2
The switch control signal is supplied by microcontroller. The limiter and SIF PLL output pin # 15 which is connected to
the deemphasis capacitor drives audio out of the scart connector and AV jack. The external inputs of IC, which are pin
# 12 for video and pin # 16 for audio, are connected to the related scart connector inputs or AV RCA jack inputs.
The internal-external signal selection for monitoring is made by control voltage at pin # 8 produced by STV 2118.
The selected video at pin # 11 drives video processing stage. On the other hand, the selected audio is processed in
volume control stage in according with DC control voltage at pin # 13 and then, it is sent from pin # 14 to sound
output stage.
The IF block produces control signals, which are AGC for tuner at pin # 23 in according to voltage of pin
# 24 controlled by microcontroller pin # 2.
2.2 Multi-Standard Video and Sound IF System & Switch IC STV 8224 and AM SIF IC STV 8225 :
STV 8224 is a picture and sound IF processor for negative and positive applications. The architecture of the
video and sound demodulator, which are both based on PLL structure, allows a very good linearity of the Intermediate
Frequency Response and an application with very few external components and adjustments.
The volume on the audio signal is controlled through a DC level (pin #13) which is provided from
STV 2112/2116/2118 where all the I
2
C decoding is realized.
STV 8224 also provides the audio and video switches with STV 8225 for one scart applications.
The IF inputs at pins # 19 and # 20 are driven by Tuner via SAW filter (Z101) which is K6256 for B/G - L/L
applications. This saw filter is switchable according to the control signals produced by microcontroller.
The baseband signal at pin # 3 is produced from these IF inputs according to the control signal at pin # 4.
This control signal comes from STV 2118 Multi-Switch Output according to the selected system standard.
The baseband signal is used to produce sound trapped video signal and on the other hand to produce intercarrier
sound signal for B/G standard. This internal video signal is fed to pin # 10 of this IC for internal-external
(coming from scart) video switching and to the base of the buffer transistor Q102 for the video output of the scart
connector. For the internal and external video switching, external video enters from pin #12, and then, selected video
is sent from pin # 11 to video processing part.
The intercarrier signal used for reproducing sound, passes through the band pass filter Z103 with center frequency
5.5MHz for B/G standard and resultant signal is applied to the pin # 8 of the IC STV 8224 which is the input of limiter
and SIF PLL stage. The limiter and SIF PLL output pin # 15 which is connected to the deemphasis capacitor drives
pin # 5 of the IC STV 8225 as FM Sound Input in order to be able to select the appropriate (FM or AM) audio signal
for producing audio out of the scart connector. The selected signal at pin # 9 of STV8225 drives external audio
output connections. On the other hand, the audio switch in STV 8224 is used to select the internal FM signal or the output
of STV 8225 (pin # 7), which selects the internal AM signal or External audio signal (at pin # 11) coming from scart socket.
The selected audio is processed in volume control stage in according with DC control voltage at pin # 13 and then, it is sent
from pin # 14 to sound output stage. The IF signal is filtered at Z102 saw filter which is switchable according to the
control signals produced by microcontroller for L or L mode. Output of SAW is connected to pin # 1 and 14 of SIF IC
STV 8225(IC 102) for AM sound producing. The audio switches in STV 8225 selects the signals according to DC voltage
at # 10 of STV 8225 .
The IF block produces control signals, which are AGC for tuner at pin # 23 in according to voltage of pin # 24
controlled by microcontroller and AFC for microcontroller at pin # 2.
There are two main switching circuits. First one is the VCO Frequency Switching circuit and the second one is
the Saw Filter Switching circuit which covers both Video and Sound saw filters.
The control voltage for VCO switching circuitry is the L/L signal produced by microcontroller. The logic of this signal,
which is defined by MAC-15, is as given in Table 2.1.
Table 2.1 The Logic of the L/L Switching Voltage (based on MAC-15)
Selected Mode L/L
Switching Voltage
Adjusted Frequency
Modes other than L
HIGH
38.9 MHz
L
LOW
33.9 MHz
This control voltage is applied to R121. When this voltage is LOW, diode D105 is ON and VC101, C129 and
C130 are connected in parallel to the reference tank circuit VL101. On the other hand, This circuit is disconnected
when L/L voltage is HIGH since D105 is OFF.
In according to selected system configuration, it must be used multistandard switched type SAW filters.
For this reason an optional circuitry is applied this stage by D101, D102, (and if using L/L, D103 ,D104 for SIF SAW Z102).
3. SIGNAL PROCESSING PART :
This part processes video coming from IF part, RGB coming from Scart, Teletext and Controller in order to
drive Horizontal and Vertical deflection stages and CRT baseboard and to produce some control voltages by means of
Bus-controlled PAL-SECAM-NTSC TV Processor IC, STV 2112/2116/2118 and 64msec Delay Line IC, STV2180.
The detailed functional descriptions of these ICs are given in the following subsections.
