TDA4780 Datasheet, PDF (7/44 Pages) NXP Semiconductors – RGB video processor with automatic cut-off control and gamma adjust

Philips Semiconductors
RGB video processor with automatic
cut-off control and gamma adjust
Preliminary specification
TDA4780
FUNCTIONAL DESCRIPTION
Signal input stages
The TDA4780 contains 3 sets of input signal stages for:
1. Luminance/colour-difference signals:
a) Y: 0.45 V (p-p) VBS or 1.43 V (p-p) VBS,
selectable via I²C-bus.
b) −(R − Y): 1.05 V (p-p).
c) −(B − Y): 1.33 V (p-p).
The capacitively coupled signals are matrixed to
RGB signals by either a PAL/SECAM or NTSC matrix
(selected via I²C-bus).
2. (RGB)₁ signals (0.7 V (p-p) VB), capacitively coupled
(e.g. from external source).
3. (RGB)₂ signals (0.7 V (p-p) VB), capacitively coupled
(e.g. videotext, OSD).
All input signals are clamped in order to have the same
black levels at the signal switch input. Displayed signals
must be synchronous with the sandcastle pulse.
Signal switches
Both fast signal switches can be operated by switching
pins (e.g. SCART facilities) or set via the I²C-bus. With the
pin FSW₁ the Y-CD signals or the (RGB)₁ signals can be
selected, with pin FSW₂ the above selected signals or the
(RGB)₂ signals are enabled. During the vertical and
horizontal blanking time an artificial black level equal to the
clamped black level is inserted in order to clip off the sync
pulse of the luminance signal and to suppress hum during
the cut-off measurement time and eliminate noise during
these intervals.
Saturation, contrast and brightness adjust
Saturation, contrast and brightness adjusts are controlled
via the I²C-bus and act on Y, CD as well as on RGB input
signals. Gamma acts on the luminance content of the input
signals.
Adaptive black (ADBL)
The adaptive black stage detects the lowest voltage of the
luminance component of the internal RGB signals during
the scanning time and shifts it to the nominal black level.
In order to keep the nominal white level the contrast is
increased simultaneously.
Blue stretch (BLST)
The blue stretch channel gets additional amplification if the
blue signal is greater than 80% of the nominal signal
amplitude. In the event the white point is shifted towards
higher colour temperature so that white parts of a picture
seem to be brighter.
Measurement pulse and blanking stage
During the vertical and horizontal blanking time and the
measurement period the signals are blanked to an ultra
black level, so the leakage current of the picture tube can
be measured and automatically compensated for.
During the cut-off measurement lines (one line period for
each R, G or B) the output signal levels are at cut-off
measurement level.
The vertical blanking period is timed by the sandcastle
pulse. The measurement pulses (leakage, R, G and B) are
triggered by the negative going edge of the vertical pulse
of the sandcastle pulse and start after the following
horizontal pulse.
The IC is prepared for 2f_H (32 kHz) application.
Output amplifier and white adjust potentiometer
The RGB signals are amplified to nominal 2 V (p-p), the
DC-levels are shifted according to cut-off control.
The nominal signal amplitude can be varied by ±50% by
the white point adjustment via the I²C-bus (individually for
RGB respect).
Gamma adjust
The gamma adjust stage has a non-linear transmission
characteristic according to the formula y = x^gamma, where x
represents the input and y the output signal. If gamma is
smaller than unity, the lower parts of the signal are
amplified with higher gain.
1997 Feb 06
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