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AN1290
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STMicroelectronics Confidential
In the next stage, the ramp and the S and C corrections are added, then a variable gain is applied
together with EHT compensation, which has its own sensitivity setting. A second offset voltage and
the Moire compensation are added, and a buffer transmits the signal to the output on pin 23.
Notice that this section does not have the “tracking” feature. Adjusting amplitude with “After Gain”
VSAG (1D), or position VPOF (1E), linearity as well as E/W will not follow, and the display aspect
will be changed. Therefore, the correct adjustment process is as follows:
●
Set VPOS to medium value, VSIZE to medium value or the preset value found convenient for
end-user commodity;
●
Set VSAG and VPOF for the raster to fill in the screen conveniently (for instance, if several tube
types are used with same chassis, the settings will be different depending on tube type); set
linearity and E/W corrections.
The aim is that the center of the sawtooth (different from center of video!) correspond to the
center of the screen and end of the sawtooth to bottom of the screen. At top of the screen there
should be a black stripe corresponding to video blanking!
●
From that moment, to preserve good linearity and geometry, use only VPOS and VSIZE for
final adjustments (including for the various VGA modes).
5.1.9
ON/OFF Function
A condition that disables H output (namely supplying less than 8.5 V when ramping up or supplying
less than 6.5 V when ramping down) will also disable the V output. Thresholds are 8 V and 6.8 V for
the TDA9112A. With a normal supply, the Disable and Enable functions can also be obtained by I²C
programming (Sad17h/d1).
5.1.10 V Lock/Unlock
The “Vertical Lock/Unlock” data is available through the I²C bus in Read mode. Strictly speaking and
as already explained, the vertical oscillator immediately locks with the incoming frequency; what
requires a delay is the establishment of the correct amplitude. Vertical locking is indicated as
completed when the peak amplitude is within
±
50 mV of the assigned 5 V voltage.
Figure 11: Variable Gain and EHT Compensation
( )²
x
To E/W corrections
(Vramp)
2
(Vramp)
3
VSIZE
VPOS
VSAG
VPOF
3.5V
VMoire
Buffer
23
22
18
4V
VCoGain
One-quadrant
variable gain
Two-quadrant
variable gain
SCOR
CCOR
