BSL PRO Software Guide - Biopac

Acquisition Functions 81Sample RateThe “Sample Rate” value indicates how many samples the MP UNIT should acquire per second on each channelduring data acquisition. The default Sample Rate is 1,000 samples/sec, but can be changed by clicking on the pulldownmenu. Use the pull-down menu to select a sample rate between 1 sample/sec. and 100,000 samples/sec. Youmust select one of the menu options — you cannot enter a custom sample rate. You need to scroll up to see thehigher sample rates.MP30 users: It is important to note that Sample Rates above 2,000 samples/sec are considered “high speed” rates and thesoftware behaves differently in High Speed mode (see page 684).Depending on the nature of the data being acquired, the “best” choice for Sample Rate will vary.Technically speaking, the minimum sampling rate should be at least twice the highest frequency component of interest. Thismeans that if the phenomenon you are interested in observing has frequency components (of interest) of 100 Hz, you shouldsample at least 200 times per second. Fourier analysis (FFT) can be used to determine what frequency components are present inthe data (see page 6222 for a more detailed description of the FFT function). Applications that typically involve higher samplingrates are ECG, EEG and evoked response acquisitions. In less technical terms, slower sampling rates can be used for data withslowly changing values (respiration, GSR, and the like), whereas higher sampling rates should be set for data where valueschange markedly (either in magnitude or direction).The disadvantage of acquiring data at high sampling rates is that each sample point takes up memory, whether it is RAM or harddisk space. Moreover, once the file is saved, it will require more hard disk space than a file of similar duration sampled at aslower rate.The maximum allowable sampling rate is 100,000/second, but rates over 2,000/second are considered “high speed” and do havelimitations (see below). Set the Sample Rate from the pull-down menu options. TIP A good rule of thumb is to select a sampling rate at least four times the highest frequency of interestfor the signal.Notes: 1) A waveform is considered “compressed” when more than three sample points are plotted per pixel onthe screen. Using the default horizontal scale (which plots eight seconds of data on the screen), anydata sampled at more than 250 samples per second would be considered compressed.Standard VGA displays are 640 pixels wide, so a compressed waveform on this type monitor would beany type of waveform displaying more than 2000 samples (approximately) on the screen at any onetime. Use the Draft mode for compressed waves option to plot compressed waveforms in draft mode,which results in faster plotting time, although the display is not exact.2) It is possible to set a Sample Rate that is too high.