Audio Source Separation Using Perceptual Principles for Content-Based Coding and Information Management

ing access to the data in this way has a number of powerful advantages. Separating the link structure from the nodes themselves allows multiple references to a particular node to be made with only a single copy of the data itself being stored. This has obvious benefits both from the point of view of expediting access and optimising storage requirements. Abstracting the presentation layer from both of the lower levels has similar advantages in that it allows the same nodes, and even link structures, to be presented in any number of user defined, context-dependent manners. Thus, the basic requirements of any hypermedia system are: • Node definition: a means to chunk the data into self-contained conceptual units that can contain anchor points for the source or destination of links; • Links: a means to define and manage relationships between the nodes; and • Interface: a presentation engine that is independent of the node-link structure. Despite its simplicity, the hypermedia data model is a powerful tool for information management. Representing a given document, for example, as conceptual units (nodes) rather than lexical units (letters or words) allows it to be viewed as a structured collection of related ideas and not just as a string of letters or words. Thus, the focus can be on what message is being conveyed rather than on how it is being conveyed. Further, the information structure can be manipulated without having to deal with the underlying data. The power of this level of abstraction becomes even more apparent in the case of 34 Melih, K. Audio Source Separation.for Content-based Coding and Management audio data where the basic lexical units (the instantaneous amplitude values of a waveform) convey very little useful content information and where manipulation of the raw data representation is tedious and requires a high level of user expertise. 1.2. 3. The State of the Art in Hypermedia Systems Thus, hypermedia systems would obviously be a powerful information management resource. Yet the question remains, does a true hypermedia system exist? In the case of textual and graphical data, the WWW offers a partial answer in the affirmative. However, as far as audio data is concerned, the situation has progressed little from the position it occupied in Bush’s day. A single audio file can serve only as a complete node at the destination of a link. No mechanism is available to either source or sink a link from within a node and the only controls that are available over the access and presentation of the node are those that are found on a standard tape player, that is start, stop, pause, etc. This is because audio data in its raw form is highly unstructured and gives very little indication of the inherent structure or semantic content of the underlying signal. Because of this obvious deficiency, the WWW would more accurately be described as a “multimedia enhanced” hypertext system than a hypermedia system. Given the vast amounts of audio data now hidden away in electronic repositories, it is simply infeasible to expect that extracting nodes from, as well defining potential source and destination link anchor points in, this data be done manually. Indeed, this is a nearly impossible task for even a single audio recording because of the temporal nature of audio signals and the fact that several nodes and/or anchor points would commonly coexist in time. Hence, there is a clear need for automated mechanisms to decompose audio data into conceptual units that might form nodes in a hypermedia system. 1.2. 4. What is an Audio Node? Before we can devise mechanisms to extract nodes from a given audio recording, we must first determine exactly what constitutes an audio node. Referring to the definition of a general hypermedia node, we note that it must be a self-contained unit embracing a single concept. The basic idea of a self-contained audio unit is encapsulated in the audio objects of auditory scene analysis (ASA) [1]. The primary aim of ASA is to determine how we, as humans, separate out, without necessarily identifying, each of the constituent sounds that make up an auditory “scene”. Each of the constituent sounds identified by the perceptual system is termed an auditory object or stream. The exact nature of an auditory object is context dependent. For example, a recording of a piece of music played back through a speaker may at one time form the primary focus Chapter 1: Introduction 35 of attention and be considered as a single object that is distinct from the background noise “object”. At another instant in time this object may be combined into the background noise object to make way for a new object that represents the voice of a person that has just entered the room. In both instances, the listener will likely be aware that the background noise object is actually composed of several sub-objects. However, they will generally want to ignore this for the rather pragmatic reason of reducing information processing requirements. This concept of context dependence leads to a recursive, hierarchical definition for an audio node. However, the nodes at each level will still embrace a single concept. These concepts will range from general to specific between the lower and higher levels of the hierarchy. Thus this hierarchical definition of an audio node is in keeping with that of a general hypermedia node. This concept is illustrated in Figure 1-2.