The Structured Audio metamodel

Structured audio is a way of representing audio information in which semantic information and high-level algorithmic models are used. Examples of existing and well-known structured audio formats include MIDI, any music synthesis language and the linear-prediction coding (LPC) model for speech signals. The term Structured Audio was introduced by Vercoe as a way of interrelating research on sound synthesis, audio coding, and sound recognition [Vercoe et al., 1998].

A structured media representation encodes a signal according to a model making assumptions about the input signal and deriving a parameter space. This property, though, does not suffice to make a representation structured. As a matter of fact any media representation has an implicit or explicit model but the less dimensions its related parameter space has and the more meaning these parameters have, the more structured the representation. Semantic parameters that represent high-level attributes give control over perceptual and structural aspects of the sound and provide for more interesting manipulations. Structured audio representations are parametric in the sense that they are based on a model by which two sounds can be distinguished according to some parameter values.

Audio compression or encoding technology mostly relies on two kind of coders: entropic or lossless coders and perceptual or lossy coders. Entropic coders exploit information-theoretic or Shannon's redundancy and perceptual coders exploit perceptual redundancy or irrelevancy (if sound X and sound Y are perceptually indistinguishable, it does not matter which one is transmitted).

Structured audio coders exploit yet another kind of redundancy: structural redundancy. Most sound signals contain structural redundancy in different ways. Many notes, for example, in a musical track sound the same or nearly the same. Not only a middle C note may be substituted by its model but neighboring notes such as C# may be obtained by transforming the original model algorithmically [Scheirer, 2001]. Another example of structural redundancy is that many sounds are more simply represented as processes than as waveform. A reverberated speech signal, for instance, may be better transmitted by encoding separately the flat speech and the description of the reverberation algorithm)[Vercoe et al., 1998].

Although structured audio is not perceptual encoding, a fundamental issue in structured audio is how listeners perceive the sound and thus how structural parameters affect perception. Structured audio is not interested in usual engineering properties such as perfect reconstruction. A minimum squares measure of the error is not useful because humans do not perceive sound this way.

So, in a structured audio application sound is coded not based on perceptual or information-theory related compression but rather representing its structure. This structural description of sound is then transmitted to a receiver which reconstructs the sound by executing real-time synthesis algorithms. All audio signals are more or less structured and neither entropy nor perceptual encoders exploit this feature. It is interesting to note, as Scheirer suggests in [Scheirer, 2001], that using structured audio it is possible to transmit data at a lower rate than that suggested by the Shannon rate distortion theory, which only becomes an unsurpassable limit if really random signals were to be transmitted and that is in practice seldom the case.

A coding format consists on two parts: a bitstream description that specifies the syntax and semantics of data sequences and a decoding process, which is an algorithm that describes how to turn the bitstream into a sound. A bitstream is a sequence of data that follows some particular coding format and represents a compressed sound when its length is shorter than the sound it represents.

What makes structured audio coding different is that the model is not fixed but rather dynamically described as part of the transmission stream. As a matter of fact, structured audio can be considered a framework or metamodel that can be used to implement all other coding techniques [Scheirer and Kim, 1999]. It can be proven that the best-known performance of a fixed audio coding method serves as the worst-case estimate of structured audio coding [Scheirer, 2001].

Structured Audio allows ultra-low bitrate transmission of audio signals. It also provides perceptually driven access to audio data. Bearing in mind these two main benefits, many applications may be envisioned: low bandwidth transmission; sound generation from process models (e.g. as in video games or virtual reality applications); flexible music synthesis by allowing the composer to create and transmit synthesis algorithms along with the event list; interactive music applications; content-based transformations and manipulations; and content-based retrieval.

Arguably, out of these applications the most important one is the transmission of ultra-low bitrate audio signals. Structured audio provides excellent compression when models that can be controlled with few parameters are available. This may be so, for example, if the space of sounds to encode is reduced. For example, if only plucked strings are to be transmitted, a plucked string model can be transmitted first and then send only the parameters that control this model. The more structured sounds are the more they can be compressed5.6. Compression rates of up to 10000:1 can be accomplished with structured audio on some particular signals [Scheirer, 2001].

The structured audio metamodel made its way into the MPEG4 standard mainly thanks to the work of Eric D. Scheirer from the MIT's Machine Listening Group. Now seen in some perspective it seems to us that Structured Audio was not mature enough to make it into an standard. It has hardly found any practical application and it has become outdated very soon after. Furthermore, its limitations and too strict specifications make it hard to adapt to future needs.

It is far beyond the scope of this document to give a thorough overview of the standard and we refer the reader interested in structured audio in MPEG-4 to [Scheirer et al., 1998,Scheirer, 1999a,Scheirer and Kim, 1999,Scheirer, 1998b,Scheirer, 1999b,Scheirer, 1998a,Scheirer et al., 2000]. Nevertheless, it is important to highlight that there are five major elements in the MPEG-4 Structured Audio (SA for short) toolset: a Structured Audio Orchestra Language (SAOL), a Structured Audio Score Language (SASL), a Structured Audio Sound Bank Format (SASBF), a normative scheduler, and a normative reference to several MIDI standards that can be used in addition or instead of SASL. englishNote that the two most important components, SAOL and SASL, were already introduced in section 2.6.1.