Philips DVDR890 Service Manual page 49

EN 152 9. DVDR880-890 /0X1 Circuit-, IC Descriptions and List of Abbreviations
tion, an additional circuit for suppressing the audio carrier of
the adjacent channel is used. This circuitry is adjusted by coil
[5710] for maximum suppression at 40.4MHz.
IF demodulator
The signal from the tuner and IF-selection circuit is processed
by the demodulator IC TDA 9818/9817 [7710]. The signal
"PSS" to pin 3 switches between demodulation of positive
(SECAM only) or negative modulated video carriers. A QSS-
audio-IF signal SIF1 is generated for demodulation in the
sound processor [7600]. The audio-IF carrier is selected in the
audio SAW filter [1702]. This filter is switched for SECAM L'. If
the signal "SB1" is "high", the switch [7714] is closed and the
diode [6705] is not conducting. For all other standards the
diode [6705] is conducting and the switch [7714] is open. The
output signal of this SAW filter is firstly processed in the TDA
9818. Audio carriers are converted from the tuner IF level to
the audio IF position and further processed in the audio
demodulator [7600]. The AFC coil [5711] on the TDA 9818/
9817 is adjusted so that when a frequency of 38.90 MHz is
supplied to the IF output of the tuner, the AFC voltage on pin
17 of [7710] is 2.5V. The setting of the picture carrier
frequency for SECAM L in the TDA 9818 is achieved by
connecting pin 7 of the IC via a resistor [3710] to ground. The
switch [7701] and the signal "SB1" do this. The HF-AGC is set
using the potentiometer [3724] so that, with a sufficiently large
antenna input signal (74 dB ), the voltage at the IF output of
µ V
the tuner [1705] pin 11 is 500 mV . This setting must be
pp
carried out when the audio carrier is switched off. The
demodulated video signal appears on pin 16 of [7710]. The
AGC voltage at pin 4 is used to determine the antenna signal
strength after a buffer [7717] with the signal "AGC" and an
analog input port of the CC-µP. The trap [1704] reduces the
sound carrier remainders in the video for BG standards. The
trap [1706] works in the same way for the Pal-I standard only.
For all other standards the switch [7713] is closed via [7706]
and "SFS_TS"-line set "high" to bypass this trap. In these
cases the selectivity of the SAW filter [1701] is sufficient. The
coil [5713] for non-BG standards realizes a frequency
response correction. This correction is not desired for SECAM
L' and therefore short-circuited by [7716] (signal SB1 is "high"
and [7702] has on-status). The demodulated video signal
"VFV" is available after the buffer and limiting stage for noise
peaks [7711]. The FM-PLL demodulator function of TDA 9818
is not necessary and therefore deactivated by the resistor
[3739].
Audio demodulator
The sound demodulation is done by the MSP3415 [7600],
which is also fully controlled via I²C-bus by the CC-µP (deter-
mination of bandwidth, amplitude, standard, ...).The audio sig-
nals are available at pin 30 and pin 31 of [7600] and fed as
"AFER"- & "AFEL"-line to the audio-I/O for further processing.