9
Circuit description new circuits
35
L7.3A
9.6.3
Video controller (IC7225-5C, see diagram A4)
The main functions are :
–
Y/(B-Y)/(R-Y) signal processing/matrix
–
RGB signal selection/processing/output stages
–
Black stretcher
–
Beam current limiter (BCL,PWL)
–
Automatic black current stabilization (ABS) .
Y/(B-Y)/(R-Y) signal processing/matrix
The luminance at pin 27 is internally AC coupled to the
luminance clamp, which is operational during burst key period.
The luminance signal is supplied to the RGB matrix circuit via
the black stretcher. The (B-Y)/(R-Y) signals supplied to pins 31
and 32 are clamped during the burst key period. The signals
are then supplied to the colour difference matrix to select the
desired matrix formv either PAL, NTSC or Japanese NTSC
matrix. The selected matrix is controlled via the 12C bus. For
RGB matrix the signal outputs from the colour difference
matrix are added with the luminance signal in colour to
generate the internal RGB signals.
RGB signal selection/processing/output stages
Within the RGB signal selection there are three modes:
–
RGB (internal)
–
Fast RGB insertion (R_TXT G_TXT B_TXT on pins 23, 24
and 25)
–
OSD-mode
Teletext
Two different types of microprocessors IC7600/7601 are used:
one with teletext(TXT) and one without teletext.
The micro-controllers are:
The oscillator to be used is 12MHz; a crystal oscillator will be
used for all the micro-controllers.
•
In case of TXT this function is integrated together with the
control part in one and the same uC. This uC is indicated
in the diagrams with the outer pin numbering.
•
In case of no TXT another (C is used with less pins. This
uC is indicated in the diagrams with the internal pin
numbering.
In the description below, the pin numbers are the numbers
mentioned outside the housing of IC 7600 (IC7600 with TXT
and IC7601 without TXT). In case of the (C with integrated
teletext function the CVBS-TXT signal is fed to pin 23, the TXT
and OSD- information are combined at pins 32,33 and 34.
Before the RGB signal selection the RGB signals are clamped
to similar DC level during burst key period. The selection is
controlled by the RGB insertion input voltage at pin 26.
The contrast, brightness and white-point RGB adjustment on
the selected RGB signals are controlled by 12C commands.
After the adjustment, three RGB black levels are added to the
RGB signals. These three signals are blanked when the RGB
blank signal is active. The RGB output stages supply the
buffered RGB signals pins 21, 20 and 19.
Black Stretcher
The black stretcher circuit, which is only operational during line
scan, extends the gray signal level towards the actual black
level (= actual black level measured during burstkey). The
amount of extension is dependent on the difference between
actual black level and the darkest part of the incoming video
signal. The darkest part of the videosignal is registered in the
capacitor at pin 39 by means of an internal discharge current
of approximately 130uA. The black stretcher is made inactive if
the voltage at pin 39 is set to ground. This is done by turning on
transistor 7220, the command blackstraw coming from the
microprocessor (if power button on the remote control unit is
activated).
Beam Current Limiter
The beam current limiter circuit functions as an average white
limiter as well as peak white limiter. The average white limiter
needs external circuitry comprising of 7265, and associated
components to function; the peak white limiter is an internal
detection circuit. The function of the beam current limiter
(average white / peak white limiter) reduces the contrast and
brightness of RGB signals. For beam current limiting, the
difference in beam current (1BCL) and an internal charge
current (1CHARGE) is stored in capacitor 2460 on pin 22. If
1BCL > 1CHARGE voltage (VBCL) at pin 22 will decrease.
For beam current limiter contrast reduction begins if VBCL <
3.5V and brightness reduction begins if VBCL < 2.5V. VBCL is
normally about 4V if the beam current limiter is not active .
The contrast and/or brightness reduction of the RGBout is
proportional to the voltage decrease at pin 22.
9.6.4
Horizontal synchronization IC 7225-5D and the line output
stage (see diagram A3)
Protections:
The line deflection incorporates protections like:
P83Cx66
42 pins
Normal
ROM size
varies
SAA529x
52 pins
Control
withTeletext
ROM size
varies
P83CX69
52 pins
Control with
Close caption
ROM size
varies
Type of
micro-
controller
Type of tuning
System
Countries
P83Cx66
VST
PAL/SECAM
AP
PLL
NTSC
Latam
SAA529x
VST
PAL/SECAM
AP
P83Cx169
PLL
BiNorma/
TriNorma
Latam
Voltage at pin 26
selected RGB signals
< 0.3V
RGB(internal)
0.9V < V < 3V
(R_TXT G_TXT B_TXT (fast
insertion at pins 23, 24 and 25)
> 4V
OSD can be inserted at the
RGB-out pins
