DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

field rate, however; at 1 ⁄3 of its rotation through the
wrap angle, the head is reading information located 1 ⁄3
of the way down the picture. Playback in shuttle
produces what is structured as a field of video, but with
lines collected from many adjacent fields. This video
forms a useful picture.
A consumer VCR is designed to play the same field
(always first, or always second) in pause and in slowspeed
trick modes. This avoids field-tearing in pause,
and vertical judder in slow play, which would be judged
as annoying by the consumer.
Recording
Head rotation during recording is synchronized to the
field rate of the input video. The scanner is equipped
with a tachometer (sometimes called a frequency generator,
FG) based upon optical or magnetic pickup from
the rotating scanner. Tach (or FG) pulses are presented
to one input of a phase comparator; the other input is
derived from the field rate of input video. The phase
error is filtered, and the error signal drives the head
motor: If tach pulses are arriving more frequently than
field sync pulses, the head motor is sped up; if less
frequently, the motor is slowed down. Once frequency
lock is established, the circuit seeks phase lock: If tach
pulses are arriving later than field sync pulses, the
motor is sped up momentarily; if earlier, it is slowed
down momentarily. This process constitutes the head
servo; it keeps playback video locked to reference video.
Tape is pulled through the scanner by a capstan. In lowcost
VTRs, the capstan has a small diameter, and mates
with a pinch roller on the opposite side of the tape. The
pinch roller is released when the VTR is placed in
shuttle mode (rewind or fast forward); tape motion is
then obtained by driving the supply and takeup reel
motors. In high-quality VTRs, the capstan has a larger
diameter; it contacts sufficient tape area that the
resulting friction makes a pinch roller unnecessary. This
type of capstan can drive the tape in shuttle modes
without requiring use of the supply and takeup reel
motors. This yields improved tape control compared to
a pinch roller.
414 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES