The C3D file header contains the frame numbers of the first and last frames of trial data stored in the C3D file while the total number of frames is stored in the POINT:FRAMES parameter. The C3D format defines these as unsigned integer values which limits these to a maximum value of 65535.
When using high-speed video cameras, this equates to a trial of just over two minutes at 240Hz. To escape from the existing 16-bit integer limits and allow more than 65535 frames to be stored, two parameters (ACTUAL_START_FIELD and ACTUAL_END_FIELD) have been created as 32-bit values in a new TRIAL group. It is proposed that C3D applications should, in future, read these parameters and only use the header values and POINT:FRAMES parameter values if these new TRIAL parameters are not present.
The choice of parameter name is unfortunate as many video systems sample in the interlaced mode where a 60Hz sample rate is obtained by sampling odd and even fields at 30Hz.
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