bandwidth of the FM signal is greater than that of the original video because of the side bands, the span
is less than one octave.
b. To render the gray scale accurately and avoid chrominance distortion, a high degree of
linearity is required in the overall record-playback characteristic. High frequency bias, as used in audio
recording, is not practical since the bias frequency required for video recording is too high. Fortunately,
the use of FM solves the problem, since the symmetrical non-linearity of the tape does not change the
time between crossing of the zero axis and this permits demodulating the signal without distortion.
2.
In addition to the preceding benefits, use of FM permits increases to the signal-to-noise ratio by
recording the signal at a constant level high enough to saturate the tape.
Learning Event 5:
PRINCIPLES OF LONGITUDINAL RECORDING WITH ROTATING HEADWHEEL
1.
High frequencies can be recorded by moving the tape longitudinally at high speed past a head
with a narrow gap. This method, however, is not feasible for television recording because of the
excessive length of tape required.
2.
The difficulty is overcome by moving the head at high speed across the tape while the tape itself
moves at a much lower speed between the supply and take-up reels. This is accomplished by scanning
the width of the tape with a rotating wheel, or headwheel, containing two tiny heads, mounted at 180
degrees apart. Each head starts across the bottom edge as the preceding head approaches the top edge.
Since the tape moves slowly at small angles to the wheel motion, the signal is recorded on a series of
parallel longitudinal tracks.
3.
To permit the reassembling of information recorded in the series of tracks into a smooth
continuous signal, a slight overlap of information is allowed to occur between the end of each track and
the start of the next track. In the playback process, the overlap is eliminated by an electronic switcher
which makes the transfer between head signals invisible in the video information.
4.
Use of two video heads introduces other problems such as dihedral errors and differences in
sensitivity and frequency response between heads.
a. Dihedral errors consist of slight deviations in spacing of two adjacent heads from a true 180-
degree arc. If uncorrected, the errors cause a breakup of the vertical lines in the picture. These timing
errors are corrected by mechanically adjusting the heads in the playback system.
b. Differences in sensitivity and response of the heads are minimized by individual gain
adjustments in the record amplifier system and individual equalization controls in the playback system.
7
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