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The Secret Life of XY Monitors

By
Jed Margolin�

�����
During my time at Atari/Atari Games I worked on several XY games. This
article represents what I know about XY Monitors. XY was Atari's name
for what the Computer Graphics industry calls '"Random Scan"
and the Video Game Community calls "Vector Games." The major
parts of the XY Monitor are the Cathode Ray Tube (CRT), the Deflection
Amplifiers, and the High Voltage Supply.

XY Monitors - Contents

1. CRT Electron
Optics

2. Deflection Types

��� A. Electrostatic

��� B. Magnetic

3. XY - Deflection Amplifiers

4. High Voltage Power Supplies

5. Screen Phosphors

6. Color CRTs

7. Alternatives

8. References

_________________________________________________________________________________________

CRT Electron Optics

The CRT is an evacuated
container with an Electron Gun that shoots a focused electron beam at
a phosphor-coated screen which absorbs the energy of the electrons and
re-emits it as visible light. The High Voltage Supply produces the voltage
needed to accelerate the electron beam toward the screen. The Deflection
Amplifiers move the electron beam in the desired pattern.

We will start with the
CRT shown in Figure 1.

When metal is heated to
incandescence in a vacuum the electrons near the surface are given enough
energy to fly off into the surrounding space.

The Electron Gun starts
with the Heater (or Filament) which uses an electric current in order
to heat it up to incandescence. However, instead of using the Heater�
to directly produce electrons, it heats up a separate element, the Cathode,
to produce the electrons. One reason for doing it this way is that it
allows the Heater supply to be electrically isolated from the Cathode.�
This keeps the Cathode voltage out of the Heater supply, allowing it
to be used for other things as well. It's also a safety issue, since
the Cathode voltage might be

substantial. I suspect that another reason for using
an indirectly heated cathode is that it allows the Heater to be optimized
for� long life and the Cathode optimized for emitting electrons.

The Control Grid (called
G1) is operated with a negative bias with respect to the Cathode (typically
around 50V.) and acts to repel the electrons coming from the Cathode.

The reason for doing this
is that the Screen Grid (G2) is operated at a voltage that is higher
than the Cathode (typically several hundred volts). The physics works
out that a small change in the Control Grid voltage has a large effect
on the electron beam current.

The electron beam is further
accelerated by Focus Grid (G3) which is operated at an even higher voltage
(typically several thousand volts). As you may have guessed, the Focus
Grid also focuses the beam.

The final acceleration
comes from an even higher voltage on the Anode (typically several times
the voltage on the Focus Grid).

The resulting beam strikes
the phosphor-covered screen which absorbs the energy of the electrons
and re-emits it as visible light.

The equations that describe
the motion of electrons in a potential field take the same form as those
used to describe the behavior of optical lenses. The shape of the electron
beam on the screen is an image of the electron beam at what is called
the crossover point, which is the place in the electron gun where the
grids cause the electrons� to change velocity.� The ratio
of the voltage at the Focus Grid (G3) relative to the voltage at the
Anode controls the crossover point and therefore the point at which
the image is focused, much the same way as the curvature of an optical
lens determines its focal point.

This is why it is called electron optics.

�

A further note about the
grid structures. While standard vacuum tubes (both receiving and transmitting
types) use grids in the form of a mesh, the grids in a CRT are cylinders.�
In addition, the Control Grid (G1) also contains a disk with a hole
in the middle called the Aperture Disk. ( See Figure 2). This helps
produce a narrow electron beam

Now that we have a nice
focused electron beam hitting the phosphor we should probably move it
around before we burn the screen.

Electrostatic Deflection

Two pairs of parallel plates
are mounted in the CRT. A voltage applied across the Vertical Plates
will deflect the electron beam vertically as shown in Figure 3. A voltage
applied across the Horizontal Plates will deflect the electron beam
horizontally.� (One of the Horizontal Plates in Figure 3 is hidden
by its mate.)�

The deflection of the beam
in an electrostatically deflected CRT is given (from Whitaker) by:

���������������������
Vd * dl

tan(da) =��� __________�����

������� 2 * dp * Vb���

where:���
Vd = the voltage between the deflection plates

��������������
dl =� the length of the plates

��������������
dp = the distance between the plates

��������������
Vb = beam voltage

Since
the construction of the plates (dl and dp) is determined by the CRT
manufacturer we will simplify things by letting�� K1 =�
dl/(2*dp)�� so that:

tan(da)
=��� K1 * Vd / Vb�����

Therefore, the tangent
of the deflection angle is proportional to the voltage between the plates
and inversely proportional to the beam voltage (Anode voltage).

From Figure 3:

tan(da) = db/L

L is another value determined
by the CRT manufacturer, so that the deflection of the beam is given
by:

db
= K2 * Vd / Vb������ where
K2 = L * dl/(2*dp)

The good news is that the
screen deflection of the beam is proportional to the voltage between
the plates.

The bad news is that it
is inversely proportional to the Anode voltage. This means that if we
double the Anode voltage we have to double the voltage between the plates
for the same beam deflection. If the voltage between the deflection
plates is already 200V, then we would need to increase it to 400V for
the same deflection. Although that would not be too much of a problem
with today's components, the deflection amplifiers would now also require
twice the slew rate if we are to move the beam at the same speed.

Why not just use a low
Anode voltage?

First, Anode voltages have
to be high just to get acceptable screen brightness.

And second, the Anode voltage
has an effect on the beam focusing.� As the Anode voltage is increased,
the electron beam can be focused to a smaller spot size.