Daewoo DTZ-29U7/U3 Functional Description of Video Processor, Vision if amplifier, FM demodulator

CP-520/520A/520F Service Manual

5.2FUNCTIONAL DESCRIPTION OF VIDEO PROCESSOR

5.2.1 Vision IF amplifier

The vision IF amplifier can demodulate signals with positive and megative modulation. The PLL demodulator is completely alignment-free.

The VCO of the PLL circuit is internal and the grequency is fixed to the required value by using the clock ftequency of the TCG u-Controller as a reference. The setting of the various frequencies (e.g. 38, 38.9, 45.75 and 58.75MHz) can be made via the control bits IFA-IFC in subaddress 2FH. Because of the internal VCO the IF circuit has a high immunity to EMC interferences.

The output of the AFC detector can be read from output byte o4H and has a resolution of 7bit(25kHz per step). By means of this information a fast tuning algorithm can be designed.

The IC contains a group delay correction circuit which can be switched between the BG and a uncompensated group delay response characteristic. This hasthe advantage that in multi- standard receivers no compromise has to be made for the choice of the SAW filter. This group delay corection is realised for the demodulated CVBS output signal. The IC contains in addition a sound trap circuit with a switchable centre frequency.

5.2.2 QSS sound circuit

The sound IF amplifier is similar to the vision IF amplifier and has an external AGC decoupling capacitor.

ThesinglereferenceQSSmixerisrealisedbyamultiplier. In this multiplier the SIF signal is converted to the intercarrier frequency by mixing it with the regenerated picture carrier from the VCO. The mixer output signal is suppliedtotheoutputviaahigh-passfilterforattenuation of the residual video signals. With this system a high performance hi-fi stereo sound processing can be achieved.

TheAMsounddemodulatorisrealisedbyamultiplier.The modulated sound IF signal is multiplied in phase with the limited SIF signal. The demodulator output signal is supplied to the output via a low-pass filter for attenuation of the carrier harmonics.

SwitchingbetweentheQSSoutputandAMoutputismade by means of the AM bit in subaddress 33H.

5.2.3 FM demodulator

TheFMdemodulatorisrealisedasnarrow-bandPLL with internal loop filter, which provides the necessary selectivity without using an external band-pass filter. To obtain a good selectivity a linear phase detector and a constant input signal amplitude are required. For this reason the intercarrier signal is internally supplied to the demodulator via a gain controlled amplifier and AGC circuit. To improve the selectivity an internal bandpass filter is connected in front of the PLL circuit.

The nominal frequency of the demodulator is tuned to the required frequency (4.5/5.5/6.0/6.5 MHz) by means of a calibration circuit which uses the clock frequency of the TCG(1)-Controller as a reference. It is also possible to frequencies of 4.72 and 5.74Mhz so that a second sound channel can be demodulated. In the latter application an external bandpass filter has to be

DTV R&D Europe 35