814 Chapter 28
restart another trace. It takes a little time—about 12 μ s. This period, which includes duration
of the 4.7- μ s line-sync pulse during which time the beam current is controlled “ blacker
than black, ” is known as the line-blanking period. A similar, much longer, period exists to
allow the scan spot to return to the top of the screen once a whole vertical scan has been
accomplished; this interval is known as the fi eld blanking or vertical interval.
Looking now at Figure 28.2 , a whole 625 lines are shown, in two fi elds of 312.5 lines.
Note the wider sync pulses that appear between each fi eld. In order that a monitor may
distinguish between horizontal and vertical sync, the duration of line-sync pulses is
extended during the vertical interval (the gap in the picture information allowing for the
fi eld retrace), and a charge-pump circuit combined with a comparator is able to detect
these longer pulses as different from the shorter line-sync pulses. This information is sent
to the vertical scan generator to control the synchronism of the vertical scan.
28.5 Color Perception ...................................................................................................
Sir Isaac Newton discovered that sunlight passing through a prism breaks into the band
of multicolored light, which we now call a spectrum. We perceive seven distinct bands
in the spectrum: red, orange, yellow, green, blue, indigo, and violet. We see these bands
distinctly because each represents a particular band of wavelengths. The objects we
perceive as colored are perceived thus because they too refl ect a particular range of
wavelengths. For instance, a daffodil looks yellow because it refl ects predominantly
wavelengths in the region 570 nm. We can experience wavelengths of different color
because the cone cells, in the retina at the back of the eye, contain three photosensitive
chemicals, each of which is sensitive in three broad areas of the light spectrum. It
is easiest to think of this in terms of three separate but overlapping photochemical
processes: a low-frequency (long-wavelength) RED process, a medium-frequency
GREEN process, and a high-frequency BLUE process. (Electronic engineers might
prefer to think of this as three, shallow-slope band-pass fi lters!) When light of a particular
frequency falls on the retina, the action of the light reacts selectively with this frequency-
discriminating mechanism. When we perceive a red object we are experiencing a high
level of activity in our long wavelength (low-frequency) process and low levels in our
other two. A blue object stimulates the short wavelength or high-frequency process and
so on. When we perceive an object with an intermediate color, say the yellow of the egg
yoke, we experience a mixture of two chemical processes caused by the overlapping
nature of each of the frequency-selective mechanisms. In this case, the yellow light from