Philips Semiconductors
Preliminary specification
YUV one chip picture improvement based on luminance
vector-, colour vector- and spectral processor
Smart sharpness controller
The smart sharpness controller (see Fig.16) is a fader
circuit that fades between peaked luminance and
step-improved luminance, controlled by the output of a
step discriminating device known as the step detector.
It also contains a variable coring level stage.
The step detector is basically a differentiator, so both
amplitude of the step and its slope add to the detection
criterion. The smart sharpness controller has four user
controls:
•
Steepness control, performed by the I
2
C-bus DAC:
bits SP5 to SP0
•
Peaking control, performed by the
bits PK5 to PK0
I
2
C-bus
DAC:
TDA9178
The smart peaking algorithm has been designed such that
the luminance output amplitude will never exceed 110% of
the luminance input signal amplitude. Therefore the
normal peaking range (12 dB) will be reduced at large
transients, and in case of colour dependent sharpness
there is even more reduction.
However, by setting bit OSP (Overrule Smart Peaking)
one can undo the extra peaking reduction in case of colour
dependant sharpness. It must be emphasized that setting
OSP may lead to unwanted large luminance output
signals, for instance in details in red coloured objects.
C
OLOUR TRANSIENT IMPROVEMENT
The colour transient improvement circuit (see Fig.17)
increases the slope of the colour transients of vertical
objects. Each channel of the CTI circuit basically consists
of two delay cells: an electronic potentiometer and an edge
detector circuit that controls the wiper position of the
potentiometer. Normally the wiper of the potentiometer will
be in position B (mid position), so passing the input
signal B to the output with a single delay. The control
signal is obtained by the signals A and C.
When an edge occurs the value of the control signal will
fade between +1 and
−1
and finally will become zero
again. A control signal value of +1 fades the wiper in
position C, passing the two times delayed input signal to
the output. A control signal of
−1
fades the wiper in
position A, so an undelayed input signal is passed to the
output. The result is an output signal which has steeper
edges than the input signal. Contrary to other existing
CTI algorithms, the transients remain time correct with
respect to the luminance signal, as the algorithm steepens
edges proportionally, without discontinuity.
S
CAVEM
A luminance output is available for SCAVEM processing.
This luminance signal is not affected by the spectral
processing functions.
Colour vector processor
The colour processing part contains skin tone correction,
green enhancement and blue stretch. The colour vector
processing is dependent on the amplitude and sign of the
colour difference signals. Therefore, both the polarity and
the nominal amplitude of the colour difference signals are
relevant when using the colour vector processor facility.
•
Video dependent coring, switched on or switched off by
the I
2
C-bus bit VDC
•
Coring level control, performed by the I
2
C-bus DAC:
bits CR5 to CR0.
The steepness setting controls the amount of steepness in
the edge-correction processing path.
The peaking setting controls the amount of contour
correction for proper detail enhancement. The envelope
signal generated by the step improvement processor
modulates the peaking setting in order to reduce the
amount of peaking for large sine wave excursions.
With video dependent coring, it is possible to have more
reduction of the peaking in the black parts of a scene than
in the white parts, and therefore automatically reducing the
visibility of the background noise.
The coring setting controls the coring level in the peaking
path for rejection of high-frequency noise.
All four settings facilitate reduction of the impact of the
sharpness features, e.g. for noisy luminance signals.
C
OLOUR DEPENDENT SHARPNESS
The colour dependent sharpness circuit increases the
luminance sharpness in saturated red and magenta parts
of the screen. Because of the limited bandwidth of the
colour signals, there is no need to increase the high
frequencies of the colour signals. Instead, the details in the
luminance signal will be enhanced. In this circuit a limited
number of colours are enhanced (red and magenta).
Contrary to normal peaking algorithm, extra gain is applied
for low frequencies (2 MHz at 1f
H
). This is needed,
because the information that is lacking below 2 MHz (at
1f
H
) is most important. In large coloured parts the normal
peaking is still active to enhance the fine details.
1999 Sep 24
8
PHILIPS [ NXP SEMICONDUCTORS ]
