Linear (or Longitudinal) Timecode (LTC) is an encoding of SMPTE timecode
data as a Manchester-Biphase encoded audio signal. The audio signal is
commonly recorded on a VTR track or other storage media.
libltcsmpte is a LGPL library which provides functionality to both
encode and decode LTC from/to SMPTE and can perform framerate conversion
Source, documentation & more info: