Learning Event 3:
DESCRIBE HOW FREQUENCIES ARE RECORDED ON VIDEOTAPE
1.
Frequencies. Before we go any further into helical scan videotape recorders,
we must understand how frequencies are recorded on tape.
The video signal
frequency range for television tape extends from very low frequencies, approaching
DC, to the upper limit of 4mHz. The size of the head gap becomes very important in
the magnetic recording of higher frequencies.
a. For a given gap size and tape speed, there is a frequency above which
recording is impossible. This condition occurs when the recorded wavelength of the
signal frequency just equals the width of the head gap.
b. In Figure 1-3, note that the positive and negative portions of the cycle
appear across the gap simultaneously and cancel each other.
Maximum output is
obtained when the recorded wavelength is twice the width of the head gap.
Figure 1-3.
Gap effect at high frequencies
2.
The relationship between recorded wavelength, frequency and velocity of the
tape is expressed by the formula: = V
F
where = recorded wavelength (in), V = tape speed (ips) and F = frequency (Hz) of
the signal.
From this formula it may be determined that as the velocity of the
tape increases, the recorded wavelength also increases.
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