5.1 Requirements

Preciseness 

Naturally, the representation has to describe the spectral envelope obtained from estimation as precisely as possible. Methods which don't fulfill this basic requirement have not been considered here.

Stability

The requirement of stability  mandates that the representation be resilient to small changes in the data to be represented. Small changes, e.g. in the presence of noise, must not lead to big changes in the representation, but must result in equally small changes.

There is also a desirable aspect of uniqueness  in that the same input data should result in the same representation every time.

The requirement of stability is of great importance if we consider that the data to be represented can result from various different estimation methods, like cepstral or LPC analysis, or even from manual input, and that some noise is always present.

Locality

The locality  requirement states that it be possible to achieve a local change of the spectral envelope, i.e. without affecting the intensity of frequencies further away from the point of manipulation.

Ideally, the representation would fulfill the requirement of orthogonality , where one component of the spectral envelope can be changed without affecting the others at all.

Flexibility and ease of manipulation

    The representation must be chosen such that manipulations with an exactly defined desired outcome can be easily specified, e.g. a certain formant location that has to be reached in voice synthesis. For the manipulative abilities to be really useful for musical applications, the relationship between the parameters of the manipulation and the effect on the spectrum has to be easily understandable.

Speed of synthesis 

The representation should be usable for sound synthesis as directly as possible, without first having to be converted to a different form at high computational costs. This requirement is heavily dependent on the type of synthesis, e.g. additive synthesis or filtering. This means that no ideal solution can be presented, but a compromise which is not the fastest choice for each synthesis type, but which doesn't penalize too much, even in the worst case.

Space 

It is required that the representation not take up too much space, especially with the file representation in mind.

Manual input 

Finally, the representation should be easy to specify manually, e.g. by drawing a curve or placing primitive shapes, or by textual input of parameters.