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The human eye is an incredibly adaptable device that can focus on distant
objects and immediately refocus on something close by. It can look into the
distance or at a wide angle nearby. It can see in bright light or at dusk,
adjusting automatically as it does so. It also has a long 'depth of field';
therefore, scenes over a long distance can be in focus simultaneously. It
sees colour when there is sufficient light, but switches to monochrome vision
when there is not. It is also connected to a brain that has a faster updating
and retentive memory than any computer. Therefore, the eyes can swivel from
side to side and up and down, retaining a clear picture of what was scanned.
The brain accepts all the data and makes an immediate decision to move to
a particular image of interest, select the appropriate angle of view and
refocus. The eye has another clever trick in that it can view a scene of
great contrast and adjust only to the part of it that is of interest.
Introduction
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The predominant method of recording video pictures at the time of publication
of this book is by analogue video recording. In analogue recording, the voltages
that make the composite video signal are recorded on to magnetic tape; the
changes in voltage magnetise and demagnetise the tape. To play back the recording
the changes in magnetism on the tape are converted back in to voltages and
the composite video signal is re-created for connection to a video monitor.
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A video tape recorder is a complex
integration of electronics and extremely high precision mechanics. There
have been several types of recording systems in recent years, the main contenders
being 'Betamax' from Sony, 'Video 2000' from Philips and 'VHS' from Matsushita.
They are all based around a tape contained in a cassette with a supply spool
and a take up spool. However, there were both electronic and mechanical
differences that prevented one tape being used on another make. The one to
emerge as the standard throughout the world is the VHS system. VHS means
Video Home System and was developed by the JVC Company in Japan.
The VHS Video Recorder
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All video tape recorders follow the same principles as an audiocassette recorder.
That is, a tape containing thousands of tiny magnets, each with a north and
a south pole is passed through a varying magnetic field. The magnetic field
is generated in a revolving drum from the video signal. This reproduces the
video signal onto the tape.
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The tape is stored in a sealed cassette with a flap at the front protecting
the tape. When the tape is loaded into the recorder, a mechanism draws the
cassette into and down the machine.
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The catch holding the front cover is released and the cover opened. The cassette
drops over two threading posts as shown in the first diagram. When one of
the functions such as play or record is operated the tape is drawn around
the head drum as shown in the second diagram.
Diagram 7. 1 VHS Tape Cassette.
Principles of Video Recording
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The descriptions give here are of necessity over simplified and are intended
to illustrate the basic principles of recording. As stated before, the two
essential elements of a video tape recorder are a rotating head assembly
and the tape passing around a drum and head. The head consists of a ferrite
ring with its continuity broken by a small gap. A coil is wound round the
ring which, when energised, creates a magnetic field. The magnetic field
in the ring concentrates in the gap. An essential aspect of design is that
head gap is in the order of 0.3 microns. A micron is one-millionth of a metre.
Therefore, 0.3 microns is about one-hundredth the thickness of a human
hair.
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The video signal is fed to the magnetic coil and creates an analogue version
in the form of a magnetic field. As the tape passes the gap in the head the
magnetic field causes the 'internal magnets' to align according to the signal
passing through the head.
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This makes a magnetic copy of the signal on the tape. The tape passes the
drum at a fixed speed, therefore low frequencies will create long 'magnets'
in the tape, and high frequencies will create short 'magnets'.
Tracks On Tape.
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The tape consists of an insulated base material with a fine oxide coating.
For various reasons, the head is displaced at an angle to the tape. This
is known as helical scanning and is standard for all recorders. The magnetic
information is recorded at an angle across the tape.
Diagram 7. 2 Tracks on Video Tape
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The width of tape for standard VHS is 12.65mm (1/2"). The speed for standard
real time recording is 23.39 mm/sec.
Early video recorders and some domestic VHS recorders still available today
had two coils, or heads, on each head cylinder. This worked well while the
tape was moving, producing moving pictures on playback. However, when the
pause function on the recorder was activated to view a single still picture
horizontal noise bars would appear on the picture because the head was not
moving fast enough to capture the single picture from the tape accurately.
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The solution to this problem was introduced when the first four-head video
recorders were made. These use four coils or heads, two each on opposite
sides of the head cylinder. By using four heads instead of two twice the
amount of information could be written to or read from the tape. Four head
video recorders can replay still images without any noise bars and this has
led to their general use in domestic and CCTV video recorders, replacing
the older two-head design.
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The heads are spaced 65 microns apart for a standard VHS time-lapse recorder
and these lay down tracks on to the tape, which are 58 microns wide. Head
cylinders of this design are known as type SP heads.
