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AN1290 Vertical
Section
5.1.11 V Moire
Depending on the definition mode, vertical amplitude etc., the vertical line step may be very close to
the tube vertical pitch. This will create an array of interference fringes, just as for the Horizontal
section, and a succession of more or less horizontal stripes, successively brighter and darker.
“Vertical Moire causes horizontal stripes” (which may be strongly distorted).
This unpleasant aspect will disappear if every other frame is slightly vertically shifted. The total shift
must be approximately 1/2 line step. This very small quantity must be set by the user because it
depends on the mode and the amplitude settings.
If the V-Moire control of the TDA9112 is activated, every other frame will be vertically shifted by an
amount that can be set through I²C bus (0Bh), allowing V-Moire cancellation.
5.1.12 Vertical Breathing Function (EHT Compensation)
In electron tube displays, and especially in monitors, the display amplitude tends to increase when
beam current increases. This effect is mostly noticeable when there is a common stage for EHT and
scanning, it depends on how the feedback voltage is performed:
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EHT may be directly regulated (through a special bleeder inside the EHT transformer). In this
case, when the beam current increases, B+ has to increase to provide the beam energy, and
consequently H width will increase. On the other hand, display height will have an almost
negligible variation.
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Another possibility is to regulate the peak voltage on a secondary of the EHT transformer. In
this case, because of the internal impedance of the EHT generator, EHT will decrease when
beam current increases. This, in turn, increases the scanning efficiency and the display
dimensions (both horizontal and vertical).
In all cases, for a same scanning current, display amplitude will increase almost proportionally to
the beam current increase. On the Vertical components, the effect is much more visible in the
second case than in the first one. It has been named “breathing” because the whole display seems
to inflate or deflate.
Since the beam current data is normally available from the Automatic Beam Limiter (ABL), it may be
used to compensate the size (vertical and horizontal, as needed). Alternatively, the variation of EHT
(available through the bleeder) may be used for the same purpose: a description of this system will
be provided later.
Here we describe the Vertical compensation. A similar system is available for Horizontal (See
Geometry and Focus control).
5.1.13 Implementing the Breathing Function
In a multi-frequency monitor, there is no direct way to change the V sawtooth amplitude by a
constant amount at various frequencies. In the TDA9112 family, pin 18 has been designed for this
purpose (
Usually, in order to make the beam current data available, the low end of the EHT winding of the
flyback transformer is connected, not to Ground, but to a constant voltage (5V or 12V) through a
resistor. When the beam current increases, the voltage drop across the resistor increases, and the
voltage on the connection point goes down. Normally, this point is filtered to GND, and its voltage is
representative of the mean beam current.
In the TDA9112, applying a more negative voltage on pin 18 will cause the amplitude of the vertical
sawtooth to decrease. This way, the breathing effect may be compensated if a proper fraction of the
ABL voltage variation is applied to pin 18.
The “breathing” input is active below 8V (like other characteristic voltages in the data sheet, this
value is derived from the internal voltage reference and has the same spread) and should not be
