Dynamic Focus Concept; Static Focus; Dynamic Focus - Sony CPD-G400 Training Manual

Dynamic Focus Concept

Static Focus

An electron beam within the picture tube consists of many electrons that
are slowed down by the focus electrode. After passing through the focus
electrode, the accelerating electrode brings the beam to a fine point on
the screen. This focus point is positioned by adjusting the voltage at the
focus electrode relative to the accelerating voltage. The accelerating volt-
age is usually fixed at the HV potential from the flyback secondary.
CRT Electrodes:
Focus Accelerating
Electron
beam
Picture Tube Electron Beam
As the electron beam is moved from side to side (swept) by the magnetic
field created by the external horizontal deflection yoke, the focus points
form an arc as shown by the arrowheads. Early picture tube glass screens
were made into a similar arc to maintain focus at the left and right sides of
the screen.
Yoke
Electron
beam
Picture Tube
Focus
point
Right
side
Focus Arc
Flat CRT
screen
Left
side
Top View
61
Modern picture tube screens are flat to reduce annoying room glare. This
means the focus point must be moved up at the left and right sides to
meet the flat picture tube screen.

Dynamic Focus

The job of the dynamic focus circuit is to change the focus points to meet
the flat picture tube screen. This is done by increasing the static focus
voltage when the beam at the left and right sides of the screen. This
correction voltage is in the shape of a parabola to match the focus arc.
H Dynamic focus voltage
Focus correction must be done in the vertical plane for the same reason.
A vertical dynamic focus waveform is needed as well.
Digital
V
Processing
DF
Unit (DPU)
H
DF
V H
CPU
(OSC)
right
left
Flyback
D F
drivers
V & H
DF
drivers
Dynamic Focus Block
Diagram
No correction
To
CRT
HV
Focus
FV
Focus
FC