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FUNCTIONAL DESCRIPTION
For SECAM L and L' the TDA9361/TDA9381 is switched to positive modulation via I2C bus. SECAM L' transmission only
occur in VHF band I and have their picture and sound carrier interchanged, compared to SECAM L and PAL B/G
channels. For SECAM'L the picture carrier is situated at 33.9 MHz and the AM sound carrier at 40.40 MHz. The IF PLL
reference is tuned from 38.9 to 33.9 MHz, this is done via I2C Bus and the SIF filter is switched from channel 2 to channel
1;this is done by pin 4 of TDA 9361.
The tuner AGC time constant is slower for positive than for negative modulation, because the TDA9361 reduces its AGC
current. To make the AGC time constant even slower an extra series resistor R103 is added. To prevent IF overload
when jumping from a very strong transmitter to a weak transmitter a diode D101 has been added
The SAW filter ( SF1 ) has a double Nyquist slope at 38.9 MHz and 33.9 MHz needed for this multistandard
application. The disadvantage of this choice is that a 5.5 MHz trap filter ( Z501 ) is needed to suppress the residual
sound carrier in the video for B/G signals.
3. Source switching
Video :
The TDA9361/TDA9381 has only one external video input. The Rear RCA video in pin (#20) is connected to the front
RCA video input.
The controlling software via I2C bus selects the signal source :
- Video signal from tuner ( Pin 40 ).
- External video.
The sound source switching is done in the video processor part and in the AM demodulator by the -Controller via I2C
bus.
The video processor pin 28 has multiple functions and provides in this applicationbus.
- Deemphasis time constant
- Audio monitor output
- External AM input
- Deemphasis time constant : The time constant is given by the capacitor C504,needer to obtain the 54 s time time
constant for standarad PAL signal.
- Audio monitor output : the nominal output signal is 500 mVrms, for all standards. The signal is also internally
connected through to the audio switch. This signal is not controlled by the volume setting and can be used for
SCART audio output.
The signal is buffered to avoid influencing the deemphasis time constant and to adjust the output level.
- External AM input : By software the deemphasis pin can be converted into an input pin. External AM signal for
SECAM L/L' is directly connected to this pin. In this configuration the FM sound is internally muted, DC level
remains at 3Vdc.
An external sound signal of 500mVrms is applied to pin 35 via a coupling capacitor. The input impedance of this pin is
25k‰ typical. Switching between internal FM, external AM or external audio from SCART is controlled internally by
software.
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APPENDIX
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