frequency as a reference. Under bad-signal conditions (e.g. VCR-playback in feature mode), it may
occur that the colour killer is activated although the colour PLL is still in lock. When this killing action
is not wanted it is possible to overrule the colour killer by forcing the colour decoder to the required
standard and to activate the FCO-bit (Forced Colour On) in subaddress 21H. The IC contains an
Automatic Colour Limiting (ACL) circuit which is switchable via the I2C-bus and which prevents
that oversaturation occurs when signals with a high chroma-to-burst ratio are received. The ACL circuit
is designed such that it only reduces the chroma signal and not the burst signal. This has the advantage
that the colour sensitivity is not affected by this function. The SECAM decoder contains an auto-
calibrating PLL demodulator which has two references, viz: the divided 12 MHz reference frequency
of the crystal oscillator which is used to tune the PLL to the desired free-running frequency and the
bandgap reference to obtain the correct absolute value of the output signal. The VCO of the PLL is
calibrated during each vertical blanking period, when the IC is in search or SECAM mode. The
reference frequency of the colour decoder is fed to the Fsc output (pin 49) and is used to tune an
external comb filter. The base-band delay line is integrated. The demodulated colour difference signals
are internally supplied to the delay line. The colour difference matrix switches automatically between
PAL/SECAM and NTSC, however, it is also possible to fix the matrix in the PAL standard.
2.7. PICTURE IMPROVEMENT FEATURES
In the TDA 8885 series various picture improvement features have been integrated. These features are:
-Video dependent coring in the peaking circuit. The coring can be activated only in the low-light parts
of the screen. This effectively reduces noise while having maximum peaking in the bright parts of the
picture.
-Colour Transient Improvement (CTI). This circuit improves the rise and fall times of the colour
difference signals. The function is realised by using delay cells with a length of 300 ns.
-Black-stretch. This circuit corrects the black level for incoming video signals, which have a deviation
between the black level and the blanking, level (back porch). The time constant for the black stretcher
is realised internally
-Blue-stretch. This circuit is intended to shift colour near ?white? with sufficient contrast values
towards more blue to obtain a brighter impression of the picture
-White-stretch. This function adapts the transfer characteristic of the luminance amplifier in a non-
linear way dependent on the picture content. The system operates such that maximum stretching is
obtained when signals with a low video level are received. For bright pictures the stretching is not
active.
-Dynamic skin tone (flesh) control. This function is realised in the YUV domain by detecting the
colourss near to the skin tone. The correction angle can be controlled via the I2 C-bus.
2.8. RGB output circuit and black-current stabilisation
This IC has a very flexible control circuit for RGB input signals, which has the following features:
? Including 3 RGB input, one is dedicated for OSD signals one for scart1,the other one is for RGB
applications.
? The RGB-1 input which is intended for OSD signals and which can be controlled on contrast and
brightness. By means of the IE1 bit the insertion blanking can be switched on or off. Via the IN1 bit it
can be read whether the insertion pin has a high level or not
? To obtain an accurate biasing of the picture tube the ?Continuous Cathode Calibration? system has
2
been included in this IC. Via the I C-bus a black level offset can be made with respect to the level
which is generated by the black current stabilisation system. In this way different colour temperatures
can be obtained for the bright and the dark part of the picture.
The black current stabilisation system checks the output level of the 3 channels and indicates whether
the black level of the highest output is in a certain window (WBC-bit) or below or above this window
2
(HBC-bit). This indication can be read from the I C-bus and can be used for automatic adjustment of
the Vg2 voltage during the production of the TV receiver.
The control circuit contains a beam current limiting circuit and a peak white limiting circuit. The peak
2
white level is adjustable via the I C-bus. To prevent that the peak white limiting circuit reacts on the
high frequency content of the video signal a low-pass filter is inserted in front of the peak detector. The
circuit also contains a soft-clipper, which prevents that the high frequency peaks in the output signal
become too high. The difference between the peak white limiting level and the soft clipping level is
2
adjustable via the I C-bus in a few steps.
9 |
ACEC
