BSL PRO Software Guide - Biopac

Acquisition Functions 109Calculation Preset: ExpressionThe online Expression evaluator can be used to perform complex computations that cannot be managed with theMath or Function calculations. The Expression evaluator uses standard mathematical notation and will symbolicallyevaluate complex equations involving multiple channels and multiple operations.Unlike the Math and Function Calculations—which can only operate on one or two channels at a time—theExpression calculation can combine data from multiple analog channels and allows you to specify other calculationchannels as input channels for an Expression. Computations performed by the Expression evaluator eliminate theneed for “chaining” multiple channels together to produce a single output channel. For example, you can square onechannel, multiply it by the sum of two other channels, and divide the product by the absolute value of anotherwaveform on a single Expression Calculation channel, which is more efficient than chaining five Math and FunctionCalculation channels together.While the Expression evaluator is more powerful than other Calculation channels, each Expression Calculationrequires more system resources than other Calculation channels do. This essentially means that acquisitions thatutilize Expression Calculations are limited to a lower maximum sampling rate than acquisitions without Expressioncalculations.For each expression, you need to specify at least one Source, the Function(s) to be performed, and any Operators tobe used. Make a selection from the pull-down options or type directly into the Expression box.To measure skin resistance (SCR) using the SS3LA transducer, use an Expression calculation channel to take thereciprocal of conductance and then apply proper scaling.Note:It is important to keep in mind that while different channels, functions, and operators can bereferenced, this Expression Calculation cannot reference past or future sample points. That is,data from waveform one can be transformed or combined in some way with data from waveformtwo at the same point in time, but data from one point in time (on any channel) cannot becombined with data from another point in time (on any channel). See the section on postacquisitionExpression commands (beginning on page 6218) for ways around this limitation.