Chapter 4. Using event filters

Event filters are a SpecTcl object that can drastically improve performance if your analysis requires several passes over the data and/or progressively reduces the set of parameters and events you care about. Performance is improved by writing an event file that can usually be decoded much more efficiently than most raw event files.

This chapter:

A filter is a list of parameters, an output file and a gate. When a filter is specified and enabled (filters can be enabled or disabled), for every event that satisies the gate, the parameters specified by the filter are written to disk. The output format is one which is self describing and easy to process back ito parameters.

The filter command creates and manipulates filters. Making a filter usable is a multi step operation:

  1. The filter must be created, specifying the gate that will determine which events are output (note you can use a True gate to output all events), and the set of parametesr to be output.

  2. The filter must be associated with an output file. This is the file to which the filter will write event data.

  3. The filter must be enabled.

Note that the filter does not respect run boundaries. It's up to you or your analysis script to disable a filter attach to a diferent file and re-enable as desired. This allows the data from several runs to be collected into a filter if desired. This is an intentional design choice.

To create a filter:

filter -new name gate parameters


Is the name given to the filter. In all subsequent commands, this name must be used to select this filter.


Name of the gate that determines which events produce output. To output all events you can use a True gate.


This is a well formed Tcl list of the names of the parameters to output to file for each event that satisfies the gate. See Scripting with SpecTcl for information about how to script SpecTcl so that you don't have to type this in. See Using the 'standard' SpecTcl GUI for a description of the SpecTcl default user interface whih includes a GUI for defining and manipulating filters.

Once the filter has been created it must be associated with an output file:

filter -file filename filtername

filename is the file to which filter data will be written (any existing file is overwritten). filtername is the name of the filter that will write that file.

Once the filter is defined and connected with a file, enabling the filter will cause it to output events that satisfy its gate until it is disabled or SpecTcl exited:

filter -enable filtername
filter -disable filtername

For a complete description of the filter command, see the command reference.