Figure 1-9.The waveform symbol equipments (PA/SG) interface path,
signals: X-axis waveforms, Y-axis waveforms, and
Z (unblinking) signals.
The symbol is painted in a timing period called
P-time. The symbol generator, using a series of
P-time interval signals, develops the proper
waveforms to be sent to the CRT deflection amplifiers
and video amplifiers.
When the symbol generator
starts its P-time counter, a signal is sent to the display
console that starts an identical counter in the console.
This ensures that both equipments are synchronized.
Figure 1-10 shows the development of a symbol using
the X, Y, and Z waveforms.
The symbol is formed
during the unblanked P-time intervals. The symbol
shown in figure 1-10 is actually a combination of
three symbols: air unknown, rocker, and full upper
bar. The unblank times have been given reference
letter designations to aid you in following the
During unblank time A, the rocker is formed.
Figure 1-11 illustrates this process on an X/Y plot.
When the unblank signal is high, the X sine wave is at
its negative point and transitions to its positive point.
At the same time, the Y waveform is at zero and
transitions its negative cycle, and returns to zero.
During unblank B, the Y waveform provides the
proper position for the upper bar, while the X
waveform transitions from negative to positive. The
unknown air symbol is formed during unblank time C.
Note that with the trapezoid waveform, X remains
at a constant negative level, while Y goes from zero to
the positive level. This draws the left vertical side of
the symbol. When Y reaches its positive level, X
starts a transition from negative to positive to draw
the top of the symbol. The right side of the symbol is
formed when the Y waveform goes in a negative
direction to zero, while X remains at a constant
positive level. The dot is formed at unblank time D
by unblinking X and Y at the zero level. At the
completion of the P-time, the console returns control
of the CRT beam to the radar scan logic.
The symbol remains displayed on the screen as
long as the persistence of the screen phosphor permits.
For the symbols to remain flicker free, they must be
periodically refreshed, or repainted, by repeating the
process just described. Symbols are refreshed 15 to
20 times per second.