Changing parameter values

The parameter values of model instances are edited using the property editor. When selecting the model instance to be changed, MLDesigner activates the Instance Properties plane. In this plane, the property editor shows the actual parameters of the selected model instance. By clicking the value field of the relevant parameter, it becomes editable. To open a multiple line text editor, you can click on the icon to the right of the edit field. Parameter properties that cannot be edited have gray text.

Parameter Expressions

Parameter values set through MLDesigner can be arithmetic expressions. This is particularly useful for propagating values from a system model parameter to primitive parameters lower down in the hierarchy. An example of a valid parameter expression is:

PI/(2*$order)

where order is a formal parameter defined in the module or system model. The basic arithmetic operators are

+

addition,

-

subtraction,

*

multiplication,

/

division, and

^

exponentiation.

These operators work on integers and floating-point numbers. Currently all intermediate expressions are converted to the type of the parameter being computed. Hence, it is necessary to be very careful when, for example, using floating-point values to compute an integer parameter. In an integer parameter specification, all intermediate expressions will be converted to integers.

The Parameter Value Expression dialog helps you to build parameter expressions and can be reached by clicking the ellipsis button on the right of a parameter value.

Using Tcl expressions in parameters

Arbitrary PTcl expressions can be embedded in a parameter expression by preceding the expression with the tcl keyword as in the following example:

Firstly, parameters in the form of {parameter} appearing in the expression are replaced by their values. Then the string is sent to the built-in interpreter for evaluation. Finally, the result is spliced into the parameter expression and re-parsed.

This facility is generalized and supports both numeric and symbolic computing of expressions. Through PTcl, you can access all of its math functions, which generally behave the same as ANSI C functions of the same name: abs, acos, asin, atan, atan2, ceil, cos, cosh, double, exp, floor, fmod, hypot, int, log, log10, pow, round, sin, sinh, sqrt, tan, and tanh.The availability of these function may vary in future version of MLDesigner. To see which functions are actually supported you may consult the Parameter Value Expression dialog, it provides a drop-down menu with the available functions.

A parameter expression could be

for the amplitude of the oscillators in a binary frequency shift keying system, in which BitDuration is a parameter. The expr command is a PTcl command that treats its arguments as a single mathematical expression that must evaluate to a number.

There are several PTcl commands embedded in MLDesigner that help support parameter calculations. They are: listApplyExpression, max, min, range, rangeApplyExpression, and sign. For example,

first evaluates to min 3 -1 and then to -1. The procedure range returns a consecutive sequence of numbers:

returns 0 1 2 3 4 5. The rangeApplyExpression procedure generates a sequence of values by applying a consecutive sequence of numbers to a PTcl expression that is a function of i. For example, you can generate the taps of an FIR filter that is a sampled sinusoid by using

This generates one period of sinusoidal function and returns

The listApplyExpression is similar to rangeApplyExpression except that it only takes two arguments: the second argument is a list of numbers to substitute for i in the expression. The command

is equivalent to the previous example of the rangeApplyExpression function.

You can receive help on these PTcl procedures by typing

at the prompt.

Array parameters

When defining arrays of integers, floats, complex numbers, fixed-point numbers, or strings, the basic syntax is a simple list separated by spaces. For example,

defines an integer array with five elements. The elements can be expressions if they are surrounded by parentheses:

Repetition can be indicated using the following syntax:

where n evaluates to an integer. An array or portion of an array can be an input from a file using the < symbol as in the following example:

Here the first two elements of the array will be 1 and 2, the next element(s) will be read from file filename, and the last two elements will be 3 and 4. The latter capability can be used in combination with the WaveForm primitive to read a signal from a file.

String Parameters

There are complications when you wish to set a string parameter or stringarray parameter equal to the value of a module or system model parameter. This is because a distinction must be made between a sequence of characters that give the name of a symbol and a sequence of characters to be interpreted literally. The syntax to use is:

Here $word represents the value of a string module or system model parameter. This capability is especially useful for constructing labels for output plots. When using string parameter to specify options to execute a command, as in the options parameter in Xgraph primitives. You need either double quotes or single quotes to include white space:

String arrays have a few more special restrictions. Each word (separated by whitespace) is a separate entry in the array. To include white space in an element of the array, use quotation marks. Thus, the following string array

has three elements in it. The string array

has ten separate copies of the string repeat in 10 separate entries in the array.

Analogous to string you can use the dollar sign to refer to parameters. Thus, in

paramname must be the name of either a string array or a scalar-valued parameter (an integer, float, or complex array, for example, is not permitted). If it is a string array, then each element of paramname becomes an element of the parameter. If it is some other kind of parameter, the value becomes a single element of the string array.

String array values may also be read from files using the < symbol. Note that for string arrays, the filename can be a literal string such as

as well as a string that refers to parameters such as

in this case the value of the parameter filename would be substituted. MLDesigner does not perform expansion of filenames such as file{1,2} into file1 file2 as a Bash might do.

Complex-valued parameters

When defining complex values, the basic syntax is

where real and imag evaluate to integers or floats.

Fixed-point parameters

Fixed-point parameters may be directly assigned a precision. To do this, the parameter is given in the syntax (value, precision), where value is an ordinary number and precision is given by either of two syntaxes:

Syntax 1

As a string like 3.2, or more generally m.n, where m is the number of integer bits (to the left of the binary point) and n the number of fractional bits (to the right of the binary point). Thus length is m+n.

Syntax 2

A string like 24/32 which means 24 fraction bits from a total length of 32. This format is often more convenient because the word length often remains constant while the number of fraction bits changes with the normalization being used.

In both cases, the sign bit counts as one of the integer bits, so this number must be at least one. Thus, for example, a fixed-point parameter might be defined as (0.8, 2/4). This means a 4-bit word will be used with two fraction bits. Since the value 0.8 cannot be represented in this precision, the actual value of the parameter will be rounded to 0.75.

A fixed-point parameter can also be given a value without a precision. In this case, the default precision is used. This has a total word length of 24 bits with the number of integer bits set as required to store the value. For example, the number 1.0 creates a fixed-point object with precision 2.22, and a value of 0.5 would create one with precision 1.23.

The precision of internal computations in a primitive is typically given by a parameter of type precision. A precision parameter has a value specified using either of the two syntaxes above.

Inserting comments in parameters

Comments are supported for non-string parameters. A comment is specified with the hash (#) symbol. Everything after the hash until the end of the line is ignored when the parameter is evaluated. Comments are especially useful in combination with files, as they remind you about which module or primitive parameter the code refers to. Comments are, however, not supported for the string or stringarray parameter types. In fact, when the image processing primitives use string states to represent a filename, the hash character is used to denote the frame number of the image being processed.