BSL PRO Software Guide - Biopac

238 Part DEnabling Auto threshold detect enables Noise rejection and Window options.NoiserejectionWindowBiopac Student Lab constructs an interval around the threshold level when Noise rejection is checked.The size of the interval is equal to the value in the noise rejection text box, which by default is equal to5% of the peak-to-peak range. Checking this option helps prevent noise “spikes” from being countedas peaks.The Window section prompts you to specify an upper and lower limit for the rate calculation. Bydefault, these are set to 40 BPM for the lower criterion and 180 for the upper criterion, but may beadjusted as necessary. Setting the upper and lower bounds for the “window” tells Biopac Student Labwhen to start looking for a peak. Biopac Student Lab will try to locate a peak that matches theautomatic threshold criteria within the specified window. If no peak is found, the area outside theenvelope will be searched and the criteria (in terms of peak value) will be relaxed until the next peak isfound.For instance, once the first peak is found, Biopac Student Lab will look for the next peak in an intervalthat corresponds to the range set by the upper and lower bounds of the window. The interval withassociated with the upper band of 180 BPM is 0.33 seconds (60 seconds ÷ 180 BPM), and the intervalfor the lower band is 1.5 seconds (1 minute ÷ 40 BPM). If a second peak is not found between .33seconds and 1.5 seconds after the first peak, then Biopac Student Lab will look in the area after 1.5seconds for a “smaller” peak (i.e., one of lesser amplitude). For those rate functions that require awindow interval in seconds, you will probably want to enter numbers like .33 seconds and 1.5 seconds(which correspond to the BPM defaults of 40 and 180). These numbers will be suitable for detectingthe heart rate of an average subject.A simple peak detector uses what is called a threshold-crossing algorithm, whereby each time theamplitude (vertical scale) value exceeds a given value, the peak detector “remembers” that point andbegins searching for the next event where the channel crosses the threshold. The interval between thetwo occurrences is then computed and usually rescaled in terms of BPM or Hz. This is how the BiopacStudent Lab Rate Calculation functions when all options are unchecked.Windowing Units are related to the selected “Function.”Rate Functions related to peak or peak time specify the unit after the Function. This is a fixed unit, sothe Windowing Units menu will be disabled.Rate Functions related to data within a cycle can be specified in units of frequency (Hz) or time (BPMor seconds). The Windowing Units menu will be enabled,a nd when a unit is selected the software willconvert the Min and Max settings to the selected units.In the sample waveform here, the threshold wasset to 390 mVolts to detect of the peaks of thewaveform and provide an accurate ratecalculation. Since it only recognizes signalsgreater than 390 mVolts as a peak, this 390-mVolt threshold is referred to as an “absolutethreshold.”Most waveforms are not so well behaved,however, and artifact can be introduced as aresult of movement, electrical interference, and soforth.Combined with actual variability in the signal ofinterest, this can result in “noise” being includedwith the signal, as well as baseline “drift” whichcan render absolute threshold algorithms useless.Visit the online support center at www.biopac.com