Reference Information
Samsung Electronics
2-3
2-2 Description of Dynamic Focus
Most large-screen video display devices that are using CRT (including CDT) usually apply the Dynamic
Focus (hereinafter D/F) circuit.
As CRT has non-spherical surface (perfect spherical surface = 1, non-spherical surface R>1), the distance
that the electron beam emitted from the electron gun reaches to the center of CRT is different from the one
that the electron beam reaches to the corners. (See Figure 1.)
Only the beam, which has the equal distance as the beam from the electron gun to the center of CRT
surface, can maintain the optimum focus.
By this reason, focus dagradation at corners occurs inevitably.
To recover this, the speed of the electron beam injected into the corners of CRT should increase and the
focus dagradation by the difference of distances can be compensated.
Increasing the voltage is used as a method of increasing the speed of the electron beam at the corners of
screen.
In this case, an ideal D/F voltage waveform is the form of parabola where the center of screen has low
voltage and the corners has the highest voltages.
The horizontal D/F waveform compensates the focus dagradation at left and right sides, but the vertical
D/F waveform does at top and bottom sides.
The horizontal D/F and vertical D/F waveforms are separately created and mix two signals to
compensate the focus of the whole screen.
And the vertical Dynamic Focus waveform is composed of the horizontal Dynamic Focus waveforms as
much as the number of scanning lines. (See Figure 2.)
Fig. 1 Dynamic Focus Diagram (Horizantal)
Fig. 2 H/V Dynamic Focus Waveform
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