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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