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C3D Recordings
This section will discuss the gain calibration of sample recordings made with the
MA-100 and MA-300 EMG systems using a commercial motion capture system with
a proprietary ADC card. For the purposes of this tutorial, the two EMG systems
differ in their treatment of the EMG signal as the MA-100 system has a continuously
variable signal gain while the MA-300 system offers discrete switched gain levels.
This section assumes that the reader is familiar with the stand-alone calibration
protocols described in the preceding pages and as a result, descriptions of the
individual features of the example EMG systems will not be repeated.
Full details of the public C3D
file format description
together with common
parameter information is
available from the C3D web
site at http://www.c3d.org
C3D files are a common biomechanics standard that allows analog and 3D data to be
recorded in data files, together with various parameters that describe the data. A
standard analog channel consists of a number of analog samples together with two
gain factors that are applied to the analog signal to transform the binary samples into
real-world values. These gain factors are called GEN_SCALE and SCALE. There
is a single GEN_SCALE parameter that is applied to every analog channel – the
value of the GEN_SCALE parameter is usually set when the data capture system is
installed. Since this scale factor is applied to all analog data it is usually not changed
very often – we will need to know its value but will not change it when we calibrate
our individual analog channels.
The calibration for each analog data channels in a C3D file is controlled by an
SCALE factor parameter – this parameter has an individual value for each channel
that is directly related to the gain or input range of the channel in combination with
the GEN_SCALE value. While the GEN_SCALE and individual SCALE
parameters can take any value, it is normal to choose these parameters such that
results in the analog channel are scaled in some known value – normally this will
default to Volts. The C3D analog parameter UNITS should normally describe the
measurement system for each channel.
Common values for GEN_SCALE are:
•
0.0048828 – the value of a single bit of data from a 12-bit ADC that is
measuring a ± 10V input range. An individual channel SCALE value
would then be 1.00 to obtain the analog data scaled in Volts.
•
0.0024414 – the value of a single bit of data from a 12-bit ADC that is
measuring a ± 5V input range. An individual channel SCALE value
would then be 1.00 to obtain the analog data scaled in Volts.
•
1.000000 – individual channel SCALE values must be set to 0.0048828
or 0.0024414 to obtain analog data scaled in Volts when sampled by a
12-bit ADC that is measuring a ± 10V or ± 5V input range respectively.
Many commercial motion capture systems use a standard naming procedure for data
recording trials that may not allow you to specify the standard names used by the
EMG Testing software. If you are using one of these systems then it may be
necessary to copy and rename the C3D files after recording to facilitate the use of the
Whisper EMG test software.
Variable Gain Systems
As described earlier, the calibration procedure is for an MA-100 system is simple. A
calibration recording is made using the internal calibration source to apply a known
calibration source to each EMG input. Since we know the input signal level (200μV
RMS sine wave) all we need to do is directly measure the recorded level of the
resulting output signal to calculate a scaling factor to record data scaled in μV. The
exact method of generating a C3D file will vary depending on your data collection
software however it is important that the analog parameters GEN_SCALE and
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Whisper User Manual