DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Heads for other longitudinal tracks,
such as audio or timecode, are typically
located in the same head stack
(or head block) as the CT head.
When recording blank tape, the capstan is driven at
fixed, precise speed. A longitudinal control track (CT) is
recorded with a regular pulse sequence that is locked to
the video field rate (and thereby to scanner rotation).
Playback
In a studio VTR, playback video is ordinarily locked to
a reference video signal. This allows a playback VTR to
be locked to local video; allows several playback VTRs
to be locked together; and allows a VTR to switch into
record mode to accomplish a videotape edit. To lock
playback to a reference signal, the capstan is equipped
with a servo. Off-tape CT pulses during playback are
compared to field pulses from reference video; the
capstan servo uses the capstan motor to drive the tape
to the correct speed and positional relationship.
If it were mechanically possible, the control track head
would be mounted right at the scanner, so that a CT
pulse would be recorded at a position on tape immediately
adjacent to the start of each recorded track. In
practice, mechanical constraints demand that the CT
head be located some distance away from the scanner.
The distance is set to an integer multiple of the distance
that the tape advances during one field. In locked playback,
with no tape stretch, this yields the same result as
if the CT head were located at the scanner. In practice,
some mechanical uncertainty and some tape stretch
occurs, and tracking adjustment – either manual or
automatic – is necessary to align the head trace with
the recorded tracks. Automatic tracking slews the tape
position, searching for maximum playback signal
magnitude.
Tape motion during normal play and record, and in
slow-speed trick modes, is controlled by the capstan.
However, tape is delivered from the supply reel and is
taken up by the takeup reel. Obviously, these need to
be driven. The performance of a studio VTR strongly
depends upon tight control of tape tension. In a studio
VTR, each reel has a motor that is driven by a reel servo
that is controlled by tape tension. (Tension can be
measured by an arm with a spring.) In a consumer VCR,
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