PV152
2-27
PRO1200 CIRCUIT DESCRIPTION
CIRCUIT DESCRIPTION
Pin 10 accepts information concerning the color system
to the sync processor. The color decoder IC 11 delivers
this information. This pin is at a high level in NTSC3.58
and low level for PAL and NTSC4.43.
This switching voltage activates either the oscillator
around XT11 (4 x 3.58Mhz = 14.318 Mhz) in NTSC3.58
or the oscillator around XT10 (4 x 4.43Mhz =
17.734Mhz).
The video signal from input 26 is amplified by 6db and
via the pin 30 output capacitively coupled to the AD
input pin 25 of the comb filter chip IC16.
The VID/LUMA undergoes a frequency limiting to 6Mhz
with FI12 driven by Q49. This frequency limiting will
avoid saturation later in the AD and DA convertors of
the Comb Filter IC16.
Comb Filter CXD2024AQ.
The above IC can be supplied either with digital or ana-
log signals. In our application the analog inputs and
outputs are used. As pin 10 INSL is here at ground level
for Video PAL, N3 and N4, we use (activate) the analog
inputs and outputs. For the other modes (see table ‘Fil-
ter switching’) pin 10 is at high level, setting the out-
puts at a DC level.
Processing for filtering the chrominance out of the lu-
minance is done with digital circuits. The analog video
is converted to digital using the sampling clock gener-
ated in the sync processor.
Note that the signal passing through this IC undergoes
a delay of approximately 1 and 2 lines in NTSC and in
PAL. This can cause synchronization and clamping prob-
lems if not taken care of. (see Input switching - syn-
chronization).
The A
Y
O (luminance) output at pin 31 passes through a
low pass filter FI11 and is then buffered with Q41 to
feed the 4-channel Analog Multi-/Demultiplexer IC18,
pin 1 Y_Comb. This filter prevents the clock frequencies
of the DA converter from passing through.
The A
C
O (Chrominance) output undergoes a frequency
limitation with the filter FI10 before supplying the pin
12 Chr_Comb of the 4-channel Analog Multi-/
Demultiplexer IC18.
The voltage at pin 21 determines the DC clamping level
of the ADC.
The voltage at pin 41 (CVRF) is the gain of the chromi-
nance output and pin 33 (YVRF) of the luminance out-
put.
Pin 70 tells the IC whether the color system is PAL or
NTSC3.58; This determines the number of line delays for
the adaptive digital filtering.
Synchronization.
The first selection of the sync signal is done in the source
input selection. This produces the SYNC-EX which is avail-
able at pin 7 of the J313 connector.
a) Video / S-Video
(Page 6-37 Sect. G-8)
When a video composite or S-Video source is selected
the +VID voltage is high
and the transistor Q42 gets forward biased just like Q43.
Pin 7 of connector J314 receives the signal via these
transistors. This signal has undergone some delays de-
pending on the standard.
To ensure that the SYNC-EX does not get through, Q36 is
saturated to short the SYNC-EX line to ground. The col-
lector DC voltage of Q43 is blocked with D24.
b) RGB .
The selection of an RGB source simply means that the
SYNC-EX signal is passed through D24 and feeds the
Input+Selection board.
Decoder - Sharpness.
This presentation is limited to the differences with the
former version.
a) Comb filtering in NTSC3.58
(Page 6-36 Sect. D&E-6)
As for NTSC3.58 comb filtering is done by the digital
comb filter, the dual baseband CCD delay IC must be by-
passed. This happens with Q13 - Q15 and Q12- Q14 when
NT3 is at a high level.
b) Sharpness control.
In this version, sharpness is done with discrete compo-
nents. A DAC TDA8444 provides the adjustable voltage:
SHARPNESS
.
The sharpness control is based on combining two sig-
nals, one with a ‘max negative’ sharpness together with
an amplitude adjusted sharpness signal.
The signal is delayed twice by two delay lines of 180ns.
A combination of signals produce the max. neg. sharp-
ness signal on the junction R62, R65 and R68 and the
sharpness correction signal on the junction R52, R55
and R61. The latter signal is amplitude adjusted in IC12
by the sharpness voltage. The two formed signals are
combined on junction R73/R78 and this signal is fur-
ther used as Luma signal.
