DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 35.1 A videotape
recorder (VTR) or videocassette
recorder (VCR) wraps
magnetic tape around a drum
that is tilted at several degrees.
The video head scans a diagonal
swath across the slow-moving
tape. This sketch shows
a scanner having 180° wrap and
two heads (on opposite sides of
the drum). Some scanners have
two stationary drum halves
separated by a rotating headwheel;
some scanners affix the
heads to a rotating upper drum.
Videotape recording 35
The last decade has seen an astonishing improvement
in the data capacity of magnetic disks. It is now feasible
to record many hours of highly compressed digital
video on fairly inexpensive disks. However, magnetic
tape remains the mainstay of video recording.
Magnetic tape cannot be transported past a recording
or playback head fast enough to accommodate the high
bit rate of digital video recording (or the high frequencies
of analog video recording). Instead of moving the
tape rapidly, videotape tape recorders (VTRs) move the
head rapidly, across slow-moving tape. In modern VTRs,
the tape is wrapped around a drum that is tilted several
degrees with respect to the edge of the tape. The drum
incorporates a rotating head that scans a helical path
across the wrapped tape, as sketched in Figure 35.1
below. (The tape exits the scanner at a different elevation
than it enters.) The technique is known as helical
scanning; it is used in all modern videotape recorders –
consumer analog (e.g., VHS), consumer digital (e.g.,
DVC), and professional.
TAPE
MOTION
HEAD B
HEAD
ROTATION
HEAD A
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