User guide

Charnwood Dynamics Ltd. Coda cx1 User Guide – Coda Basics I - 2
CX1 USER GUIDE - COMPLETE.doc 26/04/04 54/162
values are displayed as percent of the expanded section, allowing the data to be analysed
in terms of percent of one gait cycle. When the graphs are zoomed in, only the expanded
section is shown in any stick figure views. When a data file which has been expanded is
saved, the position of the expanded section is stored. Note, however, that none of the
data outside the expanded section is lost in this process, and it can be viewed again at
any time by zooming out.
To expand a section of a graph:
Position the left cursor at the start and the right cursor at the end of the section to be
expanded. (See Working with cursors for information about moving the cursors.)
Choose Zoom Graphs To Cursors from the Cursors menu or select the Zoom Graphs
To Cursors/Zoom Graphs Out button on the toolbar.
To restore the data file to its original length:
Choose Zoom Graphs Out from the Cursors menu or select the Zoom Graphs To
Cursors/Zoom Graphs Out button on the toolbar.
Using data filtering
When the Codamotion system is acquiring marker position data, it interrogates each
marker for only a very short period of time (about 40µs), whatever the overall sampling
rate. This gives very precise time resolution and prevents any ambiguous ‘smearing’ of
fast movements. The residual high frequency noise in the position measurement arises
mainly from photo-detector current noise in the cameras and a small effect due to the AC
component of room lighting arising principally at double the mains frequency. The rms
amplitude of noise from these sources is typically less than 0.1mm in the X and Z axes.
The marker distance data (Y-axis) is more susceptible to noise, as it is derived from a
triangulation calculation using the signals from the two outer cameras. When markers are
attached to a subject’s skin or clothing, further mid-frequency ‘noise’ will be introduced by
movement of the latter. These fluctuations will not normally appear significant on a graph
of marker position data, but will become more significant on derived data such as angles
and marker velocities and accelerations especially.
The higher frequency components of noise and other fluctuations may be smoothed out
by applying low-pass filtering to the data, using an adjustable cut-off frequency. The
filtering control and cut-off frequencies are set in the Data filtering dialogue box which is
opened by selecting the Data Filters... item in the Setup Menu.
Different filters may be set for each co-ordinate of marker position data, and for velocity
and acceleration data. You should be careful not to over-filter the position data, as
important detail may be removed, and the amplitudes of peaks in the data will be reduced.
For data acquired at 200Hz, a filter cut-off frequency of 100Hz is recommended. For most
biomechanics applications, such as gait analysis, it is not recommended to set the filter to
less than 20Hz.
Velocities and accelerations are calculated from filtered position data, so will be smoothed
somewhat as soon as the latter is filtered. However, you will normally need to set these
filters much lower — typically 20Hz — to remove noise without affecting the genuine detail
in the data.