We now define AFL expressions that yield a new speech state -these have no side-effects. The simplest expressions are the names of the nine predefined voices (e.g., afl:paul) and the names of variables to which states have been assigned. Using these expressions to move in the speech space makes the space a collection of discrete points. In addition, AFL provides four operators for generating new points in speech space. Each of these ``move operators'' expresses a change along a single dimension of the state space. While one move operator would have been sufficient, having multiple operators makes AFL easier to use. These operators allow us to express relative changes to the speech state. To give some intuition, they provide the same ability as scaling a font in the visual setting.
yields a state that is the same as point except that offset has been added to dimension dimension. For example, the following statement adds 50% to the assertiveness of 'afl:paul.
yields a new state, point with the value of dimension dimension multiplied by factor.
yields state point with the value of dimension dimension set to value.
yields state point with the value for dimension dimension changed by steps steps. Each dimension has a default step size, which specifies the minimum change needed to be perceptible. The step sizes for the MultiVoice parameters are shown in the ``step size'' column of ta:par. Using step-by, one can have the value of a dimension changed by a multiple of the step size.
The step-size for a particular dimension can also be changed by supplying the additional keyword afl:step-size to any of the AFL operators. For example, while the expression
yields a new state with the value of afl:average-pitch changed, the expression
yields a new state with the step size for afl:average-pitch changed to 2. Note that this expression makes use of named parameters in Common Lisp.
The four move operators are shown in their simple form. In general, these operators take a point and a list of dimension-value pairs specifying how to move.