The ANALOG:USED parameter is a single signed integer value that records the number of analog channels that are contained within the C3D file.  The value stored in ANALOG:USED is used to compute the analog data frame rate from the total number of analog data words collected during each 3D frame.  The total number of ADC samples stored per 3D sample frame must be an integer multiple of ANALOG:USED.

The value of the ANALOG:USED parameter is not stored in the C3D file header but can be calculated from two values that are stored in the C3D file header.  The ANALOG:USED parameter value is equal to C3D header word 3 divided by C3D header Word 10.

As a signed integer, the ANALOG:USED parameter has a possible range of –32768 to +32767.  Since there is no conceivable requirement to store negative values for this parameter, this range could be extended by interpreting this parameter as an unsigned integer with a range of 0 to +65535.  The use of an unsigned value for this parameter would be flagged if the parameter returned a negative value when read as a signed integer. 

This means that in theory, C3D files that contain more than 32,767analog channels may not be readable by some older software applications.  In practice, it is unusual to find analog hardware systems collecting more than 64 channels of analog data.