Philips Semiconductors
Preliminary specification
YUV one chip picture improvement based on luminance
vector-, colour vector- and spectral processor
FUNCTIONAL DESCRIPTION
Y input selection and amplification
The gain of the luminance input amplifier and output
amplifier can be adjusted to signal amplitudes of
0.315 and 1.0 V typically (excluding sync) by I
2
C-bus
bit AMS. The sync part is processed transparently to the
output, independently of the feature settings.
The Y, U and V input signals are clamped during the
burstkey period, defined by the sandcastle reference and
should be DC-coupled (the circuit uses internal clamp
capacitors). During the clamp pulse (see Figs 7, 8, 9
and 10) an artificial black level is inserted in the Y input
signal to correctly preset the internal circuitry.
Luminance vector processor
In the luminance vector processor the transfer is controlled
by a black stretch, the histogram processing and a gamma
control circuit. The luminance vector processor also
creates the cue flash signal.
B
LACK STRETCH
A black detector measures and stores the level of the most
black part of the scene within an internal defined fixed
window in each field into a time constant. The time
constant and the response time of the loop are internally
fixed. Any difference between this value and the value
measured during the clamp is regarded as black offset.
In a closed loop offsets until a predefined value of the full-
scale value are fed back to the input stage for
compensation. The loop gain is a function of the histogram
and variable gamma settings. The black offset correction
can be switched on and off by the I
2
C-bus bit BON.
Related to the corrected black offset the nominal signal
amplitude is set again to 100% full scale through an
amplitude stretch function. Luminance values beyond full
scale are unaffected. Additionally, the measured black
offset is also used to set the adaptive black stretch gain
(see also Section “Adaptive black stretch”).
H
ISTOGRAM PROCESSING
For the luminance signal the histogram distribution is
measured in real-time over five segments within an
internally defined fixed window in each field. During the
period that the luminance is in one segment, a
corresponding internal capacitor is loaded by a current
source. At the end of the field five segment voltages are
stored into on-board memories. The voltages stored in the
memories determine the non-linear processing of the
luminance signal to achieve a picture with a maximum of
information (visible details).
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TDA9178
Each field the capacitors are discharged and the
measurement starts all over again.
Parts in the scene that do not contribute to the information
in that scene, like sub or side titles, should be omitted from
the histogram measurement. No measurements are
performed outside the internal fixed window period.
Very rapid picture changes, also related to the field
interlace, can result in flicker effects. The histogram values
are averaged at the field rate thus cancelling the flicker
effects.
Adaptive black stretch
The so-called adaptive black stretch gain is one of the
factors that control the gamma of the picture. This gain is
controlled by the measured black offset value in the black
stretch circuit and the I
2
C-bus adaptive black stretch DAC:
bits BT5 to BT0. For pictures with no black offset the black
stretch gain equals unity so the gamma is not changed and
the DAC setting has no influence. In case of a black offset,
the black stretch gain is increased so the gamma of the
picture is reduced. This procedure results in a maximum of
visible details over the whole range of luminances.
However, depending on personal taste, sometimes higher
values of gamma are preferred. Therefore the amount of
gamma reduction can be adjusted by the DAC.
Adaptive white-point stretching
For pictures with many details in white parts, the histogram
conversion procedure makes a transfer with large gain in
the white parts. The amount of light coming out of the
scene is reduced accordingly. The white stretcher
introduces additional overall gain for increased light
production, and so violating the principle of having a
full-scale reference. The white-point stretching can be
switched on or off by means of the I
2
C-bus bit WPO.
Standard deviation
For scenes in which segments of the histogram distribution
are very dominant with respect to the others, the non-linear
amplification should be reduced in comparison to scenes
with a flat histogram distribution. The standard deviation
detector measures the spread of the histogram distribution
and modulates the user setting of the non-linear amplifier.
Non-linear amplifier
The stored segment voltages determine the individual gain
of each segment in such a way that continuity is granted
for the complete luminance range.
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