Emotional excitement and physical activity are reported to lead to faster and deeper respiration. Peaceful rest and relaxation lead to slower and shallower respiration[Fri86]. A state of stress would therefore be indicated by frequent respiration; however, sudden stressors such as a startle, tend to cause momentary cession of respiration[Fri86].
Respiration is currently measured by a Hall effect sensor which detects the amount of expansion of the chest cavity as the wearer breathes in and out. This signal, sampled at 20Hz, is processed by first convolving with a low-pass filter and then taking the first difference. A threshold is applied to the smoothed slope and the rising and falling edges are detected. The time between breaths is calculated using the trigger for successive inhalations. Breathing rate is then recorded as the inverse of the inter-breath interval for that period. A heuristic for detecting the confounding activity of talking was also developed. If thresholds for the both the inter-breath interval and the average value of the raw signal during that interval exceeded certain thresholds, a flag indicating talking was triggered. Figure 9 shows the respiration signal and the results of the algorithms detecting inhalation, exhalation and talking.
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