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Fig. 4.7
Labeling interface: visualization of inertial signals and video
inspect them. The occurrence of the first PT was used as a reference event to find
the time shift between the video and inertial signals. A verification step was made
by examining the last PT of each experiment. Figure
4.7
shows the labeling interface
with an example of the synchronization process.
After synchronization, the start and end times were manually set for each per-
formed activity (BAs and PTs) using the labeling application. All the labels for all
the experiments were collected in a file (labels file) which was used as one of the
inputs for the dataset generation process.
4.3.2 Signal Processing
Raw sensor signals from the accelerometer (
a
r
(
)were
preprocessed by the application of a series of filters for conditioning. First,
noise
reduction
was achieved with a third-order median filter and then followed by a third-
order low-pass Butterworth filter with a cutoff frequency of 20Hz. This frequency
threshold was selected from the work presented in (Karantonis et al.
2006
) which
states that the energy spectrum of the human body motion lies mainly within the
range of 0-15Hz. From these processes, a clean triaxial total acceleration (
a
t
)
) and the gyroscope (
ω
r
(
t
)
˄
(
t
)
)
and an angular velocity (
ω
t
(
t
)
) signals were obtained. The application of these filters
is represented by the
H
1
()
transfer function. The example presented in Fig.
4.8
shows
the raw triaxial acceleration while a subject is walking and then walking-upstairs.
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