Daewoo DTK-28 Series, Руководство по обслуживанию, Страница: 49 / 71

CP-850/F Service Manual
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5.2.5 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. 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.
5.2.6 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.
5.2.7 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.
5.2.8 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.
5.2.10 INTERNAL VOLTAGE STABILISER
The internal bandgap reference ensures constant performance independent of supply voltage
and temperature.
5.3 VIDEO / RGB
5.3.1
FRONT END
5.3.1.1 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 and SECAM operation a baseband delay line is used for U and V
signals. This can be used as comb filter in NTSC operation (only for chrominance).