Five sample C3D files that contain prescaled force plate data from a Vicon Nexus system

07/25/2007  08:52 AM         5,026,528 analogfpscale01.c3d
07/25/2007  08:52 AM         4,508,512 analogfpscale02.c3d
07/25/2007  08:52 AM           470,112 analogfpscale03.c3d
07/25/2007  08:52 AM           514,304 analogfpscale04.c3d
07/31/2007  09:11 AM           260,608 analogfpscale04i.c3d
07/31/2007  09:11 AM           213,477 int_conversion.JPG

The 01 and 02 trials feature POINT and ANALOG data from a subject dancing on the plate,
while the 03 and 04 trials are simple walks over the force plates producing recognisable
force plate gait data.  Each of these files is stored in the C3D floating point (REAL) 
format using the PC/DOS byte order.

In each case, the manufacturer claims that a force plate calibration matrix has been
applied to the force plate data before the data is written to the C3D file which explains
why the ANALOG:SCALE parameters for the force plate channels are all set to 1.00.

All trials appear to contain filtered POINT data (residuals are all set to 0.0).  
Trials 1 and 2 appear to contain corrupted analog data - forces that exceed the FP system
limits or the ADC system input range.

The 04i trial is a copy of the 04 trial converted from the original (REAL) floating point
format into DEC integer.  This demonstrates the problems caused when inappropriate ANALOG
SCALE factors are chosen - note the corruption in the Mx1 channel as the REAl values exceed
the range of the 16-bit integer format - see int_conv.jpg (int data at the top, with the
original floating point data below).

The choice of the analog scaling factors (GEN_SCALE = 1.0 and SCALE = 1.0) has effectively
forced all analog data into the range of –32768 +32767 causing data corruption when the file
is converted to INTEGER.

Five sample C3D files from Vicon that contain prescaled force plate data. The 01 and 02 trials 
feature POINT and ANALOG data from a subject dancing on the plate, while the 03 and 04 trials 
are simple walks over the force plates producing recognizable force plate gait data.
Although the C3D file is correctly formatted, the poor choice of the analog scaling factors 
(GEN_SCALE = 1.0 and SCALE = 1.0) has the potential to cause data corruption when the analog 
data in the file is converted to INTEGER format.