Rectifier / Integrator for EMG
When recording EMG it can be advantageous to perform some signal processing before sampling the EMG measure. EMG energy has a relatively wide frequency range, from 30Hz up to 500Hz. This would require high data acquisition rate if the BIOAmplifier is linked directly to the A-D converter, three times the maximum frequency, with the anti-aliasing filter set to 500Hz. At this rate, huge volumes of data will be generated - a single channel will produce at least 200kB per minute of recording. This volume of data will then require to be processed by software rectifier / integration methods, which will be slow even on the fastest computers, and largely a waste of effort.
SAM has a special Rectifier/Integrator mode in which the A-D converter runs at a high data rate (~10kHz), but each reading is rectified and added to a sum variable. This variable is then divided by the number of samples that have been read in the time defined for each sample sent to the data file, which can be any period. SAM sample rate may be set from 78Hz up to 4,500Hz (10kHz in special mode). Thus while data may be stored at say 200Hz, the amplifier filter may be st to 500Hz or higher, allowing all the EMG energy to be recorded. This mode is only appropriate for measures like EMG where rectification may be used. SAM rectifier/Integrator mode complies with the standard sited by Fridlund, Psychophysiology, Vol 16, No. 5, 1979.
Comparison of raw (upper) and rectified EMG.