the control structure simple in the current implementation, the precondition of every KS is run at the beginning of each control loop. Each KS is asked to rate how "beneficial" its action will be (a necessarily nebulous and arbitrary rating!), and the action belonging to the KS with the highest rating is fired.
Currently (as of April 17, 1996), I am in the process of implementing the proposed system from scratch in C++. As of this writing, the basic class structure has been defined, the "track", "partial", and "note" hypothesis objects have been implemented, three knowledge sources have been coded and debugged (a fourth is in the works), and a first run at the basic control structure has been implemented.
Classes that have not yet been implemented are in italics
Currently, as can be seen from the C++ Class Hierarchy, I have implemented three different types of hypothesis objects.
So far, I have worked on four knowledge sources, as described below:
The control structure of the blackboard system as currently implemented is extremely simple.