Changing Filter Parameter Values and Constraints
MSO allows you to manually adjust filter parameters and also use the optimizer to find the optimum parameter values. For any parameter, you can specify whether or not the optimizer is allowed to alter it. If you permit the optimizer to modify it, you can specify the range of permissable values (the constraints) for the parameter. If not, the parameter will retain its user-chosen value at all times. You perform these operations using the Properties window.
Using the Properties Window
Try adding a Parametric EQ to the Filters category of a channel. After doing so, select the filter node in the Config View on the left, and you'll see its properties displayed in the Properties window on the right. The Properties window is where you can examine and alter filter parameter values and their constraints. The first group of properties shown in the Properties window is Filter Information, which is just a description of the type of filter, its reference designator, and the configuration and filter channel to which it belongs. You cannot change this information. The second and subsequent groups are the filter parameters, which can be changed. In the case of the Parametric EQ, the parameters are Center frequency in Hz, boost in dB, and Q. Each parameter has four properties: its value, minimum and maximum allowed values, and whether or not the optimizer is allowed to adjust the parameter's value. You can alter the properties of any parameter by clicking in the field containing the value and editing it directly. After doing so, press Enter to finalize the change. The text of the value will then change to bold. In addition, when clicking on the Value property, a spin-button control will appear, allowing you to adjust the value by pressing and holding the left mouse button down on a spin button arrow as well. This can be useful (or gimmicky, depending on your perspective) if a graph is currently being displayed having traces that depend on the value of the parameter. In this case, pressing and holding the spin button causes a "tuning" effect and the graph updates continually in real time to show the effect. The Windows spin-button control takes a while before its action speeds up enough for this to be easily visible.
Here's another example. Select the Parametric EQ you've added. Then right-click on it in the Config View, and while glancing at the Properties Window, choose Lock Filter Parameters from the context menu. You'll see the Optimization allowed property of all three parameters change from True to False. The filter's icon will also turn gray to indicate that the optimizer will not alter its parameter values. Then right-click on the filter again and choose Unlock Filter Parameters to change the Optimization allowed property back to True for all the filter's parameters. The filter's icon will turn back to a white color to indicate that the optimizer can alter the filter's parameter values. "Lock" in this context means "require the optimizer to respect the user's value for the filter parameters". The user can still modify any parameter value or other editable properties using the Properties Window. To lock a single parameter, it's necessary to do so using the Properties Window, by setting its Optimization allowed property to True. If you need to alter many filters at once, locking and unlocking can be used at different levels of the tree hierarchy in the Config View (such as all filters in a channel, all subwoofer filters, all the way up to locking all filters in the configuration).
The Importance of Constraints
Constraints are the means by which you prevent MSO from using parameter values that are impractical or that your DSP hardware does not support. The optimizer used by MSO is a general-purpose one, so it does not automatically constrain itself to avoid, say, boosting a null in a frequency response. You impart this information to MSO by applying constraints to filter parameters. This can be done on an individual filter basis using the Properties window as described above, or you can define constraints for different parameters of newly-created filters of various types choosing Tools, Application Options from the main menu. The Application Options property sheet has a tree view on its left, allowing different filter types to be chosen. Choosing a filter type will cause a property page for the selected filter type to be displayed on the right.
Example: Maximum Boost for Parametric EQ
By default, the maximum value for the center-frequency boost of newly-created PEQ filters is 0 dB. Suppose you want this default maximum value to be 3 dB. Launch the Application Options dialog by choosing Tools, Application Options from the main menu and select Parametric EQ under the Filters category in the tree view at the left of the dialog. At the upper right of the associated property sheet, check the checkbox labeled Use custom value for maximum allowable gain, and enter the number 3 in the Custom maximum gain, dB edit field. Click OK to close the Application Options property sheet. This change will only affect newly-created parametric EQ filters, not existing ones. If you wish to alter the maximum allowable boost of PEQ filters that you already added before making this change, you'll need to do so by selecting each filter and using the main Properties window.
Advanced Example: Ensuring High Output With Dissimilar Subwoofers
When using dissimilar subwoofers, ensuring the highest possible maximum output is more difficult than it appears. Although MSO only deals with frequency responses and does not optimize large signal parameters, by using constraints and other techniques, you can steer the solutions of MSO toward those that are favorable to a high maximum output. This is discussed in the separate Dealing With Dissimilar Subwoofers topic.