Codebook.txt

Codebook

This codebook is a modified exceprt of the codebook provided in the original data. Note that only the mean and standard deviation columns are the only ones included int eh final output produced by this analysis. Variables names in the discussion below have been updated to make them more descriptive.

The features selected for this database come from the accelerometer and gyroscope 3-axial raw signals tAcc-XYZ and tGyro-XYZ. These time domain signals (prefix 't' to denote time) were captured at a constant rate of 50 Hz. Then they were filtered using a median filter and a 3rd order low pass Butterworth filter with a corner frequency of 20 Hz to remove noise. Similarly, the acceleration signal was then separated into body and gravity acceleration signals (tBodyAcc-XYZ and tGravityAcc-XYZ) using another low pass Butterworth filter with a corner frequency of 0.3 Hz. 

Subsequently, the body linear acceleration and angular velocity were derived in time to obtain Jerk signals (transBodyAccJerk-XYZ and transBodyGyroJerk-XYZ). Also the magnitude of these three-dimensional signals were calculated using the Euclidean norm (transBodyAccMag, transGravityAccMag, transBodyAccJerkMag, transBodyGyroMag, transBodyGyroJerkMag). 

Finally a Fast Fourier Transform (FFT) was applied to some of these signals producing fourierBodyAcc-XYZ, fourierBodyAccJerk-XYZ, fourierBodyGyro-XYZ, fourierBodyAccJerkMag, fourierBodyGyroMag, fourierBodyGyroJerkMag. (Note the 'frequency' to indicate frequency domain signals).