Helical scan videotape transport
a. These features include the tape supply reel, which is provided with either
a friction brake or an active back torque.
The back torque is supplied by the
drive system of a torque motor.
Back tension (torque) is necessary to keep the
tape from becoming tangled due to the inertia of the tape reel.
b. The tension idler ((c), fig 1-9a) holds a certain amount of tape in its
loop. This spare amount of tape is temporarily let out during quick starts.