Input Processing - Commodore 1901 Service Manual

PAL/RGBI MODULE
INPUT PROCESSING
This module enables the set to operate with either RGBI or PAL signal. Voltage supply for RGBI mode is by the
regulation network that comprises of TI04, DI09, RI30 and CI06, whereas for PAL it is tap directly from the 13V
rail. Selection of each mode is done by a switch on the front panel with switch at RGBI position, pin 1 of module is
set to 2V by RV21, RV22, RV35 and RV36 on the main board. This enables the video processing 1C TDA 3506 to
accept RGB signals of 1 Vp-p.
Due to the input requirements of TDA 3506, the RGBI signals from the computer (pins 6, 8, 10 & 15 of module)
must be transformed into RGB signals of 1Vp-p. This is done by IC01 SN7407 and its surrounding components. The
intensity signal (pin 5 of module) is integrated into the 3 RGB signals and together with the loading resistors RV12,
RV13 and RV14 on the main board, RGB signals of 1 Vp-p are obtained for the video processing 1C TDA 3506. PV04
from the main board is wired to pin 13 of module, and together with transistor TI01, it controls the level of intensity
from zero to maximum.
An additional feature of the module enables brown colour wherever the computer produces red and green signals
only. This is done by 1C02 SN7420 (Dual 4 input NAND gates) and transistor TI02. The task is to reduce the green
output level by RI21 and DI06 when the correct combination appears at the input lines.
Picture synchronization is achieved by IL01 TEA 2017 on the main board, and it accepts only composite synchroniza¬
tion pulses. Hence, the separate horizontal and vertical pulses from the computer have to be combined before being
fed to IL01. This is done by transistor Ti03. In order to obtain a bigger range of starting position for the display,
the horizontal pulses is shaped and made variable in position by the monostable multivibrator IC03 SN74LS123. The
RC network controlling this is at pin 15 of IC03 and is wired directly to PV07 on the main board. Transistors TI05
and TI06 differentiate the horizontal pulse to allow only negative spilces into pin 3 of IC03, this is to avoid any false
triggering of the multivibrator.
Consider now the switch at PAL position, a voltage at BA01 drives transistor TI10 into saturation, as such pin 1 of
module is pulled to OV and the video processing 1C TDA 3506 is switched to composite mode. At the same time,
DM7, DI22 to DI25 are activated and the signals of synchronization and RGBI from the 'D' connector are inhibited.
The chroma signal is fed to pin 1 of the PAL decoder 1C, IC04 AN5620. Here, the signal is processed with the burst
gate pulse at pin 3 and the line retrace pulse at pin 12. The outcome is the colour difference signals (R-Y) and (B-Y) at
pins 14 and 15 of module. This is in turn supplied to the video processing 1C TDA3506 at pins 17 and 18 for matrixing
into RGB signals suitable to drive the video amplifier.
Synchronization signal for the PAL mode is wired to BV01 on the main board and fed directly to IL01 TEA2017.