Daewoo DTL- 25G6F Service Manual download pdf (Page 39)

AM Demodulator

The alignment-free AM demodulator is realized by a synchronous detector. The modulated SIF signal from the SIF

amplifier output is multiplied in phase with the limited SIF signal(AM is removed). The AF signal of the demodulator

output is fed to the output amplifier and to the SIF-AGC. For all TV standards with negative video modulation(e.g., B/G

standard), the AF output signal (Pin 24) is switched off by the standard switch.

Quasi-Parallel-Sound(QPS) Mixer

The QPS mixerj is realized 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.

Standard Switch

To have equal polarity of the video output signal the polarity can be switched in the demodulation stage in

accordancewith 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 stamdard) 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.

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 possile via a potentiometer. The L’ switch is only active for positive

modulated video IF-signals(standard switch in L mode).

AFC Switch

The AFC output signal at Pin 22 can be controlled by a switching voltage at Pin 19. It is possible to select an AFC output

signal with a rising-or falling AFC curve and to switch off the AFC.

Source select switch

TDA8844 input switch can select one of the following sources ;

pin 13 front-end : CVBS l int

pin17 : CVBS 2 ext

pin 11, pin10 : Y (S-VHS), C (S-VHS)

Selected signal is available at the CVBS output pin 38, in case of Y/C input Y+C are added.

It drive teletext.

For S-VHS applications, the Y,C input can be selected, independent of the CVBS source switch.

Y,C inputs are selected, while the source switch outputs CVBS l int or CVBS 2 ext on CVBS out.

Horizontal synchronization and protection

The synchronization separator adapts its slicing level in the middle between top-sync and black level of the CVBS signal.

The separated synchronization pulses are fed to the first phase detector and to the coincidence detector.

The ¥Õ-1 loop gain is determined by the components at pin 43 (C+RC).

The coincidence detector detects whether the horizontal line oscillator is synchronized to the incoming video.

The line oscillator is a VCO-type, running at twice the line frequency.

It is calibrated with the X-tal oscillator frequency of the colour decoder and has a maximum deviation of 2% of the

nominal frequency, so no alignment is-needed.

The second phase detector ¥Õ-2 locks the phase of the horizontal driver pulses at output pin 40 to the horizontal

flyback pulse at input pin 41 .

This compensates for the storage time of the horizontal deflection transistor.

The ¥Õ- 2 loop filter (C) is externally connected to pin 42.

The horizontal phase can be given a static off set via I2C-but (HSH “horizontal shift”)

A dynamic correction is possible by current feedback into the ¥Õ- 2 loop filter capacitor.

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Daewoo DTL- 25G6F Service Manual Page 39

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