8
The additional controls for these types are:
•
EW
width
•
EW parabola width
•
EW upper and lower corner parabola correction
•
EW
trapezium
correction
•
Vertical zoom
2A.6: CHROMA AND LUMINANCE PROCESSING: -
The chroma band-pass and trap circuits
(including the SECAM cloche filter) are realised by means of gyrators and are tuned to the right
frequency by comparing the tuning frequency with the reference frequency of the colour
decoder. The luminance delay line and the delay cells for the peaking circuit are also realised
with gyrators. The circuit contains a black stretcher function which corrects the black level for
incoming signals which have a difference between the black level and the blanking level.
2A.7: COLOUR DECODER :-
The IC can decode PAL, NTSC and SECAM signals. The
PAL/NTSC decoder does not need external reference crystals but has an internal clock
generator which is stabilised to the required frequency by using the 12 MHz clock signal from
the reference oscillator of the micro controller/Teletext decoder. 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 Automatic Colour Limiting (ACL) circuit
(switchable via the ACL bit in subaddress 20H) prevents that over saturation 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
(obtained from the microcontroller) 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 base-band delay line (TDA 4665 function) is integrated. This delay line
