STV6889 datasheet(39/56 Pages) STMICROELECTRONICS

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STV6889

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9.5 EW drive section

The goal of the EW drive section is to provide, on

pin EWOut, a waveform which, used by an external

DC-coupled power stage, serves to compensate

for those geometry errors of the picture that are

symmetric versus vertical axis across the middle

of the screen.

The waveform consists of an adjustable DC value,

corresponding to horizontal size, a parabola of 2nd

order for “pin cushion” correction, a linear for “key-

stone” correction, independent half-parabolas of

4th order for top and bottom corner corrections, S-

shape for “S” correction and W shape for “W” cor-

rection. All of them are adjustable via I²C-bus, see

Chapter 8 - page 25

Refer to Figure 14, Figure 15 and chapter Chapter 7 -

page 21

. The adjustments of these correction

waveforms have no effect in the middle of the ver-

tical scan period (if the VPOS control is adjusted to

its medium value). As they are summed, the re-

sulting waveform tends to reach its maximum span

at top and bottom of the picture. The voltage at the

EWOut

is top and bottom limited (see parameter

VEW). According to Figure 15, especially the bottom

limitation seems to be critical for maximum hori-

zontal size (minimum DC). Actually it is not critical

since the parabola component must always be ap-

plied to obtain a picture without pin cushion distor-

tion. As all the components of the resulting correc-

tion waveform are generated from an internal line-

ar vertical sawtooth waveform bearing VSIZE and

VPOS adjustments, they all track with vertical am-

plitude and position, thus being fixed vertically on

the screen. They are not affected by C- and S-cor-

rections, by prescale adjustments (VSAG and

VPOF), by vertical breathing compensation and by

vertical moire cancellation. The sum of compo-

nents other than DC is conditionally affected by

value in HSIZE I²C-bus control in reversed sense.

Refer to electrical specifications for value. This

tracking with HSIZE can be switched off by

EWTrHSize I²C-bus bit. The DC value, adjusted

via HSIZE control, is also affected by voltage on

HEHTIn

input, thus providing a horizontal breathing

compensation. The effect of this compensation is

controlled by HEHTG. The resulting waveform is

conditionally multiplied with voltage on HPLL1F,

which depends on frequency. Refer to electrical

specifications for values. This tracking with fre-

quency provides a rough compensation of varia-

tion of picture geometry with frequency and allows

to fix the adjustment ranges of I²C-bus controls

throughout the operating range of horizontal fre-

quencies. It can be switched off by EWTrHFr I²C-

bus bit (off by default). The functionality is ex-

plained in Figure 13. The upper part gives the influ-

ence on DC component, the lower part on AC

component, showing also the tracking with HSIZE.

Grey zones give the total span of breathing correc-

tion using the whole range of input operating volt-

age on HEHTIn input and whole range of adjust-

ment of HEHTG register.

The EW waveform signal is buffered by an NPN

emitter follower, the emitter of which is directly

routed to EWOut output. It is internally biased (see

electrical specifications for current value).

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