LESSON 3
PRINCIPLES OF HELICAN SCAN VCR CIRCUITS IN RECORD MODE
TASK
Describe the principles of the record mode of helical scan video recorders.
CONDITIONS
Given information and illustration on the operation and circuitry of the helical scan record mode of
VCRs.
STANDARDS
Demonstrate competency of the task skills and knowledge by correctly responding to 80 percent of the
multiple-choice test questions covering playback circuits of a helical scan VCR in the playback mode.
REFERENCES
Overall block diagram (Figure 3-1, foldout (fo) at end of lessons).
Learning Event 1:
FUNCTION AND OPERATIONAL CHARACTERISTICS OF A VCR
1.
The video signal processing is fundamentally the same as in all VCRs. In the record electronics,
the luminance signal is separated from composite color video input signal. The amplitude of the
luminance signal is then adjusted either manually or automatically by the AGC circuit. The luminance
signal is then frequency modulated and recorded on the tape via video heads.
2.
When the 3.58 MHz chroma signal is separated from the incoming video signal, it is passed
through the ACC circuit where its amplitude is regulated to a constant value. Following the ACC circuit
the chroma signal is frequency down converted to 688 KHz and recorded on tape by the video heads
along with the Y-FM signal.
3.
In the playback, the video head output signal is first amplified by a high gain preamplifier (CH-A
and CH-B HEAD AMP) and the output signals from the two heads are mixed by switching circuit to
form a continuous signal. The luminance FM and 688 KHz chroma signals are then separated by high
and low pass filters, respectively. The separated luminance FM carrier is then passed through a limiter
and dropout compensator which eliminates short duration noise components caused by tape dropout.
The signal then passes through a sideband equalizer, another limiter stage and to the original AM
luminance signal. It then passes through deemphasis and noise-cancelling circuits.
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