Video Demodulation And Amplifier; Sound If Amplifier And Sif-Agc; Quasi-Parallel-Sound (Qps) Mixer; Standard Switch - Daewoo DTF-2950-100D Service Manual

CP-850FX Service Manual

5.3.5 VIDEO DEMODULATION AND AMPLIFIER

The video IF signal, which is applied from the gain controlled IF amplifier, is multiplied with the
inphase component of the VCO signal. The video demodulator is designed for low distortion and
large bandwidth. The demodulator output signal passes an integrated low pass filter for
attenuation of the residual vision carrier and is fed to the video amplifier. The video amplifier is
realised by an operational amplifier with internal feedback and 8 MHz bandwidth (–3 dB). A
standard dependent dc level shift in this stage delivers the same sync. level for positive and
negative modulation. An additional noise clipping is provided. The video signal is fed to VIF-AGC
and to the video output buffer. This amplifier with a 6 dB gain offers easy adaptation of the sound
trap. For nominal video IF modulation the video output signal at Pin 12 is 2 Vpp.

5.3.6 SOUND IF AMPLIFIER AND SIF-AGC

The SIF amplifier is nearly identical with the 3-stage VIF amplifier. Only the first amplifier stage
exists twice and is switchable by a control voltage at Pin 3. Therefore with a minimal external
expense it is possible to switch between two different SAW filters. Both SIF inputs features
excellent cross-talk attenuation and an input impedance which is independent from the switching
condition. The SIF-AGC is related to the average level of AM- or FM-carrier and controls the SIF
amplifier to provide a constant SIF signal to the AM demodulator and QPS mixer.

5.3.7 QUASI-PARALLEL-SOUND (QPS) MIXER

The QPS mixer is realised by a multiplier. The SIF signal (FM or NICAM carrier) is converted to
the intercarrier frequency by the regenerated picture carrier (quadrature signal) which is provided
from the VCO. The intercarrier signal is fed via an output amplifier to Pin 24.

5.3.8 STANDARD SWITCH

To have equal polarity of the video output signal the polarity can be switched in the demodulation
stage in accordance with the TV standard. Additional a standard dependent dc level shift in the
video amplifier delivers the same sync. level. In parallel to this, the correct VIF-AGC is selected
for positive or negative modulated VIF signals. In the case of negative modulation (e.g., B/G
standard) the AM output signal is switched off. For positive modulation (L standard) the AM
demodulator and QPS mixer is active. This condition allows a parallel operation of the AM sound
signal and the NICAM-L stereo sound.

5.3.9 L' SWITCH

With a control voltage at Pin 14 the VCO frequency can be switched for setting to the required L'
value (L' standard). Also a fine adjustment of the L'-VCO center frequency is possible via a
potentiometer. The L' switch is only active for positive modulated video IF-signals (standard
switch in L mode).

5.3.10 INTERNAL VOLTAGE STABILISER

The internal bandgap reference ensures constant performance independent of supply voltage
and temperature.

5.4 VIDEO / RGB

5.4.1 FRONT END

5.4.1.1 CVBS Front-End

The CVBS front-end consists of the colour-decoding circuit itself, a sync processing circuit for
generation of H/V signals out of the CVBS signal, and the luminance processing. The main task
of the luminance processing is to remove the colour carrier by means of a notch filter. For PAL
Europe R&D 48
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