Philips Semiconductors
Preliminary specification
YUV one chip picture improvement based on luminance
vector-, colour vector- and spectral processor
S
KIN TONE CORRECTION
Skin tones are very sensitive for transmission (hue) errors,
because we have an absolute feeling for skin tones.
To make a picture look free of hue error, the goal is to
make sure that skin tones are put at a correct colour.
The dynamic skin tone correction circuit achieves this goal
by instantaneously and locally changing the hue of those
colours which are located in the area in the UV plane that
matches skin tones (see Fig.4).
The correction is dependent on luminance, saturation and
distance to the preferred axis and can be done towards
two different angles. The preferred angle can be chosen by
bit ASK in the I
2
C-bus settings. The settings are
123° (ASK = 0) and 117° (ASK = 1). The enclosed
correction area can be increased to 140% with the I
2
C-bus
bit SSK (so-called: Size). The enclosed detection ‘angle’ of
the correcting area can be increased to 160% with the
I
2
C-bus bit WSK (so-called: Width). The skin tone
correction can be switched on or off with the I
2
C-bus
bit DSK.
G
REEN ENHANCEMENT
The green enhancement circuit (see Fig.5) is intended to
shift low saturated green colours towards more saturated
green colours. This shift is achieved by instantaneously
and locally changing those colours which are located in the
area in the UV plane that matches low saturated green.
The saturation shift is dependent on the luminance,
saturation and distance to the detection axis of 208°.
The direction of shift in the colour is fixed by hardware.
The amount of green enhancement can be increased to
160% by the I
2
C-bus bit GGR. The enclosed detection
‘angle’ of the correcting area can be increased to 160%
with the I
2
C-bus bit WGR (so-called: Width). The enclosed
correction area can be increased to 140% with the I
2
C-bus
bit SGR (so-called: Size). The green enhancement can be
switched on or switched off with the I
2
C-bus bit DGR.
B
LUE STRETCH
The blue stretch circuit (see Fig.6) is intended to shift
colours near white towards more blueish coloured white to
give a brighter impression. This shift is achieved by
instantaneously and locally changing those colours which
are located in the area in the UV plane that matches
colours near white. The shift is dependent on the
luminance and saturation. The direction of shift (towards
an angle of 330°) in the colour is fixed by hardware.
The amount of blue stretch can be increased to 160% by
the I
2
C-bus bit GBL.
TDA9178
The enclosed correction area can be increased to 140% by
the I
2
C-bus bit SBL (so-called: Size). The blue stretch can
be switched on or off by the I
2
C-bus bit DBL.
S
ATURATION CORRECTION
The non-linear luminance processing done by the
histogram modification and variable gamma, influences
the colour reproduction; mainly the colour saturation.
Therefore, the U and V signals are linear processed for
saturation compensation.
Noise measuring
A video line which is supposed to be free from video
information (‘empty line’) is used to measure the amount of
noise. The measured RMS value of the noise can be used
for reducing several features, by the I
2
C-bus interface,
such as luminance vector processing and spectral
processing. For the TDA9178 the empty line is chosen
three lines after recognition of the vertical blanking from
the sandcastle pulse input. Figures 7, 8, 9 and 10 show
the measurement locations for different broadcast norms.
The noise detector is capable of measuring the
signal-to-noise ratio between
−45
and
−20
dB. The output
scale runs linearly with dB. The noise samples are
averaged for over 20 fields to reduce the fluctuations in the
measurement process. It is obvious, that for signal
sources (like VCR in still picture mode) that re-insert the
levels of the retrace part, the measurement is not reliable
(see Section “Feature mode detector”). The result of the
averaging process will update the contents of the I
2
C-bus
register: bits ND5 to ND0 at a rate of
1
⁄
32
of the field
frequency. If a register access conflict occurs, the data of
the noise register is made invalid by setting the flag bit DV
(Data Valid) to zero.
Feature mode detector
A detector is available for detecting signal sources (like
VCR in still picture mode) that re-inserted the levels of the
retrace part. For this kind of signals the noise
measurement of the TDA9178 is not reliable, but this
detector sets bit FM in the ND-register to logic 1.
For normal video signals bit FM is set to logic 0.
This circuit measures transients (like synchronization
pulses) on the luminance input during the internal V-pulse.
The feature mode detector is setting bit FM to logic 1 when
no transients are present during 2 lines in the vertical
retrace part over 3 fields (like the synchronization pulses).
1999 Sep 24
9
PHILIPS [ NXP SEMICONDUCTORS ]
